W i r
2JD
WASHOVER TRANSPORT ON SANDBARS
Literature study
N.A.M. van den Wollenberg
970130
Delft University of Technology
Frederic R. Harris Inc.
November 1993
Washover Transport on Sandbars
TABLE OF CONTENTS
page
1.
INTRODUCTION
1
2.
ORIGIN OF BARRIER ISLANDS
2
2.1
2.2
General
B a r r i e r I s l a n d Development
2
2
2.2.1 G e n e r a l
2.2.2 F o u r m a i n t h e o r i e s
2
2
2.3
3.
THE
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
4.
5.
of b a r r i e r i s l a n d genesis
B a r r i e r Types
4
2.3.1
2.3.2
2.3.3
2.3.4
4
4
5
6
General
Bay B a r r i e r s
Barrier Spits
Barrier Islands
COASTAL SEDIMENTARY ELEMENTS OF BARRIER ISLAND SYSTEMS
9
General
M a i n l a n d Element
9
9
3.2.1 L i t h o l o g y
3.2.2 S l o p e
3.2.3 D r a i n a g e
9
10
B a c k b a r r i e r Lagoon
I n l e t and I n l e t D e l t a s
S a l t Marshes and T i d a l F l a t s
Washover Fans
Barrier Flats
Shoreface/Beach
10
11
12
12
13
13
BARRIER ISLAND EVOLUTION
15
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
15
15
16
17
18
19
19
19
General
Sea l e v e l R i s e / S u b s i d e n c e
Storm Events
I n l e t Dynamics
Overwash Processes
A e o l i a n Processes
Littoral Drift
R i v e r Sediment D i s c h a r g e
SEDIMENT TRANSPORT RATES AND BARRIER ISLAND MIGRATION;
A QUANTITATIVE AND A QUALITATIVE ANALYSIS
5.1
5.2
General
Q u a l i f i c a t i o n and Q u a n t i f i c a t i o n o f Sediment
Rates and E n v i r o n m e n t a l C o n d i t i o n s
5.2.1 Q u a l i t a t i v e A n a l y s i s
5.2.2 Q u a n t i t a t i v e A n a l y s i s
21
21
Transport
21
21
22
Washover Transport on Sandbars
page
5.3
Barrier Migration
5.3.1
5.3.2
5.3.3
5.3.4
5.3.5
5.3.6
6.
25
Introduction
Barrier Migration:
Barrier Migration
Barrier Migration
Barrier Migration
Barrier Migration
S p i t s and O f f s h o r e B a r r i e r I s l a n d s
& Sea L e v e l R i s e
& Tidal Inlets
a n d Washover E v e n t s
& Longshore Sediment S u p p l y
WAI SAN TING SANDBAR
6.1
6.2
28
Introduction
Morphological Description
28
28
o f Study Area
6.2.1 Y u n l i n C o a s t
6.2.2 Wai San T i n g S a n d b a r
6.3
6.4
28
29
M o r p h o l o g i c a l Development o f t h e Y u n l i n
Coast
29
6.3.1 R e c e n t D e v e l o p m e n t
6.3.2 C o a s t l i n e M o b i l i t y
6.3.3 Q u a l i t a t i v e H y p o t h e s i s
29
30
31
Evolution
33
6.4.1
6.4.2
6.4.3
6.4.4
6.4.5
6.4.6
6.4.7
6.4.8
6.4.9
7.
25
25
26
27
27
27
o f t h e Wai San T i n g S a n d b a r
S e d i m e n t T r a n s p o r t Mechanisms
Longshore T r a n s p o r t
Cross Shore T r a n s p o r t
Washover
T i d a l Channel T r a n s p o r t
Subsidence
Remaining Processes
Wai San T i n g Sand B a l a n c e M e a s u r e m e n t s
Summary - W a i - S a n - T i n g P r e s e n t a n d F u t u r e D e v e l o p m e n t
CONCLUSIONS
33
33
34
34
35
36
36
36
38
39
L I S T OF LITERATURE
41
APPENDIX 1 : B a r r i e r m i g r a t i o n
a n d sea l e v e l r i s e : A c a l c u l a t i o n method
APPENDIX 2: The e r o s i o n r a t e o f b a r r i e r i s l a n d s ;
J i m e n e z (1993)
45
Sanchez-Arcilla and
47
APPENDIX 3: E s t i m a t i o n o f e r o s i o n due t o o v e r w a s h ; S a n c h e z - A r c i l l a a n d
Jiménez (1993)
48
APPENDIX 4: E s t i m a t i o n
sandbar
50
APPENDIX 5: C a l c u l a t i o n
channel
APPENDIX 6: Y u n l i n
o f washover t r a n s p o r t
o f sediment t r a n s p o r t
Sand b a l a n c e
o f t h e W a i San T i n g
capacity
of a barrier
51
52
Washover Transport on Sandbars
L I S T OF
FIGURES
page
Figure
Figure
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2
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3
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35
Figure
Figure
36
37
Figure
38
Figure
39
Figure
40
Figure
Figure
Figure
41
42
43
Figure
44
: Hoyt's theory of b a r r i e r i s l a n d f o r m a t i o n
: B a r r i e r i s l a n d f o r m a t i o n by s p i t a c c r e t i o n and i n l e t
breaching
: Cape Cod b a r r i e r i s l a n d e v o l u t i o n
: D e v e l o p m e n t o f an i s l a n d b y u p b u i l d i n g s u b m a r i n e b a r
: Bayhead b a r r i e r s
: Baymouth b a r r i e r s
: Tombolo
: Simple s p i t
: Double s p i t
; Recurved s p i t
: Complex s p i t
: R e l a t i o n s h i p o f s a n d s u p p l y and t i m e t o b a r r i e r g r o w t h
and decay
: Microtidal transgressive barrier island
: Microtidal regressive barrier island
: Mesotidal transgressive b a r r i e r island
: Mesotidal regressive b a r r i e r island
: Cape
: Sea i s l a n d
: Chenier
: Schematic diagram o f b a r r i e r i s l a n d system: r e l a t i o n s h i p
between environmental elements
: B a c k b a r r i e r l a g o o n : v a r i a b l e w a t e r s u r f a c e a r e a due t o
flood/ebb
: B a c k b a r r i e r l a g o o n : w a t e r l e v e l s and t i d a l c u r r e n t s
: D e v e l o p m e n t o f f l o o d and ebb t i d a l d e l t a s
: A e r i a l v i e w o f t h e Ancao I n l e t f l o o d t i d a l d e l t a P o r t u g a l
: Fuzeta I n l e t a t low s p r i n g t i d e P o r t u g a l
: Ebb t i d a l d e l t a s o f t i d a l i n l e t s : r e l a t i v e f o r c e s o f t h e
o f f / o n and l o n g s h o r e c u r r e n t
: O v e r w a s h s u r g e s d u r i n g s t o r m c o n d i t i o n s and r e s u l t i n g
washover f a n
: O v e r w a s h b r e a c h i n h i g h d u n e l i n e o f Ancao p e n i n s u l a
(Portugal)
: I l l u s t r a t i o n of i n t e r n a l structure of e x t e n s i v e l y
o v e r w a s h e d s p i t A t k i n s o n P o i n t (U.S.)
: The p r i m a r i l y c o a s t a l l a n d f o r m t y p e s
: Schematic diagram o f a b a r r i e r i s l a n d : r e l a t i o n s h i p
among m a j o r s e d i m e n t a r y e n v i r o n m e n t s and l a n d f o r m s
: B e a c h t e r m i n o l o g y ( f r o m U.S.A CERC)
: P r i n c i p a l f a c t o r s a f f e c t i n g b a r r i e r beach m i g r a t i o n
: A b a r r i e r i s l a n d r e t r e a t s , remains i n place o r
moves t o w a r d s t h e sea
: E v o l u t i o n o f b a r r i e r i s l a n d s b y sea l e v e l r i s e a n d s p i t
extension
_
: B a r r i e r i s l a n d m i g r a t i o n and sea l e v e l r i s e
: G e n e r a l i z e d model f o r dune d e v e l o p m e n t r e l a t e d t o
storm/hurricane
: Example o f r a p i d c y c l e o f d e s t r u c t i o n ( h u r r i c a n e )
+ reconstruction
^
: D e v e l o p m e n t o f e x t e n s i v e s a l t m a r s h e s on a f l o o d t i d a l
delta
: Sequential diagrams of i n l e t breaching, m i g r a t i o n ,
c l o s i n g and d e v e l o p m e n t o f a l a r g e f l o o d t i d a l d e l t a
: O v e r w a s h m o d e l b a s e d on C o r e Banks (U.S., b y G o d f r e y )
: S p i t f o r m a t i o n a l o n g a c o a s t l i n e b y o b l i q u e wave a p p r o a c h
: Formation o f t h e i s l a n d o f S a l a k i n i n f r o n t o f t h e mouth
o f t h e Danube ( f r o m J . P e t r e s c u , 1957)
: B a t h y m e t r y o f t h e Y u n l i n Coast
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Washover Transport on Sandbars
page
F i g u r e 45
F i g u r e 46
F i g u r e 47
F i g u r e 48
F i g u r e 49
F i g u r e 50
F i g u r e 51
F i g u r e 52
F i g u r e 53
Figure
Figure
Figure
Figure
Figure
Figure
Figure
54
55
56
57
58
59
60
F i g u r e 61
F i g u r e 62
F i g u r e 63
: D e p o s i t i o n a l model f o r a t r a n s g r e s s i v e b a r r i e r s h o r e l i n e
development i n t h e M i s s i s s i p p i R i v e r d e l t a p l a i n
: H i s t o r i c a l map t i m e s e r i e s s h o w i n g c o a s t a l c h a n g e s a s s o c i a t e d w i t h Bayou L a f o u r c h e b a r r i e r s h o r e l i n e ( 1 8 8 7 - 1 9 7 8 )
: G e n e r a l i z e d f l o w d i a g r a m f o r s a n d movements f r o m b a r r i e r
s o u r c e s t o dune and o v e r w a s h d e p o s i t i o n a l l a n d f o r m s
: S c h e m a t i c o f f a c t o r s a f f e c t i n g s e d i m e n t b u d g e t s on
washover fans
_
: The c o n t i n u u m o v e r t o p p i n g / o v e r w a s h i n g modes b y w h i c h
g r a v e l b a r r i e r m i g r a t i o n may o c c u r
: S c h e m a t i c v i e w o f b a r r i e r c r e s t a l s t a b i l i t y d o m a i n s as a
f u n c t i o n o f seaward and b a c k - b a r r i e r s h o r e l i n e m i g r a t i o n
: W i n t e r a n d summer b e a c h p r o f i l e s , Duck, U.S.
: Schematic diagrams o f beach p r o f i l e s : volumes o f sand
a s s o c i a t e d w i t h s h o r e l i n e r e c e s s i o n a n d sea l e v e l r i s e
: Geomorphic model o f s p i t and b a r r i e r i s l a n d m i g r a t i o n
(Canada): m a j o r sediment t r a n s p o r t p a t h s
: Study area; Taiwan
: S t u d y a r e a ; Y u n l i n Coast
: B a t h y m e t r y o f t h e Wai-San-Ting
: L o c a t i o n s o f t h e Wai-San-Ting i n d i f f e r e n t y e a r s
: C r o s s s h o r e p r o f i l e m e a s u r e m e n t s ; f r o m 1990 t o 1992
: L o c a t i o n s o f p r o f i l e s Wai-San-Ting
; S p r i n g and neap t i d e : s u r f a c e e l e v a t i o n s n o r t h and s o u t h
s i d e o f Wai-San-Ting
: Overwash e f f e c t on Wai-San-Ting: s p r i n g t i d e
: Cross s e c t i o n a l d e f . ' s o f " e r o s i o n s u s c e p t i b i l i t y i n d e x "
: P r o f i l e l o c a t i o n s T r a b u c a d o r Bar, S p a i n
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Washover Transport on Sandbars
1.
INTRODUCTION
1
Washover Transport on Sandbars
2.
ORIGIN OF
2.1
General
ISLANDS
B a r r i e r b e a c h e s { i s l a n d s and s p i t s ) a r e f o u n d a l o n g c o a s t s a l l o v e r t h e
w o r l d . T h e s e e l o n g a t e d , n a r r o w l a n d f o r m s a r e composed o f s a n d a n d o t h e r
l o o s s e d i m e n t s t r a n s p o r t e d b y waves, c u r r e n t s , s t o r m s u r g e s a n d w i n d .
The t e r m " b a r r i e r " i d e n t i f i e s t h e s t r u c t u r e as one t h a t p r o t e c t s o t h e r
f e a t u r e s , s u c h as l a g o o n s and s a l t m a r s h e s , f r o m d i r e c t wave a t t a c k o f
t h e o p e n o c e a n . A l s o human i n t e r f e r e n c e i n c o a s t a l z o n e s ( r e c l a m a t i o n
p r o j e c t s , h a r b o u r development
plans, coastal protection) require a
c l e a r v i e w i n b a r r i e r b e a c h m o r p h o l o g y . T h e r e f o r due t o t h i s i n t e n s e
d e v e l o p m e n t p r e s s u r e on b a r r i e r b e a c h e s , t h e r e has b e e n a s u r g e i n
r e s e a r c h o v e r t h e p a s t t w e n t y y e a r s t o d e t e r m i n e t h e i r g e o l o g i c and
e c o l o g i e c h a r a c t e r i s t i c s i n o r d e r t o more c l e a r l y d e f i n e
barrier
c a r r y i n g c a p a c i t i e s a n d s u s c e p t i b i l i t y t o s t o r m damage.
S i n c e t h e y a r e p r o d u c e d b y one o f t h e f u n d a m e n t a l c o a s t a l p r o c e s s e s ,
v i z . t h e d i s p l a c e m e n t o f m a t e r i a l , a c c u m u l a t i o n ' forms p r o v i d e an
e x c e l l e n t i n d i c a t i o n o f c o a s t a l d y n a m i c s . F o r an a c c u m u l a t i o n t o f o r m ,
m a t e r i a l m u s t be moved t o a g i v e n l o c a l i t y a n d d e p o s i t e d t h e r e . To
e x p l a i n t h e o r i g i n o f f o r m s i t i s t h e r e f o r n e c e s s a r y t o a s c e r t a i n how
t h e m a t e r i a l moves and t h e r e a s o n s f o r any c e s s a t i o n o r w e a k e n i n g o f
t h i s process (Zenkovich, 1967).
I n p a r a g r a p h 2.2 t h e d e v e l o p m e n t o f c o a s t a l f e a t u r e s w i l l be d i s c u s s e d
and p a r a g r a p h 2.3 d e s c r i b e s t h e d i f f e r e n t f o r m s a n d t y p e s o f t h e s e
features.
2.2
B a r r i e r I s l a n d Development
2.2.1
General
The
type o f b a r r i e r i s l a n d found i n a g i v e n geographical r e g i o n
reflects the geological history of that particular region,
sediment
s u p p l y , o c e a n i c f o r c e s , and r e l a t i v e s e a - l e v e l c h a n g e s . Man has t o be
c a u t i o u s as i n f o r m a t i o n a b o u t t h e d e v e l o p m e n t o f t h e b a r r i e r i s l a n d s i s
o n l y p r e s e r v e d w h e r e t h e c o a s t l i n e has n o t u n d e r g o n e m a j o r r e t r e a t
(Balkema, 1988) .
2.2.2
Four main t h e o r i e s o f b a r r i e r
island
genesis
B a r r i e r c o a s t s i m i l a r t o t h o s e o f t h e N o r t h Sea and t h e E a s t C o a s t o f
t h e U n i t e d S t a t e s a r e f o u n d i n v a r i o u s p a r t s o f t h e w o r l d . As i n t h e
American l i t e r a t u r e (Leatherman, 1982), f o u r d i f f e r e n t b a s i c concepts
o f b a r r i e r i s l a n d g e n e s i s h a v e n b e e n c o n s i d e r e d f o r b o t h t h e N o r t h Sea
c o a s t a n d t h e E a s t C o a s t o f t h e U.S.:
(1) t h e d r o w n i n g o f c o a s t a l d u n e
r i d g e s ; (2) t h e f o r m a t i o n o f s p i t s , w h i c h w e r e l a t e r b r e a c h e d b y t h e
sea;
(3) i s l a n d d e v e l o p m e n t
f r o m emerging s h o a l s and
(4) b a r r i e r
i s l a n d s f o r m a t i o n caused by r i v e r d e p o s i t s .
1 . DrownincT o f c o a s t a l dune r i d g e s :
Hoyt
{1967) p r o p o s e d t h a t as sea l e v e l r o s e , d u n e r i d g e s o n t h e s e a w a r d
'Relief features built by action of waves and currents at a given level and under a given hydrodynamic regime.
2
Washover Transport on Sandbars
edge o f t h e m a i n l a n d became t h e b a r r i e r i s l a n d s . The l o w e r a r e a s
b e h i n d s t h e s e r i d g e s w e r e f l o o d e d , c r e a t i n g l a g o o n s ( F i g . 1) . T h i s
a p p e a r s t o b e t h e m a j o r means o f b a r r i e r f o r m a t i o n a l o n g t h e s o u t h e a s t
A t l a n t i c and perhaps t h e Gulf Coasts.
2. F o r m a t i o n o f s p i t s ,
transport:
w h i c h were l a t e r b r e a c h e d b y t h e sea:
Littoral
Gripp(1944) promoted t h e idea t h a t t h e chain o f i s l a n d s a l o n g t h e N o r t h
Sea c o a s t h a d d e v e l o p e d f r o m a c h a i n o f s p i t s . C o a s t p a r a l l e l s a n d
t r a n s p o r t w o u l d h a v e c r e a t e d t h e b a r r i e r . However, w i t h i n t h i s a r e a
major s p i t f o r m a t i o n i s o n l y observed on t h e I s l e o f S y l t . There i s an
imminent danger t h a t t h e e x i s t i n g s p i t s w i l l be breached, b o t h b e i n g
h e a v i l y e r o d e d on t h e i r w e s t e r n s i d e s . Wether such a b r e a c h d u r i n g a
storm t i d e would a c t u a l l y lead t o a separation o f t h e s p i t from t h e
i s l a n d , o r w e t h e r t h e n e w l y f o r m e d gap w o u l d be r a p i d l y c l o s e d a g a i n b y
c o a s t - p a r a l l e l sand t r a n s p o r t r e m a i n s a n open q u e s t i o n (Balkema, 1 9 8 8 ) .
A l o n g t h e n o r t h e a s t c o a s t o f t h e U n i t e d S t a t e s , l i k e t h e N o r t h Sea
coast,
spit
accretion
by l i t t o r a l
drift
appears
t o be t h e
c h a r a c t e r i s t i c manner i n w h i c h b a r r i e r i s l a n d s h a v e f o r m e d . B a r r i e r
beaches f o r m as sand i s t r a n s p o r t e d f r o m a s o u r c e , such as sea c l i f f s
and a s s o c i a t e d beaches, t o w a r d a r e g i o n o f a c c r e t i o n and s e d i m e n t a t i o n
i n o p e n w a t e r . The r e s u l t may b e a s p i t : a n e l o n g a t e , f i n g e r l i k e r i d g e
composed o f b e a c h s e d i m e n t t h a t i s a t t a c h e d a t o n e e n d t o t h e m a i n l a n d
but
t e r m i n a t e s i n o p e n w a t e r . Some b a r r i e r s c l e a r l y g r e w l a t e r a l l y as
long
spits
a n d were c o n v e r t e d i n t o
barrier
islands
with
inlet
t r u n c a t i o n ( P i g . 2 and 3 ) .
3. I s l a n d d e v e l o p m e n t
f r o m emergina s h o a l s : Onshore
transport:
The t h i r d m a j o r m e c h a n i s m f o r b a r r i e r i s l a n d f o r m a t i o n i s b y s u b m a r i n e
b a r u p b u i l d i n g ( F i g . 4 ) . Ordemann (1912) a p p e a r s t o h a v e b e e n t h e f i r s t
t o a r g u e t h a t t h e i s l a n d s h a d f o r m e d as a r e s u l t o f e m e r g i n g sandbanks.
Behrmann (1922) a l s o f a v o u r e d t h i s c o n c e p t .
I n t h e course o f h i s g e o l o g i c a l mapping o f t h e East F r i s i a n I s l a n d s
a l o n g t h e D u t c h c o a s t , K e i l h a c k (1925) came t o t h e same c o n c l u s i o n t h a t
the
i s l a n d c h a i n had been b u i l t up b y sediment s u p p l y f r o m t h e sea.
From t h e f o r m a t i o n o f t h e I s l e o f Memmert he i n v o k e d t h e f o l l o w i n g
model o f b a r r i e r i s l a n d f o r m a t i o n :
1.
M a t e r i a l from t h enearshore bottom i s transported
l a n d w a r d b y waves f o r m i n g o f f s h o r e b a r s n e a r t h e s h o r e .
2. I n c r e a s i n g s a n d s u p p l y c a u s e s t h e s h o a l s t o r i s e u n t i l
t h e y f i n a l l y emerge d u r i n g l o w t i d e .
3. I f a c c u m u l a t i o n c o n t i n u e s . They e v e n t u a l l y r e m a i n d r y
even d u r i n g o r d i n a r y f l o o d s .
4. U n d e r t h e i n f l u e n c e o f t h e w i n d , p a r t s o f t h e f o r m e r
s h o a l f i n a l l y grow above s p r i n g t i d e l e v e l , f o r m i n g a n
island.
4. B a r r i e r
i s l a n d s f o r m a t i o n a t t h e mouths o f m a j o r
rivers:
B a r r i e r i s l a n d may a l s o f o r m a t t h e m o u t h s o f m a j o r r i v e r s w h i c h
d e p o s i t l a r g e q u a n t i t i e s o f sediment i n t h e ocean. F o r i n s t a n c e , s m a l l
b a r r i e r s have d e v e l o p e d on t h e M i s s i s s i p p i R i v e r d e l t a a n d on s m a l l
d e l t a s a l o n g t h e west c o a s t o f Taiwan
t h r o u g h wave e r o s i o n a n d
r e w o r k i n g o f t h e s e r i v e r i n e s e d i m e n t s . Some r i v e r s a r e c u t o f f some 80
y e a r s ago. B a r r i e r s i s l a n d s w h i c h w e r e f o r m e d b y s e d i m e n t d e p o s i t s f r o m
3
Washover Transport on Sandbars
t h o s e r i v e r s a r e e r o d e d a n d d e f o r m e d h e a v i l y b y waves, c u r r e n t s a n d
wind, s u f f e r i n g g r e a t sand l o s s e s a t t h e d i s t a l end o f t h e b a r r i e r .
A s i m i l a r t y p e o f b a r r i e r beach can d e v e l o p where r i v e r s empty i n t o t h e
sea w i t h o u t f o r m i n g l a r g e d e l t a s . A l o n g t h e E a s t C o a s t o f t h e U.S.
b a r r i e r b e a c h e s c a n be f o u n d n e a r t h e m o u t h o f some m a j o r r i v e r s .
Marshy p l a t f o r m s o r c h e n i e r s can o r i g i n a t e i n a low-energy e n v i r o n m e n t ,
where a l a r g e p o r t i o n o f sediment i n t r a n s p o r t i s s i l t , c l a y a n d v e r y
f i n e s a n d . R i d g e s a r e c r e a t e d when s t o r m s u r g e s p u s h s a n d y / s h e l l y
sediment l a n d w a r d f r o m t h e s h o r e l i n e f o r d e p o s i t i o n on t h e marsh
surface.
Modern (Holocene) b a r r i e r i s l a n d s c a n a l s o be w e l d e d o n t o m a i n l a n d
f e a t u r e s , w h i c h a r e c a l l e d c o a s t a l p l a i n r e m n a n t s . Thus t h e c o r e o f
these sea i s l a n d c o n s i s t s o f P l e i s t o c e n e sediment w h i c h were d e p o s i t e d
p r i o r t o t h e l a s t I c e Age. These s e c t i o n s o f t h e c o a s t w e r e c u t o f f
f r o m t h e m a i n l a n d b y s e a - l e v e l r i s e a n d a r e now i s o l a t e d a s i s l a n d s .
A d d i n g t o t h i s c o m p l e x i t y , b a r r i e r s h a v e b e e n r e s h a p e d b y waves a n d
currents since t h e i r
initial
f o r m a t i o n and m i g r a t e d l a n d w a r d i n
r e s p o n s e t o a r i s i n g s e a l e v e l . A l t h o u g h a n i s l a n d may h a v e d e v e l o p e d
b y a n y o f t h e a b o v e m e c h a n i s m s , s u b s e q u e n t c h a n g e s due t o l a n d w a r d
m i g r a t i o n have r e s u l t e d i n major a l t e r a t i o n s t o i t s o r i g i n a l geomorphic
structure.
2.3
Barrier
2.3.1
General
Types
I n t h i s c h a p t e r two forms o f c l a s s i f i c a t i o n o f b a r r i e r i s l a n d w i l l be
d i s c u s s e d . The f i r s t c l a s s i f i c a t i o n i s b a s e d u p o n t h e e x a m i n a t i o n o f
the
geomorphic e x p r e s s i o n o f c o a s t a l f e a t u r e s
( L e a t h e r m a n , 1982) .
Discussing t h e d i f f e r e n t types o f b a r r i e r islands, a c l a s s i f i c a t i o n can
be made r e g a r d i n g t h e i s l a n d m o r p h o l o g y as a f u n c t i o n o f t i d a l a n d wave
r e g i m e (Hayes, 1 9 7 5 ) .
2.3.2
Bay B a r r i e r s
B a r r i e r s v a r y c o n s i d e r a b l y w i t h r e g a r d t o t h e i r geomorphic s t r u c t u r e .
Leatherman
(1982) d i s t i n g u i s h e d t h r e e t y p e s o f b a r r i e r s :
(1) b a y
b a r r i e r s ; ( 2 ) b a r r i e r s p i t s a n d (3) b a r r i e r i s l a n d s .
T h i s f i r s t s e p a r a t i o n i s dependent upon t h e n a t u r e o f a t t a c h m e n t o f t h e
b a r r i e r t o t h e mainland, i f detached a t a l l ! Class d i v i s i o n s can then
be made a c c o r d i n g t o b a r r i e r shape.
B a r r i e r beaches, t h a t a r e c o n n e c t e d a t b o t h ends t o h e a d l a n d s , a r e
c a l l e d bay barriers.
T h e s e b a r r i e r s a r e t y p i c a l l y s m a l l a n d common
a l o n g h i g h l y i n d e n t e d c o a s t l i n e s , where r o c k o r g l a c i a l l y - f o r m e d
h e a d l a n d s j u t i n t o t h e s e a . Bay b a r r i e r s u s u a l l y do n o t h a v e p e r m a n e n t
i n l e t s , b u t t e m p o r a r y openings c a n o c c a s i o n a l l y o c c u r . Bayhead b a r r i e r s
are
f o u n d i n t h e upper reaches o f a bay and f o r m where t h e a d j a c e n t
c l i f f s are highly resistant t o erosion.
( F i g . 5 ) . W h i l e some s a n d y
s e d i m e n t s may be s u p p l i e d f r o m s e a c l i f f e r o s i o n , much o f t h e m a t e r i a l
necessary t o form t h e b a r r i e r feature i s derived from other sources,
such as i n - p l a c e b i o l o g i c a l
( s h e l l ) p r o d u c t i o n , u p l a n d r u n o f f and
adjacent r i v e r s .
4
Washover Transport on Sandbars
B a r r i e r beaches t h a t connect headlands t o g e t h e r a l o n g t h e o u t e r reaches
o f a n embayment a r e c a l l e d baymouth barriers
( F i g . 6 ) . These b a r r i e r s
f o r m where headlands
a r e e a s i l y erodable and c o n t a i n s u f f i c i e n t
q u a n t i t i e s o f sand and g r a v e l f o r b a r r i e r c o n s t r u c t i o n .
Baymouth
b a r r i e r s o r i g i n a t e d as s p i t s m e r g i n g f r o m a d j a c e n t , e r o d i n g h e a d l a n d s ,
the end r e s u l t b e i n g a f a i r l y s t r a i g h t s h o r e l i n e . Baymouth b a r r i e r s a r e
common i n m i c r o t i d a l a r e a s , w h e r e l o w t i d a l r a n g e i n h i b i t s
inlets
except immediately f o l l o w i n g breaching by a major storm. W i t h t h e
r e s t r i c t i o n o f s a l t w a t e r i n p u t , f r e s h w a t e r o r b r a c k i s h marshes w i l l
develop.
A tombolo
c a n be r e g a r d e d as a s p e c i a l k i n d o f b a y b a r r i e r , b u t
p r o b a b l y d e s e r v e s r e c o g n i t i o n as a s e p a r a t e f e a t u r e ( F i g . 7 ) . T o m b o l o s
a t t a c h a n i s l a n d t o t h e m a i n l a n d f o l l o w i n g s p i t g r o w t h . The i s l a n d ,
w h i c h may be e r o d a b l e o r n o n e r o d a b l e , s e r v e s t o a n c h o r t h e b a r r i e r .
T o m b o l o s a r e m o s t common a l o n g t h e N o r t h e a s t g l a c i a l c o a s t o f t h e
U n i t e d S t a t e s , where t h e a t t a c h e d i s l a n d s a r e e r o d a b l e and p r o v i d e sand
f o r b a r r i e r development.
2.3.3 B a r r i e r
Spits
B a r r i e r b e a c h e s t h a t a r e a t t a c h e d a t one e n d t o a s o u r c e o f s e d i m e n t m a i n l a n d o r l a r g e i s l a n d - a n d e x t e n d i n t o o p e n w a t e r , a r e c a l l e d spits.
B a r r i e r s p i t s a r e f o r m e d as l o n g s h o r e c u r r e n t s move a b u n d a n t s a n d a n d
g r a v e l f r o m e r o d i n g c l i f f s i n t o open w a t e r . Since t h e s e s e d i m e n t - l a d e n
c u r r e n t s a r e d i r e c t e d s t r a i g h t along t h e shore, m a t e r i a l i s d e p o s i t e d
o f f s h o r e o f s h o r e l i n e embayments. W i t h t h e c o n t i n u e d d e l i v e r y o f s a n d
i n c o n j u n c t i o n w i t h overwash and wind, a b a r r i e r s p i t f i n a l l y f o r m s .
S p i t s e l o n g a t e i n t h e g e n e r a l d i r e c t i o n o f t h e l i t t o r a l d r i f t and
t h e r e f o r r e p r e s e n t t h e d i r e c t movement o f s a n d a l o n g a b e a c h . B a r r i e r
s p i t s can develop i n t o bay b a r r i e r s i f t h e y grow c o m p l e t e l y across a
b a y , o r b a y b a r r i e r s become s p i t s i f a p e r m a n e n t i n l e t i s c r e a t e d .
L e a t h e r m a n (1986) d i v i d e d s p i t s i n t o f o u r t y p e s b a s e d o n t h e i r o v e r a l l
shape and p o s i t i o n r e l a t i v e t o each o t h e r : s i m p l e s p i t s , d o u b l e s p i t s ,
r e c u r v e d s p i t s and convex s p i t s .
S p i t s w h i c h a r e r e l a t i v e l y s t r a i g h t a n d n a r r o w a r e c a l l e d simple spits
(Fig.
8 ) . Examples o f s i m p l e s p i t s i n c l u d e O g u n q u i t Beach, Maine, and
Sandy Neck, M a s s a c h u s e t t s .
A double
spit d e v e l o p s when t h e r e i s s i g n i f i c a n t l o n g s h o r e
sand
t r a n s p o r t i n b o t h d i r e c t i o n s a l o n g t h e s h o r e l i n e ( F i g . 9) . The i n l e t
between t h e two s p i t segments i s o f t e n u n s t a b l e , m i g r a t i n g l a t e r a l l y i n
response t o t h e p r e v a i l i n g d i r e c t i o n o f n e t l i t t o r a l d r i f t . I f t h e
i n l e t c l o s e s , t h e d o u b l e s p i t can be c o n v e r t e d i n t o an baymouth
b a r r i e r . The i n l e t u s u a l l y r e m a i n s o p e n a t a l l t i m e s s i n c e t h e e n c l o s e d
bay d i c t a t e s t i d a l exchange w i t h t h e open ocean.
JJecurved spits a r e e s s e n t i a l l y s i m p l e s p i t s w h i c h a r e r e c u r v e d o r
s i g n i f i c a n t l y b e n t s h o r e w a r d ( F i g . 1 0 ) . The d i s t i n c t i v e r e c u r v e d dune
ridges represent l i n e s of accretion, i n d i c a t i n g successive b a r r i e r
g r o w t h . R e c u r v e d s p i t s f o r m when t h e b a r r i e r i s g r o w i n g s e a w a r d f r o m a
h e a d l a n d t e r m i n u s s u c h t h a t t h e b a r r i e r i s s u b j e c t t o waves f r o m a w i d e
r a n g e o f d i r e c t i o n s . I n some c a s e s s p i t s c a n f o r m a t t h e d o w n d r i f t e n d
of a b a r r i e r i s l a n d ; these s p i t s can e x h i b i t l a r g e r e c u r v a t u r e s , such
as F i s h i n g P o i n t a t t h e s o u t h e r n e n d o f A s s a t e a g u e I s l a n d , V i r g i n i a .
Complex spits a r e uncommon a n d o n l y f o r m when b o t h s i d e s o f a m a i n l a n d
p e n i n s u l a a r e e r o d i n g a n d s u b j e c t t o l a r g e e n o u g h waves t o p r o d u c e
s i g n i f i c a n t l o n g s h o r e s a n d t r a n s p o r t ( F i g . 11) . The o n l y d o c u m e n t e d
5
Washover Transport on Sandbars
example o f a complex s p i t i s t h e P r o v i n c e Lands f o r m e d as a l a r g e
r e c u r v e d s p i t f o l l o w e d b y t h e g r o w t h o f a n e n c l o s e d s i m p l e s p i t . The
e x t r e m e l y l a r g e s i z e o f t h i s b a r r i e r s p i t and t h e p r e s e n c e o f m u l t i p l e
dune r i d g e s on t h e r e c u r v e d f l a n k make i t t h e m o s t s t a b l e o f a l l s u c h
features.
2.3.4
Barrier
Islands
B a r r i e r i s l a n d s , w h i c h o c c u r p r i m a r i l y on c o a s t a l p l a i n s h o r e l i n e s
l o c a t e d o n t h e t r a i l i n g edges o f c o n t i n e n t s a n d o n m a r g i n a l s e a s , v a r y
i n m o r p h o l o g y i n r e s p o n s e t o t h e i n t e r a c t i o n o f t i d a l r a n g e a n d wave
energy e f f e c t s . B a r r i e r i s l a n d s are r e s t r i c t e d t o those c o a s t a l p l a i n
shore-lines with
tidal
ranges
less
t h a n a p p r o x i m a t e l y 4m.
This
c o n c l u s i o n i s based on a s t u d y o f c o a s t a l c h a r t s o f t h e w o r l d ,
c o n d u c t e d a t t h e Defence Research L a b o r a t o r y , U n i v e r s i t y o f Texas ( i n
1 9 6 3 - 6 4 ; d i s c u s s e d b y Hayes 1965 a n d Hayes a n d Kana, 1 9 7 6 ) . The l a c k o f
e x t e n s i v e b a r r i e r i s l a n d s i n a r e a s w i t h l a r g e t i d e s has b e e n p o i n t e d
o u t b y s e v e r a l o t h e r a u t h o r s ( P r i n c e 1955, G i e r l o f f - E m d e n 1 9 6 1 a n d K i n g
1 9 7 2 ) . G l e a s e r (1978) n o t e d t h a t " o n l y 10% o f t h e w o r l d ' s b a r r i e r
i s l a n d s a r e p r e s e n t a l o n g c o a s t l i n e s w h e r e t h e t i d e r a n g e e x c e e d s 3 m".
I n t h e f o l l o w i n g t e x t t h e two most i m p o r t a n t p a r a m e t e r s i n v o l v e d i n
b a r r i e r i s l a n d f o r m a t i o n a n d c o a s t a l m o r p h o l o g y w i l l be d i s c u s s e d .
F o l l o w i n g t h e w o r k o f W.A.
P r i c e ( 1 9 5 5 ) , i t i s c o n c l u d e d t h a t t h e most
i m p o r t a n t c o n t r o l o f t h e geomorphology o f d e p o s i t i o n a l c o a s t s i s t h e
t y p e a n d amount o f h y d r a u l i c e n e r g y e x p e n d e d w i t h i n a n a r e a . F u r t h e r more, t h e t w o e n e r g y f a c t o r s o f m o s t s i g n i f i c a n c e a r e wave e n e r g y a n d
t i d a l c u r r e n t e n e r g y , w h i c h c a n be r e l a t e d d i r e c t l y t o t i d a l
range.
D a v i e s (1964) c l a s s i f i e d s h o r e l i n e s o n t h e b a s i s o f t i d a l r a n g e ( s e e
t a b l e 2.1).
T a b l e 2.1:
Shoreline
classification
Tidal
Class
range
0-2
m
mesotidal coasts
2-4
m
macrotidal coasts
> 4 m
microtidal
coasts
I n a r e a s o f a v e r a g e m a r i n e wave c o n d i t i o n s , c o a s t s w i t h s m a l l t i d a l
r a n g e s ( m i c r o t i d a l ) a r e u s u a l l y d o m i n a t e d b y wave e n e r g y , a n d c o a s t s
w i t h l a r g e t i d a l ranges (macrotidal) are u s u a l l y dominated by t i d a l
c u r r e n t s and t i d a l - l e v e l f l u c t u a t i o n s . Coasts w i t h i n t e r m e d i a t e t i d e s
( m e s o t i d a l ) show i n f l u e n c e s o f b o t h waves a n d t i d e s a n d a r e t h u s t e r m e d
m i x e d - e n e r g y c o a s t s (Hayes, 1 9 6 5 ) .
The r e a s o n f o r t h e e m p h a s i s o n t i d a l r a n g e i s t h e f a c t t h a t t h e
e f f e c t i v e n e s s o f wave a c t i o n d i m i n i s h e s ( i . e . , waves c a n n o t b r e a k i n a
c o n c e n t r a t e d a r e a f o r a l o n g p e r i o d o f t i m e ) , and t i d a l
current
a c t i v i t y i n c r e a s e s as t h e v e r t i c a l t i d a l r a n g e i n c r e a s e s . S m a l l waves
a r e m o s t e f f e c t i v e o n c o a s t s w i t h s m a l l t i d a l r a n g e s . On t h e o t h e r
h a n d , i n a r e a s o f s m a l l waves, a s m a l l e r t i d a l r a n g e i s r e q u i r e d t o
p r o d u c e t i d a l - i n f l u e n c e d m o r p h o l o g y t h a n on c o a s t s w i t h medium o r h i g h
wave e n e r g y .
An o t h e r v a r i a b l e w h i c h p l a y s a p r i n c i p a l r o l e i n d e t e r m i n i n g b a r r i e r
expression
i s sand
supply.
Utilizing
Hayes'
(1979)
micro-meso
c l a s s i f i c a t i o n coupled w i t h sediment supply c o n d i t i o n s y i e l d s
four
6
Washover Transport on Sandbars
basic
types
-
of
islands:
Microtidal
Microtidal
Macrotidal
Macrotidal
transgressive
regressive
transgressive
regressive.
T r a n s g r e s s i v e i n d i c a t e s s a n d d e f i c i e n c y and t h e t e n d e n c y f o r s h o r e l i n e
e r o s i o n . R e g r e s s i v e d e n o t e s a p a s t p e r i o d o f a c c r e t i o n as e v i d e n c e d b y
m u l t i p l e dune r i d g e s . P a s t a c c r e t i o n does n o t p r e c l u d e p r e s e n t
or
future
erosion;
a
regressive
barrier
can
be
converted
to
a
t r a n s g r e s s i v e one w i t h t h e d e m i s e o f s u f f i c i e n t s a n d s u p p l i e s ( F i g .
12) .
* F o r m a c r o t i d a l c o n d i t i o n s ( t i d a l r a n g e s a b o v e 4m), b a r r i e r s a r e n o t
e x p e c t e d , a l t h o u g h i t i s p o s s i b l e t h a t some s m a l l , e p h e m e r a l f e a t u r e s
may e x i s t . I n a c t u a l l y , b a r r i e r i s l a n d m o r p h o l o g y d e p e n d s u p o n t i d a l
e n e r g y i n r e l a t i o n t o wave e n e r g y so t h a t e x a c t b o u n d a r i e s c a n n o t be
s e t by knowledge of t i d a l range alone. M a c r o t i d a l s h o r e l i n e s are a l s o
c h a r a c t e r i z e d by a b r o a d e x t e n s i o n s
of i n t e r t i d a l
flats
and
salt
m a r s h e s . Sand d e p o s i t s a r e u s u a l l y r e s t r i c t e d t o l i n e a r s a n d s h o a l s , o r
t i d a l c u r r e n t ridges, i n the o f f s h o r e areas
* Microtidal barriers, c o r r e s p o n d i n g t o w a v e - d o m i n a t e d s h o r e l i n e s , a r e
e x p e c t e d a l o n g sandy c o a s t l i n e s w i t h a t i d a l range l e s s t h a n 2 m e t r e s .
L e a t h e r m a n (1979) d e s c r i b e s t h a t t h e r e i s a s i g n i f i c a n t d i f f e r e n c e
b e t w e e n m i c r o t i d a l b a r r i e r i s l a n d m o r p h o l o g y and m a c r o t i d a l b a r r i e r
i s l a n d m o r p h o l o g y . These d i f f e r e n c e s a r e o u t l i n e d i n t a b l e 2.2, d e r i v e d
from a study of b a r r i e r i s l a n d f o r m a t i o n a t the East Coast o f the
United States.
M i c r o t i d a l t r a n s g r e s s i v e b a r r i e r s ( F i g . 13) i n a r e a s o f medium wave
e n e r g y a r e t h e l e a s t s t a b l e and m o s t v u l n e r a b l e t o s t o r m - i n d u c e d
changes o f t h e f o u r i s l a n d t y p e s . T i d a l i n l e t s and t i d a l d e l t a s a r e o f
r e l a t i v e l y minor s i g n i f i c a n c e , i n c o n t r a s t w i t h mesotidal
barriers,
w h e r e t i d a l i n l e t s and t i d a l d e l t a s a r e l a r g e and s i g n i f i c a n t . T h e s e
i s l a n d s , w h i c h a r e l o n g and n a r r o w , a r e c h a r a c t e r i z e d b y
low-lying
t o p o g r a p h y and numerous w a s h o v e r s , i n d i c a t i n g d e f i c i e n t s e d i m e n t s u p p l y
a n d r e l a t i v e r a p i d s h o r e l i n e r e t r e a t . F i g u r e 14 i s a n e x a m p l e o f a
microtidal regressive barrier.
* M e s o t i d a l shorelines
are c h a r a c t e r i z e d by b a r r i e r s w i t h a s h o r t ,
s t u n t e d n a t u r e and b y t h e a b u n d a n c e o f t i d a l i n l e t s . M e s o t i d a l b a r r i e r
island
can
a l s o be
subdivided
into
transgressive
( F i g . 15)
and
r e g r e s s i v e types (Fig. 16), r e f l e c t i n g sediment supply. T i d a l i n l e t s
b e t w e e n m e s o t i d a l b a r r i e r s t e n d t o be more s t a b l e t h e n t h o s e i n a
m i c r o t i d a l s e t t i n g . As a r u l e , t h e e b b - t i d a l d e l t a s a r e l a r g e r t h e n t h e
flood-tidal
deltas
(the opposite
i s true f o r m i c r o t i d a l areas).
F i t z G e r a l d (1977) s u g g e s t e d t h a t t h e ebb d o m i n a n c e o f t h e m a i n c h a n n e l
at P r i n c e I n l e t , South C a r o l i n a , r e s u l t s from g r e a t e r i n l e t e f f i c i e n c y
a t l o w w a t e r t h a n a t h i g h w a t e r , w h i c h s h o r t e n s t h e t i m e o f ebb f l o w
and l e n g t h e n s
t h e t i m e o f f l o o d f l o w , hence p r o d u c i n g s t r o n g e r
ebb
c u r r e n t s . The l a r g e t i d e n e c e s s i t a t e s many b r e a k s t h r o u g h t h e b a r r i e r
c h a i n t o a l l o w d a i l y exchange o f w a t e r between t h e e n c l o s e d bays and
t h e open ocean.
T a b l e 2.2 shows t h e m a i n b a r r i e r
and m e s o t i d a l b a r r i e r i s l a n d s .
7
c h a r a c t e r i s t i c s comparing m i c r o t i d a l
Washover Transport on Sandbars
T a b l e 2.2:
1976)
Barrier
Barrier island characteristics (after
Microtidal
Type
Mesotidal
(3-20
km)
long
Shape
elongated hot
Washover
features
abundant:
washover t e r r a c e s
& washover f a n s
numerous
minor: beach r i d g e s
o r washover
t e r r a c e s ; washover
fans rare
Tidal
infrequent
numerous
Flood-tidal
deltas
l a r g e , commonly
coupled w i t h
washovers
moderate s i z e
absent
Ebb-tidal
deltas
small
large with strong
wave r e f r a c t i o n
effects
Three o t h e r
i s l a n d s and
km)
stunted
Length
inlets
(30-100
Leatherman,
dog
t o absent
drum
stick
to
t y p e s o f b a r r i e r s t r u c t u r e s deserve a t t e n t i o n : capes,
cheniers.
sea
B a r r i e r i s l a n d s t h a t p r o j e c t i n t o t h e sea
to form a r i g h t - a n g l e d
s h o r e l i n e a r e c a l l e d c u s p a t e f o r e l a n d s o r capes
( F i g . 17) . C u s p a t e
f o r e l a n d s t y p i c a l l y c o n s i s t o f t r a n s g r e s s i v e and r e g r e s s i v e
sections.
Sea islands, l i k e c h e n i e r s , a r e p r i n c i p a l l y f o u n d i n s p e c i f i c r e g i o n s
a l o n g t h e U.S.
c o a s t l i n e . These l a r g e i s l a n d s c o n s i s t o f two p a r t s : a
r e c e n t o r H o l o c e n e o u t e r b a r r i e r and a P l e i s t o c e n e c o r e ( F i g . 1 8 ) .
The
o u t e r b a r r i e r may be w e l d e d o n t o t h e P l e i s t o c e n e c o r e o r s e p a r a t e d f r o m
i t by i n t e r v e n i n g s a l t marshes and t i d a l c r e e k s . O n l y t h e o u t e r b a r r i e r
s t r u c t u r e b e h a v e s i n a d y n a m i c manner. The i s l a n d c o r e i s P l e i s t o c e n e
i n age ( a p p r o x i m a t e l y 100,000 y e a r s o l d ) , i s q u i t h i g h ( o f t e n a b o v e t h e
100
y e a r and
s o m e t i m e s a b o v e t h e 500
year f l o o d p l a i n level) ,
and
c o n v e r t e d by a mature, t h i c k c o n t i n e n t a l s o i l w h i c h s u p p o r t s a c l i m a x
m a r i t i m e f o r e s t . T h u s , t h e P l e i s t o c e n e c o r e , w h i c h c a n be q u i t e l a r g e
i n a r e a l d i m e n s i o n s , i s a c t u a l l y a v e r y s t a b l e f e a t u r e and i s m e r e l y an
emersed p i e c e o f t h e m a i n l a n d .
Cheniers, w h i c h a r e t h e l o w e s t a n d m o s t v u l n e r a b l e t o s t o r m s u r g e s o f
a l l b a r r i e r types, are g e n e r a l l y s t r a n d p l a i n s , i n s t e a d of t r u e b a r r i e r
s t r u c t u r e s ( F i g . 19) . C h e n i e r s c o n s i s t l a r g e l y o f o r g a n i c
deposits
( s a l t marshes) w i t h r i d g e s o f sand and s i l t o v e r l a y i n g t h i s b a s a l
s t r u c t u r e (Leatherman 1979,1982).
8
Washover Transport on Sandbars
3.
THE
COASTAL SEDIMENTARY ELEMENTS OF
3.1
General
BARRIER ISLAND SYSTEMS
The f u n d a m e n t a l e l e m e n t s i n d e s c r i p t i v e d e f i n i t i o n s o f b a r r i e r i s l a n d s
u s u a l l y i n c l u d e t h e t e r m s e l o n g a t e , n a r r o w l a n d f o r m a t i o n s , composed o f
unconsolidated
materials,
lying
parallel
to
the
shoreline,
and
s e p a r a t e d f r o m t h e m a i n l a n d c o a s t s by l a g o o n s a n d / o r b a y s . O p e r a t i o n a l
d e f i n i t i o n s u s e d b y a v a r i e t y o f a u t h o r s and w o r k i n g g r o u p s ( B e r r y h i l l
e t a l . , 1969;
C r o m w e l l , 1971)
may
i m p o s e o t h e r l i m i t a t i o n s on
the
d e f i n i t i o n s s u c h as " g e n e r a l l y b e i n g l e s s t h e n 10 m a b o v e sea l e v e l " .
O t h e r o p e r a t i o n a l r e s t r i c t i o n s t h a t have been u s e d i n c l u d e a l e n g t h t o
w i d t h r a t i o o f l e s s t h a n 1 0 : 1 , and commonly b e i n g s u b j e c t e d t o wave,
t i d a l and w i n d e n e r g i e s . W h i l e t h e above d e f i n i t i o n s a r e g e n e r a l l y
geomorphically
o r i e n t e d , several authors
have a t t e m p t e d t o
define
barrier
i s l a n d s as
being
composed o f s e v e r a l
major
depositional
e n v i r o n m e n t s . One
o f t h e m i s G.F.
O e r t e l ( 1 9 8 4 ) . He c o n s i d e r e d t h e
b a r r i e r i s l a n d as t h e f o c a l e l e m e n t o f a much l a r g e r s y s t e m t h a t i s
t e r m e d t h e barrier island system.
The b a r r i e r i s l a n d s y s t e m c o n s i s t s o f s e v e n m a j o r c o a s t a l e n v i r o n m e n t s .
The s e v e n e n v i r o n m e n t s a r e a l s o r e q u i r e d e l e m e n t s n e e d e d t o i m p o s e t h e
d e s i g n a t i o n " b a r r i e r i s l a n d " t o a l i t t o r a l s a n d b o d y . The e l e m e n t s a r e
(see F i g . 2 0 ) :
1.
2.
3.
4.
5.
6.
7.
3.2
Mainland
Backbarrier lagoon
I n l e t s and i n l e t d e l t a s
S a l t m a r s h e s and t i d a l f l a t s
Washover f a n s
Barrier flats
Shoreface.
Mainland Element
The m a i n l a n d e l e m e n t o f t h e b a r r i e r i s l a n d s y s t e m e s t a b l i s h e s an i s l a n d
as a b a r r i e r a n d i s a n e c e s s a r y r e q u i r e m e n t f o r t h e d e s i g n a t i o n o f
b a r r i e r i s l a n d . Three major c h a r a c t e r i s t i c s of the mainland determine
how t h i s e l e m e n t i n t e r a c t s w i t h t h e b a r r i e r i s l a n d s y s t e m ; t h e s e a r e :
1. l i t h o l o g y
2 . slope
3 . drainage.
3.2.1
Lithology
L i t h o l o g i c a l c h a r a c t e r i s t i c s of the mainland s t r o n g l y a f f e c t the r a t e
o f l a g o o n a l e x p a n s i o n by i n f l u e n c i n g t h e r e t r e a t r a t e o f t h e m a i n l a n d
s h o r e l i n e o f t h e b a c k b a r r i e r l a g o o n . S t u d i e s on t h e e r o s i o n ( s h o r e l i n e
recession rate) of the e s t u r i n e shoreline i n the Albemarle-Pamlico
r e g i o n o f N o r t h C a r o l i n a ( B e l l i s e t a l . , 1975)
i l l u s t r a t e how
shore
t y p e i n f l u e n c e t h e e r o s i o n r a t e s . Mean e r o s i o n r a t e s f o r f i v e c o u n t i e s
b o r d e r i n g t h e m a i n l a n d s h o r e s o f t h e l a g o o n r a n g e d f r o m 0.3 t o 1.4 m
y r ' . H o w e v e r , l o c a l r e c e s s i o n r a t e s o f 0.6-0.9 m y r ' w h e r e t y p i c a l o f
many a r e a s a n d r a t e s o f o v e r 6 . 1 m y r " ' h a d b e e n r e p o r t e d . B e l l i s e t a l .
(1975) showed t h a t e x p o s e d , l o w s h o r e l i n e s o f s a n d and c l a y w e r e l e s s
s u s c e p t i b l e t o e r o s i o n t h a n marshy s h o r e l i n e s (0.9-2.4 v e r s u s 1.8-6.1
9
Washover Transport on Sandbars
m yr'S r e s p e c t i v e l y ) . Under s i m i l a r e n e r g e t i c c o n d i t i o n s i t w o u l d
anticipated that
indurated material
along the mainland shore
b a c k b a r r i e r l a g o o n s w o u l d be f a r l e s s s u s c e p t i b l e t o e r o s i o n .
3.2.2
be
of
Slope
Regional slope i s also a very important
factor controlling
the
recession
rate
of
l a g o o n a l and
barrier
island
shorelines.
In
b a c k b a r r i e r l a g o o n s wave e n e r g y i s g e n e r a l l y c o n s i d e r a b l y l o w e r t h a n i n
t h e open seas. T h e r e f o r e , r e c e s s i o n o f m a i n l a n d s h o r e s i s l a r g e l y
c o n t r o l l e d by submergence. Assuming a c o n s t a n t r a t e o f s e a - l e v e l r i s e
during periods of b a r r i e r island evolution, the rate of shoreline
r e c e s s i o n w o u l d b e : (1) c o n s t a n t a t c o n s t a n t s l o p e s ; (2) i n c r e a s e a t
d e c r e a s i n g s l o p e s a n d (3) d e c r e a s e a t i n c r e a s i n g s l o p e s .
The m a i n l a n d e l e m e n t o f t h e V i r g i n i a (USA) b a r r i e r i s l a n d s y s t e m has a n
a v e r a g e s l o p e o f a p p r o x i m a t e l y 0.5° a n d t h e e s t i m a t e d l o c a l r a t e o f
s e a - l e v e l r i s e i s a b o u t 30 cm y r " ' ( H i c k s , 1972; Prommer, 1980) . T h e s e
c o n d i t i o n s w o u l d p r o d u c e a r e c e s s i o n r a t e o f 34.4
cm y r ' ' due
to
submergence.
3.2.3
Drainage
D r a i n a g e c h a r a c t e r i s t i c s have a g r e a t e r i n f l u e n c e upon t h e shapes o f
e v o l v i n g b a r r i e r s t h a n on t h e r a t e s o f development. C o a s t a l a r e a s w i t h
t r e l l i s d r a i n a g e p a t t e r n ( i . e . , r e l i c t c o a s t a l r i d g e s and swales) o f t e n
produce b a r r i e r systems w i t h w e l l - d e v e l o p e d s h o r e - p a r a l l e l
trends.
C o a s t a l areas w i t h w e l l - d e v e l o p e d d e n d r i t i c d r a i n a g e topography a r e
more a p t t o h a v e h e a d l a n d b a r r i e r s y s t e m s t h a n h a v e w e t l a n d s a n d
e s t u a r i e s w i t h shore-normal trends.
W h i l e t h e i n f l u e n c e o f l i t h o l o g y , s l o p e and d r a i n a g e p a t t e r n a r e
r e a d i l y apparent along t h e mainland shore o f a b a r r i e r lagoon, t h e
o v e r w h e l m i n g i n f l u e n c e t h a t t h e m a i n l a n d s u r f a c e has o n a r e t r e a t i n g
b a r r i e r s y s t e m has o n l y r e c e n t l y b e e n g i v e n a t t e n t i o n b y
several
c o a s t a l w o r k e r s ( K r a f t , 1 9 7 1 ; Kayan and K r a f t , 1 9 7 9 ; H a l s e y , 1 9 7 9 ) .
3.3
The
B a c k b a r r i e r Lagoon
Lagoons o r bays a r e l a r g e b o d i e s o f open w a t e r t h a t a r e p r o t e c t e d f r o m
o c e a n i c f o r c e s by a b a r r i e r beach; f r e s h w a t e r i n p u t i s g e n e r a l l y
l i m i t e d t o l a n d s u r f a c e r u n o f f . The m o s t o b v i o u s c h a r a c t e r i s t i c s o f
b a c k b a r r i e r l a g o o n s a r e s u r f a c e - a r e a changes r e l a t e d t o e b b i n g and
f l o o d i n g t i d e s ( O e r t e l and Dunstan, 1 9 8 1 ) .
Two
endpoints exist:
- open-water
- expandable
lagoons
tidal l a g o o n s .
Regardless of t i d a l stage, open-water lagoons are bodies o f w a t e r t h a t
have a r e l a t i v e l y c o n s t a n t w a t e r s u r f a c e area between t h e main l a n d and
t h e b a r r i e r i s l a n d . A l b e m a r l e a n d P a m l i c o Sounds i n N o r t h C a r o l i n a a r e
examples o f open-water-type l a g o o n s . W a t e r - s u r f a c e areas o f expandable
l a g o o n s i n c r e a s e b y g r e a t e r t h a n 15% b e t w e e n l o w a n d h i g h t i d e . Many
b a r r i e r lagoons i l l u s t r a t e w a t e r - s u r f a c e areas (and p r i s m s ) t h a t v a r y
s i g n i f i c a n t l y during a t i d a l cycle. Surface-area v a r i a b i l i t y u s u a l l y
r e s u l t s f r o m p a r t i a l o r c o m p l e t e emergence o f t h e seabed d u r i n g l o w
w a t e r s t a g e s o f t h e t i d e ( P i g . 2 1 a n d 22) . Emergence i s r e l a t e d t o
10
Washover Transport on Sandbars
t i d a l water levels w i t h respect t o a v a r i e t y o f i n t e r t i d a l
lagoonal
f e a t u r e s (marshes, o y s t e r r e e f s , mangroves, t i d a l f l a t s , f l o o d d e l t a s ,
swash b e r m s , a n d b a r s , e t c . ) . The c o a s t a l m a r s h e s o f G e o r g i a (USA) a r e
e x a m p l e s o f w e l l - d e v e l o p e d e x p a n d a b l e t i d a l l a g o o n s whose w a t e r s u r f a c e
areas i n c r e a s e by g r e a t e r than 50% between s p r i n g l o w and h i g h t i d e s
( O e r t e l , 1984).
3.4
I n l e t s and I n l e t D e l t a s
Separating t h e i n d i v i d u a l islands o f a b a r r i e r chain are t i d a l i n l e t s
t h r o u g h w h i c h t i d e s f l o w t o b a c k b a r r i e r l a g o o n s . These f e a t u r e s a r e
n e c e s s a r y t o e s t a b l i s h t h e b a r r i e r s as i s l a n d s r a t h e r t h e n b a r r i e r
s p i t s o r b a y b a r r i e r s . I n l e t s a r e t h e p r i m a r y means b y w h i c h s a n d i s
t r a n s p o r t e d l a n d w a r d a c r o s s a m i g r a t i n g b a r r i e r beach system. They open
and c l o s e i n r e s p o n s e t o c h a n g i n g c o n d i t i o n s a n d may m i g r a t e
long
d i s t a n c e s a l o n g a b a r r i e r s h o r e l i n e . An i n l e t may a p p r o a c h e q u i l i b r i u m
c o n d i t i o n s i f a balance i s a c h i e v e d between t i d a l i n f l o w s and o u t f l o w s
t h a t s c o u r a channel, and t h e l o n g s h o r e t r a n s p o r t o f sand t h a t tends t o
close an i n l e t .
I n l e t s t a b i l i t y i s r e l a t e d t o t h e strength o f t h e longshore current
versus t i d a l j e t f l u s h i n g capacity. I n l e t s tend t o close unless there
i s s u b s t a n t i a l o u t f l o w o f w a t e r f r o m a m a j o r r i v e r . When o p e n , a n i n l e t
a c t s as a complete o r p a r t i a l b a r r i e r t o l o n g s h o r e sand t r a n s p o r t .
Depending on e q u i l i b r i u m c o n d i t i o n s ,
an i n l e t
may t r a p s a n d o r
n a t u r a l l y bypass a l a r g e p o r t i o n o f t h e l o n g s h o r e sediment t r a n s p o r t
( l i t t o r a l d r i f t ) . Under t h e c o n d i t i o n s o f s m a l l t i d a l f l o w s a n d h i g h
r a t e s o f l i t t o r a l d r i f t , an i n l e t w i l l e v e n t u a l l y c l o s e .
T i d a l b a r r i e r i n l e t s have b a t h y m e t r i c a l and s e d i m e n t o l o g i c a l
patterns
t h a t r e s u l t f r o m t h e i n t e r a c t i o n o f ( 1 ) i n l e t c u r r e n t s ; (2) b a c k b a r r i e r
e n e r g y a n d (3) o p e n s e a e n e r g y s o u r c e s . Two t y p e s o f b a r r i e r i n l e t s c a n
be c o n s i d e r e d : o p e n - e n d e d b a r r i e r i n l e t s a n d T i d a l b a r r i e r i n l e t s :
When t h e l a t e r a l d i s t a n c e b e t w e e n b a r r i e r i s l a n d s e x c e e d s t h e l e n g t h o f
t h e i s l a n d b y s e v e r a l m u l t i p l e s , i n l e t f l o w w i l l b e more c h a r a c t e r i s t i c
o f t h e c i r c u l a t i o n o f a d j a c e n t b a c k b a r r i e r l a g o o n o r t h e open sea
r a t h e r t h a n an o p e n i n g between t o d i f f e r e n t b o d i e s o f w a t e r . These
open-ended b a r r i e r i n l e t s have d i s t i n c t s e d i m e n t a t i o n and b a t h y m e t r i c
p a t t e r n s t h a t a r e o f t e n r e l a t e d t o s p i t development.
T i d a l b a r r i e r i n l e t s , o r fluvial b a r r i e r i n l e t s a r e a s s o c i a t e d
with
r i v e r systems and a r e c h a r a c t e r i z e d b y m u l t i p l e d i s t r i b u t a r y channels
t h a t d i v e r g e and converge around shoals and i s l a n d s i n t h e e n t r a n c e
t h r o a t area. Also c h a r a c t e r i s t i c f o rt i d a l b a r r i e r i n l e t s i s t h a t t h e
i n l e t w i d t h i s l e s s than s e v e r a l m u l t i p l e s o f an i s l a n d l e n g t h ( O e r t e l ,
1984) .
W i t h a r i s i n g t i d e , sand t h a t has been moving a l o n g t h e b e a c h as
l i t t o r a l d r i f t i s interrupted by t h e t i d a l current; a p o r t i o n o f t h i s
m a t e r i a l w i l l be t r a n s p o r t e d t h r o u g h t h e i n l e t f o r d e p o s i t i o n i n t h e
b a y . T h i s s e d i m e n t a t i o n p r o d u c e s a n e x t e n s i v e flood tidal delta t h a t
e x h i b i t s a d e l t a i c p a t t e r n when f u l l y d e v e l o p e d ( F i g . 2 3 ) . F i g u r e 24
and 25 show t w o t i d a l i n l e t s o f t h e A l g a r v e B a r r i e r I s l a n d s , P o r t u g a l .
On t h e s e a w a r d s i d e o f t h e i n l e t a r e a s e r i e s o f s h o a l s c a l l e d t h e e b b
t i d e d e l t a , m a r k e d b y b r e a k i n g w a v e s . The ebb tidal delta i s f o r m e d
f r o m t h e s a n d t h a t moves t h r o u g h t h e i n l e t w i t h t h e e b b i n g t i d e . O e r t e l
(1975b) a n d l a t e r Hubbard e t a l . (1979) used t h e g e n e r a l c o n f i g u r a t i o n
o f e b b t i d a l d e l t a s a s a means o f c l a s s i f y i n g d i f f e r e n t t y p e s o f e b b
d e l t a s , b y r e l a t i n g d e l t a c o n f i g u r a t i o n t o t h e r e l a t i v e magnitudes o f
t h e f o r c e s o f dynamic d i v e r s i o n ( F i g 2 6 ) .
11
Washover Transport on Sandbars
3.5
S a l t m a r s h e s and
tidal
flats
As a b a r r i e r s p i t g r o w s i n t o o p e n w a t e r , i t p r o v i d e s p r o t e c t i o n f o r
e c o s y s t e m s t h a t r e q u i r e q u i e t w a t e r , s u c h as tidal flats and
salt
marshes. T h e s e i n t e r t i d a l e n v i r o n m e n t s a r e b u i l t f r o m s a n d s , muds, a n d
clays transported through t i d a l i n l e t s or across the b a r r i e r f o r
d e p o s i t i o n i n t h e e n c l o s e d bay.
Through
t i m e t h e sea b o t t o m i s
e v e n t u a l l y e l e v a t e d e x t e n d i n g t h e i n t e r t i d a l zone and p r o v i d i n g
new
s u b s t r a t e f o r s l a t marsh expansion. Marsh g r a s s e s g r a d u a l l y s t a b i l i z e
t h i s s u r f a c e and s p r e a d s l o w l y o u t w a r d i n t o t h e bay.
When i n l e t s o p e n , s a n d y s h o a l s d e v e l o p i n t h e b a y . These s h o a l s , w h i c h
h a v e a c h a r a c t e r i s t i c d e l t a i c p a t t e r n , p r o v i d e s \ i b s t r a t e f o r new s a l t
marshes a f t e r t h e i n l e t c l o s e s o r m i g r a t e s d o w n d r i f t . Marsh t h a t grows
c l o s e t o a c t i v e i n l e t s o r a c c r e t i n g s p i t s i s m o s t p r o d u c t i v e due t o
c o n t i n u a l f l u s h i n g by t i d a l c u r r e n t s . A l s o m a j o r washover d e p o s i t s can
serve i n t h i s c a p a c i t y , c a r r y i n g sand d i r e c t l y i n t o a lagoon.
3.6
Washover
fans
Washover f a n s a r e c r e a t e d b y t h e f l o w o f w a t e r t h r o u g h t h e p r i m a r y d u n e
l i n e w i t h d e p o s i t i o n o f t h e sand on t h e b a r r i e r f l a t s , marsh, o r i n t o
t h e l a g o o n , d e p e n d i n g on t h e s t o r m m a g n i t u d e and t h e i s l a n d w i d t h ( F i g
2 7 ) . O v e r w a s h i s d e f i n e d as any s w a s h u p r u s h t h a t c r o s s e s a dune l i n e
( o r s t o r m b e r m , i f no dunes a r e p r e s e n t ) . U s u a l l y o v e r w a s h o c c u r s
d u r i n g c o a s t a l storms, b u t a m a r g i n a l e v e n t can o c c u r d u r i n g e x t r e m e l y
h i g h s p r i n g t i d e c o n d i t i o n s a t l o w a r e a s i n t h e b a r r i e r dune l i n e .
S t o r m o v e r w a s h c a n o c c u r t h r o u g h n a r r o w dune c a p s , o v e r w i d e s e c t i o n s
w i t h l o w dune t o p o g r a p h y , o r o v e r an e n t i r e s t r e t c h o f a b a r r i e r i s l a n d
(Fig.
28) . The
s h a p e and d i m e n s i o n o f t h e w a s h o v e r d e p o s i t i s
d e t e r m i n e d by t h e b a c k s h o r e t o p o g r a p h y and t h e volume o f sand b e i n g
i n t r o d u c e d by t h e overwash surges ( F i g . 2 9 ) . Large overwash fans can
t e r m i n a t e i n t h e l a g o o n o r bay b e h i n d t h e b a r r i e r beach. I f s c o u r o f
t h e b a r r i e r p r o c e e d s t o an e l e v a t i o n b e l o w mean sea l e v e l d u r i n g t h e
o v e r w a s h p e r i o d , an i n l e t can r e s u l t . O v e r w a s h g e n e r a l l y o c c u r s w h e r e
t h e d u n e s y s t e m has b e e n weakened, e i t h e r n a t u r a l l y b y b l o w n o u t s , o r
a r t i f i c i a l l y b y man.
Where t h e b a r r i e r i s w i d e w i t h e x t e n s i v e b a r r i e r f l a t s o r s a l t marshes,washovers r a t h e r t h a n i n l e t s w i l l g e n e r a l l y o c c u r . Overwash s u r g e s
q u i c k l y l o s e t h e i r i n i t i a l , h i g h v e l o c i t i e s due t o p e r c o l a t i o n a n d
f r i c t i o n a l e f f e c t s as t h e b o r e o f w a t e r c r o s s e s t h e b a r r i e r f l a t s .
Hence e r o s i o n i s m i n i m i z e d and s a n d d e p o s i t i o n o c c u r s i n t h e f a n ( F i g
27) as t h e s u r g e e n e r g y d r o p s t o a l o w l e v e l .
For a s e c t i o n o f t h e L o u i s i a n a c o a s t l i n e t o t h e e a s t o f t h e I s l e s
D e r n i e r e s , R i t c h i e a n d P e n l a n d (1988) d e f i n e d f o u r g e n e r a l b a r r i e r
i s l a n d m o r p h o l o g i e s : w a s h o v e r f l a t s , w a s h o v e r t e r r a c e s , dune t e r r a c e s ,
and c o n t i n u o u s d u n e s ( F i g . 3 0 ) .
R e v e g e t a t i o n o f b a r r e n washover f l a t s p r o g r e s s e s r a p i d l y a f t e r
l i n e has b e e n r e s t o r e d . From f i e l d i n s p e c t i o n i t i s s o m e t i m e s
t o i d e n t i f y o l d washover areas, b u t t h e i r e x i s t e n c e can
v e r i f i e d by u s i n g h i s t o r i c a l a e r i a l photography (Leatherman,
12
t h e dune
difficult
o f t e n be
1982) .
Washover Transport on Sandbars
3.7
Barrier
Flats
B a r r i e r f l a t s a r e l o c a t e d on t h e l a g o o n a l s i d e o f b a r r i e r i s l a n d s a n d
have n e g l e c t a b l e r e l i e f
( g e n e r a l l y l e s s t h e n 2 m a b o v e mean w a t e r
l e v e l ) . These a r e a s may r e s u l t f r o m t h e d e s t r u c t i o n o f d u n e a r e a s b y
overwash and i n l e t p r o c e s s e s ( F i g . 3 1 ) .
Dunes a r e t h e m a j o r t o p o g r a p h i c f e a t u r e o f t h e i n t e r i o r o f n o r t h e a s t
b a r r i e r beaches. T h e r e f o r e , b a r r i e r f l a t s are q u i t r e s t r i c t e d i n a r e a l
e x t e n t . I n some c a s e s , dunes and s a l t m a r s h e s s h a r e a common b o r d e r ,
and no t r u e f l a t s e x i s t , as i s t h e c a s e on p a r t s o f N a u s e t S p i t , Cap
Cod, M a s s a c h u s e t t s . However, m i d - A t l a n t i c and s o u t h e a s t e r n U.S.
coast
b a r r i e r s a r e more t y p i c a l l y b r o a d and l o w and h a v e e x t e n s i v e f l a t s .
B a r r i e r f l a t s c o v e r e d w i t h g r a s s l a n d o r meadow v e g e t a t i o n a r e a d a p t e d
t o f r e q u e n t sand b u r i a l and f l o o d i n g by overwash. W i t h
decreasing
o v e r w a s h f r e q u e n c y , e i t h e r b e c a u s e o f i n c r e a s i n g d\me g r o w t h o r l a c k o f
s t o r m s , b a r r i e r f l a t s w i l l s u p p o r t t h i c k e t s , f o l l o w e d b y w o o d l a n d s and
finally forest.
3.8
Shoreface/Beach
A b e a c h c a n be d e f i n e d as an a c c u m u l a t i o n o f wave-washed,
loose
sediment t h a t e x t e n d s between t h e o u t e r m o s t b r e a k e r s and t h e l a n d w a r d
l i m i t o f wave and s w a s h a c t i o n . A g e n e r a l i z e d b e a c h p r o f i l e a n d r e l a t e d
t e r m i n o l o g y a r e shown i n f i g u r e 32. The b e a c h c o n s i s t s o f t h e b a c k s h o r e
and f o r e s h o r e . The b a c k s h o r e , w h i c h i n c l u d e s t h e b e r m , i s s u b j e c t t o
wave a c t i o n o n l y d u r i n g s t o r m
c o n d i t i o n s . The
foreshore
is
the
i n t e r t i d a l a r e a t h a t l i e s seaward o f t h e berm c r e s t .
Where waves b r e a k o f f s h o r e , s u b m e r g e d r i d g e s o r b a r s o f c o a r s e s e d i m e n t
a r e p r e s e n t . M o s t waves b r e a k a t t h e i n n e r b a r t h a t p a r a l l e l s t h e s h o r e
o f a b a r r i e r i s l a n d , b u t some s m a l l e r waves may
not break
until
r e a c h i n g t h e w a t e r ' s edge. B a r s , by d e f i n i t i o n , a r e submerged a t a l l
t i m e s . A l o n g s h o r e f e a t u r e t h a t becomes e x p o s e d a t l o w t i d e i s c a l l e d
a r i d g e . The
t r o u g h b e t w e e n t h e r i d g e and b e a c h b e r m i s t e r m e d a
r u n n e l . R i d g e s and r u n n e l s a r e commonly f o u n d a l o n g m e s o t i d a l
coasts.
Waves a r e t h e p r e v a i l i n g s o u r c e o f e n e r g y d e l i v e r e d t o t h e s h o r e f a c e
and a r e p r i m a r i l y t h e a g e n t s r e s p o n s i b l e f o r s h a p i n g an " e q u i l i b r i u m "
u p p e r s h o r e f a c e p r o f i l e . I n g e n e r a l , h i g h waves w i t h s h o r t p e r i o d s
c a u s e t h e b e a c h t o e r o d e , and t h e b e r m s a n d i s s h i f t e d o f f s h o r e . Low
waves w i t h l o n g e r p e r i o d s , p r i m a r i l y i n summer, move s a n d f r o m t h e b a r
and r e t u r n i t t o t h e b e r m . The a v e r a g e wave h e i g h t v a r i e s o n a s e a s o n a l
b a s i s , r e s u l t i n g i n t y p i c a l " w i n t e r " ( h i g h e n e r g y ) a n d "summer" ( l o w
e n e r g y ) p r o f i l e s . The w i n t e r p r o f i l e i s c h a r a c t e r i z e d b y a n a r r o w
b a c k s h o r e , f l a t f o r e s h o r e , and a l a r g e o f f s h o r e b a r . The summer p r o f i l e
has a w i d e b a c k s h o r e , a s t e e p f o r e s h o r e , and a w e l l - d e v e l o p e d b e r m .
W i t h b r e a k i n g , waves a r e t r a n s l a t e d i n t o swash, w h i c h i s a t h i n s h e e t
o f w a t e r f l o w i n g o v e r t h e b e a c h f a c e . Swash c o n s i s t s o f t w o m o t i o n s :
(1) u p r u s h , c r e a t e d b y i n i t i a l wave b r e a k and (2) backwash, w h i c h i s
t h a t p o r t i o n o f t h e w a t e r t h a t r e t u r n s s e a w a r d . The
swash p r o c e s s
r e s u l t s i n s e d i m e n t a c c r e t i o n , c r e a t i n g a b e a c h t e r r a c e known as a b e r m
( F i g . 3 2 ) . The c r e s t o f a b e r m may r i s e s e v e r a l m e t e r s a b o v e mean sea
l e v e l . The h e i g h t l i m i t a t i o n f o r a b e r m i s d e t e r m i n e d b y t h e s i z e o f
t h e waves; l a r g e s w e l l waves c a n b u i l t s u b s t a n t i a l b e r m s . S e v e r e
s t o r m s , however, o f t e n r e s u l t i n beach e r o s i o n w i t h berm f l a t t e n i n g .
A l t h o u g h a d j u s t m e n t t o an e q u i l i b r i u m p r o f i l e t a k e s p l a c e c o n t i n u o u s l y
w i t h t h e t i d a l c y c l e and t h e s e a s o n a l l y , l a r g e - s c a l e changes a r e most
13
Washover Transport on Sandbars
e v i d e n t d u r i n g a c o a s t a l storm. W i t h i n t e n s s t o r m s , t h e b a r r i e r dunes
may be e r o d e d and b r e a c h e s b y o v e r w a s h s u r g e s . B e a c h c h a n g e s t a k e p l a c e
w i t h i n a very short time i n t e r v a l , l a s t i n g from o n l y one-half a t i d a l
c y c l e t o s e v e r a l d a y s . S w e l l u s u a l l y f o l l o w s sea c o n d i t i o n s as a l o w
p r e s s u r e c e l l ( c o a s t a l s t o r m ) moves a l o n g a n d / o r o f f s h o r e . The
swell
moves t h e s a n d b a c k o n s h o r e , o f t e n b y t h e p r o c e s s o f r i d g e and r u n n e l
migration.
From t h e b e r m c r e s t t o t h e t o e o f t h e p r i m a r y dune i s t h e z o n e t e r m e d
the beach backshore. This area i s r a r e l y s u b j e c t t o h y d r a u l i c (water)
a c t i o n , except during extremely h i g h s p r i n g t i d e s or c o a s t a l
storm
a c t i v i t y (Fig. 32).
Sand a c c r e t i o n can o n l y p r o c e e d as h i g h as t h e s t o r m s u r g e s c a n r e a c h
w i t h o u t t h e development o f dunes. V e g e t a t i o n
is critical
i n this
b u i l d i n g and s t a b i l i z a t i o n p r o c e s s ; o t h e r w i s e , t h e sand w o u l d c o n t i n u e
m o v i n g as i t does i n t h e d e s e r t . A f t e r s a n d i s d e p o s i t e d o n t h e b e a c h
b a c k s h o r e b y waves, t h e w i n d s t a k e o v e r and e i t h e r move t h e s a n d i n l a n d
o r b a c k o u t t o sea, d e p e n d i n g on t h e p r e v a i l i n g w i n d d i r e c t i o n . W i n d
v e l o c i t i e s o f 12 mph o r g r e a t e r a r e c a p a b l e o f m o v i n g f i n e , d r i e d b e a c h
sand (Leatherman, 1982).
A c c o r d i n g t o L e a t h e r m a n (1979) , coastal
r o l e s d u r i n g storms, f u n c t i o n i n g as:
dunes p l a y a t l e a s t t h r e e m a j o r
(1) s a n d r e s e r v o i r s
(2) e n e r g y d i s s i p a t o r s
(3) b a r r i e r s t o s t o r m waves and
swash.
The
p o s i t i o n and h e i g h t o f b a r r i e r dunes d e p e n d s on a number o f
f a c t o r s . The h e i g h t o f t h e dune i s c o n t r o l l e d p r i m a r i l y b y s a n d s i z e
and w i n d v e l o c i t y . S e a w a r d dune v e g e t a t i o n g r o w t h i s c o n t r o l l e d b y wave
a c t i o n a n d s o i l s a l i n i t y . The s e a w a r d dune p o s i t i o n a l s o d e p e n d s u p o n
t h e f r e q u e n c y o f s t o r m s e r o d i n g t h e dune f a c e and t h e r a t e a t w h i c h
t h e s e s c a r p s can h e a l b y w i n d t r a n s p o r t and v e g e t a t i o n g r o w t h .
Sand t r a n s p o r t e d o v e r t h e dune c r e s t r e s u l t s i n l a n d w a r d movement o f
t h i s f e a t u r e . On e r o d i n g s h o r e l i n e s , dune s y s t e m s u r v i v a l d e p e n d s u p o n
t h e r e l a t i v e r a t e s o f dune m i g r a t i o n and s h o r e l i n e r e t r e a t .
During
p e r i o d s o f i n c r e a s e d s h o r e l i n e e r o s i o n , t h e dunes may be e r o d e d a n d
breached d u r i n g storm c o n d i t i o n s .
14
Washover Transport on Sandbars
4.
PROCESSES OF
4.1
General
BARRIER ISLAND EVOLUTION
The l o n g - t e r m b e h a v i o u r o f a b a r r i e r i s l a n d d e p e n d s p r i n c i p a l l y u p o n
t h e r a t e o f sea l e v e l r i s e , s a n d s u p p l y , and sea e n e r g y ( F i g 3 3 ) . Human
i n t e r v e n t i o n c a n a l s o be an i m p o r t a n t f a c t o r when m o d e r n t e c h n o l o g y i s
a p p l i e d . Sand s u p p l y i s o f t e n t h e k e y t o a b a r r i e r i s l a n d ' s e v o l u t i o n .
W i t h a r e l a t i v e c o n s t a n t s u p p l y o f s e d i m e n t , t h e i s l a n d can
maintain
i t s e l f i n p l a c e and b u i l d u p w a r d w i t h sea l e v e l r i s e ( F i g 3 4 ) . W i t h a n
excess sand supply, t h e b a r r i e r can a c t u a l l y a c c r e t e seaward.
B a r r i e r i s l a n d s a r e s u b j e c t e d t o e r o s i o n a l and d e p o s i t i o n a l p r o c e s s e s .
The m o s t g e n e r a l o f t h e s e p r o c e s s e s a r e t h e r i s e o f sea l e v e l ,
a
process t h a t r e s u l t s i n b a r r i e r i s l a n d transgression,
or
onshore
m i g r a t i o n . Other processes i n c l u d e t i d a l - i n l e t m i g r a t i o n which r e s u l t s
i n reworking
o f l a r g e p o r t i o n s o f b a r r i e r i s l a n d s . Storms p l a y
an
i m p o r t a n t r o l e i n washover s e d i m e n t a t i o n ,
windblown ( a e o l i a n ) sand
t r a n s p o r t , i n l e t f o r m a t i o n , and b a y i n f i l l i n g . F l o o d - d o m i n a t e d i n l e t
sedimentation
i s responsible f o r p a r t i a l i n f i l l i n g of shallow
water
lagoons. Flood-tide deltas weld onto u p d r i f t portions of
adjacent
b a r r i e r s r e s u l t i n g i n wide, t h i c k accumulations of sediments.
Nummedal (1983) p o i n t e d o u t t h a t w a s h o v e r d e p o s i t s and t h e s e d i m e n t s i n
t h e m i g r a t i n g i n l e t c h a n n e l s r e m a i n a p a r t o f t h e l a n d w a r d and u p w a r d
m o v i n g b a r r i e r i s l a n d as t h e y a d j u s t t o r i s i n g sea l e v e l . The w a s h o v e r
and i n l e t d e p o s i t s a r e t h e p r e d o m i n a n t m a t e r i a l i n a l l t h e m i d - A t l a n t i c
b a r r i e r s . These d e p o s i t s r e m a i n i n t h e b a r r i e r complex a l t h o u g h
they
a r e f r e q u e n t l y f o u n d i n t h e l a g o o n s b e h i n d t h e i s l a n d s as s u b m e r g e d
p a r t s of the m i g r a t i n g b a r r i e r s . I f t h i s " r o l l o v e r " process of b a r r i e r
i s l a n d t r a n s g r e s s i o n were n o t t r u e t h e i s l a n d s w o u l d have d i s a p p e a r e d
l o n g ago ( F i g . 3 5 ) .
In
the
f o l l o w i n g paragraphs
p r o c e s s e s w i l l be d i s c u s s e d .
4.2
Sea
five
major
barrier
island
evolution
level rise
As p l e i s t o c e n e g l a c i e r s m e l t e d a n d sea l e v e l r o s e , t h e r i v e r v a l l e y s
w e r e f l o o d e d a n d much o f t h e r i v e r i n e s e d i m e n t s h a v e b e e n t r a p p e d i n
c o a s t a l e s t u a r i e s i n s t e a d o f r e a c h i n g b a r r i e r s h o r e l i n e s . W i t h no
new
s e d i m e n t o r s e d i m e n t l o s s , a b a r r i e r i s l a n d m u s t r e t r e a t l a n d w a r d up
t h e c o a s t a l p l a i n i n o r d e r t o m a i n t a i n a c o n s t a n t e l e v a t i o n and p r e v e n t
s u b m e r g e n c e ( d r o w n i n g ) w i t h sea l e v e l r i s e ( F i g 36) . Sand f r o m t h e
o c e a n s i d e o f t h e b a r r i e r i s t r a n s p o r t e d b y w a t e r and w i n d t o w a r d t h e
b a c k s i d e o f t h e i s l a n d so t h a t t h e w h o l e l a n d f o r m g r a d u a l l y c h a n g e s i t s
location.
F o r c o a s t s w i t h a b r o a d , g e n t l e s l o p i n g s u r f a c e , a s m a l l r i s e i n sea
l e v e l w i l l r e s u l t i n a dramatic h o r i z o n t a l r e t r e a t of the land (Fig.
36) . F o r a g i v e n r i s e i n sea l e v e l , a b a r r i e r i s l a n d m u s t
migrate
l a n d w a r d a d i s t a n c e o f two o r t h r e e o r d e r s o f m a g n i t u d e o f t h i s v a l u e
( v e r t i c a l r i s e ) , as shown i n f i g u r e 36. From t h e e v i d e n c e a v a i l a b l e , i t
i s c l e a r t h a t t h e b a r r i e r s l o c a t e d on t h e c o a s t a l p l a i n s o f t h e
U.S.
a r e m i g r a t i n g l a n d w a r d o v e r t h e l o n g t e r m as sea l e v e l r i s e s .
T h u s , r i s i n g sea
l e v e l i s the primary
d r i v i n g force f o r landward
movement o f t h e s h o r e l i n e . T h r o u g h g e o l o g i c a l t i m e sea l e v e l has a l w a y s
b e e n r i s i n g o r f a l l i n g r e l a t i v e t o t h e l a n d s u r f a c e . The l a s t m a j o r
15
Washover Transport on Sandbars
change i n s e a l e v e l o c c u r r e d d u r i n g t h e m o s t r e c e n t I c e Age, when sea
l e v e l was a p p r o x i m a t e l y 100 m e t e r s l o w e r t h a n a t p r e s e n t . The v o l u m e o f
w a t e r i n t h e sea i n c r e a s e d v e r y r a p i d l y a f t e r i c e m e l t i n t e n s i f i e d
a b o u t 15,000 y e a r s ago, w i t h sea l e v e l r i s i n g a t a f a s t e r r a t e t h e n
d u r i n g t h e p a s t 5,000 y e a r s . P r e s e n t l y , t h e r a t e o f sea l e v e l r i s e ,
r e l a t i v e t o l a n d , a v e r a g e s 40 cm p e r c e n t u r y a l o n g t h e N o r t h e a s t c o a s t
o f t h e U.S. Where t h e l a n d i s a l s o s x i b s i d i n g , r e l a t i v e sea l e v e l r i s e
has b e e n much g r e a t e r .
W h i l e m e l t i n g o f t h e p o l a r i c e c a p c o n t i n u e s t o be a m a j o r c a u s e o f
r i s i n g sea l e v e l , t h e r a t e o f r i s e i s e x p e c t e d t o a c c e l e r a t e i n t h e
f u t u r e due t o i n c r e a s i n g g l o b a l t e m p e r a t u r e s , d r i v e n b y h i g h e r l e v e l s
o f c a r b o n d i o x i d e i n t h e a t m o s p h e r e . Due t o t h e " g r e e n h o u s e e f f e c t " ,
h i g h e r a v e r a g e g l o b a l t e m p e r a t u r e s c o u l d r e s u l t i n as much as a 2 m
r i s e w i t h i n t h e n e x t 100 y e a r s , w h i c h c o u l d be c a t a s t r o p h i c f o r m a j o r
c i t i e s l o c a t e d i n l o w - l y i n g c o a s t a l areas
(e.g. Washington
D.C,
London).
O r f o r d e t a l . (1991) d e t e r m i n e d t h e r e l a t i o n s h i p b e t w e e n sea l e v e l r i s e
and g r a v e l b a r r i e r m i g r a t i o n f o r t h e S t o r y Head b a r r i e r . Nova S c o t i a ,
Canada. The r e s u l t s a r e shown i n a p p e n d i x 1 .
I t i s c l e a r l y t h a t t h e s u b m e r g e n c e o f b a r r i e r i s l a n d due t o v a r i o u s
p r o c e s s e s a l o n g t h e c o a s t has t h e same e f f e c t o n t h e m i g r a t i o n o f
b a r r i e r i s l a n d s as sea l e v e l r i s e .
4.3
Storm
Events
Sea l e v e l r i s e b y i t s e l f w o u l d n o t be s u f f i c i e n t t o d r i v e t h e b a r r i e r
b e a c h s y s t e m l a n d w a r d . The m a j o r amount o f s a n d t r a n s p o r t o c c u r s i n a
quantum f a s h i o n d u r i n g extreme e v e n t s . Storms t h a t e f f e c t t h e c o a s t l i n e
f a l l i n t o t w o b a s i c c a t e g o r i e s : (1) h u r r i c a n e s a n d (2) " N o r m a l " s t o r m s .
Near t h e c e n t r e o f a h u r r i c a n e , w i n d s may g u s t t o more t h a n 2 00 m i l e s
p e r h o u r . W h i l e i n t e n s i v e winds a r e t h e most o b v i o u s t h r e a t t o l i f e and
p r o p e r t y a n d do much damage, m a s s i v e s t o r m s u r g e s a r e b y f a r t h e
g r e a t e s t cause o f d e a t h s and d e s t r u c t i o n on b a r r i e r beaches. S i x
t h o u s a n d p e o p l e d i e d on a b a r r i e r i s l a n d - G a l v e s t o n I s l a n d - d u r i n g a
1900 h u r r i c a n e s u r g e w h i c h s t i l l s t a n d s as t h e w o r s t n a t u r a l d i s a s t e r
i n U.S.
history.
A s t o r m s u r g e i s a s u p e r - e l e v a t e d mound o f w a t e r t h a t sweeps a c r o s s t h e
c o a s t l i n e n e a r t h e a r e a w h e r e a h u r r i c a n e p a s s e s o r makes l a n d f a l l .
S u r g e a n d h u r r i c a n e d r i v e n waves a c t i n d e a d l y c o m b i n a t i o n t o hammer
t h e s h o r e , s w e e p i n g a c r o s s any l o w - l y i n g c o a s t a l a r e a s . B a r r i e r i s l a n d s
being the o u t e r shorelines, are forced t o absorb the storm brunt,
r e d u c i n g t h e f o r c e o f wave and w i n d b e f o r e t h e y r e a c h t h e m a i n l a n d .
R i t c h i e & Penland
(1987) i n v e s t i g a t e d t h e i m p a c t s o f s t o r m s
and
h u r r i c a n e s o n t h e Caminada-Moreau b a r r i e r h e a d l a n d , a l o n g t h e d e l t a i c
coast o f South L o u i s i a n a . This coast i s a storm-dominated, M i c r o t i d a l
e n v i r o n m e n t w i t h a mean d i u r i n i a l r a n g e o f 36 cm. N o r m a l l y , t h e p a s s a g e
o f f r o n t a l s y s t e m s e l e v a t e s w a t e r l e v e l s up t o 90 cm, 10 t o 30 t i m e s
p e r y e a r ( B o y d a n d P e n l a n d , 1 9 8 1 ) . These s t o r m s o f t e n p r o d u c e waves 2
t o 3 m h i g h ; t h e l o c a l a v e r a g e wave h e i g h t s a r e a b o u t 60 cm. T r o p i c a l
s t o r m s ( w i n d s o v e r 63 km/h) h a v e a r e c u r r e n c e i n t e r v a l o f 1.6 y r . , a n d
h u r r i c a n e s ( w i n d s o v e r 118 km/h) h a v e a l o n g e r r e c u r r e n c e i n t e r v a l o f
4.1 y r . (Sympson & L a w r e n c e , 1 9 7 1 ) . T h e s e s t o r m s r a i s e w a t e r l e v e l s
c o n s i d e r a b l y h i g h e r t h a t t h e more common c o l d f r o n t p a s s a g e , n o r m a l l y
2 t o 7 m a b o v e mean w a t e r l e v e l . ( T a b l e 4 . 1 ) . D u r i n g t h e s e s t o r m s t h e
f r e q u e n c y and i n t e n s i t y o f overwash i s a f u n c t i o n o f t h e w a t e r l e v e l
16
Washover Transport on Sandbars
e l e v a t i o n p r o d u c e d b y a c o m b i n a t i o n o f s t o r m s u r g e , wave s e t - u p , wave
r u n - u p and a s t r o n o m i c a l t i d e s e t t i n g s a g a i n s t t h e v a r i a b l e ,
pree x i s t i n g e l e v a t i o n s o f t h e c o a s t l i n e . Measurements o f o v e r w a s h e v e n t s
has shown t h a t t h e r e g i o n a l o v e r w a s h t h r e s h o l d i s 1.42 m a b o v e mean sea
l e v e l , w h i c h w i l l p r o d u c e o v e r w a s h a l o n g 72% o f t h e c o a s t l i n e
an
a v e r a g e o f 15 t i m e s p e r y e a r ; a t a l e v e l o f 2.50
m most o f t h e
c o a s t l i n e w i l l be i n u n d a t e d .
T a b l e 4.1: P o t e n t i a l o v e r w a s h e l e v a t i o n s i n c o a s t a l
L o u i s i a n a ( m o d i f i e d f r o m B o y d a n d P e n l a n d , 1981)
Event
Overwash
elevation
Minor c o l d
front
1.42-1.73
Major cold
front
Frequency
(m)
10-15
per
yr
per
yr
1.73-2.02
5-10
hurricane
1.73-2.92
1 per
8 yrs
Force 2 h u r r i c a n e
2.66-4.46
1 per
10
yrs
Force 3 h u r r i c a n e
3.90-7.00
1 per
32
yrs
Force 1
A g e n e r a l i z e d c o n c e p t u a l m o d e l f o r s a n d dune d e v e l o p m e n t a l o n g t h e
Caminada-Moreau c o a s t i n L o u i s i a n a has b e e n s e t up b y R i t c h i e
and
P e n l a n d (1987) ( F i g . 3 7 ) , as i t r e l a t e s t o t h e r o l e o f s t o r m s a n d t h e
r e t u r n p e r i o d o f h u r r i c a n e i m p a c t . The m o d e l r e p r e s e n t s a n i n c r e a s i n g
v o l u m e o f s u p r a t i d a l s a n d s t o r a g e l e a d i n g t o d u n e d e v e l o p m e n t and
revegetation, i n c r e a s i n g the s t a b i l i t y of the b a r r i e r s h o r e l i n e . Major
storms c o r r e l a t e t o h u r r i c a n e s and m i n o r s t o r m s c o r r e l a t e t o c o l d
fronts.
F i g u r e 38 shows t h e e f f e c t o f h u r r i c a n e Bob
(1979) on t h e
c r o s s - s e c t i o n a l a r e a o f a b a r r i e r i s l a n d a l o n g t h e Caminada-Moreau
coast.
4.4
Inlet
Dynamics
I n l e t s are i m p o r t a n t f o r b a r r i e r i s l a n d m i g r a t i o n by d e v e l o p i n g f l o o d
t i d a l d e l t a s . A p o r t i o n o f s e d i m e n t moving a l o n g a c o a s t i s swept
t h r o u g h a n i n l e t a n d i n t o a l a g o o n b y t h e f l o o d c u r r e n t s ( F i g . 23, 24
and 2 5 ) . S i n c e b a y w a t e r i s l e s s t u r b u l e n t t h a n t h a t a l o n g t h e o c e a n
s h o r e , s a n d s e t t l e s o u t t o f o r m a t i d a l d e l t a . Some s e d i m e n t i s moved
b a c k o u t t o sea as t h e c u r r e n t r e v e r s e s on t h e ebb t i d e , b u t u s u a l l y
t h e n e t c h a n g e r e s u l t i n b a y s e d i m e n t d e p o s i t i o n . The g r o w t h r a t e o f a
f l o o d t i d a l d e l t a i s a m e a s u r e o f t h e amount o f s a n d b e i n g t r a p p e d f r o m
the l i t t o r a l d r i f t by t h e i n l e t . F l o o d t i d a l d e l t a s s e r v e as p l a t f o r m s
u p o n w h i c h s a l t m a r s h e s c a n d e v e l o p o n c e an i n l e t c l o s e s o r m i g r a t e s
d o w n d r i f t ( F i g . 23 a n d 3 9 ) .
There i s evidence t h a t a c y c l e o f i n l e t f o r m a t i o n , marsh development,
a n d o v e r w a s h o c c u r s on m o s t b a r r i e r b e a c h e s as t h e y m i g r a t e l a n d w a r d
( L e a t h e r m a n , 1982) . An i n l e t w i l l o c c u r a t a n a r r o w p o i n t a l o n g a
b a r r i e r beach, t h e n m i g r a t e d o w n d r i f t u n t i l i t c l o s e s o r reaches t h e
end o f t h e s p i t . ( F i g . 4 0 ) . W i t h t i m e , t h e b a r r i e r w i l l m i g r a t e o v e r
o l d e r i n l e t or overwash d e p o s i t s .
I t i s known t h a t a g i v e n volume o f w a t e r i n a t i d a l p r i s m t e n d s t o
maintain a constant size cross-section i n a t i d a l i n l e t or i n a series
o f i n l e t s . The s i t u a t i o n seems a n a l o g o u s t o t h a t f o r r i v e r s , w h e r e t h e
s e c t i o n area i s r e l a t e d t o t h e stream d i s c h a r g e . For t i d a l
inlets,
t h e r e a p p e a r s t o be a b a l a n c e b e t w e e n t h e s c o u r i n g a c t i o n o f t h e t i d a l
17
Washover Transport on Sandbars
c u r r e n t s t h a t keep t h e c h a n n e l open, a n d t h e l o n g s h o r e t r a n s p o r t o f
beach sand t h a t tends t o c l o s e them (Johnson, 1919), see a l s o f i g u r e
40. B o t h O ' B r i e n
(1969) a n d J a r r e t t
(1976) h a v e shown a b i v a r i a t e
r e l a t i o n s h i p between c r o s s - s e c t i o n a l area o f channel and t i d a l p r i s m o f
the form:
A = n\r
Where A =
V =
n =
m =
minimum c r o s s - s e c t i o n a l a r e a o f t h e i n l e t
m e a s u r e d b e l o w M.S.L.
volume o f t i d a l p r i s m between s p r i n g t i d e s
constant
constant.
channel
The v a l u e s o f t h e c o n s t a n t s f o r A t l a n t i c c o a s t i n l e t s a r e m =
1.05 a n d n = 3.04 X 10"^ when A a n d V a r e i n m^ a n d m' (USAGE CERC,
1 9 8 4 a ) . F o r O r e g o n I n l e t , J a r r e t t (1976) l i s t s t h e s p r i n g t i d a l
p r i s m a s V = 133 x 10' m' a n d t h e c h a n n e l c r o s s - s e c t i o n a l a r e a a s
A = 6190 m^ w i t h a h y d r a u l i c r a d i u s o f 4.1 m.
4.5
Overwash p r o c e s s e s
The s e c o n d p r i n c i p a l m e t h o d b y w h i c h s e d i m e n t i s t r a n s p o r t e d a c r o s s t h e
b a r r i e r i s l a n d i s o v e r w a s h . O v e r w a s h may b e c o n s i d e r e d a s a h i g h e r
m a g n i t u d e e x t e n s i o n o f o v e r t o p p i n g , i n t h a t t h e v o l u m e o f swash t h a t
moves o v e r t h e c r e s t i s s u f f i c i e n t t o c r e a t e a u n i d i r e c t i o n a l f l o w o n
the b a c k s l o p e , e r o d i n g t h e upper back beach, l o w e r i n g t h e c r e s t , a n d
d e p o s i t i n g m a t e r i a l on t h e landward s i d e ( C a r t e r & O r f o r d , 1981).
The m a g n i t u d e a n d f r e q u e n c y o f o v e r w a s h d e p e n d s o n t h e f o l l o w i n g
elements:
- b a r r i e r exposure and o r i e n t a t i o n
- s h o r e - p a r a l l e l v a r i a t i o n o f t h e beach c r e s t l i n e .
- frequency o f major storms
- wave e n e r g y
- t i d a l range
- beach sediment
- e c o l o g i c a l response o f v e g e t a t i o n t o t h e overwash process
I n p a r a g r a p h 5.2.2 e x a m p l e s o f t h e q u a n t i f i c a t i o n o f o v e r w a s h c o m p a r e d
w i t h t h e t o t a l sand l o s s o f a b a r r i e r i s l a n d w h i c h i s exposed t o t h e
above mechanisms w i l l be d i s c u s s e d .
Normally overwashing would occur d u r i n g high-magnitude, low-frequency
s t o r m s w h e r e swash f l o w s c r e a t e a n d o c c u p y n a r r o w c h a n n e l s t h r o u g h
b e a c h o r dune r i d g e s , w h i c h a c t a s c o n d u i t t r a n s f e r r i n g s e d i m e n t i n t o
c h a r a c t e r i s t i c fan-shaped s i n k s . This process p r o v i d e s an i m p o r t a n t
mechanism
f o r shoreline
migration.
Where
tide
ranges
are low
( m i c r o t i d a l ) and storm f r e q u e n c y h i g h , overwash i s a r e g u l a r event on
barrier islands without naturally high or a r t i f i c i a l l y - s t a b i l i z e d
dvines. C o n v e r s e l y , w h e r e t i d e r a n g e s a r e l a r g e ( m e s o t i d a l ) , t h e c h a n c e
t h a t a storm w i l l a r r i v e a t t h e highest t i d e l e v e l s and overtop t h e
b a r r i e r i s c o n s i d e r a b l y l e s s , a n d hence overwash i s a n i n f r e q u e n t e v e n t
(Leatherman, 1982).
The e c o l o g i c a l r e s p o n s e t o o v e r w a s h i s d i f f e r e n t o n N o r t h e r n a n d
S o u t h e r n b a r r i e r beaches
( L e a t h e r m a n 79) . G o d f r e y
(1969) made a n
o v e r w a s h m o d e l b a s e d o n C o r e B a n k s , N o r t h C a r o l i n a , t h a t shows t h e
e c o l o g i c a l response t o overwash on a r e t r e a t i n g b a r r i e r i s l a n d ( F i g .
41) .
18
Washover Transport on Sandbars
4.6
Aeolian
processes
Overwash s e d i m e n t a t i o n i s e p i s o d i c , o c c u r r i n g d u r i n g d i s c r e t e s t o r m
e v e n t s o f v a r y i n g m a g n i t u d e a n d f r e q u e n c y . Aeolian processes
(windb l o w n p r o c e s s e s ) , w h i c h o p e r a t e on a more c o n t i n u a l b a s i s , can d e p o s i t
o r e r o d e s a n d f r o m w a s h o v e r f a n s u r f a c e s . L e a t h e r m a n (1976a) c o n c l u d e d
t h a t a e o l i a n p r o c e s s e s were s l i g h t l y more d o m i n a n t t h a n o v e r w a s h i n
a c c o u n t i n g f o r changes i n sediment b u d g e t s a l o n g n o r t h e r n A s s a t e a g u e
I s l a n d , V i r g i n i a - M a r y l a n d , U.S. d u r i n g t h e m i d - 1 9 7 0 ' s . L e a t h e r m a n a l s o
o b s e r v e d t h a t a e o l i a n d e f l a t i o n b e t w e e n o v e r w a s h e v e n t s was s i g n i f i c a n t
enough t o r e s u l t i n n e t l o w e r i n g o f f a n s u r f a c e s by d e f l a t i o n o v e r t h e
p e r i o d o f h i s s t u d y . S i m i l a r h i g h d e f l a t i o n r a t e s were o b s e r v e d
by
F i s h e r a n d S t a u b l e (1977) on N o r t h A s s a t e a g u e f o l l o w i n g a p e r i o d o f
e x c e s s i v e o f f s h o r e w i n d s d u r i n g t h e w i n t e r o f 1976-1977.
A l t h o u g h mean w i n d v e l o c i t y a n d f r e q u e n c y o f w i n d y d a y s i s i m p o r t a n t
f o r d e f l a t i o n , t h e v a r i a b i l i t y i n w i n d seems t o be l e s s i m p o r t a n t t h a n
the
m a g n i t u d e and f r e q u e n c y o f f a n w e t t i n g e v e n t s . P e r i o d s o f h i g h
a c c u m u l a t i o n c a n o c c u r when w a s h o v e r f a n s r e m a i n w e t , c o r r e l a t i n g w i t h
p e r i o d s o f f r e q u e n t overwash and p r e c i p i t a t i o n . S t u d i e s o f t h e r e l a t i v e
r o l e o f o v e r w a s h and a e o l i a n p r o c e s s e s on washover f a n s ( K o c h e l and
W a m p f l e r , 1 9 8 8 ) showed t h a t t h e a e o l i a n e r o s i o n o f f a n s u r f a c e s i s
g r e a t l y e f f e c t e d b y how wet t h e f a n r e m a i n s , due t o t h e c o m b i n a t i o n o f
overwash
flooding
and
precipitation,
because wet
fans are
more
resistant to deflation.
4.7
Littoral
Drift
The movement o f s a n d a l o n g a b e a c h i s a c c o m p l i s h e d b y
longshore
c u r r e n t s . As waves a p p r o a c h a b e a c h a t a n a n g l e , a c u r r e n t i s g e n e r a t e d
a l o n g t h e s h o r e , r e s u l t i n g i n s a n d movement i n t h i s d i r e c t i o n .
B a r r i e r b e a c h e s c a n f o r m i f s a n d i s t r a n s p o r t e d f r o m a s o u r c e , s u c h as
an e r o d i n g g l a c i a l h e a d l a n d ,
f o r d e p o s i t i o n i n t o open w a t e r .
The
l o n g s h o r e t r a n s p o r t o f s e d i m e n t b y wave a c t i o n t e n d s t o l e n g t h e n t h e
b e a c h i n t h e d i r e c t i o n t h a t waves g e n e r a l l y s t r i k e a c o a s t l i n e . T h e s e
c u r r e n t s f l o w p a r a l l e l t o s h o r e , and t h u s t e n d t o b u i l d r e l a t i v e l y
s t r a i g h t sediment d e p o s i t s a l o n g t h e c o a s t , even i f t h e s h o r e l i n e i s
i n d e n t e d t o f o r m a b a y ( F i g . 42) . The c o n f i g u r a t i o n o f t h e c o a s t c a n be
e x t r e m e l y v a r i e d a n d p r o j e c t i o n s n e e d n o t n e c e s s a r i l y be f o u n d a t b a y
m o u t h s , b u t t h e f l o w o f m a t e r i a l may be s l o w e d down a t a n y p o i n t w h e r e
the d i r e c t i o n o f t h e c o a s t changes s h a r p l y .
S p i t f o r m a t i o n t h r o u g h t h e mechanism o f l i t t o r a l d r i f t i s t h e p r i n c i p l e
p r o c e s s s h a p i n g b a r r i e r b e a c h e s . As a s p i t b u i l d s a c r o s s o p e n w a t e r t h e
e a r l y s t a g e s a r e d o m i n a t e d b y o v e r w a s h t h a t r e g u l a r l y moves s a n d f r o m
t h e o c e a n t o t h e b a y . A t t h e same t i m e , t i d a l c u r r e n t s a r e m o v i n g h u g e
q u a n t i t i e s o f sediment a r o u n d t h e end o f an a c c r e t i n g s p i t .
This
m a t e r i a l i s deposited i n the q u i t waters i n the lee of the p r o t e c t i v e
barrier.
4.8
R i v e r sediment
discharge
An i m p o r t a n t f a c t o r i n t h e s t u d y o f d e l t a s i s t h a t a l a r g e a m o u n t o f
m a t e r i a l i n s u s p e n s i o n may be b r o u g h t down b y r i v e r s i n t i m e o f f l o o d
a n d d e p o s i t e d o f f s h o r e . T h i s m a t e r i a l b u i l d s up t h e b o t t o m a n d h i n d e r s
t h e p r o d u c t i o n o f a p r o f i l e o f e q u i l i b r i u m b y wave a c t i o n . I f t h e r e i s
r e l a t i v e l y l i t t l e a l l u v i a l m a t e r i a l , t h e waves a r e a b l e t o move a l l t h e
m a t e r i a l shorewards and t h u s cause t h e c o a s t t o advance seawards. S m a l l
19
Washover Transport on Sandbars
river mouth b a r s o f t e n f o r m i n t h e s h a l l o w w a t e r i n f r o n t o f d e l t a s .
D e l t a s t h a t a r e g r o w i n g a c t i v e l y and a d v a n c i n g i n t o r e l a t i v e l y d e e p
seas a r e f r o n t e d b y a s h a l l o w zone as d e s c r i b e d a b o v e . T h i s z o n e , w h i c h
i s u s u a l l y f a i r l y narrow, i s bounded by a s t e e p s l o p e (perhaps
20°).
E x a m p l e s i n c l u d e t h e d e l t a s o f t h e Danube ( f i g . 4 3 ) , a n d t h e Rhone.
Submarine
furrows,
often
smoothed
and
gently
sloping,
are
c h a r a c t e r i s t i c f e a t u r e s i n t h e s h a l l o w w a t e r o f f f o r m e r and e x i s t i n g
r i v e r m o u t h s . A f t e r a r i s e o f sea l e v e l t h e y may
be r e g a r d e d
as
submerged r i v e r
courses,
but
their
true nature
can
usually
be
e s t a b l i s h e d ( Z e n k o v i c h , 1958) .
The g e n e r a l l y h e l d v i e w t h a t t h e b a r s always d e v e l o p f r o m a l l u v i a l
m a t e r i a l i s n o t t r u e . On t h e c o n t r a r y , t h e y a r e o f t e n composed e n t i r e l y
o f b e a c h m a t e r i a l , and more o f t e n on a m i x t u r e o f b o t h . B a r r i e r i s l a n d
w h i c h a r e f o r m e d by t h e d e p o s i t i o n o f r i v e r s , and a r e l a y i n g i n t h e
l o n g s h o r e c u r r e n t z o n e , h a v e an e r o s i o n a l p a t t e r n w h i c h v a r i e s f r o m
a l m o s t z e r o a t t h e a t t a c h e d end o f t h e i s l a n d t o a maximum e r o s i o n
s e a w a r d , w h e r e t h e wave e n t r a n c e i s m o s t o b l i q u e ,
and
thus
the
l o n g s h o r e c u r r e n t , i s maximum. When t h e r e i s n o t e n o u g h s e d i m e n t
s u p p l y , t h e s e b a r r i e r s p i t s r o t a t e due
t o the eroding
processes
l a n d w a r d , a n d t h e s e d i m e n t i s d e p o s i t e d a t t h e l e e s i d e a t t h e end o f
t h e b a r r i e r , f o r m i n g a b a c k b a r r i e r l a g o o n (Wai San T i n g s a n d b a r a t t h e
west c o a s t o f Taiwan, f i g u r e 4 4 ) .
The
coastal
plain,
near
the
river
mouth,
has
morphological
c h a r a c t e r i s t i c s r e l a t e d s t r o n g l y t o the hydrodynamic regime which i s
p r e s e n t a t t h e m o u t h o f t h e r i v e r . P a r a g r a p h 3.4 f i g u r e 26 showed t h e
v a r i o u s f o r m s o f ebb t i d a l d e l t a s on w h i c h b a r r i e r i s l a n d s c a n f o r m .
They d e p e n d o n
the r e l a t i v e magnitudes of the
f o r c e s o f dynamic
d i v e r s i o n o f t h e h y d r a u l i c c o n d i t i o n s . When b a r r i e r i s l a n d s o r s p i t s
f o r m o n t h e s e ebb
t i d a l d e l t a s , t h e r e s h a p e d e p e n d s on t h e m a j o r
c u r r e n t d i r e c t i o n a n d t h e s u p p l y o f s a n d . When t h e t i d a l c u r r e n t i s
a l m o s t e q u a l i n ebb and f l o o d d i r e c t i o n , t h e r i v e r s e d i m e n t i s s p r e a d
o u t l i k e a f a n o v e r t h e sea bed,
w i t h a c h a r a c t e r i s t i c "umbrella
shape".
N o t o n l y a l l u v i a l m a t e r i a l f r o m t h e r i v e r s e r v e s as b u i l d i n g m a t e r i a l
f o r t h e d e l t a , a l s o s e d i m e n t t r a n s p o r t e d a l o n g s h o r e c a n be a l a r g e
s u p p l y a c c u m u l a t i n g on t h e d e l t a . E v e n i f t h e r e i s a s t r o n g c o a s t a l
f l o w o f m a t e r i a l , n o t a l l o f i t w i l l p a s s a r i v e r m o u t h . The
river
w a t e r w i l l s l o w down t h e f l o w o f m a t e r i a l ; waves w i l l b r e a k somewhat
f a r t h e r f r o m t h e s h o r e , and t h e i r e f f e c t on t h e c o a s t a n d on t h e b a r
will
be
w e a k e r t h a n t h a t on
adjacent
sectors.
I f there
is
an
u n i d i r e c t i o n a l movement o f m a t e r i a l , some o f i t may be h e l d i n f r o n t o f
t h e r i v e r m o u t h and g i v e r i s e t o a c c r e t i o n i n t h e f o r m o f a t r i a n g u l a r
f o r e l a n d . I t i s a l s o p o s s i b l e t h a t r i v e r waters f o r c e back the beach
m a t e r i a l t h a t moves t o w a r d s t h e r i v e r m o u t h and c a r r y i t t o t h e
sea
f l o o r . Waves c a n n o t r e t u r n t h i s m a t e r i a l and p i l e i t up, s i n c e t h e y a r e
w e a k e n e d when t h e y e n c o u n t e r t h e r i v e r c u r r e n t . Wave c o n t i n u e
to
d i s p l a c e m a t e r i a l a l o n g t h e b o t t o m , a n d a s u b m a r i n e s p i t i s b u i l t up i n
t h e z o n e w h e r e t h e e f f e c t o f t h e waves on t h e b o t t o m i s s t r o n g e s t . A
b a y - m o u t h b a r i s s e m i c i r c u l a r b e c a u s e i t f o r m s a l o n g t h e edge o f t h e
f a n where t h e r i v e r w a t e r f l o w s o u t , t h e t i d a l c u r r e n t i s a c t i v e , and
t h e w a v e s b r e a k (see a l s o f i g u r e 44; t h e d e l t a i c c o a s t o f Y u n l i n ) .
Figure
45
shows a d e p o s i t i o n a l m o d e l o f a t r a n s g r e s s i v e
barrier
shoreline
(the
Caminada-Moreau b a r r i e r h e a d l a n d
coast
of
South
L o u i s i a n a ) i n t h e M i s s i s s i p p i R i v e r d e l t a p l a i n ( f i g . 46) . R a p i d
c o a s t l i n e r e t r e a t , due t o s u b s i d e n c e , s e d i m e n t d e f i c i e n c y , and o v e r w a s h
activity,
i s c h a r a c t e r i s t i c o f t h e e n t i r e c o a s t l i n e . The
barrier
c o n s i s t s o f t h r e e m a i n f o r m s ; an e r o s i o n a l h e a d l a n d a n d t w o f l a n k i n g
barriers w i t h recurved spits.
20
Washover Transport on Sandbars
5.
5.1
SEDIMENT
TRANSPORT
PROCESSES
AND
BARRIER
QUANTITATIVE AND QUALITATIVE ANALYSIS.
ISLAND
MIGRATION:
A
General
B a r r i e r i s l a n d s and b a r r i e r
s p i t s a r e deformed by e n v i r o n m e n t a l
p r o c e s s e s ( w i n d , waves, c u r r e n t ) . T h e s e e n v i r o n m e n t a l p r o c e s s e s a r e t h e
d r i v i n g f o r c e s f o r t h e m o r p h o l o g i c a l processes (sediment t r a n s p o r t by
water)
and a e o l i a n processes
(sediment
transport
b y w i n d ) . The
m o r p h o l o g i c a l and a e o l i a n processes
on t h e i r
term occasion t h e
geodynamic processes
(barrier migration,
deformation)
of barrier
i s l a n d s . I n t h i s c o n t e x t , k n o w i n g t h e s e d i m e n t t r a n s p o r t q u a n t i t i e s due
t o t h e d i f f e r e n t mechanisms, i s e s s e n t i a l .
I n t h e former c h a p t e r s t h e main element o f b a r r i e r i s l a n d s and s p i t s
and t h e main e n v i r o n m e n t a l c o n d i t i o n s a r e examined, even as t h e
processes o f b a r r i e r i s l a n d e v o l u t i o n . I n t h i s chapter t h e sediment
t r a n s p o r t processes a r e d i s c u s s e d which cause b a r r i e r m i g r a t i o n and
b a r r i e r e r o s i o n . An a t t e m p t i s made t o q u a n t i f y t h e c o n t r i b u t i o n o f t h e
d i f f e r e n t " m o r p h o d y n a m i c " m e c h a n i s m s , d i s c u s s e d i n c h a p t e r 4.
5.2
Q u a n t i f i c a t i o n and q u a l i f i c a t i o n o f s e d i m e n t t r a n s p o r t
e n v i r o n m e n t a l p r o c e s s e s a c t i n g on b a r r i e r i s l a n d s .
5.2.1
Qualitative analysis
rates
due t o
Processes by which b a r r i e r i s l a n d a r e deformed v a r y from p l a c e t o
p l a c e . As d i s c u s s e d i n c h a p t e r 2 b a r r i e r i s l a n d s f o r m i n a r e a s w h e r e
t h e t i d a l r a n g e d o e s n o t e x c e e d t h e +4m M.S.L. A l s o t h e s e d i m e n t s u p p l y
i s o f g r e a t importance, f o r i t i s t h e b u i l d i n g m a t e r i a l , o r w i t h a
n e g a t i v e sand b a l a n c e , forms t h e b a s i s f o r t h e decay o f c o a s t a l
e l e m e n t s . The s e d i m e n t
sources
a r e r e l a t e d t o each o t h e r and a
q u a l i t a t i v e f l o w d i a g r a m o f t h e c o a s t a l sediment budget can be p r o d u c e d
t o i l l u s t r a t e t h e c o n t e x t u a l p o s i t i o n o f t h e sand i n v o l v e d i n t h e
beach/dune changes ( F i g . 4 7 ) .
E s p e c i a l l y f o r w a s h o v e r f a n s , t h e r e a r e a l a r g e number o f p r o c e s s
v a r i a b l e s l i k e l y t o e x e r t i n f l u e n c e o n s e d i m e n t b u d g e t s . F i g u r e 48
summarizes
the trends, resulting
from m u l t i v a r i a t e
analysis of
v a r i a b l e s , l i k e l y t o a f f e c t sediment budgets and t h e data a c q u i r e d from
twelve surveys
( K o c h e l a n d W a m p f l e r , 1 9 8 9 ) . The d i f f e r e n t
factors
a f f e c t i n g t h e sediment budgets on washover f a n s ( f a n s u r f a c e dynamics)
are:
- Overwash f r e q u e n c y
- Overwash m a g n i t u d e
- Rainfall
- High wind
frequency
frequency
- R e l i e f near
the fan
- Depth t o t h e water
table.
The s c a l e t i p s t o w a r d s a c c r e t i o n o r e r o s i o n d e p e n d i n g
f a c t o r s toward t h e i r end v a l u e s .
21
on symmetry o f
Washover Transport on Sandbars
5.2.2
Quantitative analysis
To p r o d u c e a q u a n t i t a t i v e a n a l y s i s o f s e d i m e n t t r a n s p o r t r a t e s o f
d i f f e r e n t b a r r i e r i s l a n d s a r o u n d t h e w o r l d , a c o m p a r i s o n c a n be made
between t h e sediment t r a n s p o r t r a t e s / r e t r e a t r a t e s o f these b a r r i e r
i s l a n d s due t o t h e d i f f e r e n t e n v i r o n m e n t a l p r o c e s s e s . H o w e v e r , m a k i n g
a p r o p e r c o m p a r i s o n i s n o t e a s y . The s e v e r a l s t u d i e s o f b a r r i e r i s l a n d
use
different
units
and
m e t h o d s when d e t e r m i n e
barrier
island
migration.
An a p p r o a c h o f q u a n t i f y i n g s e d i m e n t t r a n s p o r t m e c h a n i s m s w i l l
be
p r e s e n t e d by t h e d i s c u s s i o n o f a few case s t u d i e s o f b a r r i e r i s l a n d
m i g r a t i o n along f o u r d i f f e r e n t coasts around the w o r l d .
T a b l e ' s 5.1 and 5.2 shows f i v e d i f f e r e n t mechanisms w h i c h c a u s e a l o s t
o r a n a c c u m u l a t i o n a r e a f o r d u n e s o n O l d N o r t h Beach, a t t h e e a s t c o a s t
o f t h e U n i t e d S t a t e s . The o c e a n has t h e l a r g e s t e f f e c t o n d u n e l o s s e s ,
b u t i t i s a l s o t h e m a i n e l e m e n t t h a t r e s t o r e s t h e dune damage ( s a n d
b a l a n c e ) . H a r d l y no a r e a i s l o s t due t o t h e e f f e c t s o f t h e o c e a n
(waves,
currents,
tides).
The
second l a r g e s t mechanisms
causing
sediment t r a n s p o r t i s washover t r a n s p o r t . A t a washover event a l o t o f
m a t e r i a l f r o m t h e dunes i s t r a n s p o r t e d t o t h e b a c k s i d e o f t h e m a t e r i a l ,
c a u s i n g t h e l o s s o f 2 1 % o f t h e d u n e a r e a . The 5% g a i n e d a r e a f r o m
w a s h o v e r c a n be e x p l a i n e d b y t h e s a n d t r a n s p o r t e d w i t h t h e b a c k f l o w i n g
w a t e r t o t h e sea a f t e r t h e s e a l e v e l l o w e r e d . The n e t r e s u l t o f l o s t o r
g a i n e d a r e a f o r dunes c o n c e r n i n g w a s h o v e r s i s a l o s s o f d u n e a r e a o f
16% ( i n t h i s s i t u a t i o n ) , w h i c h i s t h e l a r g e s t n e t s e d i m e n t l o s s o f t h e
f i v e e n v i r o n m e n t s and s h o w i n g t h a t t h e c o n t r i b u t i o n o f w a s h o v e r e v e n t s
t o d u n e l o s s e s (and t h e r e b y b a r r i e r m i g r a t i o n ) i s s i g n i f i c a n t .
For s a l t marshes a d i f f e r e n t s i t u a t i o n o c c u r s . Here washover i s t h e
m a i n e l e m e n t c o n s e r v i n g t h e amount o f a r e a f o r s a l t m a r s h e s . New d u n e s
(and w a s h o v e r s ) d e v e l o p e d l a n d w a r d o f t h e " 1 8 6 8 - f e a t u r e s " , p r i m a r i l y on
w a s h o v e r s a n d on s a l t m a r s h e s t h a t h a d b e e n b u r i e d b y o v e r w a s h . T w e l v e
p e r c e n t o f t h e s l a t m a r s h l o s s b e t w e e n 1868 a n d 1978 was a t t r i b u t e d t o
b a y s i d e e r o s i o n . T i d a l c u r r e n t s b e h i n d O l d N o r t h B e a c h do n o t g e n e r a l l y
move l a r g e v o l u m e s o f s e d i m e n t o r e r o d e c o h e s i v e s l a t m a r s h p e a t . M o s t
o f t h e s a l t m a r s h e s d e v e l o p e d o n o l d w a s h o v e r s (51%) a n d b y
rhizome
e x t e n s i o n i n t o s h a l l o w P e a s a n t Bay ( 3 0 % ) .
T a b l e 5.1: L o c a t i o n o f a r e a s l o s t o r g a i n e d f o r dunes o n
N o r t h B e a c h , U.S.
( f r o m 1868-1978, a f t e r Leatherman and
Zaremba, 1986)
Lost t o or
from:
gained
% lost
area
% gained
ocean
63
61
washover
21
5
salt
10
35
side
10
0
terminus
1
0
bay
marsh
22
Old
area
Washover Transport on Sandbars
T a b l e 5.2: L o c a t i o n o f a r e a s l o s t o r g a i n e d f o r s a l t m a r s h e s
on O l d N o r t h Beach, U.S.
( f r o m 1 8 6 8 - 1 9 7 8 , a f t e r L e a t h e r m a n and
Zaremba,
1986)
Lost t o or
from:
gained
% lost
area
% gained
washover
43
51
bay
12
30
46
18
0
0
side
dune
terminus
area
An o t h e r way o f s h o w i n g t h e i n f l u e n c e o f s e d i m e n t t r a n s p o r t p r o c e s s e s
e f f e c t i n g t h e m i g r a t i o n b a r r i e r i s l a n d s i s t o compare t h e l a n d w a r d
s e d i m e n t t r a n s f e r due t o t i d a l i n l e t s and o v e r w a s h ( t a b l e 5 . 3 ) . T h r e e
i s l a n d s a t t h e s o u t h e a s t c o a s t o f t h e U n i t e d S t a t e s were s t u d i e d . I n
t h i s m e s o t i d a l a r e a t i d a l i n l e t s a r e n u m e r o u s and c a u s e t h e l a r g e s t
amount o f s e d i m e n t t r a n s p o r t t o t h e b a c k s i d e o f t h e b a r r i e r s . O v e r w a s h
e v e n t s a r e l e s s numerous, b u t t h e y a l s o c a r r y a l o t o f s e d i m e n t t o t h e
b a c k b a r r i e r lagoon, causing the b a r r i e r t o migrate.
T a b l e 5.3:
Barrier
Cap
Landward sediment t r a n s f e r i n d i f f e r e n t
system
Tidal
Hatteras
Assateague
72 . 4
Island
Malpeque b a r r .
Inlets
syst.
environments
Overwash
14.2
82
12
52
24
I n t a b l e 5.4 t h e r e s u l t s o f a l a r g e b a r r i e r m i g r a t i o n s t u d y a t t h e w e s t
coast o f P o r t u g a l (Algarve) are p r e s e n t e d (mesotidal s h o r e l i n e ; t i d a l
r a n g e 2-4m). T h i s t a b l e shows t h e v a r i o u s d o m i n a n t p r o c e s s e s on
an
i s l a n d by
i s l a n d basis,
and
s e p a r a t e s f r o n t - s i d e and
lagoon-side
p r o c e s s e s . The l a t e r a l v a r i a t i o n i n i m p o r t a n c e o f i s l a n d p r o c e s s e s i s
assumed t o be
due
primarily to variations i n island orientation
r e l a t i v e t o b o t h w i n d and wave e n e r g y . S e c o n d a r y f a c t o r s i n c l u d e s h e l f
m o r p h o l o g y , i s l a n d s h a p e / e l e v a t i o n c o n t r o l on t h e v u l n e r a b i l i t y t o
e r o s i o n by s p r i n g t i d e s , sediment g r a i n s i z e , p r o x i m i t y t o i n l e t s , and
t h e i m p a c t o f man due t o c h a n n e l d r e d g i n g a n d j e t t y c o n s t r u c t i o n s . I t
appears t h a t t h e v u l n e r a b i l i t y o f b a r r i e r i s l a n d s t o washover e v e n t s i s
d i r e c t r e l a t e d t o the presence
and l e n g t h o f a s i n g l e dune r i d g e a t
t h e sea s i d e o f t h e b a r r i e r . No d u n e r i d g e means 100% v u l n e r a b l e
to
washover a t the f r o n t s i d e . At t h e l a g o o n s i d e o f the b a r r i e r i s l a n d s
t h r e e p r o c e s s e s a r e c o n s i d e r e d : Overwash, i n c o r p o r a t i o n o f t h e r e c u r v e d
s p i t w i t h t h e l a g o o n s i d e , and
t i d a l delta incorporation with
the
l a g o o n s i d e . When no o v e r w a s h o c c u r s and no t i d a l i n l e t s a r e
present,
a l l t h e s e d i m e n t w i l l be t r a n s p o r t e d a l o n g t h e b a r r i e r , c a u s i n g s p i t
a c c r e t i o n a t t h e end o f t h e b a r r i e r . However when t i d a l i n l e t s
and
washover e v e n t s t r a n s f e r sediment d i r e c t l y t o the back s i d e o f t h e
b a r r i e r , a l m o s t no s e d i m e n t i s l e f t f o r t h e r e c u r v e d s p i t s t o a c c r e t e .
23
Washover Transport on Sandbars
T a b l e 5.4: C o m p a r i s o n dominant p r o c e s s e s a f f e c t i n g
and l a g o o n s i d e b a r r i e r i s l a n d
Algarve B a r r i e r Islands,
Portugal
Lagoonside
processes
Frontside
Processes
Island
Overwash
Total
length
single
Dune
Ridge
(m)
%
Ancao
Barreta
frontside
Overwash
o,
o
%
10,000
6
94
100
6, 850
48
52
67
Recurved
Spit
Incorporation
%
Tidal
Delta
Incorpor
ation %
-
-
33
-
Culatra
5, 750
55
35
-
100
-
Armona
8, 750
40
60
-
40
60
Tavira
10,250
-
100
15
-
85
Cabanas
4, 500
100
-
100
-
-
Cacela
4,400
100
-
100
-
-
T a b l e 5.5 shows t h e w a s h o v e r v o l u m e s p e r u n i t b e a c h l e n g t h a l l due t o
one same s t o r m , a l s o a t t h e s o u t h e a s t c o a s t o f t h e U n i t e d S t a t e s . The
numbers show t h e v a r i e t y o f i m p a c t due t o w a s h o v e r s .
TcüDle 5.5: Washover v o l u m e s / u n i t b e a c h l e n g t h due
E a s t C o a s t U.S.
( a f t e r L e a t h e r m a n , 1976)
Barrier
Caffey
Biaxton,
v o l . /meter beach
system
Inlet,
2.7
N.C.
38 .2
N.C.
Cape Cod,Mass. t o L u d l a m
Jones Beach,
t o 1 storm.
Isl.,N.Y.
N.Y.
Sandy Hook t o B a r n e g a t L i g h t ,
10.8
24 . 9
N.J.
24
101. 7
(mVm)
Washover Transport on Sandbars
5.3
Barrier
migration
5.3.1
Introduction
B a r r i e r i s l a n d s and b a r r i e r s p i t s m i g r a t e t o w a r d s t h e c o a s t l i n e due t o
s e v e r a l c o a s t a l p r o c e s s e s , w h i c h w e r e d e s c r i b e d i n c h a p t e r 4. Some o f
t h e m ( o v e r w a s h , sea l e v e l r i s e ) p l a y a more i m p o r t a n t r o l e i n t h e
m i g r a t i o n process than others
( a e o l i a n t r a n s p o r t , on- a n d
offshore
t r a n s p o r t by waves). I n t h i s p a r a g r a p h t h e most i m p o r t a n t processes
causing b a r r i e r m i g r a t i o n are discussed.
5.3.2
Barrier migration:
island
A
spit,
a detached s p i t
and
an
offshore barrier
I n t h i s paragraph the d i f f e r e n c e s i n m i g r a t i o n r a t e s between B a r r i e r
i s l a n d s , s p i t s and d e t a c h e d s p i t s a r e e x a m i n e d , i n t e r m s o f s e d i m e n t
s u p p l y , m o r p h o l o g y and p h y s i c a l p r o c e s s e s r e s p o n s i b l e f o r t h e l a n d w a r d
m i g r a t i o n o f t h e c o a s t a l l a n d f o r m s . H e q u e t t e a n d Ruz (1990) s t u d i e d t h e
m i g r a t i o n r a t e s o f d i f f e r e n t s p i t s and b a r r i e r i s l a n d s a l o n g t h e c o a s t
of
the
southeastern
Canadian
Beaufort
Sea.
Comparison
of
air
p h o t o g r a p h s showed t h e f o l l o w i n g m i g r a t i o n r a t e s o f t h r e e
coastal
f e a t u r e s , b e t w e e n 1950 an
1985:
T a b l e 5.6: A v e r a g e r e t r e a t r a t e s t a k e n f r o m 9 b a r r i e r
i s l a n d s , 19 s p i t s and 5 d e t a c h e d s p i t s o f t h e
s o u t h e a s t e r n C a n a d i a n B e a u f o r t Sea ( a f t e r Héquette
and Ruz,
1990)
Coastal
Offshore
Retreat rates
landforms
(m/yr)
3.1
barrier islands
Spits
1.7
Detached s p i t s
2.0
R e g r e s s i o n a n a l y s e s o f r e t r e a t r a t e s as a f u n c t i o n o f
longshore
sediment supply revealed t h a t sediment a v a i l a b i l i t y i s a s i g n i f i c a n t
p a r a m e t e r c o n t r o l l i n g s p i t and b a r r i e r i s l a n d l a n d w a r d m i g r a t i o n .
A n a l y s e s o f r e t r e a t r a t e s as a f u n c t i o n o f i n c o m i n g d e e p - w a t e r wave
p o w e r showed t h a t b a r r i e r i s l a n d s a r e r e t r e a t i n g more r a p i d l y t h a n
s p i t s e v e n i f t h e y a r e e x p o s e d t o t h e same l e v e l o f wave e n e r g y . D i r e c t
s e d i m e n t s u p p l y f r o m a d j a c e n t b l u f f s and t h e p r e s e n c e o f c o a s t a l d u n e s
e f f e c t i v e l y l i m i t s p i t e r o s i o n and r e t r e a t . I n t h e c a s e o f b a r r i e r
islands,
the
lack
of
significant
longshore
sediment
supply
is
f a v o u r a b l e t o a n e t s e d i m e n t d e f i c i t and r e s u l t s i n h i g h e r r e t r e a t
r a t e s . S p i t s a r e r e t r e a t i n g i n r e s p o n s e t o b e a c h f a c e e r o s i o n and s t o r m
o v e r w a s h i n g where low c r e s t a l e l e v a t i o n p e r m i t s swash i n c u r s i o n a c r o s s
the
c r e s t . B a r r i e r i s l a n d s are
more o f t e n a n d
more
extensively
o v e r w a s h e d and
overwashing represents
the
main p h y s i c a l
process
responsible f o r t h e i r migration.
5.3.3
Barrier migration
& sea
level
rise
Sea l e v e l r i s e p e r se i s n o t a m e c h a n i s m d i r e c t l y i n f l u e n c i n g b a r r i e r
migration.
Clearly,
sea
level
i s the p a s s i v e p l a n e upon
which
m e c h a n i s m s o c c u r and i n t u r n g e n e r a t e t h e d y n a m i c p r o c e s s e s o f b a r r i e r
alteration.
25
Washover Transport on Sandbars
O r f o r d , C a r t e r and F o r b e s (1989) s t u d i e d t h e r e l a t i o n s h i p b e t w e e n
g r a v e l b a r r i e r m i g r a t i o n and sea l e v e l r i s e . T h e y p o i n t e d o u t t h a t t h e
process through which g r a v e l b a r r i e r s migrate i s r a t h e r d i f f e r e n t t o
t h e way s a n d b a r r i e r s m i g r a t e . The d o m i n a n t p r o c e s s b y w h i c h g r a v e l
b a r r i e r m i g r a t e i s t h r o u g h rollover. B e a c h f a c e s e d i m e n t i s p a s s e d o v e r
t h e b a r r i e r c r e s t by storm-wave a c t i v i t y . I t t h a n remains p a s s i v e l y on
t h e b a c k s l o p e b e f o r e b u r i a l by subsequent s t o r m - g e n e r a t e d washover
s e d i m e n t . I n t h i s way t h e b a r r i e r s h i f t l a n d w a r d , so t h a t i n t i m e t h e
backbarrier
sediments w i l l
emerge t h r o u g h
the beach face
to
be
i n c o r p o r a t e d o n c e more i n t h e o v e r p a s s i n g c y c l e . I n g e n e r a l ,
the
c a p a c i t y f o r sediment r e t u r n t o the seaward face o f a g r a v e l system i s
r a d i c a l l y below t h a t of a sand system, where t i d a l l y
maintained
b r e a c h e s i n t h e b a r r i e r and w a s h o v e r f a n s a r e common and e f f e c t i v e i n
c r o s s - b a r r i e r sediment r e c y c l i n g . ( C a r t e r e t a l . , 1 9 8 9 b ) .
The r a t e o f b a r r i e r m i g r a t i o n i s d e p e n d e n t on t h a t p a r t o f t h e s p e c t r u m
o f run-up volume t h a t c r o s s e s t h e b a r r i e r c r e s t
(Fig. 49).
The
d i f f e r e n t i a l r e s p o n s e o f m i g r a t i n g s e a w a r d b a r r i e r and
back-barrier
s h o r e l i n e s i s a measure o f b a r r i e r volume change (per u n i t l e n g t h o f
b a r r i e r ) r e l a t e d t o b a r r i e r e l e v a t i o n changes. I f t h e b a r r i e r ' s seaward
shoreline r e t r e a t s at a f a s t e r r a t e than the back-barrier
shoreline
t h e n t h e b a r r i e r m u s t be b u i l d i n g up w i t h c r e s t a l e l e v a t i o n s r i s i n g
t h r o u g h c o n c e n t r a t e d o v e r t o p p i n g ( F i g . 5 0 ) . The r e v e r s e s i t u a t i o n w i t h
t h e b a c k - b a r r i e r edge m i g r a t i n g f a s t e r t h a n t h e s e a w a r d f a c e i n d i c a t e s
t h a t t h e b a r r i e r c r e s t m u s t be f a l l i n g w i t h o v e r w a s h
predominating.
When m i g r a t i o n r a t e s a r e e q u a l t h e n c r e s t a l e l e v a t i o n i s h e l d d e s p i t e
migration.
The b a l a n c e b e t w e e n b a r r i e r c r e s t b u i l t - u p due t o wave o v e r t o p p i n g
and
c r e s t b r e a k down b y wave o v e r w a s h i n g d i c t a t e s t h e r a t e o f r o l l o v e r a n d
m i g r a t i o n . The b a l a n c e o f o v e r t o p p i n g t o o v e r w a s h i n g may be a l t e r e d b y
t h e r a t e o f sea l e v e l change a s s u m i n g t h a t t h e s t o r m i n t e n s i t y r e m a i n s
c o n s t a n t . Any i n c r e a s e i n mean sea l e v e l w i l l change t h e
overtopping/
overwashing
ratio t o w a r d o v e r w a s h i n g . T h i s r a t i o can o n l y be
held
c o n s t a n t u n d e r r i s i n g sea l e v e l b y t h e p r o v i s i o n o f s e d i m e n t t o b u i l t
t h e c r e s t up. I t i s l i k e l y w i t h h i g h r a t e s o f sea l e v e l r i s e t h a t
overwashing w i l l predominate, causing a r e d u c t i o n i n the b a r r i e r c r e s t
e l e v a t i o n a n d an i n c r e a s e i n t h e b a c k - b a r r i e r m i g r a t i o n r a t e .
I n m a n a n d D o l a n (1989) t r i e d t o d e s c r i b e t h e s h o r e l i n e r e t r e a t due
t w o g e n e r a l b e a c h c h a n g e s c a n be
considered:
(1)
Seasonal beach changes (by m e t e o r o l o g i c a l
=> Summer and w i n t e r p r o f i l e ( F i g . 51)
(2)
L o n g - t e r m s h o r e l i n e c h a n g e s (sea l e v e l r i s e )
=> S h o r e l i n e c h a n g e s due t o l o c a l a c c r e t i o n / e r o s i o n .
=> S h o r e l i n e c h a n g e s due t o sea l e v e l r i s e .
to
conditions)
F o r t h e c a s e o f c o n s t a n t sea l e v e l i t i s assumed t h a t a c c r e t i o n o r
e r o s i o n c a u s e t h e b e a c h p r o f i l e t o be d i s p l a c e d a h o r i z o n t a l d i s t a n c e
Ax
( F i g . 52a) . R e s p o n s i b l e f o r t h e l o c a l a c c r e t i o n / e r o s i o n i s t h e
c h a n g e i n t h e l o n g s h o r e c u r r e n t , d r i v e n b y waves a r r i v i n g t h e c o a s t a t
an o b l i q u e a n g l e , and c a u s e d b y l o c a l f e a t u r e s t h a t d i s t u r b t h e f l o w
field.
For
rising
sea
level
i t i s assumed t h a t t h e p r o f i l e
i s raised
v e r t i c a l l y a d i s t a n c e Az, t h e n moved h o r i z o n t a l l y a d i s t a n c e Ax s u c h
t h a t t h e volume eroded from the upper beach i s equal t o the volume
deposited
f u r t h e r offshore along
the shore r i s e
( F i g . 52b) .
The
s h o r e l i n e r e c e s s i o n o b t a i n e d by e q u a t i n g
t h e onshore and
offshore
v o l u m e s d u r i n g sea l e v e l r i s e i s k n o w n as t h e B r u u n r u l e ( A p p e n d i x 1 ) .
26
Washover Transport on Sandbars
5.3.4
Barrier migration & tidal
inlets
L a n d w a r d d i s p l a c e m e n t o f a b a r r i e r e c o s y s t e m seems t o d e p e n d l a r g e l y
upon i n l e t dynamics. Temporary and m i g r a t i n g permanent i n l e t s p r o v i d e
t h e bases - t h a t i s , t h e l a r g e f l o o d t i d a l d e l t a s upon w h i c h t h e i s l a n d
e n v i r o n m e n t s a r e e s t a b l i s h e d . T h e s e s u b s t r a t e s s e r v e as p l a t f o r m s f o r
m a r s h d e v e l o p m e n t a n d hence l a n d w a r d e x t e n s i o n o f t h e b a y s h o r e l i n e .
S u b s e q u e n t l y , w i n d - b l o w n and o v e r w a s h s e d i m e n t s c a n be d e p o s i t e d o n t o p
o f t h i s a c c r e t i o n a r y b a s e . These l a t t e r t w o f o r m s o f s e d i m e n t movement
are r e s p o n s i b l e f o r the v e r t i c a l growth o f a b a r r i e r i s l a n d .
I n a r e a s w h e r e t h e t i d a l r a n g e i s g r e a t (2-4 m, m e s o t i d a l ) ,
tidal
i n l e t s a n d a s s o c i a t e d c o n s t r u c t i o n o f f l o o d d e l t a s may a c c o u n t f o r m o s t
o f t h e l a n d w a r d s e d i m e n t t r a n s f e r s (Armon a n d Mccann, 1979) c o m p a r e d t o
overwash, w i n d and l i t t o r a l d r i f t .
5.3.5
B a r r i e r M i g r a t i o n & washover
events
I n m i c r o t i d a l a r e a s ( t i d a l r a n g e 0-2 m) o v e r w a s h i s t h e m a i n p r o c e s s
r e s p o n s i b l e f o r l a n d w a r d m i g r a t i o n o f b a r r i e r i s l a n d s . O v e r w a s h has
c o n t r i b u t e d t o t h e m i g r a t i o n a l p r o c e s s , l a r g e l y by making a v a i l a b l e
f r e s h s a n d f o r dune d e v e l o p m e n t
( v e r t i c a l g r o w t h ) and t o a l e s s e r
d e g r e e b y a d d i n g new s e d i m e n t t o t h e b a c k s h o r e . ( l a t e r a l e x p a n s i o n ) . I t
i s i n t h i s f a s h i o n t h a t h i g h a r e a s ( d u n e s ) become l o w a r e a s ( w a s h o v e r s )
i n a c y c l i c f a s h i o n . T h e r e f o r e , t h e o c c u r r e n c e o f overwash has t h e
e f f e c t o f d i s p l a c i n g t h e b a r r i e r l a n d w a r d , and d i f f e r e n t s e c t i o n s o f
t h e b a r r i e r r e p r e s e n t v a r i o u s s t a g e s i n t h i s c y c l i c phenomenon.
I n a p p e n d i x 3 t h e o v e r w a s h m a g n i t u d e s w i l l be e s t i m a t e d o f
dominated b a r r i e r s p i t
(The T r a b u c a d o r B a r , n o r t h e a s t e r n
coast)
5.3.6
a waveSpanish
B a r r i e r M i g r a t i o n & longshore sediment supply
Héquette a n d Ruz (1991) d e s c r i b e d i n t h e i r s t u d y o f s p i t a n d b a r r i e r
m i g r a t i o n i n t h e s o u t h e a s t e r n C a n a d i a n B e a u f o r t Sea t h e r o l e o f
l o n g s h o r e s e d i m e n t s u p p l y . I n t h e i r s t u d y r e g r e s s i o n a n a l y s i s showed a
l i n e a r r e l a t i o n s h i p between r e t r e a t r a t e s and p o t e n t i a l l o n g s h o r e
sediment supply. I f t h e r e t r e a t i n g b l u f f s i n t h e u p d r i f t d i r e c t i o n o f
a s p i t p r o v i d e a l a r g e sediment s u p p l y , c o a s t w i s e p r o p a g a t i o n o f t h e
s p i t w i l l o c c u r w i t h l i t t l e e r o s i o n o n t h e b e a c h f a c e . However, i f t h e
amount o f s e d i m e n t e n t e r i n g t h e s y s t e m i s d e c r e a s i n g due t o a r e d u c t i o n
of a v a i l a b l e sediments a t the source (e.g. lower b l u f f h e i g h t o r b l u f f
sediment t o o f i n e t o remain i n t h e l i t t o r a l system) , wave-generated
e r o s i o n o f t h e seaward s i d e o f t h e s p i t w i l l o c c u r . I n t h a t case, most
of the sediments are transported alongshore leading t o a c c r e t i o n a t the
d i s t a l p a r t o f t h e s p i t w h i l e t h e s p i t i s p r o g r e s s i v e l y t h i n n i n g as a
r e s u l t o f b e a c h f a c e e r o s i o n due t o t h e r e d u c t i o n o f s e d i m e n t i n p u t i n t o
t h e s y s t e m . F o r e s h o r e and b a c k s h o r e e r o s i o n r e s u l t s i n t h e l a n d w a r d
m i g r a t i o n o f t h e s p i t c r e s t a n d o f t h e s e a w a r d edge o f t h e f o r d u n e ( i f
any) a n d t h u s i n t h e l a n d w a r d d i s p l a c e m e n t o f t h e s h o r e l i n e ( F i g . 5 3 ) .
27
Washover Transport on Sandbars
6.
6.1
THE WAI SAN TING SANDBAR
Introduction
I n t h e f o r m e r c h a p t e r e l e m e n t s o f b a r r i e r i s l a n d s and mechanisms b y
w h i c h b a r r i e r i s l a n d s a r e e f f e c t e d were examined and w i l l be a p p l i e d i n
t h i s c h a p t e r f o r a s p e c i f i c f e a t u r e along t h e west coast o f Taiwan: t h e
Wai San T i n g s a n d b a r . The Wai San T i n g s a n d b a r i s l o c a t e d a l o n g t h e
mid-west c o a s t o f Taiwan ( F i g . 5 4 ) , between t h e Choshui R i v e r and
Kuosheng Kang ( F i g . 44 a n d 5 5 ) , a n d i s f o r m e d o n a n o l d r e l i c t r i v e r
d e l t a . The Wai San T i n g i s a c t u a l l y a b a r r i e r s p i t , d e t a c h e d f r o m t h e
mainland b y a l a r g e t i d a l i n l e t . T h i s t r a n s g r e s s i v e b a r r i e r i s moving
( r o t a t i n g ) t o w a r d s t h e c o a s t l i n e a t a s p e e d o f 40 m / y e a r a t t h e s e a w a r d
end o f t h e b a r r i e r , measured f r o m s e v e r a l s a t e l l i t e c h a r t s . T h i s
f e a t u r e i s chosen f o r e x a m i n a t i o n f o r i t s f u n c t i o n as a sea defence,
p r o t e c t i n g t h e c o a s t l i n e i n t h e l e e s i d e o f t h e sandbar. E s t i m a t i o n s
show t h a t , a b o u t o n e c e n t u r y , when t h e Wai San T i n g r e a c h e s t h e
c o a s t l i n e , t h e p o o r p r o t e c t e d c o a s t b e h i n d t h e b a r w i l l t h e n be e x p o s e d
d i r e c t l y t o t h e wave a n d c u r r e n t s , w h i c h c a n c a u s e s e v e r e f l o o d i n g i n
the overcrowded area b e h i n d t h e c o a s t .
I n t h i s c h a p t e r a d e s c r i p t i o n w i l l be g i v e n o f t h e s t u d y a r e a i n
g e n e r a l a n d t h e Wai San T i n g s a n d b a r i n p a r t i c u l a r . ^ P a r a g r a p h 6.3
describes t h e m o r p h o l o g i c a l development o f t h e Y u n l i n coast and a
q u a l i t a t i v e hypothesis i sgiven t o define thed i f f e r e n t features o f the
Yunlin coast.
I n p a r a g r a p h 6.4 t h e d i f f e r e n t mechanisms ( d i s c u s s e d i n c h a p t e r 4)
c a u s i n g t h e m o r p h o l o g i c a l a n d g e o l o g i c a l c h a n g e s o f t h e Wai San T i n g
s a n d b a r w i l l be d e t e r m i n e d a n d i n 6.5 a n e s t i m a t i o n w i l l b e made o f t h e
c o n t r i b u t i o n o f t h e d i f f e r e n t mechanisms t o t h e t o t a l s e d i m e n t l o s s e s
of t h e b a r .
6.2
M o r p h o l o g i c a l d e s c r i p t i o n of t h e study area'.
6.2.1
Y u n l i n Coast
The s t u d y a r e a i s b o r d e r e d b y t h e H a i f e n g b a r i n t h e n o r t h a n d t h e Wai
San T i n g s a n d b a r i n t h e s o u t h ( F i g . 4 4 ) . The d i s t a n c e b e t w e e n t h e s e t w o
f e a t u r e s i s 15 km. The c e n t r e a n d t h e n o r t h e r n h a l f o f t h e r e g i o n
( Y i a n l i n c o a s t ) i s c h a r a c t e r i z e d b y a 3 t o 4 km w i d e s h a l l o w i n t e r t i d a l
s a n d a r e a i n f r o n t o f t h e p r e s e n t d y k e s a n d s e a d e f e n c e . The Wai San
T i n g b a r , t o g e t h e r w i t h some o t h e r b a r r i e r s e x t e n d f r o m t h e s e a w a r d
edge o f t h e i n t e r t i d a l a r e a i n a SSW d i r e c t i o n , t h e l a r g e s t o f t h e s e
b e i n g t h a t e x t e n d i n g f r o m T a i H s i w h i c h i s r e f e r r e d t o as t h e H a i f e n g
b a r . B e t w e e n H a i f e n g a n d Wai San T i n g t h e c o a s t h a s t h e n a t u r e o f a n
embayment w h i c h i s s h e l t e r e d f r o m t h e p r e d o m i n a n t wave d i r e c t i o n b y t h e
Haifeng sandbar.
The huge b u t i n t e r m i t t e n t s e d i m e n t i n p u t s f r o m t h e C h o s h u i R i v e r i n t h e
n o r t h and t h e m i g r a t i o n and reshaping o f t h e sandbars along t h e
s h o r e l i n e p r e s e n t s a complex m o r p h o l o g i c a l s c e n a r i o which i n c r e a s e s t h e
d i f f i c u l t y o f making p r e d i c t i o n s o f impacts o f r e c l a m a t i o n on t h e
shoreline evolution.
'After the Yunlin Offshore Industrial Estate Development; Working Paper No. 3: Hydraulic and Morphological Studies; Frederic R.
Harries B.V., Sinotech Engineering Consultants Inc., 1993.
28
Washover Transport on Sandbars
6.2.2
Wai
San
T i n g sandbar
The Wai San T i n g s a n d b a r i s p a r t l y e m e r g e n t a t h i g h t i d e w i t h t h e +2m
TD^ l e v e l b e i n g r e a c h e d o n l y i n a s m a l l a r e a c l o s e t o t h e s e a w a r d e n d .
The s e a w a r d s l o p e o f t h e s a n d b a r e x p e r i e n c e s f u l l e x p o s u r e t o t h e
monsoon waves, p a r t i c u l a r l y n e a r t h e s o u t h e r n e n d o f t h e f e a t u r e w h e r e
t h e wave a t t a c k i s m o s t o b l i q u e . D o u b l e and o f t e n t r i p l e s u b m e r g e d
b r e a k e r b a r s o f f t h e n o r t h f a c i n g c o a s t a r e v i s i b l e on t h e s a t e l l i t e
photograph. Several channels connect the foreshore w i t h the backshore
a r e a . P a r t i c u l a r l y i n t h e n o r t h e r n p a r t , s m a l l w a s h o v e r d e l t a s c a n be
seen a t t h e backshore s i d e o f t h e s e channels w h i c h i n d i c a t e s sediment
t r a n s p o r t t h r o u g h t h e c h a n n e l s t o t h e b a c k s h o r e . The l e e s i d e o f t h e
sand b a r i s a g e n t l y s l o p i n g and w i d e i n t e r t i d a l a r e a , p r o t e c t e d f r o m
t h e waves a n d c u r r e n t s { F i g . 5 6 ) .
The s o u t h w e s t e r n t i p o f t h e Wai
San T i n g a n d o t h e r s a n d b a r s
and
s u b m a r i n e f e a t u r e s show a s h a r p t u r n t o t h e n o r t h , o f t e n f o l l o w i n g a
s t e e p b a n k . T h i s c a n be i n t e r p r e t e d as t h e e f f e c t s o f t h e n o r t h f l o w i n g
f l o o d t i d e r e s h a p i n g t h e ends o f t h e s e f e a t u r e s a n d / o r as t h e f o r m e r
seaward e x t e n t o f r e l i c t r i v e r d e l t a s .
6.3
M o r p h o l o g i c a l development of t h e Y u n l i n
6.3.1
Recent development
coast
The Y u n l i n c o a s t has e x p e r i e n c e d s e v e r e i n t e r f e r e n c e b y man
even
t h r o u g h i t i s l a r g e l y "undeveloped" i n t h e i n d u s t r i a l sense. Dykes and
r e t a i n i n g s t r u c t u r e s h a v e b e e n e r e c t e d and t h e " b a c k s h o r e " has b e e n
f i x e d b y t h e s e s t r u c t u r e s a l o n g t h e e n t i r e l e n g t h o f t h e c o u n t y . These
dykes were b u i l t a t o r above t h e h i g h w a t e r mark and t o d a y t h e seabed
l e v e l a t t h e t o e o f t h e d y k e s i s m o s t l y b e t w e e n - 1 a n +2m a b o v e TD.
The
s h a l l o w i n t e r t i d a l a r e a s i n f r o n t o f t h e dykes and b e t w e e n t h e dykes
and t h e sandbars a r e h e a v i l y e x p l o i t e d f o r o y s t e r c u l t i v a t i o n i n
p a r t i c u l a r l y b e t w e e n -2 a n d Om TD.
A l s o i n t h e s h a l l o w i n t e r t i d a l a r e a s a number o f v e r y n a r r o w d e e p (5 t o
10m deep) c h a n n e l s a r e m a i n t a i n e d b y t h e l o c a l f i s h i n g b o a t t r a f f i c ,
t i d a l f l o w s and s m a l l s c a l e d r e d g i n g e f f o r t s . The l a r g e s t o f t h e s e
channels i s s i t u a t e d o f f S a n t i a o l u n i n t h e l e e o f t h e H a i f e n g sandbar.
I t i s a b o u t 8m deep a t i t s d e e p e s t p o i n t a n d c r o s s e s t h e H a i f e n g
sandbar o p p o s i t e t h e f i s h i n g harbour o f P o t z u l i a o . South o f P o t z u l i a o ,
t h e e x i s t i n g c o a s t i s s h e l t e r e d f r o m wave a t t a c k a n d t i d a l c u r r e n t s
b o t h b y t h e H a i f e n g s a n d b a r a n d t h e Wai San T i n g s a n d b a r . H a i f e n g a c t s
as a v e r y e f f e c t i v e b a r r i e r t o t h e NNE wave a t t a c k .
Wai San T i n g i s u n d e v e l o p e d e x c e p t f o r a s m a l l l i g h t h o u s e n e a r t h e
s o u t h e r n end where t h e e l e v a t i o n e n s u r e s d r y l a n d a t h i g h w a t e r o v e r a
s m a l l a r e a . Many s m a l l b o a t s u s e t h e c h a n n e l s w h i c h c u t a c r o s s Wai San
T i n g c l o s e t o i t s n o r t h e r n end.
'Taiwan Datum
29
Washover Transport on Sandbars
6.3.2
Coastline
mobility
A r e l i a b l e r e c o r d o f l a r g e - s c a l e s a n d b a r movements has b e e n g a i n e d f r o m
s a t e l l i t e i m a g e r y and a e r i a l p h o t o g r a p h y . A l t h o u g h t h e r e i s i n h e r e n t
inaccuracy
due
t o the water
level
differences
at the
various
o b s e r v a t i o n t i m e s t h e g r o s s movement a n d f e a t u r e s a r e c l e a r . A summary
o f t h e l o c a t i o n s o f t h e Wai San T i n g s a n d b a r i n d i f f e r e n t y e a r s i s
g i v e n i n F i g u r e 57.
The
f o l l o w i n g are clear trends from these
figures:
1.
I t c a n be s e e n t h a t f r o m 1932 t o 1972 t h e movement was d r a m a t i c
a n d a c o m p l e t e r e s h a p i n g o f t h e Wai San T i n g s a n d b a r has t a k e n
p l a c e f r o m t h e c o n v e x s h a p e i n 1932 t o t h e c o n c a v e s h a p e i n 1972.
The a v e r a g e r a t e o f movement o f t h e n o r t h w a r d f a c e o f t h e s a n d b a r
has b e e n a b o u t 100 t o 150m s o u t h w a r d s o v e r t h i s p e r i o d . A t t h i s
r a t e t h e sandbar's n o r t h e r n f a c e i s r e t r e a t i n g a t a r a t e o f about
10km p e r c e n t u r y . The c o n t i n u a t i o n o f t h i s t r e n d a t t h e p r e s e n t
r a t e w o u l d r e s u l t i n t h e r e t r e a t o f Wai San T i n g b a c k t o t h e
s h o r e l i n e i n a b o u t 100 y e a r s . However m o r e r e c e n t l y t h e r a t e has
slowed.
2.
From 1972 t o 1992 t h e r e s h a p i n g has b e e n l e s s d r a m a t i c a n d t h e
m a i n movement has b e e n a n a p p a r e n t r o t a t i o n o f t h e Wai San T i n g
as a r e s u l t o f more s e v e r e r e t r e a t a t t h e s e a w a r d e n d c o m p a r e d t o
t h e l a n d w a r d end. The l a n d w a r d e n d i s s i t u a t e d i n t h e l e e o f t h e
H a i f e n g s a n d b a r and i s s h e l t e r e d f r o m t h e p r e d o m i n a n t wave
a t t a c k . Some a c c r e t i o n c a n e v e n be s e e n a t t h e l a n d w a r d e n d
b e t w e e n t h e 1990
a n d 1992
shorelines.
The
retreat of the
s h o r e l i n e a t t h e s e a w a r d e n d has s l o w e d down a n d a r e t r e a t o f t h e
n o r t h e r n s h o r e b y a b o u t 40 t o 50m/year i s a p p a r e n t f r o m t h e
c o a s t a l o v e r l a y s . T h i s i n d i c a t e s t h a t t h e r e s h a p i n g p r i o r t o 1972
had reduced t h e t r a n s p o r t p o t e n t i a l by a l i g n m e n t o f t h e sandbar
t o p r e d o m i n a n t wave a c t i o n .
3.
The s t r i k i n g f e a t u r e o f H a i f e n g i s t h e r a t e o f movement o f t h e
s o u t h e r n m o s t t i p . The t i p h a s moved i n a S t o SSW'ly d i r e c t i o n b y
a b o u t 4 t o 6km o v e r t h e p a s t 20 y e a r s (1972 t o 1 9 9 2 ) , a r a t e o f
a b o u t 200 t o 300m p e r y e a r . T h i s r a t e w o u l d mean t h a t i n t h e
a b s e n c e o f f u r t h e r i n t e r f e r e n c e b y man t h e t i p w o u l d r e a c h t h e
p r e s e n t l o c a t i o n o f t h e n o r t h e r n s i d e o f W a i - S a n - T i n g i n a b o u t 30
y e a r s t i m e . T h i s w o u l d a c t as a " n a t u r a l " n o u r i s h m e n t f o r t h e
n o r t h e r n a n d l a n d w a r d p a r t s o f W a i - S a n - T i n g . However,^ t h e r e i s
u n c e r t a i n t y a b o u t t h e c o n t i n u e d r a t e o f movement, i t may
be
e x p e c t e d t h a t as t h e f e a t u r e a p p r o a c h e s Wai San T i n g i t becomes
m o r e d i s p e r s e d a n d "moves" s l o w e r , p e r h a p s e f f e c t i n g a c o m p l e t e
m e r g i n g w i t h Wai San T i n g o v e r a much l o n g e r t i m e s c a l e , s a y 30
t o 100 y e a r s .
4.
The t o t a l s u r f a c e a r e a o f W a i - S a n - T i n g t h a t i s o u t l i n e d
on
v a r i o u s map o v e r l a y s a p p e a r s t o be f a i r l y c o n s i s t e n t f r o m y e a r t o
y e a r a n d t h i s s u g g e s t s t h a t t h e s a n d b a r as a w h o l e may be m o v i n g
(eg.
by washover f l o w s and b r e a k t h r o u g h ) o r t h a t t h e r e i s
c o n t i n u a l b u i l d up o f f i n e m a t e r i a l due t o w e a k e r t r a n s p o r t f r o m
s o u t h t o n o r t h i n t h e l e e o f Wai-San-Ting.
30
Washover Transport on Sandbars
6.3.3
Q u a l i t a t i v e h y p o t h e s i s : Y u n l i n c o a s t a l m o r p h o l o g y , Wai
San
Ting
A l t h o u g h t h e r e i s l i t t l e q u a n t i t a t i v e c e r t a i n t y about the h i s t o r i c
e p i s o d i c sediment s u p p l i e s t o t h e c o a s t by t h e s e v e r a l h i s t o r i c a l l y
a c t i v e r i v e r s i n t h e Y u n l i n r e g i o n , i t c a n n o t be d o u b t e d t h a t t h e
m a j o r i t y o f t h e H o l o c e n e c o a s t a l s e d i m e n t s b e t w e e n s a y t h e -20 m TD
c o n t o u r and t h e p r e s e n t c o a s t l i n e have been d e p o s i t e d by t h e s e r i v e r s
over thousands o f y e a r s . U n t i l 1911, w i t h t h e c o n s t r u c t i o n o f t h e d i k e s
a t L i n n e i , t h e r e c o u l d have b e e n f i v e a c t i v e r i v e r s i n t h e c a t c h m e n t
a r e a e x t e n d i n g o v e r t h e w h o l e o f t h e l o w l a n d s o f Y u n l i n and Ch\ing Hwa
c o u n t y , v i z . M a i Yu Cho H s i , C h o s h u i H s i , O l d Hu Wei H s i , New Hu Wei
H s i a n d P e i k a n g H s i . The l a t t e r f o u r w e r e p r o b a b l y a c t i v e a l o n g t h e
coast
of
Yunlin, but
an
educated
g u e s s was
made a b o u t
their
q u a n t i t a t i v e r o l e s . I n o r d e r t o do s o , t h e f o l l o w i n g h y p o t h e s i s was
used.
O b s e r v a t i o n o f t h e p r e s e n t d e p t h c o n t o u r l i n e s ( F i g . 44) i n d i c a t e s t h a t
the p r i m a r i l y t i d e d o m i n a t e d d e p t h s a r e c o n t a i n e d i n t h e d e p t h r e g i o n s
b e l o w -20 m t o -25 m TD. The p r i m a r i l y wave d o m i n a t e d d e p t h s a r e f o u n d
i n t h e r e g i o n s s h a l l o w e r t h a n 10 m d e p t h ( e . g . , n o t e t h e p a r a l l e l
c h a r a c t e r o f t h e -10 m TD d e p t h c o n t o u r r e l a t i v e t o t h e s h o r e l i n e
p o s i t i o n ) . Between these depths t h e r e i s a t r a n s i t i o n r e g i o n o f a mix
b e t w e e n t i d e a n d wave i n d u c e d phenomena. The -10 m TD d e p t h c o n t o u r s
more o r l e s s i n s t a n t a n e o u s l y f o l l o w t h e c o a s t l i n e c h a n g e s , i n c o n t r a s t
w i t h t h e -20 m TD d e p t h c o n t o u r s . B a s e d o n t h i s , i t i s p r o p o s e d t o
a d o p t t h e h y p o t h e s i s t h a t t h e -20 m TD d e p t h c o n t o u r s t i l l r e f l e c t s t h e
seaward boundary o f the o u t e r d e l t a i c systems formed by t h e
now
sedimentologically inactive rivers.
I n d o i n g t h i s i t m u s t be c o n c l u d e d t h a t t h e l a r g e s t r e l i c t d e l t a was
f o r m e d b y t h e P e i k a n g H s i , t h e s e c o n d l a r g e s t b y t h e O l d Hu Wei H s i a n d
a m i n o r d e l t a - i f a n y - b y t h e New Hu Wei H s i . The d e p t h c o n t o u r s n e a r
and j u s t s o u t h o f t h e C h o s h u i H s i m o u t h c o n f i r m t h a t t h i s i s a s t i l l
active
delta,
although with a
fairly
weakly
pronounced
deltaic
o u t b u i l d i n g ( p o s s i b l y c o n n e c t e d w i t h t h e s e d i m e n t s i n k s due t o s t r o n g
subsidence and s a n d m i n i n g ) .
I t may be o b s e r v e d t h a t t h e p o s i t i o n o f t h e s e r e l i c t d e l t a b a s e l i n e s i s
s t r o n g l y skew t o w a r d s t h e s o u t h . T h i s may be e x p l a i n e d n o t o n l y on t h e
basis o f the r e l a t i v e l y l a r g e net southward d i r e c t e d longshore t r a n s p o r t s , b u t a l s o from the - t o a c e r t a i n degree - i n s t a b i l i t y e f f e c t s
c a u s e d b y t h e s t r o n g l y o b l i q u e a n g l e o f d o m i n a n t wave i n c i d e n c e t o t h e
c o a s t ( s e e a l s o f i g 4 5 ) . The d o m i n a n c e o f t h e i n c i d e n t waves f r o m t h e
n o r t h and n o r t h - n o r t h e a s t most p r o b a b l y i m p l i e s t h a t t h e l o n g s h o r e
s e d i m e n t t r a n s p o r t i s d r i v e n b y waves w h i c h a p p r o a c h t h e a v e r a g e
c o a s t a l o r i e n t a t i o n u n d e r a n g l e s o f more t h a n 45 d e g r e e s r e l a t i v e t o
the
normal d i r e c t i o n . I n t h e case o f a t r i a n g u l a r o u t b u i l d i n g o f t h e
outer d e l t a s (connected w i t h the d i v e r g i n g r i v e r o u t f l o w , c r e a t i n g
e d d i e s on b o t h s i d e s , w h i c h a r e i n f l u e n c e d by t h e i n t e r a c t i o n w i t h t h e
t i d e s , see a l s o f i g u r e 2 6 ) , i t i s p r o b a b l e t h a t t h e 45 d e g r e e s a n g l e i s
surpassed by t h e c o a s t l i n e n e a r t h e r i v e r m o u t h . T h i s l e a d s t o an
i n s t a b i l i t y i n t h e s h o r e l i n e e v o l u t i o n on t h e n o r t h e r n l o b o f t h e
d e l t a s . However, t h i s i n s t a b i l i t y w i l l n o t grow u n l i m i t e d l y , f i r s t l y
s i n c e t h e r e e x i s t s a l i m i t t o t h e g r o w t h where t h e c o a s t l i n e meets t h e
-20 m TD d e p t h c o n t o u r and s e c o n d l y s i n c e i t i s e x p e c t e d t h a t i n i t i a l l y
wave-asymmetry
induced
onshore
transport
and
finally
overwash
mechanisms a c t as a d d i t i o n a l s i n k s t o t h e s y s t e m (see S t i v e ,
1986,
d e s c r i b i n g a s i m i l a r e f f e c t i n t h e V o o r d e l t a , The N e t h e r l a n d s ) . The
c o m b i n a t i o n o f t h e s e mechanisms w i l l p r o m o t e a s o u t h w a r d
oriented
s k e w n e s s o f t h e d e l t a s and a n e t o n s h o r e t r a n s p o r t and r e g r e s s i o n .
31
Washover Transport on Sandbars
I n summary, t h e t r a n s i t i o n o f t h e s e r e l i c t s u b a q u e o u s d e l t a s f r o m a
p r i m a r i l y r i v e r d o m i n a t e d t o a p u r e l y wave ( f r o m n o r t h e r n i n c i d e n c e )
d o m i n a t e d s y s t e m has l e d t o t h e f o r m a t i o n o f b a r r i e r b e a c h e s o n t h e
n o r t h e r n f r i n g e s w i t h l a r g e r o b l i q u e angles t o the coast than the
d e l t a i c base, t h r o u g h a c o m b i n a t i o n o f l o n g s h o r e t r a n s p o r t mechanisms,
l o s s e s a t t h e -20 m TD d e p t h c o n t o u r and o v e r w a s h m e c h a n i s m s , f e d b y
o n s h o r e w a v e - i n d u c e d t r a n s p o r t mechanisms. T h i s i m p l i e s t h a t
the
b a r r i e r b e a c h e s ( s a n d b a r s ) o f W a i - S a n - T i n g Chou a n d H a i f e n g Chou a r e
a t t r i b u t e d t o t h e r e l i c t d e l t a s o f P e i k a n g H s i a n d O l d Hu Wei H s i .
B a s e d o n t h e above c o n c e p t s o f t h e l a r g e s c a l e h i s t o r i c a l e v o l u t i o n o f
the Y u n l i n c o a s t and i n agreement w i t h t h e g e o s t a t i s t i c a l a n a l y s i s o f
the
l o n g s h o r e c o r r e l a t i o n l e n g t h s o f sediment s i z e and s h o r e l i n e
changes, i t i s c o n c l u d e d t h a t t h e p r e s e n t c o a s t a l system n e a r Peikang
H s i i s a m o d e r a t e l y i n t e r a c t i n g r e l i c t d e l t a , c h a r a c t e r i z e d b y t h e WaiSan-Ting b a r r i e r b e a c h and i n t e r t i d a l b a r s y s t e m ( F e a t u r e s F o f f i g u r e
44) .
The P e i k a n g S h i r e l i c t d e l t a i c s y s t e m has t o a v a r y i n g d e g r e e d e g e n e r a t e d t o a s h o r e l i n e f e a t u r e because o f the n a t u r a l tendency o f c o a s t a l
systems t o smooth o u t d i s t u r b a n c e s . O b v i o u s l y , t h e l a r g e s t o f t h e
three,
t h e Peikang
Hsi
delta
w i t h W a i - S a n - T i n g Chou,
is
still
reflecting
most
clearly
the
initial
transformation
features
as
d e s c r i b e d w i t h t h e h i s t o r i c a l c o n c e p t u a l m o d e l . The t r a n s i t i o n a l s t a t e
b e t w e e n t h e s e t w o s y s t e m s i s r e f l e c t e d b y t h e r e l i c t O l d Hu Wei H s i
delta.
An i m p o r t a n t q u e s t i o n c o n c e r n s t h e d e g r e e o f i n t e r a c t i o n b e t w e e n t h e
H e i f a n g s a n d b a r a n d t h e Wai San T i n g s a n d b a r . C o n n e c t e d w i t h t h e
instability
mechanism t h e r e e x i s t s
a relatively
large
southward
d i r e c t e d , n e t a l o n g s h o r e s e d i m e n t t r a n s p o r t due t o t h e d o m i n a n c e o f
n o r t h e r n wave d i r e c t i o n s . As l o n g as t h i s t r a n s p o r t i s n o t i n t e r r u p t e d
t h e r e may n o t e x i s t l a r g e a l o n g s h o r e g r a d i e n t s , b u t i n t e r r u p t i o n s w i l l
cause l a r g e g r a d i e n t s and t h u s l a r g e e f f e c t s . I n t h i s r e s p e c t , i t can
be r e m a r k e d t h a t t h e e x i s t i n g c o a s t c o n t a i n s s u c h i n t e r r u p t i o n s and
t h e s e may be t h e c a u s e f o r t h e r a p i d e r o s i o n o f l o c a l a r e a s r e l a t i v e t o
o t h e r s a l o n g t h e c o a s t . The p r e s e n t i n t e r r u p t i o n s a r e m a i n l y n a t u r a l
s u c h as t h e H a i f e n g t i d a l c h a n n e l , H a i f e n g t i p , P e i k a n g t i d a l c h a n n e l ,
Wai San T i n g t i p e t c . The H a i f e n g s y s t e m , due t o i t s g e o m e t r y , a t
p r e s e n t s t i l l e f f e c t i v e l y b l o c k s t h e f e e d i n g o f t h e Wai San T i n g Chou
along a surf-zone route.
This i s a very important conclusion since i t r e s u l t s i n several points
of
" d i s c o n t i n u i t y " a l o n g t h e Y u n l i n coast where t h e t r a n s p o r t
and
s u p p l y o f s e d i m e n t f r o m n o r t h t o s o u t h i s c o m p l e t e l y d i s r u p t e d . The
r e s u l t i s t h a t f e a t u r e s s u c h as t h e m a i n p o r t i o n o f Wai San T i n g a r e
c o n s i d e r e d t o be d e v e l o p i n g a t p r e s e n t i n d e p e n d e n t l y f r o m t h e a d j a c e n t
coastal supply s i t u a t i o n .
An i m p o r t a n t phenomenon i n t h e e v o l u t i o n o f t h e " c o a s t l i n e " i s c a u s e d
by t h e r e l a t i v e l y l a r g e subsidence which occurs i n t h e Y u n l i n c o a s t a l
r e g i o n . The o b s e r v a t i o n s show t h a t t h e maximum r a t e s (up t o l O c m / y e a r )
may e v e n be c l o s e t o o r a t t h e a c t u a l c o a s t a l f r i n g e . S u b s i d e n c e o f t h e
c o a s t l i n e e f f e c t i v e l y a c t s as a s i n k mechanism a n d c a n a c c o u n t f o r a
c o n s i d e r a b l e amount o f v o l u m e l o s s e s o v e r p e r i o d s o f y e a r s . These
a p p a r e n t l o s s e s w i l l be s l i g h t l y i n c r e a s e d b y t h e a d d i t i o n a l r e l a t i v e
r i s e i n s e a l e v e l due t o g l o b a l w a r m i n g w h i c h i s p r e d i c t e d ( S e c t i o n
4.2) t o be i n t h e r a n g e o f 3 t o 10cm o v e r t h e n e x t d e c a d e .
32
Washover Transport on Sandbars
6.4
E v o l u t i o n o f the Wai
San
Ting
6.4.1
S e d i m e n t t r a n s p o r t mechanisms
The f o r m a t i o n o f t h e b a r r i e r b e a c h a n d i n t e r t i d a l b a r s y s t e m Wai-SanT i n g Chou i s e x p l a i n e d ( p a r a g r a p h 6.3.3) as t h e r e s u l t o f w a v e - i n d u c e d
l o n g s h o r e a n d c r o s s - s h o r e m e c h a n i s m s a c t i n g on t h e s h a l l o w o u t e r d e l t a
of the Peikang H s i , a f t e r t h i s r i v e r stopped s u p p l y i n g sediment t o the
d e l t a . I n i t i a l l y , t h e l o n g s h o r e i n s t a b i l i t y mechanism enhanced i t s
o b l i q u e o r i e n t a t i o n t o t h e s h o r e l i n e , w h i l e t h e wave a s y m m e t r y d r i v e n
c r o s s - s h o r e t r a n s p o r t s deepened and f l a t t e n e d i t s f o r e s h o r e and t h e r e b y
i n c r e a s e d i t s h e i g h t t o i n t e r t i d a l l e v e l s . A t t h a t s t a g e o v e r w a s h and
b a r r i e r b r e a k p r o c e s s e s must have grown i n i m p o r t a n c e , r e i n f o r c i n g t h e
s h a l l o w n e s s o f t h e i n t e r t i d a l f l a t s o n t h e s o u t h e r n s i d e o f Wai-SanT i n g Chou.
The mechanisms r e q u i r i n g e x a m i n a t i o n when c o n s i d e r i n g W a i - S a n - T i n g i n
d e t a i l are the f o l l o w i n g :
=> L o n g s h o r e t r a n s p o r t :
B r e a k i n g waves t r a n s p o r t s e d i m e n t
alongshore.
=» O f f s h o r e t r a n s p o r t :
S t o r m a n d t y p h o o n waves t r a n s p o r t s e d i m e n t
=> E f f e c t s o f w a s h o v e r :
Waves t r a n s p o r t s e d i m e n t
on and
over t o p sandbar a t h i g h
offshore.
tide
=> C h a n n e l s t r a n s p o r t :
T i d a l channels ( n a t u r a l and dredged) t r a n s p o r t sediment
back side.
t o the
=» R e l a t i v e s e a l e v e l r i s e :
S u b s i d e n c e , sea l e v e l r i s e due t o g l o b a l w a r m i n g a n d c o m p a c t i o n
of the d e l t a s a l l serve t o b r i n g about net l o s s e s o f m a t e r i a l
t o t h e a c t i v e c o a s t a l system.
=> R i p c u r r e n t s :
T r a n s p o r t m a t e r i a l o f f s h o r e i n l o c a l a c c e l e r a t i o n s , m a i n l y due
t o the b r e a k i n g out o f longshore f l o w s i n t h e s u r f z o n e between
bed f e a t u r e s .
=> Wind t r a n s p o r t s :
T r a n s p o r t s o f sand by
wind.
These m e c h a n i s m s h a v e b e e n a d d r e s s e d i n t h i s S e c t i o n a n d w h e r e p o s s i b l e , q u a n t i f i e d . The S e c t i o n e n d s ( S e c t i o n 6.4.9) w i t h a q u a n t i f i e d
e s t i m a t e o f t h e p r e s e n t a n d f u t u r e e v o l u t i o n o f Wai San T i n g . I n
A p p e n d i x 6 a sand b a l a n c e o f t h e Y u n l i n c o a s t i s been g i v e n .
6.4.2
Longshore
transports
The r e s u l t s o f t h e l o n g s h o r e t r a n s p o r t c o m p u t a t i o n s , e x e c u t e d w i t h
UNIBEST_LT a n d MIKE21_ST', a r e p r e s e n t e d on t h e n e x t p a g e . A l o n g Wai
San T i n g t h r e e c r o s s - s e c t i o n s ( F i g . 58 & 59) w e r e u s e d t o s c h e m a t i s e
t h e wave a n d c u r r e n t c l i m a t e a n d t h e b a t h y m e t r y , a l l o f w h i c h c h a n g e
a l o n g t h e sandbar s i g n i f i c a n t l y .
'Frederic R. Harris B.V. and Sinotecli Engineering Consultants Inc.
33
Washover Transport on Sandbars
In
summary t h e c a l c u l a t e d l o n g s h o r e
Section 7
Section 8
Section 9
transports are:
0.2 m i l l i o n mVyear
0.8 m i l l i o n mVyear
1.2 m i l l i o n mVyear
These f i g u r e s show a n i n c r e a s e i n t r a n s p o r t r a t e s m o v i n g n o r t h t o s o u t h
a l o n g t h e s a n d b a r . The a v e r a g e g r a d i e n t a l o n g W a i San T i n g i s a b o u t 0.1
m i l l i o n m3/km/year a l t h o u g h t h e t i d a l c h a n n e l s a n d b a t h y m e t r y e f f e c t s
c a n c a u s e h i g h e r a n d l o w e r t r a n s p o r t s w i t h i n some a r e a s .
The g r a d i e n t t o w a r d s
t h e southwestern
e n d o f Wai San T i n g i s i n
r e s p o n s e t o t h e g r e a t e r e x p o s u r e t o b o t h waves a n d c u r r e n t s . The
d i f f r a c t i o n o f waves a r o u n d H a i f e n g a n d t h e s h i e l d i n g f r o m s t r o n g
c u r r e n t s a t t h e l a n d w a r d p a r t o f W a i San T i n g i s t h e m a i n m e c h a n i s m f o r
t h i s g r a d i e n t . I t i s expected
that just north of Profile
7 the
t r a n s p o r t w i l l be a l m o s t z e r o a n d s t i l l f u r t h e r n o r t h a n d e a s t t h e
t r a n s p o r t w i l l become n e g a t i v e , v i z . n o r t h - g o i n g . To t h e s o u t h w e s t o f
p r o f i l e 9, t o w a r d t h e t i p o f W a i San T i n g t h e t r a n s p o r t w i l l i n c r e a s e
f u r t h e r u n t i l t h e c o a s t a n g l e becomes s o o b l i q u e t h a t t h e t r a n s p o r t
drops.
6.4.3
Crosshore t r a n s p o r t s
A n o t h e r t r a n s p o r t mechanism i s c r o s s h o r e t r a n s p o r t w h e r e b y e f f e c t s s u c h
as u n d e r t o w , wave a s y m m e t r y a n d wave g r o u p i n g (wave s u r g e s ) c a n c a u s e
the t r a n s p o r t o f m a t e r i a l i n a crosshore (onshore-offshore) d i r e c t i o n .
T y p i c a l l y , t h e s e e f f e c t s a r e m o s t s e v e r e d u r i n g h i g h s t o r m waves a n d
r e s u l t i n m a t e r i a l being eroded from the upper p a r t o f the crosshore
p r o f i l e a n d dumped o n t h e l o w e r p a r t . The s u b s e q u e n t a c t i o n o f t i d e s
and n o n - s t o r m waves may t h e n r e m o v e s u c h m a t e r i a l a l o n g s h o r e o r t o
deeper water.
C o m p u t a t i o n w i t h t h e s i m u l a t i o n m o d e l U n i b e s t - T C " w e r e made o n p r o f i l e s
b e t w e e n 7 a n d 9 ( F i g . 5 9 ) . Some l o s s d u e t o t h e t y p h o o n s e q u e n c e was
f o u n d o v e r t h e -8 m c o n t o u r a t t h e more s o u t h e r l y p r o f i l e s , b u t t h i s i s
o n l y i n t h e o r d e r o f 50 mVm. I n t h e o t h e r s e c t i o n s t h e l o s s e s d u r i n g
t h e t y p h o o n c y c l e as c o m p u t e d w i t h U n i b e s t - T C a r e n e g l i g i b l e .
On t h e b a s i s o f t h e i n d i c a t i v e c o m p u t a t i o n s o f t h e c r o s s - s h o r e t r a n s p o r t and i n s p e c t i o n o f data, i t appears t h a t t h e o v e r a l l o f f s h o r e
l o s s e s a r e n o t s i g n i f i c a n t . The m o s t l i k e l y c o n c l u s i o n i s t h a t o f f s h o r e
l o s s e s d o n o t c o n t r i b u t e s i g n i f i c a n t l y t o t h e e r o s i o n o f W a i San T i n g .
6.4.4
Washover
D u r i n g h i g h w a t e r l a r g e p a r t s o f W a i San T i n g submerge. When t h e w a t e r
l e v e l a t t h e f r o n t ( n o r t h ) s i d e o f W a i San T i n g i s h i g h e r t h a n t h e b a c k
s i d e t h e n t h e w a s h o v e r f l o w s w i l l b e s e t u p . I n f i g u r e 60 t h e w a t e r
l e v e l s on e i t h e r s i d e o f t h e sandbar have been p l o t t e d f o r a w i n t e r
s p r i n g t i d e a n d neap t i d e . I t c a n b e s e e n t h a t , a s s u m i n g t h e s a n d b a r
becomes f l o o d e d a t e l e v a t i o n s a b o v e +0.5 m TD t h e w a t e r l e v e l o n t h e
n o r t h s i d e i s a l w a y s h i g h e r t h a n t h a t o n t h e s o u t h s i d e ( t h e same i s
t r u e f o r summer t i d a l c o n d i t i o n s ) . T h i s l e a d s t o s t r o n g s h e e t f l o w f r o m
the n o r t h t o t h e s o u t h over t h e i n u n d a t e d p a r t s and t h e r e i s no r e t u r n
f l o w f r o m s o u t h t o n o r t h . The w a t e r l e v e l d i f f e r e n c e i n t h e m o d e l i s
'Performed by Frederic R. Harris and Sinotech Engineering Inc.
34
Washover Transport on Sandbars
due t o a d i f f e r e n c e i n p h a s e a n d t i d a l a m p l i t u d e o n e i t h e r s i d e o f t h e
s a n d b a r . I n r e a l i t y w i n d a n d wave s e t - u p w o u l d c a u s e a n e v e n h i g h e r
w a t e r l e v e l o n t h e n o r t h a n d more w a s h o v e r . The v e c t o r p l o t i n f i g u r e
61 d e m o n s t r a t e s t h e w a s h o v e r e f f e c t ( i n t h e m o d e l t h e t o p o g r a p h y o f Wai
San T i n g was e s t i m a t e d as f l a t a t a n e l e v a t i o n o f +0.4 m T D ) .
The f l o w o v e r t h e c r e s t w i l l c a r r y s e d i m e n t f r o m t h e s e a w a r d s i d e a n d
from t h e c r e s t across t o t h e southern s i d e . I n appendix 4 a rough
e s t i m a t e h a s b e e n made t o q u a n t i f y t h e w a s h o v e r t r a n s p o r t .
W i t h a w a t e r d e p t h o f 0.4m o n t o p o f t h e b a r t h e s e d i m e n t t r a n s p o r t i s
a b o u t 220 mVyr, w h i l e a w a t e r d e p t h o f 0.5m a t t h e c r e s t o f t h e b a r a
c a u s e s a s e d i m e n t t r a n s p o r t o f 220,000 mVyr! T h e s e e s t i m a t e s i n d i c a t e
t h a t t h e e f f e c t o f w a s h o v e r c a n b e s i g n i f i c a n t , a n d t h e waves ( a n d t h e
c o r r e s p o n d i n g wave s e t u p ) c a n i n c r e a s e t h i s w a s h o v e r c u r r e n t v e l o c i t i e s
e v e n m o r e . O v e r a l e n g t h o f 10 km t h e e s t i m a t e s s u g g e s t t h a t t h e
c o n t r i b u t i o n o f washovers c o u l d be i n t h e o r d e r o f 1 t o 2 m i l l i o n
m3/yr. H o w e v e r , t h i s i s a v e r y r o u g h e s t i m a t e a n d i t s h o u l d b e
c o n s i d e r e d as a f i r s t i n d i c a t i o n o n l y t h a t s i g n i f i c a n t l o s s e s due t o
washovers u n d e r d a i l y t i d a l c o n d i t i o n s c a n o c c u r . I t s h o u l d be n o t e d
t h a t no e x t r e m e e v e n t s have been c o n s i d e r e d . I t c a n be e x p e c t e d t h a t
one e x t r e m e e v e n t w i t h h i g h s u r g e s a n d h i g h waves c a n c o n t r i b u t e
s i g n i f i c a n t l y t o t h e y e a r l y washover.
6.4.5
The e f f e c t
of tidal
channels
T h r o u g h W a i San T i n g s e v e r a l c h a n n e l s
can be observed.
Though t h e
c h a n n e l s a r e s h a l l o w , t h e y c a n a c t as s e d i m e n t s i n k s . The l a r g e s t o f
t h e s e c h a n n e l s i s a b o u t one t h i r d o f t h e way f r o m t h e l a n d t o w a r d s t h e
s e a w a r d e n d . The l a n d w a r d d i r e c t e d f l o w i n t h i s c h a n n e l c a n t r a n s p o r t
s e d i m e n t f r o m t h e s e a f a c i n g s l o p e o f Wai San T i n g t o t h e l a n d w a r d s i d e
o f W a i San T i n g . The wave s e t - u p a t t h e s e a f a c i n g ( a n d n o s e t - u p a t t h e
l a n d f a c i n g s i d e ) o f W a i San T i n g m i g h t
also contribute t o this
"sediment
sink" during conditions with
h i g h w a v e s . No
detailed
i n f o r m a t i o n i s a v a i l a b l e f o r t h e c r e s t a n d t h e c h a n n e l . The d e p t h o f
t h e c h a n n e l i s e s t i m a t e d as -2 m TD. A p p e n d i x 5 shows a r o u g h e s t i m a t e
o f t h e s e d i m e n t t r a n s p o r t c a p a c i t y b y c h a n n e l s (see a l s o p a r a g r a p h 4.4
for
t h e c a l c u l a t i o n o f cross
sectional
areas o f i n l e t s
(tidal)
channels.
The r e s u l t s i n d i c a t e a n o r d e r o f m a g n i t u d e f o r t h e t r a n s p o r t c a p a c i t y
t h r o u g h a c h a n n e l o f 1000 t o 1500 mVyr. p e r m w i d t h o f t h e c h a n n e l . F o r
channels o f s e v e r a l hundreds o f meters t h i s c o u l d c o n t r i b u t e t o t h e
l o s s w i t h 0.1 t o 1 m i l l i o n mVyr p e r c h a n n e l . T h i s i m p l i e s t h a t t h e
t r a n s p o r t c a p a c i t y o f t h e l a r g e s t channel t h r o u g h Wai-San-Ting ( i f t h e
w i d t h i s e s t i m a t e d a t 500 m) c o u l d b e r o u g h l y e s t i m a t e d a t 0.5 m i l l i o n
mVyr. T h i s i m p l i e s t h a t a l l t h e l o n g s h o r e t r a n s p o r t c o m i n g f r o m t h e
n o r t h ( w h i c h i s a l s o i n t h e o r d e r o f 0.5 m i l l i o n mVyr a t t h e l o c a t i o n
of t h i s channel) can be absorbed c o m p l e t e l y t h r o u g h t h e channel. T h i s
would i n c r e a s e t h e t r a n s p o r t g r a d i e n t and e r o s i o n t e n d e n c i e s on t h e
s e a w a r d s i d e t o w a r d s t h e m i d d l e o f Wai San T i n g .
Though t h e above i s a r o u g h e s t i m a t e , i t i n d i c a t e s t h a t
can s i g n i f i c a n t l y c o n t r i b u t e t o t h e s e d i m e n t l o s s .
t h e channels
I t should be r e a l i z e d t h a t t h e s i n k c a p a c i t y o f t h e channels i s o n l y
d e t e r m i n e d b y t h e t i d e . The d i f f e r e n c e i n t i d a l l e v e l o v e r t h e b a r
determines t h e f l o w v e l o c i t y and t h e corresponding t r a n s p o r t through
the c h a n n e l . T h i s r e s u l t s i n a sediment s i n k c a p a c i t y o f t h e channel
d u r i n g e v e r y h i g h w a t e r . These c h a n n e l s a r e t h e r e f o r e b a s i c a l l y t i d a l
f e a t u r e s and t h e y are a sediment s i n k independent o f whether t h e r e i s
35
Washover Transport on Sandbars
a longshore t r a n s p o r t c a p a c i t y a l o n g t h e s e a f a c i n g s i d e o f Wai-San-Ting
o r n o t . A l s o , because o f t h i s ,
l o s s e s due
t o the channels
(and
overwash) a r e s u p p l e t e d n o t o n l y by s u r f z o n e sediments b u t a l s o by
s e d i m e n t s f r o m up t o 10 m d e p t h .
6.4.6
Subsidence
A t t h e n o r t h e r n , l a n d w a r d end o f Wai San T i n g t h e r e c o r d e d
subsidence
i s 0.10 m/year. A l s o t h e r a t e o f sea l e v e l r i s e due t o g l o b a l w a r m i n g
c a n a d d up t o 1cm p e r y e a r i n t h e w o r s t s c e n a r i o . T h e s e e f f e c t s c a u s e
a r e l a t i v e sea l e v e l r i s e w h i c h a c t s as a " s i n k " i n t e r m s o f a n e t l o s s
o f v o l u m e f r o m t h e a c t i v e p r o f i l e . I t i s n o t k n o w n how t h e s u b s i d e n c e
r a t e c h a n g e s a l o n g Wai
San T i n g b u t i t i s e x p e c t e d
t o decrease
s i g n i f i c a n t l y i n s e a w a r d d i r e c t i o n (see a l s o p a r a g r a p h 4.2) .
F o r a c o n s e r v a t i v e e s t i m a t e , i f i t i s assumed t h a t t h e 0 m TD c o n t o u r
s u b s i d e s b y 0.10 m/year a n d t h a t t h e s u b s i d e n c e d e c a y s w i t h d e p t h t o
z e r o a t t h e -10 m TD d e p t h c o n t o u r (an a v e r a g e d i s t a n c e o f 2 km away)
t h e a l o n g s h o r e v o l u m e l o s s i s ( 2 , 0 0 0 * 1 * 0 . 1 * 0 . 5 = ) 100 mVm/year. T h i s
f i g u r e i s a b o u t 10% o f t h e l o n g s h o r e t r a n s p o r t r a t e . T h o u g h t h i s
a n a l y s i s i s r o u g h i t s e r v e s t o show t h a t t h e " s i n k " due s u b s i d e n c e and
sea l e v e l r i s e i s r a t h e r s m a l l r e l a t i v e t o o t h e r p o s s i b l e l o s s e s .
6.4.7
Remaining
processes
The r e m a i n i n g p r o c e s s h a v e n o t b e e n q u a n t i f i e d , b u t t h e y a r e e x p e c t e d
t o have o n l y a l i m i t e d e f f e c t on t h e o v e r a l l s e d i m e n t b a l a n c e .
The
e f f e c t o f rip-currents i s p r o b a b l y l i m i t e d t o t h e u p p e r r e g i o n s ( a b o v e
t h e -10 m TD l e v e l ) . T h e y c a n l o c a l l y r e d i s t r i b u t e t h e s e d i m e n t b u t
w i l l p r o b a b l y n o t cause s i g n i f i c a n t l o s s e s t o t h e deeper r e g i o n s . T h i s
e f f e c t does t h e r e f o r e n o t a f f e c t t h e s a n d b a l a n c e o f t h e u p p e r 10 m.
Transport by wind ( a e o l i a n t r a n s p o r t ) c a n r e s u l t i n some l o s s e s , b u t
t h e s e l o s s e s a r e e x p e c t e d t o be s m a l l . The p e r c e n t a g e o f t i m e t h a t
c e r t a i n a r e a s a r e d r y i s s m a l l . The p e r c e n t a g e o f t i m e t h a t
this
c o i n c i d e s w i t h s t r o n g winds i s even s m a l l e r . I n a d d i t i o n , t h e d r y areas
o n l y have a l i m i t e d s u r f a c e a r e a . R e l a t i v e t o t h e l a r g e t r a n s p o r t
c a p a c i t i e s o f t h e o t h e r mechanisms, t h e c o n t r i b u t i o n o f w i n d t r a n s p o r t s
i s b e l i e v e d t o be n o t s i g n i f i c a n t .
6.4.8
Wai
San
T i n g sand balance
measurements
Survey data:
The I n s t i t u t e o f H a r b o u r a n d M a r i n e R e s e a r c h a n d T e c h n o l o g y (IHMRT)
c o n d u c t e d s u r v e y s o f t h e s o u t h e r n p a r t o f t h e s e a f a c i n g s l o p e o f WaiS a n - T i n g i n O c t o b e r 1990, O c t o b e r 1991 and O c t o b e r 1992. The o r i g i n a l
d a t a w e r e t r a n s f o r m e d t o a 50 b y 50 m g r i d . T h i s d a t a s e t was u s e d f o r
f u r t h e r a n a l y s i s . Two
methods were a p p l i e d , v i z . t h e a n a l y s i s o f
s e p a r a t e p r o f i l e s a n d t h e a n a l y s i s o f s u r v e y e d a r e a s . From t h i s d a t a s e t
10 p r o f i l e s w i t h a s p a c i n g o f 1 km a l o n g t h e c o a s t w e r e d e r i v e d a n d t h e
change o f t h e p r o f i l e v o l u m e was d e t e r m i n e d f o r t h e p e r i o d s 1 9 9 0 - 1 9 9 1
and 1991-1992.
The p l o t t e d p r o f i l e s a r e p r e s e n t e d i n f i g u r e 58. The l o c a t i o n s o f t h r e e
o f t h e t e n a n a l y z e d p r o f i l e s a r e i n d i c a t e d i n f i g u r e 59. From t h e s e
p l o t s an a v e r a g e y e a r l y l a n d w a r d s h i f t o f t h e p r o f i l e s b e t w e e n 4 0 a n d
100 m/yr c a n be e s t i m a t e d . I t c a n be o b s e r v e d t h a t t h i s s h i f t t a k e s
36
Washover Transport on Sandbars
p l a c e o v e r a p r o f i l e d e p t h o f more t h a n 10 m.
The s u r v e y l e v e l s w e r e b a s e d o n a LLW d a t u m w h i c h a t Wai San T i n g c a n
be t a k e n a t a b o u t -1.0 m TD f o r t h e p u r p o s e s o f t h e f o l l o w i n g d e s c r i p tions .
For t h e i n t e r p r e t a t i o n o f t h e s u r v e y r e s u l t s i t s h o u l d be m e n t i o n e d
t h a t a n e r r o r may have b e e n i n t r o d u c e d b y t h e t i d a l c o r r e c t i o n t h a t h a s
b e e n a p p l i e d . No s p e c i f i c d e t a i l s a b o u t t h e c o r r e c t i o n s a r e known,
e x c e p t t h a t t h e c o r r e c t i o n s were based o n measurements c a r r i e d o u t i n
a s h i e l d e d a n d s h a l l o w a r e a n e a r P u t a i . The m o d e l l i n g h a s shown t h a t
t h e w a t e r l e v e l s a t P u t a i c a n v a r y a t h i g h a n d l o w w a t e r b y a b o u t 0.2
t o 0.3 m f r o m t h o s e a t t h e s e a w a r d s i d e o f Wai San T i n g . The i m p a c t o f
s u c h a n e r r o r c o u l d b e s i g n i f i c a n t ( i e a v e r a g e e r r o r 0.10 m o v e r t h e
t i d e c a n a c c o u n t f o r a d i s c r e p a n c y o f 1 m i l l i o n m^ o v e r a n a r e a o f 10
km b y 1 k m ) . The m a g n i t u d e a n d t h e d i r e c t i o n o f t h e e r r o r i n a s p e c i f i c
p o i n t d e p e n d s o n t h e p h a s e o f t h e t i d e a t t h e moment t h e m e a s u r e m e n t
was t a k e n .
Results o f survey a n a l y s i s :
The b a l a n c e r e s u l t s b a s e d o n t h e p r o f i l e d a t a a r e p r e s e n t e d i n A p p e n d i x
6. The r e s u l t s f o r e a c h y e a r b o t h i n d i c a t e a l o s s f r o m t h e u p p e r 10 m
o f ±7.5 m i l l i o n mVyr, f r o m w h i c h + 5 0 % i s l o s t f r o m t h e s e c t i o n b e t w e e n
0 a n d -5 m LLW (-1 a n d -6 m TD) a n d 5 0 % f r o m t h e s e c t i o n b e t w e e n -5 m
a n d -10 m LLW (-6 a n d - 1 1 m TD). The c h a n g e s i n i n d i v i d u a l p r o f i l e s a s
p r e s e n t e d i n F i g u r e s 58 show some l a r g e v a r i a t i o n s b e t w e e n t h e t w o
p e r i o d s 1990-1991 and 1991-1992. A r e a s w i t h h i g h e r a n d l o w e r e r o s i o n
r a t e s c a n t h e r e f o r e n o t b e i n d i c a t e d a c c u r a t e l y . T h e r e seems t o b e a
s i g n i f i c a n t i n t e r a c t i o n between t h e upper and t h e lower p a r t s o f t h e
p r o f i l e s . The b a l a n c e r e s u l t s b a s e d o n t h e a r e a a n a l y s i s a g r e e d w e l l
w i t h t h o s e o f t h e p r o f i l e s a n a l y s i s and a c t e d as a good check t h a t t h e
c a l c u l a t i o n s were c o r r e c t . The a n a l y s i s o f t h e a r e a s a l s o s u g g e s t s a
d e f i c i t i n o f 7 t o 7.5 m i l l i o n mVyr f o r t h e 10 km s t r e t c h b e t w e e n 0 a n d
-10 m LLW (-1 t o - 1 1 m T D ) .
From s a t e l l i t e p i c t u r e s a n d o l d c o a s t l i n e s i t c a n be r o u g h l y e s t i m a t e d
t h a t t h e a v e r a g e r e t r e a t o f Wai San T i n g o v e r t h e l a s t d e c a d e s c a n b e
e s t i m a t e d a t 40 t o 90 m / y r . F o r t h e l a s t d e c a d e s t h e r e t r e a t seems
c l o s e s t t o t h e 50 m / y r .
From t h e p r e s e n t l o c a t i o n o f t h e -10 m TD c o n t o u r i t c a n b e c o n c l u d e d
t h a t t h e -10 m TD c o n t o u r h a s f o l l o w e d t h i s r e t r e a t t o a l a r g e e x t e n t
a l t h o u g h i n some l o c a l a r e a s t h e r e a r e d e v i a t i o n s f r o m t h i s ( e g
g u l l e y s , b r e a k e r b a r s e t c . ) . I t c a n a l s o b e c o n c l u d e d t h a t t h e -20 m TD
c o n t o u r h a s n o t r e s p o n d e d a t a l l . The c l o s u r e d e p t h w i l l t h e r e f o r e b e
somewhere b e t w e e n t h e -10 m a n d -20 m TD d e p t h c o n t o u r . I f t h e c l o s u r e
d e p t h i s c h o s e n i n b e t w e e n , s a y a t t h e -15 m TD c o n t o u r a n d a
d e f o r m a t i o n o f a p a r a b o l i c p r o f i l e w i t h a r e t r e a t o f 50 m / y r a t t h e LWl i n e i s c o n s i d e r e d , t h e n a s e d i m e n t l o s s r e s u l t s o f 0.67*15*50 = 500
mVm/yr. F o r a 10 km s t r e t c h t h i s r e s u l t s i n a l o s s o f 5 m i l l i o n mVyr.
Though t h e s e c o a s t l i n e changes c a n o n l y be e s t i m a t e d w i t h a l i m i t e d
a c c u r a c y , i t c a n be c o n c l u d e d t h a t t h e measurements a n d a n e v a l u a t i o n
o f t h e h i s t o r i c a l d e v e l o p m e n t i n d i c a t e t h e same o r d e r o f m a g n i t u d e f o r
t h e s e d i m e n t d e f i c i t o f t h e s o u t h e r n p a r t o f W a i - S a n - T i n g . F i g u r e 62
g i v e s an o v e r a l l view o f t r a n s p o r t processes a c t i n g on t h e sandbar.
6.4.9
Summary - Wai San T i n g p r e s e n t a n d f u t u r e
In
summary t h e Wai San T i n g
sandbar
37
development
i s considered
t o be a
decaying
Washover Transport on Sandbars
d e l t a f o r m a t i o n o f t h e ( o l d ) P e i k a n g R i v e r . The Wai San T i n g s h o r e l i n e
i s r e t r e a t i n g o v e r i t s c e n t r a l and s o u t h e r n p o r t i o n s a t a r a t e o f a b o u t
50m/year a t p r e s e n t . The n o r t h e r n end i s s h e l t e r e d b y H a i f e n g and i s
r e t r e a t i n g much s l o w e r , w i t h e v e n some s i g n s o f a c c r e t i o n .
L o s s e s h a v e b e e n e s t i m a t e d a t a b o u t 5 t o 7.5 m i l l i o n m 3 / y e a r o v e r a
10km s t r e t c h n e a r t h e s o u t h e r n end, b a s e d on m e a s u r e m e n t s and a n a l y s i s
o f h i s t o r i c a l c h a r t s and s a t e l l i t e i m a g e s . B a s e d on t h e q u a n t i f i c a t i o n s
o f t h e s e v e r a l m a j o r mechanisms a c t i n g o v e r t h e 10 km s e c t i o n , i t i s
e s t i m a t e d t h a t t h e l o n g s h o r e t r a n s p o r t l o s s e s t h r o u g h t h e s o u t h e r n end,
southward losses through the channels w h i c h cross the sandbar
and
w a s h o v e r l o s s e s d u r i n g h i g h w a t e r and s t o r m s a r e e a c h r e s p o n s i b l e f o r
a b o u t one
t h i r d o f t h e volume l o s s . M i n o r mechanisms a r e o f f s h o r e
t r a n s p o r t ( u n d e r t o w and r i p c u r r e n t s ) , s u b s i d e n c e a n d c o m p a c t i o n , wave
asymmetry induced onshore t r a n s p o r t , w i n d t r a n s p o r t e t c .
The H a i f e n g b a r r i e r w i l l merge w i t h Wai San T i n g w i t h i n s e v e r a l d e c a d e s
and w h i l e t h i s w i l l a c t t o f e e d t h e n o r t h e r n end o f t h e sandbar i t w i l l
mean t h a t t h e c e n t r a l p a r t o f t h e s a n d b a r w i l l be more e x p o s e d t o waves
a n d c u r r e n t s a n d t h e r i s k o f s e p a r a t i o n o f t h e s o u t h e r n and n o r t h e r n
parts w i l l increase.
38
Washover Transport on Sandbars
7
CONCLUSIONS
39
Washover Transport on Sandbars
LIST OF L I T E R A T U R E
40
Washover Transport on Sandbars
L I S T OF LITERATURE
ARMON, J.W., a n d McCann, S.B.
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ANDREWS, P.B.
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Coast Association
of Geological
Societies,
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CARR, A.P.
S h i n g l e S p i t and R i v e r Mouth: S h o r t Term
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Migration of B a r r i e r s
Overvash Processes, S.P. L e a t e r m a n 1 9 7 9 , R e p r i n t e d f r o m p a g e s 94 a n d 104-106
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Dernieres, a Louisiana Barrier Island a r c
Marine Geology, 9 1 , 195-206, 1990.
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i n r e l a t i o n to
meteorological influences
J o u r n a l of Coastal
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D i s t r i b u t i o n o f B a r r i e r I s l a n d P l a n t s i n R e l a t i o n t o Overwash D i s t u r b a n c e s :
a T e s t of L i f e H i s t o r y Theory
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Yunlin Offshore Industrial Estate Development,
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O c e a n i c Overwash and t h e O u t e r Banks o f N o r t h
Overwash Processes, S.P. L e a t e r m a n , 1 9 7 9 .
41
Carolina
Washover Transport on Sandbars
HEQUETTE, A. a n d H I L L , P.R.
Storm G e n e r a t e d C u r r e n t s and O f f s h o r e S e d i m e n t T r a n s p o r t on a Sandy S h o r e f a c e ,
T i b j a k Beach, C a n a d i a n B e a u f o r t S e a
Marine Geology, 1 1 3 , 283-304, 1 9 9 3 .
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S p i t and B a r r i e r I s l a n d M i g r a t i o n i n t h e S o u t h e a s t C a n a d i a n B e a u f o r t S e a
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The O u t e r Bank o f N o r t h C a r o l i n a :
a M i g r a t i n g B a r r i e r System
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i n Southeastern
Budget o f Sediment and I n l e t Dynamics
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5, 2, 193-237, 1 9 8 9 .
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B a r r i e r I s l a n d Dynamics: Overwash P r o c e s s e s and E o l i a n T r a n s p o r t
Overwash Processes,
S.P. L e a t e r m a n 1 9 7 9 , R e p r i n t e d f r o m p a g e s 1958-1974 o f
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LEATHERMAN, S.P.
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Story
Washover Transport on Sandbars
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43
Washover Transport on Sandbars
APPENDICES
44
Washover Transport on Sandbars
APPENDIX 1 : BARRIER MIGRATION AND
Two
SEA
LEVEL RISE: A CALCULATION METHOD:
g e n e r a l b e a c h changes c a n be
considered:
(1) S e a s o n a l b e a c h c h a n g e s ( b y m e t e o r o l o g i c a l c o n d i t i o n s )
(2) L o n g - t e r m s h o r e l i n e c h a n g e s ( s e a l e v e l r i s e ) .
Seasonal
changes:
I n p a r a g r a p h 3.8, t w o b e a c h p r o f i l e s w e r e e x a m i n e d : t h e w i n t e r a n d t h e
summer p r o f i l e .
I n h i s s t u d y o f t h e Duck P i e r , N o r t h C a r o l i n a ,
B i r k e m e i e r (1984) i d e n t i f i e d , i n a g r e e m e n t w i t h s t u d i e s e l s e w h e r e , a
s e a s o n a l o f f s h o r e movement o f m a t e r i a l f r o m t h e b e a c h a n d i n n e r b a r
d u r i n g w i n t e r w i t h a g r a d u a l r e t u r n t o t h e b e a c h f a c e d u r i n g summer. He
a l s o f o u n d t h a t t h e l a r g e s t a n d m o s t r a p i d changes i n t h e p r o f i l e
o c c u r r e d d u r i n g s t o r m s . The i n n e r b a r ( F i g . 51) moved o f f s h o r e d u r i n g
t h e s t o r m a n d o n s h o r e a g a i n f o l l o w i n g t h e s t o r m . The o u t e r b a r
r e s p o n d e d more s l o w l y a n d t h e summer p r o f i l e r e q u i r e d a b o u t s i x m o n t h s
t o recover f o l l o w i n g a storm.
The s e a s o n a l changes i n t h e s h o r e zone a r e b o t h 2 a n d 3 - d i m e n s i o n a l i n
r e s p o n s e t o c r o s s - s h o r e a n d l o n g s h o r e w a t e r m o t i o n s . The b e a c h r e s p o n d s
t o t h e i n t e r a c t i o n b e t w e e n i n c i d e n t a n d l o w e r f r e q u e n c y waves b y
f o r m i n g c r e s c e n t i c b a r s (Bowen a n d Inman, 1971) a n d c u s p s ( I n m a n a n d
Guza, 1 9 8 2 ) . Thus t h e i n n e r a n d o u t e r b a r s shown i n f i g u r e 5 1 ( w i n t e r )
may
represent p a r a l l e l
longshore bars, or crescentic bars.
The
t r a n s f o r m a t i o n from l o n g s h o r e t o c r e s c e n t i c b a r s and back t o l o n g s h o r e
b a r s a r e known t o be a s s o c i a t e d w i t h t h e o c c u r r e n c e o f s t o r m s a n d t h e
r e t u r n t o l o w e r waves ( e . g . S h o r t , 1979; W r i g h t & S h o r t , 1 9 8 3 ) .
Long-term s h o r e l i n e
changes:
I n a d d i t i o n t o t h e seasonal beach changes, t h e r e a r e t o t y p e s o f l o n g e r
t e r m s h o r e l i n e changes t h a t a r e e s s e n t i a l i n d e t e r m i n i n g t h e r a t e o f
s h o r e l i n e r e c e s s i o n . These a r e t h e s h o r e l i n e changes due t o l o c a l
a c c r e t i o n / e r o s i o n a n d t h o s e a s s o c i a t e d w i t h sea l e v e l r i s e . W i t h s e a
l e v e l r i s e constant, l o c a l net a c c r e t i o n (or erosion) of the shoreline.
Ax ( F i g . 5 2 a ) . W h i l e , f o l l o w i n g B r u u n ' s ( s e e n e x t page') r u l e ( s c h w a r t z ,
1967; B r u u n , 1983), b o t h a s h o r e l i n e r e c e s s i o n and a r e d i s t r i b u t i o n o f
s a n d i n t h e s h o r e z o n e a r e assumed t o be a s s o c i a t e d w i t h sea l e v e l r i s e ,
Az ( F i g . 52b) .
F o r t h e c a s e o f c o n s t a n t s e a l e v e l i t i s assumed t h a t a c c r e t i o n o r
e r o s i o n c a u s e t h e b e a c h p r o f i l e t o be d i s p l a c e d a h o r i z o n t a l d i s t a n c e
Ax ( F i g . 5 2 a ) . F o r r i s i n g s e a l e v e l i t i s assumed t h a t t h e p r o f i l e i s
r a i s e d v e r t i c a l l y a d i s t a n c e Az, t h e n moved h o r i z o n t a l l y a d i s t a n c e Ax
such t h a t t h e volume eroded f r o m t h e upper beach i s e q u a l t o t h e volume
deposited
further
offshore along
t h e shore
rise.
The
shoreline
r e c e s s i o n o b t a i n e d by e q u a t i n g t h e onshore and o f f s h o r e volumes d u r i n g
s e a l e v e l r i s e i s known as t h e B r u u n r u l e .
The v o l u m e change i s g i v e n b y
q,, «
AxZ
where Z i s t h e v e r t i c a l d i s t a n c e o v e r w h i c h t h e e r o s i o n o c c u r s . I n t h i s
c a s e Z = h + c, w h e r e c i s t h e h e i g h t o f t h e b e a c h berm c r e s t a b o v e MSL
45
Washover Transport on Sandbars
When s e a l e v e l r i s e s a v e r t i c a l d i s t a n c e Az t h e b e a c h p r o f i l e i s
a d j u s t e d upward such t h a t t h e area between t h e two p r o f i l e s i s g i v e n b y
t h e p r o d u c t AzX, w h e r e X i s t h e h o r i z o n t a l
distance over which t h e
a d j u s t m e n t o c c u r s , B r u u n ' assumed t h a t t h e e q u i l i b r i u m
beach p r o f i l e
would remain c o n s t a n t , and t h a t a r e d i s t r i b u t i o n o f m a t e r i a l would
o c c u r s u c h t h a t t h e amount e r o d e d f r o m t h e u p p e r b e a c h i s e q u a l t o t h a t
d e p o s i t e d o f f s h o r e . The r e s u l t i n g s h o r e l i n e r e c e s s i o n becomes
Ax = AzX/Z = Az/tan/3
w h e r e tan/S = Z/X i s t h e e x t e n d e d b e a c h s l o p e . T h i s r e l a t i o n i s known a s
Bruun's r u l e o f m a t e r i a l a s s o c i a t e d w h i t t h i s r e d i s t r i b u t i o n o f t h e
p r o f i l e i s g i v e n a p p r o x i m a t e l y as
q^ « AxZ/2 = AzX/2
where a f a c t o r o f about M e n t e r s because t h e e r o s i o n o c c u r s o n l y o v e r
a p o r t i o n o f t h e t o t a l d i s t a n c e s X a n d Z ( I n m a n & D o l a n , 1989, P i g .
52b) .
T a b l e I : S h o r e l i n e c h a n g e b e t w e e n b e a c h r e c e s s i o n Ax, s e a l e v e l r i s e
s a n d v o l u m e q f o r p r o f i l e s a t Duck, N o r t h C a r o l i n a .
X
km
Profile
a.
h
m
Az, a n d
q-
m
Ax
m
mVm
10
1. 0
10
Sealevel constant
7.5
Summer
0. 8
Winter
1. 0
10
15
1. 0
15
Extended
1. 8
12
17
1. 0
17
Extended
4. 5
18
23
1. 0
23
10
0 .3
1.6
b.
Sealevel r i s e
(0.004 m)"
7.5
Summer
0 .8
Winter
1. 0
10
15
0. 3
2.2
Extended
1. 8
12
17
0. 4
3.6
Extended
4 .5
18
23
0. 8
9.0
'Z = h + c w h e r e c i s h e i g h t o f b e r m c r e s t a b o v e MSL.
"As v o l u m e - e q u i v a l e n t f a c t o r
i n a, q' h a s d i m e n s i o n s o f mVm^-
" S e a l e v e l r i s e o f 0.004 m/yr = 40 c m / c e n t u r y ,
46
as a t P o r t s m o u t h ,
Virginia.
Washover Transport on Sandbars
APPENDIX 2: The e r o s i o n r a t e o f b a r r i e r i s l a n d s ; S a n c h e z - A r c i l l a a n d
(1993)
Jiménez
The r a t e o f e r o s i o n o f an b a r r i e r i s l a n d d e p e n d s o n s e v e r a l e l e m e n t s ,
s u c h as t h e e n v i r o n m e n t a l c o n d i t i o n s ( w i n d , w a v e s , c u r r e n t s ) , s e d i m e n t
s u p p l y and m o r p h o l o g i c a l e l e m e n t s ( n e a r s h o r e s l o p e , g r a i n s i z e , c r e s t
h e i g h t , b a r r i e r w i d t h e t c . ) . M o r g a n and S t o n e (1985) d e r i v e d a " s t o r m
wave s u s c e p t i b i l i t y q u o t i e n t " f o r i s l a n d s a l o n g t h e F l o r i d a c o a s t ,
which gives the tendency of the b a r r i e r t o eroded. Higher values of the
i n d e x i n d i c a t e a h i g h e r v u l n e r a b i l i t y . The
index i s g i v e n by the
dimensional r e l a t i o n s h i p
ES
(1)
B
w h e r e ES
Bh
Xb„
Zt
Xb
=
=
=
=
=
100
erosion s u s c e p t i b i l i t y index
barrier height
barrier width
depth a t top longshore bar i n f r o n t of b a r r i e r
d i s t a n c e between t o p l o n g s h o r e b a r and t h e s h o r e l i n e .
and a l l v a r i a b l e s a r e g i v e n i n m e t r e s (see f i g u r e 62 f o r d e f i n i t i o n s ) .
A l t h o u g h a l l v a r i a b l e s i n v o l v e d a r e o f m o r p h o l o g i c a l c h a r a c t e r , Z^ and
Xb a r e r e p r e s e n t a t i v e o f wave e n e r g e t i c s b e c a u s e t h e f o r m e r l i m i t s t h e
i n c i d e n t wave h e i g h t , and t h e s e c o n d d e t e r m i n e s t h e s u r f z o n e w i d t h i n
w h i c h e n e r g y i s d i s s i p a t e d a f t e r b r e a k i n g . T a b l e I I shows ES v a l u e s
a l o n g t h e T r a b u c a d o r B a r ( S p a i n , see f i g u r e 63 f o r p r o f i l e l o c a t i o n s )
obtained from morphological c h a r a c t e r i s t i c s surveyed before the storm
( J u l y 1990) . P r o f i l e 1 1 n e a r t h e b r e a c h e d s e c t i o n p r e s e n t s t h e h i g h e s t
ES v a l u e a l o n g t h e b a r r i e r , i n d i c a t i n g t h e h i g h e s t e r o s i o n p o t e n t i a l .
T a b l e I I : M o r p h o l o g i c a l c h a r a c t e r i s t i c s and " e r o s i o n s u s c e p t i b i l i t y i n d e x "
ES a l o n g t h e T r a b u c a d o r B a r , o b t a i n e d b e f o r e t h e s t o r m a t t a c k ( J u l y 1 9 9 0 ) .
B^: b a r r i e r h e i g h t a b o v e mean w a t e r l e v e l ; Xi,.: b a r r i e r w i d t h ; Z^: d e p t h a t
t o p o f l o n g s h o r e b a r i n f r o n t o f t h e p r o f i l e ; Xi,: d i s t a n c e b e t w e e n t o p
l o n g s h o r e b a r and s h o r e l i n e .
(m)
Xb»
(m)
(m)
(m)
ES
(-)
P09
1.11
120
1 . 97
285
0.61
PIO
1 . 02
190
1 . 71
110
0 . 62
Pll
1.05
120
2 .35
210
0 . 81
P12
0 . 87
130
1.40
135
0.59
P13
1 . 08
190
1.47
70
0.53
Profile
47
Washover Transport on Sandbars
APPENDIX 3: E s t i m a t i o n o f e r o s i o n
Jiménez ( 1 9 9 3 ) .
due t o overwash;
Sanchez-Arcilla
and
U n d e r t h e c o m b i n e d a c t i o n o f w a t e r l e v e l r i s e a n d s t o r m waves a b a r r i e r
c a n b e o v e r w a s h e d . As t h e b a r r i e r d e g r a d e d s o t h e i n t e n s i t y o f
o v e r w a s h i n g i n c r e a s e d . To o b t a i n a n i n i t i a l e s t i m a t e o f t h e o v e r w a s h
m a g n i t u d e , t h e s t o r m r u n - u p was c a l c u l a t e d u s i n g t h e e x p r e s s i o n d u e t o
Mase ( 1 9 8 9 ) :
(2)
R
where R
Hg
a,b
^
=
=
=
=
r u n - u p h e i g h t ( m e a s u r e d v e r t i c a l l y f r o m S.W.L)
d e e p - w a t e r s i g n i f i c a n t wave h e i g h t
empirical coefficients
I r i b a r r e n number, s u r f s i m i l a r i t y p a r a m e t e r
(Battjes,1974), which i s given by
5=
w h e r e Lo
tane
tan6
(3)
= d e e p - w a t e r wave l e n g t h
= beach s l o p e .
( T h i s s l o p e was o b t a i n e d f r o m t h e p r o f i l e s u r v e y e d
right
b e f o r e t h e s t o r m , a n d i t was d e f i n e d a s t h e r a t i o b e t w e e n
t h e t r o u g h ( i n t h e o u t e r b a r / t r o u g h system) p l u s berm
h e i g h t s and t h e h o r i z o n t a l d i s t a n c e between t r o u g h and
berm. T h i s s l o p e d e f i n i t i o n c o r r e s p o n d r e a s o n a b l y w e l l w i t h
the a c t u a l p r o f i l e slope shorewards o f t h e t r o u g h ) .
The r u n - u p h e i g h t o b t a i n e d f o r t h e s t o r m wave c o n d i t i o n s c a n b e
p r e s e n t e d i n t w o h e i g h t s , R„e,„ a n d R„.^, t h e mean a n d t h e maximum r u n - u p
v a l u e s r e s p e c t i v e l y . A l t h o u g h t h e r u n - u p e x p r e s s i o n was d e r i v e d f r o m
l a b o r a t o r y e x p e r i m e n t s u n d e r n o r m a l wave a c t i o n , t h e e x p e c t e d
rim-up
w i l l be q u i t s i m i l a r t o t h e e s t i m a t e d one, because t h e n e a r s h o r e a n g l e
o f wave i n c i d e n c e was l e s s t h a n 30°. From e x p e r i m e n t s o f r u n - u p i n
b r e a k w a t e r s , Owen (1980) o b t a i n e d t h a t w h e n e v e r t h e wave a n g l e i s l e s s
t h a n 30° r u n - u p h e i g h t s a r e n o t i n f l u e n c e d b y t h e a n g l e . I n a n y c a s e ,
r e s u l t s show t h a t r u n - u p d u r i n g s t o r m a c t i o n was i m p o r t a n t . T a k i n g i n t o
a c c o u n t t h a t d u r i n g t h e s t o r m t h e w a t e r l e v e l was 0.40 m a b o v e i t s
n o r m a l p o s i t i o n (due t o s t o r m s u r g e ) a n d 0.70 m i f t h e wave s e t - u p i s
i n c l u d e d , t h e t o p o f t h e b a r r i e r h a p p e n e d t o b e l o c a t e d a t a b o u t 0.60
m a b o v e t h e mean w a t e r l e v e l ( w i t h o u t wave s e t - u p ) o r 0.30 m i f t h e
wave s e t - u p i s c o n s i d e r e d . T h i s h e i g h t i s w e l l b e l o w t h e r i o n - u p h e i g h t
e s t i m a t i o n , showing t h a t d u r i n g t h e f i r s t stages o f t h e storm ( b e f o r e
b a r r i e r b r e a c h i n g ) , an i m p o r t a n t c r o s s - b a r r i e r f l o w e x i s t e d . T h i s f l o w
was a b l e t o s t a r t e r o d i n g t h e b a c k - b a r r i e r . The r e m o v e d s e d i m e n t was
t r a n s p o r t e d t o w a r d s t h e b a y , w h e r e i t was d e p o s i t e d f o r m i n g a s e m i submerged b a r landward o f t h e b r e a c h .
48
Washover Transport on Sandbars
One p r o b l e m t o be f a c e d i s how t o q u a n t i f y t h e i m p o r t a n c e o f t h i s
p r o c e s s . S e d i m e n t l o s s e s due
t o longshore
transport gradient
and
o f f s h o r e t r a n s p o r t were e s t i m a t e d i n t h e p r e v i o u s s e c t i o n s a r o u n d
1 1 , 000 m% i . e . 15% o f t h e t o t a l l o s s . On t h e o t h e r h a n d , a p p r o x i m a t e l y
60, 000 m^ o f s a n d was d e p o s i t e d o n t h e s h e l f b e h i n d t h e b a r r i e r d u r i n g
t h e s t o r m . Thus, under t h e d e s c r i b e d s t o r m c o n d i t i o n s , most o f t h e
s e d i m e n t e r o d e d f r o m t h e e x p o s e d p a r t o f t h e b a r r i e r was t r a n s p o r t e d
t o w a r d s t h e i n n e r bay, s h o w i n g t h a t u n d e r t h e s e c o n d i t i o n s o v e r w a s h
p r o c e s s e s were dominant.
49
Washover Transport on Sandbars
APPENDIX 4: E s t i m a t i o n o f w a s h o v e r t r a n s p o r t o f t h e Wai San T i n g
A rough e s t i m a t i o n o f t h e c o n t r i b u t i o n
made a s s u m i n g t h e f o l l o w i n g :
sandbar.
o f washover t r a n s p o r t
c a n be
=> Time d u r i n g w h i c h w a s h o v e r o c c u r s :
3 h o u r s p e r 12 h o u r s = 2 5 % o f a d a y .
=. C o n d i t i o n s i n t h e a r e a have been r o u g h l y s c h e m a t i z e d a s f o l l o w s :
5 0 % o f t h e y e a r : t i d e , n o waves
5 0 % o f t h e y e a r : t i d e a n d waves w i t h a wave s e t u p o f 0.1 m.
Based
sides
= 10%
= 10%
on
of
of
of
i n s p e c t i o n o f t h e w a t e r l e v e l d i f f e r e n c e s Az a t b o t h
t h e Wai San T i n g :
t h e y e a r : washover due t o o n l y t i d e :
Az = 0.15m
t h e y e a r : w a s h o v e r d u e t o t i d e a n d w a v e s : Az = 0.25m.
=> V e l o c i t i e s c a n be d e t e r m i n e d w i t h a C h e z y f o r m u l a t i o n :
V =
cW{h*±)
C = 1 8 * l o g { 1 2 * h / r ) = 45 m'^/s
Table I I I :
V e l o c i t i e s o f overwash
current
water l e v e l
diff.
Az (m)
slope o f
overwash
i(-)
overwash
depth
h(m)
current
No waves
0 .15
0 . 00008
0.4
0.25
Waves
0.25
0.00013
0.5
0.35
V(m/s)
I f waves o n t h e c r e s t a r e assumed as 11^=0.25m, Tp=6s ( d e p t h l i m i t e d )
f o l l o w i n g t r a n s p o r t s can be computed w i t h t h e B i j k e r f o r m u l a :
T a b l e I V : S e d i m e n t t r a n s p o r t c a l c u l a t i o n due t o o v e r w a s h
h
(m)
V
H3
(m/s)
(m)
(s)
(mVm/hr)
hrs/yr
(mVm/yr)
S
0.4
0 .25
0 . 00
0
0.002
850
0.5
0 .35
0 .25
6
0.300
850
50
S
(mVyr)
2, 000
220,000
then t h e
Washover Transport on Sandbars
APPENDIX 5: C a l c u l a t i o n o f s e d i m e n t t r a n s p o r t c a p a c i t y
of a barrier
channel
The a p p r o a c h f o r t h e e s t i m a t e
of the transport capacity
o f such
c h a n n e l s i s s i m i l a r t o t h e a p p r o a c h f o r w a s h o v e r . As a r e s u l t t h e
greater depth, v e l o c i t i e s increase although the water l e v e l d i f f e r e n c e s
w i l l c a u s e f l o w i n b o t h d i r e c t i o n s . However t h e w i d t h o f t h e f l o o d
( s o u t h t o n o r t h ) f l o w a t l o w w a t e r s t a g e s w i l l b e much s m a l l e r t h a n t h e
w i d t h a n d e x t e n t o f t h e f l o w d u r i n g ebb a n d h i g h w a t e r s t a g e s . A l s o a t
high water stages t h e water l e v e l d i f f e r e n c e s are g r e a t e s t .
The d u r a t i o n o f 3 h o u r s h a s t h e r e f o r e b e e n t a k e n as r e p r e s e n t a t i v e o f
the p e r i o d d u r i n g which s i g n i f i c a n t t r a n s p o r t s from n o r t h t o s o u t h can
occur.
The v e l o c i t i e s
c a n be d e t e r m i n e d a g a i n a s :
T a b l e V: V e l o c i t i e s o f f l o w t h r o u g h t i d a l
Az
and
(m)
channels
h
i
(m)
V (m/s)
no waves
0.15
0.00008
2.9
0.7
waves
0.25
0.00013
3.0
0.9
the corresponding transports
as:
Table V I : Sediment t r a n s p o r t c a l c u l a t i o n bv channel
h
(m)
V
Ha
(m/s)
(m)
flow
s
(mVm/hr)
hrs/yr
(S)
s
(mVm/yr)
2.9
0.7
0 . 00
0
0.2
850
170
3.0
0.9
0.25
6
1.3
850
1105
s i n c e t h e s i t u a t i o n w i t h wave s e t u p m a i n l y d e t e r m i n e d t h e h i g h t r a n s p o r t c a p a c i t y through t h e channel, t h e order o f magnitude o f t h e
sediment t r a n s p o r t e d
i n a seaward d i r e c t i o n w i l l be a n o r d e r o f
magnitude lower.
51
Washover Transport on Sandbars
APPENDIX 6: Y u n l i n Sand
balance
T a b l e V I I : Summary o f Y u n l i n c o a s t
sand
balance
L o s s e s t o t h e s h o r e f a c e a c t i v e p r o f i l e (down t o -10 m T D ) :
l o s s e s as n e g a t i v e , g a i n s as p o s i t i v e
( m i l l i o n mVyr)
Description
Longshore
transport
Subsidence
Haifeng
south t i p
-0.3
-0.6
0
to
Wai San
Ting
-0.1
-0.12
0
to
I t i s an o v e r s i m p l i f i c a t i o n
there are l o c a l gradients
p r e s e n t e d h e r e . However t h e
s o u t h e r n p a r t s compares w e l l
R i v e r bed Channels
in
load input
sandbars
Washover
of
sandbars
Total
Loss/Ga
in
-0.1
-0.2
-0.60
+/-0.20
-0 .15
t o -0.2
-0.40
+/-0.04
0
0
to
0
to
-0.1
-0.12
t o r e p r e s e n t Wai San T i n g as a s i n g l e c e l l a s
which can d i f f e r s t r o n g l y from t h e averages
o v e r a l l r e t r e a t o f 40m/year f o r t h e c e n t r a l a n d
w i t h a n a l y s i s and measurements.
52
Washover Transport on Sandbars
FIGURES
53
Washover Transport on Sandbars
54
Washover Transport on Sandbars
F i g u r e 2: B a r r i e r i s l a n d f o r m a t i o n b y s p i t
i n l e t breaching
{from Hoyt).
a c c r e t i o n and
ATLANTIC
Nauset
Province
£ ^
Lands
/ \
^
Cape
Cod
Monomoy
Island
^
Early Shoreline
Present Shoreline
)
ll
F i g u r e 3: Cape Cod h a s e v o l v e d t h r o u g h e r o s i o n o f g l a c i a l
sediments and a c c r e t i o n o f c o a s t a l d e p o s i t s ( a f t e r D a v i s ) .
55
VJashover Transport on Sandbars
F i g u r e 4: D e v e l o p m e n t
submarine bar.
of
an
56
island
by
upbuilding
of
a
washover Transport on Sandbars
Washover Transport on Sandbars
58
Washover Transport on Sandbars
Washover Transport on Sandbars
Q.
Q.
VJ
•o
c
effective
sand addition
to b a r r i e r
rate of sand supply r e q u i r e d
to m a i n t a i n constant elevation
of barrier above sea l e v e l
with a rising sea
O
O
<u
O
• period
barrier
growtti
barrier
decay-
time
F i g u r e 12: S u g g e s t e d r e l a t i o n s h i p o f s a n d s u p p l y a n d t i m e t o b a r r i e r
g r o w t h a n d d e c a y , when b a r r i e r i s s u f f i c i e n t l y l a r g e t o r e s i s t
washover o f sand a n d submergence o c c u r s a t a c o n s t a n t r a t e .
60
Washover Transport on Sandbars
F i g u r e
13:
M i c r o t i d a l
Transgressive Barrier Island.
F i g u r e 14: M i c r o t i d a l
Barrier Island.
Regressive
F i g u r e 15 : M e s o t i d a l T r a n s g r e s s i v e
Barrier Island.
F i g u r e 16:
Mesotidal
Barrier Island.
Regressive
61
Washover Transport on Sandbars
Washover Transport on Sandbars
63
Washover Transport on Sandbars
MAINLAND
'
F i g u r e 21: B a c k b a r r i e r l a g o o n ; v a r i a b l e
a r e a s b y f l o o d a n d ebb t i d a l c u r r e n t s .
F i g u r e 22:
currents.
Backbarrier
lagoon;
64
water
water
levels
surface
and
tidal
VJashover Transport on Sandbars
FLOOD TIDAL
DELTA
OCEAN
E B B ' - X I D A J . ' DELTA
INLET
FEATURES
F i g u r e 23 : D e v e l o p m e n t
t i d a l deltas.
of
ebb
and
flood
F i g u r e 24: A e r i a l v i e w s o f t h e A n c a o I n l e t f l o o d t i d a l d e l t a
(13
S e p t e m b e r 1 9 8 4 ) ; F l o o d t i d a l d e l t a c o m p o n e n t s a r e l a b e l e d : (1) f l o o d
ramp, (2) f l o o d c h a n n e l , (3) ebb s h i e l d , a n d (4) ebb s p i t .
S5
VJashover T r a n s p o r t on
Sandbars
F i g u r e 25: F u z e t a I n l e t (B.F.) a t l o w s p r i n g t i d e (27 Se
ptember 1984). I n c o r p o r a t e d are remnants o f f l o o d t i d a l
d e l t a s mark t h e f o r m e r p o s i t i o n s o f t h e r a p i d l y m i g r a t i n
g i n l e t (Algarve B a r r i e r Islands, P o r t u g a l ) .
66
VJashover Transport
on Sandbars
F i g u r e 26: E b b - t i d a l d e l t a s o f t i d a l i n l e t s . A r r o w s i l l u s t r a t e t h e
r e l a t i v e f o r c e s o f t h e onshore, l o n g s h o r e , and o f f s h o r e c u r r e n t s .
A:
B:
C:
D:
The p r e v a i l i n g f o r c e o f l o n g s h o r e a n d o n s h o r e c u r r e n t s i s g r e a t er than the offshore force.
The p r e v a i l i n g f o r c e o f t h e s o u t h w a r d
longshore c u r r e n t i s
g r e a t e r t h a n t h e t h r e e r e m a i n i n g component f o r c e s .
The p r e v a i l i n g f o r c e o f t h e n o r t h w a r d l o n g s h o r e c u r r e n t i s
g r e a t e r t h a n t h e t h r e e r e m a i n i n g component f o r c e s .
The p r e v a i l i n g f o r c e s o f t h e i n l e t c u r r e n t s a r e g r e a t e r t h a n t h e
f o r c e s o f n o r t h w a r d and southward l o n g s h o r e c u r r e n t s .
67
Washover T r a n s p o r t
on S a n d b a r s
PROFILE
BEACH
—
,
—T
THROAT
,
A-A'
OVERWASH
—1—
FAN
,
FLATS
—T—
F i g u r e 27: O v e r w a s h s u r g e s d u r i n g s t o r m
conditions
(above) and r e s u l t i n g washover fans
(below).
F i g u r e 28: O v e r w a s h b r e a c h i n t h e h i g h dune l i n e o f t h e Ancao
P e n i n s u l a . By 1984 t h i s 1960's p a s s was n e a r l y s e a l e d b y r e f o r m a t i o n
o f t h e v e g e t a t e d dune (28 A u g u s t 1980) .
68
Washover Transport on Sandbars
WASHOVER FLA'
FOHESET STRATIFICATION ^ ^ ^ ^ ^ f e j g S Q S ^
FORESHORE
CHOSS-STRATlFICATiONl'
0
• 5ACKSARRIER SUBHORIZONTAL OVERWASH STRATIFICATION
100
fWVa
200
'\'
"
^
^
300
ORGANIC RICH LAGOONAL SEDIMENT
FRESHWATER PEAT
F i g u r e 29: Schematic i l l u s t r a t i o n o f i n t e r n a l s t r u c t u r e o f e x t e n s i
v e l y o v e r w a s h e d s p i t a t A t k i n s o n P o i n t (U.S.) s h o w i n g b e a c h c r o s s bedded s e t s and washover p l a n a r s t r a t i f i c a t i o n .
69
Washover Transport on Sandbars
F i g u r e 31: Schematic d i a g r a m o f a b a r r i e r i s l a n d
r e l a t i o n s h i p among m a j o r s e d i m e n t a r y e n v i r o n m e n t s
illustrating the
and landforms.
COASTAL AREA
ZONE OF
NEARSHORE CURRENTS
UPLAND
BEACH OR SHORE
BACKSHORE
Dune
FORESHORE
INSHORE OR S H O R E F A C E
Berms
High
Water Level .
Beach S c a r p ^
C r e s l of Berm^ Lo
F i g u r e 32: Beach T e r m i n o l o g y
( f r o m U.S.A. CERC)
70
OFFSHORE
Washover Transport on Sandbars
F i g u r e 33: P r i n c i p a l
factors
affecting
m i g r a t i o n ( a f t e r L e a t h e r m a n , 1980) .
barrier
beach
Barrier Island
Stabilization
Retrogression
F i g u r e 34: A b a r r i e r i s l a n d may ( 1 ) r e m a i n i n p l a c e , ( 2 )
p r o g r a d e seaward, o r (3) r e t r e a t , d e p e n d i n g o n sediment
s u p p l y , sea l e v e l , and sea energy c o n d i t i o n s ( a f t e r S w i f t ,
Hoyt).
71
Washover
Transport
(a)
(b)
(c)
on
Sandbars
12,000 y e a r s ago when sea l e v e l was a b o u t 50 m l o w e r t h a n
present;
L a n d w a r d m i g r a t i o n a n d u p w a r d g r o w t h b y waves as s e a l e v e l r i s e s ,
forming a lagoon behind the b a r r i e r ;
C o n t i n u e d m i g r a t i o n t o p r e s e n t ( a f t e r D o l a n & L i n s , 1987) .
72
Washover Transport on
Sandbars
Past Locolion
of Barrier
tsiand
Presenl
Shoreline
- o f Molnland
Present Location
of Migroting
- Barrier Islond
Post Locolion of
Barrier tslond at
Post Seo Level
(D « too lo 1000 llmM H }
post s e a level
H =
rtse In seo level
D =
horiïonlol mïgrotion of
barrier
island
F i g u r e 3 6 : W i t h s e a l e v e l r i s e a b a r r i e r m u s t r e t r e a t up t h e
g r a d u a l l y c o a s t a l p l a i n over g e o l o g i c t i m e . Without m i g r a t i o n t h e
b a r r i e r c a n be d r o w n e d .
c
Minor Storms
1
i
2
I
3
i
4
rTo
10
I
Year
Coastal Erosion/SubsidcncG
F i g u r e 37 : G e n e r a l i z e d c o n c e p t u a l m o d e l
f o r s a n d dune d e v e l o p m e n t a l o n g t h e
C a m i n a d a - M o r e a u c o a s t i n L o u i s i a n a as
i t r e l a t e s t o t h e r o l e o f storms and
the r e t u r n p e r i o d o f h u r r i c a n e impact.
73
Washover
Transport
on
Sandbars
PROFILE
INNER D U N E
LAGOON
T
J A N T O M A Y 1979
PROFILE ENVELOPE
H
BACKSLOPE
(HURRICANE
WASHOVER)
COASTAL
SEP
79 (POST
—
DUNE
HURRICANE B O B )
APRIL
1980
DEC
F i g u r e 38: P r o f i l e
H example
of rapid
cycle
( h u r r i c a n e Bob) a n d r e c o n s t r u c t i o n o f h i g h c o a s t a l
I
I
PRESENT-DAY
INLET
FEATURES
|
of destruction
dune r i d g e .
|
F i g u r e 39: D e v e l o p m e n t o f e x t e n s i v e
s a l t marshes on a f l o o d t i d a l d e l t a ,
following inlet
closure.
74
1880
Washover T r a n s p o r t
on
Sandbars
F i g u r e 40: S e q u e n t i a l d i a g r a m s o f i n l e t
breaching,
m i g r a t i o n , c l o s i n g , and d e v e l o p m e n t o f l a r g e f l o o d
tidal delta.
A.
B.
C.
D.
E.
F.
G.
I n i t i a t i o n of storm:
• Wave o v e r w a s h i n g o f b a r r i e r i s l a n d a t l o w p l a c e s i n dune l i n e s .
F l o w o f s u p e r e l e v a t e d w a t e r i n t h e bay, b y s t r o n g o f f s h o r e w i n d s :
• C r e a t i o n o f i n l e t a t low, narrow p o i n t s a l o n g t h e i s l a n d .
Normal t i d a l c u r r e n t s t h r o u g h i n l e t t h r o a t :
• C r e a t i o n o f f l o o d t i d a l d e l t a i n t h e bay.
Sediment a c c u m u l a t i o n i n bay:
• Continuous growth of f l o o d t i d a l d e l t a ;
• S m a l l ebb t i d a l d e l t a due t o d i s t u r b a n c e o f w a v e s .
Net l o n g s h o r e c u r r e n t t o t h e s o u t h :
Southward m i g r a t i o n o f i n l e t ;
• I n c r e a s e d s i z e and g r o w t h f l o o d t i d a l d e l t a .
Water p a t h t r o u g h i n l e t
t h r o a t a r e a becomes v e r y l o n g
and
tortuous:
• I n l e t ' s e f f i c i e n c y g r e a t l y reduced.
Inlet
sediment
t r a n s p o r t by
tidal
c u r r e n t overpowered
by
longshore sediment t r a n s p o r t :
• I n l e t closes.
75
Washover T r a n s p o r t
on
Sandbars
F i g u r e 41: O v e r w a s h m o d e l
C a r o l i n a (Godfrey, 1969).
based
o f a low b a r r i e r
on
Core
Banks,
North
1.
Cross-section
2.
A s e v e r e s t o r m w i l l p u s h h i g h t i d e s o v e r t h e b e a c h b e r m a n d as
t h e w a t e r f l o w s back across t h e i s l a n d i t c a r r i e s beach sand i n t o
grasslands.
3.
A second, l e s s severe storm c a r r i e s a s m a l l e r q u a n t i t y o f sand
o n t o t h e e a r l i e r d e p o s i t . Abnormal h i g h t i d e s have pushed a s m a l l
q u a n t i t y o f s a n d o v e r t h e edge o f t h e b e r m . G r a s s e s
have
r e c o l o n i z e d t h e e a r l i e r l a y e r b y p u s h i n g up t h r o u g h t h e d e p o s i t .
The f r o n t o f t h e i s l a n d has r e t r e a t e d a s m a l l amount.
76
i s l a n d a t any s t a r t i n g
time.
Washover Transport on Sandbars
4.
A v e r y severe s t o r m c o m p l e t e l y i n u n d a t e s t h e i s l a n d and c a r r i e s
a g r e a t d e a l o f sand a c r o s s w h i c h adds t o t h e e a r l i e r l a y e r s o n t o
the
b a c k b a r r i e r , e l e v a t i n g t h e s u b t i d a l and t i d a l environment.
O l d d u n e s h a v e b e e n k n o c k e d down a n d t h e s a n d s p r e a d o u t . The
b e a c h f r o n t has r e t r e a t e d s u b s t a n t i a l l y .
I n some p l a c e s , t h e berm f r o n t may b e l o w e r e d as s a n d moves away
from t h i s source.
5.
A f e w y e a r s l a t e r ( l e s s t h a n 10 u s u a l l y ) , a n d a f t e r a p e r i o d o f
relative
storm
quiet,
the island
s u r f a c e has
completely
r e c o v e r e d , b u t t h e v e g e t a t i o n zones h a v e b e e n d i s p l a c e d s l i g h t l y .
Low s a l t m a r s h h a s d e v e l o p e d o n t h e new b a s e i n t h e b a c k b a r r i e r
l a g o o n , t h e f o r m e r l o w m a r s h h a s become h i g h m a r s h , a n d t h e
b a r r i e r f l a t has e x t e n d e d i n t o t h e o l d h i g h marsh b o r d e r ( a f t e r
G o d f r e y , 1969) .
F i g u r e 42: S p i t f o r m a t i o n a l o n g a c o a s t l i n e
wave a p p r o a c h a n d r e s u l t i n g l i t t o r a l d r i f t .
77
by o b l i q u e
Washover T r a n s p o r t
on
Sandbars
F i g u r e 4 3 : F o r m a t i o n o f t h e i s l a n d o f S a k a l i n i n f r o n t o f one o f t h e
m o u t h s o f t h e Danube ( f r o m J . P e t r e s c u , 1957)
78
Washover T r a n s p o r t
on
Sandbars
F i g u r e 44: B a t h y m e t r y
o f the Y u n l i n coast
{west c o a s t
Taiwan)
The s o u t h w e s t e r n t i p o f t h e Wai San T i n g s a n d b a r ( o n o l d d e l t a P e i k a n g R i v e r )
and H e i f e n g s a n d b a r
(on d e l t a f r o m Choshui R i v e r ) and o t h e r submarine
f o r m a t i o n s (F) show a s h a r p t u r n t o n o r t h . T h i s r e f l e c t s t h e i n f l u e n c e o f t h e
s t r o n g t i d a l f l o w on t h e submerged r e m a i n s o f o l d d e l t a s .
79
Washover
Transport
on S a n d b a r s
REGRESSIVE
ENVIRONMENTS
^
Distributary
Q
Frcsii Marsli
Beach Ridges
FLANKING
BARRIER
ISLAND
TRANSGRESSIVE
ENVIRONMENTS
HEADLAND
Q
Subaerial Barrier
n
Subaqueous Barrier
03
Sand Sheet
El
Salt Marsh
O
•i--
Bay-Lagoon
Recurved Spits
il;'' Shell Reef
y
Channel
F i g u r e 45: D e p o s i t i o n a l
model f o r a t r a n s g r e s s i v e
barrier
s h o r e l i n e development i n t h e M i s s i s s i p p i R i v e r d e l t a p l a i n ;
e r o s i o n a l h e a d l a n d w i t h f l a n k i n g b a r r i e r i s l a n d s {Penland and
Boyd, 1981)
VMBAUCR
F i g u r e 4 6 : A h i s t o r i c a l map t i m e s e r i e s s h o w i n g c o a s t a l c h a n g
es a s s o c i a t e d w i t h t h e Bayou L a f o u r c h e b a r r i e r s h o r e l i n e (188
7-1978)
80
Washover T r a n s p o r t
on
Sandbars
barrier sand source
beach and shoreface supply
offshorJdispersal
onshore beach accretion
I'
available for washover/aeolian processes
1
moved alongshore tost to deep vi/ater
onshore beach accretion
berm and beach ridges
storage
nearshore bars
lost to tidal inlets
lost to extension of rlanking barrier island
F i g u r e 47: G e n e r a l i z e d f l o w d i a g r a m f o r s a n d movements f r o m
r s o u r c e s t o dune a n d w a s h o v e r d e p o s i t i o n a l l a n d f o r m s .
81
barrie
Washover T r a n s p o r t
on
Sandbars
F i g u r e 48:
Schematic
of
a f f e c t i n g s e d i m e n t b u d g e t s on
fans .
82
factors
washover
Washover T r a n s p o r t
on S a n d b a r s
12
.
"f/
BARRIER BREAKDOWN R>B, Q w » » 0
SLUICING OVERWASH R>B, Qw»>0
DISCRETE OVERWASH R>B, Q w » 0
12
^ ^ ^ ^ iR
OVERTOPPING R>B, Qw>0
STABLE R<B, Qw=0
F i g u r e 49:
The
continuum
o f o v e r t o p p i n g and
o v e r w a s h i n g modes b y w h i c h g r a v e l b a r r i e r
crest
m i g r a t i o n may o c c u r ; t l a n d t 2 a r e c r e s t a l p r o f i l e s
b e f o r e and a f t e r s t o r m g e n e r a t e d o v e r t o p p i n g and/or
overwashing run-up.
R = r u n - u p e l e v a t i o n ; B = B a r r i e r c r e s t h e i g h t ; Q„=run-up v o l u m e
83
Washover
Transport
on
Sandbars
BARRIER SHORELINE LANDWARD MIGRATION R A T E
F i g u r e 50: S c h e m a t i c v i e w o f b a r r i e r
domains
as a f u n c t i o n
o f seaward
shoreline migration.
800
roo
600
500
«00
Oislonce,
300
crestal
stability
and b a c k - b a r r i e r
200
100
0
meters
F i g u r e 51: W i n t e r a n d summer b e a c h p r o f i l e s ( a b o v e ) a n d
e n v e l o p e o f p r o f i l e s f r o m J a n u a r y - A u g u s t 1984 ( b e l o w ) ,
Duck, N o r t h C a r o l i n a , U.S.A. ( a f t e r USAGE CERC, 1984) .
84
Washover
T r a n s p o r t on
Sandbars
a. Sealevel
Constant
1
MSL
h
h
_
\.
b. Sealevel
1
H
-• X
Rising
MSL + i z
—
t
—
1
1
*
z
MSL
(notching ^
r
^^^^^^^^
^
X
f
F i g u r e 52: S c h e m a t i c d i a g r a m s o f b e a c h p r o f i l e s s h o w i n g t h e v o l u m e
o f b e a c h s a n d a s s o c i a t e d w i t h (a) n e t r e c e s s i o n o f t h e s h o r e l i n e Ax,
and (b) w i t h s e a l e v e l r i s e Az. I n b o t h c a s e s i t i s assumed t h a t t h e
beach p r o f i l e remains c o n s t a n t .
85
Washover T r a n s p o r t
on
Sandbars
F i g u r e 53: C o n c e p t u a l g e o m o r p h i c m o d e l o f s p i t a n d b a r r i e r i s l a n d
migration
i n t h e s o u t h e a s t e r n Canadian
B e a u f o r t Sea,
showing
i d e a l i z e d p l a n e v i e w and c r o s s - s e c t i o n w i t h m a j o r sediment t r a n s p o r t
paths.
(1) s p i t ; ( 2 ) b a r r i e r i s l a n d . A r r o w d i m e n s i o n s a r e p r o p o r t i o n a l
v o l u m e o f s e d i m e n t t r a n s p o r t . (Héquette a n d H i l l , 1 9 8 9 ) .
86
to
Washover
T r a n s p o r t on
Sandbars
km
(Gridspacing
50
Scale
100
150
200
1:2750000
F i g u r e 54: S t u d y a r e a ;
1 Km)
Taiwan
87
250
300
350
Washover T r a n s p o r t
on S a n d b a r s
Z3
^
5
(IMJI^ÊO^
MAKUNG F
P'ENO-HUi"
H'ÜN-TAO'-
I ADMIRALTY PREDICTION POINT
MEASURED DATA AVAILABLE
F i g u r e 55: S t u d y
area; Y u n l i n
coast
88
Washover
Transport
on
Sandbars
(Gridspacing 75 m)
Scale 1:155000
F i g u r e 56: B a t h y m e t r y o f t h e W a i - S a n - T i n g
89
Washover T r a n s p o r t
on S a n d b a r s
Washover
Transport
on
Sandbars
PROFILE OF SECTION 7
5.00
i
0.00
0
500
'
1000
1500
i
20OO
2500
OISTANCE(m)
PROFILE OF SECTION 8
5.00
.20.00 i
0
i
500
1
1
1000
1500
1
1
2000
2500
DIST>WCE{m)
PROFILE OF SECTION 9
5.00 1
i1
1
i
.cm
i
1
.
i
ER DEPl
?
i
0
500
1000
1500
1990 —
2000
- — 1991
2500
1992
F i g u r e 58: C r o s s s h o r e p r o f i l e m e a s u r e m e n t s ; f r o m 1990 t o 1992
91
Washover T r a n s p o r t
on
Sandbars
Washover T r a n s p o r t
on
Sandbars
(Gridspacing 7 5
m)
Surface elevation south side
00:00
06:00
12:00
18:00
03/19
00:00
06:00
03/20
1992
F i g u r e 60: S p r i n g t i d e a n d neap
n o r t h a n d s o u t h s i d e o f WST
tide
(below): Surface
elevations
Washover
Transport
on
Sandbars
(Gridspacing 75 m)
Scale
1:175000
1 9 9 2 / 0 3 / 1 9 23:30:00
F i g u r e 61: O v e r w a s h e f f e c t
o n WST:
94
spring
tide
Washover T r a n s p o r t
on
Sandbars
CROSS-SHORE
Figure
62: C r o s s - s e c t i o n a l
index'.
Figure
63: P r o f i l e
DISTANCE
definitions
l o c a t i o n s Trabucador
95
(m)
f o r "erosion
Bar, Spain.
susceptibility