65.
USA--LOUISIANA
K. Meyer-Arendt
Department of Geography
Mississippi State University
Starkville, Mississippi 38759
United States of America
D.W. Davis
Department of Earth Science
Nicholls State University
Thibodaux, Louisiana 70301
United States of America
INTRODUCTION
I
Louisiana's 40,000 km 2 coastal zone developed over the last 7,000 years
by the progradation, aggradation, and accretion of sediments introduced
via various courses of the Mississippi River (Frazier 1967). The
deltaic plain (32,000 km 2 ), through which the modern river cuts diagonally (Fig. 1), consists of vast wetlands and waterbodies. With elevations ranging from sea level up to 1.5 m, it is interrupted by natural
levee ridges which decrease distally until they disappear beneath the
marsh surface . The downdrift chenier plain of southwest Louisiana
(8,000 kmZ) consists of marshes, large round-to-oblong lakes, and
stranded, oak covered beach ridges known as cheniers (Howe et al.
1935). This landscape is the result of alternating long-term phases of
shoreline accretion and erosion that were dependent upon the proximity
of an active sediment-laden river, and a low-energy marine environment
(Byrne et al. 1959).
Since the dyking of the Mississippi River, fluvial sedimentation
in the deltaic plain has effectively been halted. Today, most Mississippi River sediment is deposited on the outer continental shelf; only
at the mouth of the Atchafalaya River distributary is deltaic sedimentation subaerially significant (Adams and Baumann 1980). Over most of
the coastal zone, subsidence, saltwater intrusion, wave erosion,
canalization, and other hydrologic modification have led to a rapid
increase in the surface area of water (Davis 1986, Walker et al.
1987). Land loss in coastal Louisiana has been estimated to exceed 100
kmZ/yr (Gagliano et al. 1981). Some researchers have attributed up to
40% of the wetland loss to man's activities (Craig et al. 1979). The
500 km long Gulf of Mexico shoreline, which is characterized by barrier
islands and beaches in the deltaic plain and by barrier beaches and
mudflats in the chenier plain, is generally transgressive with average
shoreline erosion rates in excess of 10 m/yr (Adams et al. 1978, van
Beek and Meyer-Arendt 1982). The barrier islands, which function as
the first natural line of defense against incoming tropical storms,
have lost almost half of their surface area within the last 100 years
because of wave er osion and subsidence (Penland and Boyd 1982).
629
H.J. Walker (ed.), Artificial Structures and Shorelines, 629--040.
1988 by Kluwer Academic Publishes.
@
630
Shorelines in more inland estuarine settings are experiencing high
erosion rates because of fetch and wave action, and the unconsolidated
nature of the shore sediments (Adams et al 1978).
Human settlement in coastal Louisiana was historically confined to
beach ridges and natural levees (Davis 1983), However, with expansion
of urban/industrial land (especially around New Orleans), extension of
farmlands, development of the oil and gas industry and its associated
demand for improved navigation channels, recreational demands along
waterfronts, and continued construction of hurricane-protection levees,
man has extended his activities deep into the wetlands and along the
shoreline (Davis and Detro 1980). Three classes of structural modification are identifiable: (1) those associated with the harnessing of
the Mississippi River, (2) those within the estuarine wetlands, and 3)
those associated with the shoreline (Fig. 1).
THE MISSISSIPPI RIVER
The Mississippi River, confined within a conduit of artificial levees,
can no longer inundate its historic floodplain. River leveeing dates
from the initial European settling of New Orleans in 1718; by the
1850s, intermittent dykes flanked most of the lower Mississippi River
(Elliott 1932). However, the present comprehensive manipulation of
river flow (within levees averaging 7.6 m above ground level) is a
result of construction made after the 1927 flood that inundated 65,000
km 2 (USACE n.d.). Extensive revetments have been laid to reduce bank
collapse and stabilize levees. Formerly constructed of willow mats,
revetments are now made of interlocking concrete mats and riprap
rubble.
Complementing the levees are floodways that can re-direct flood
water and reduce river flood stages. Louisiana's floodways (Old
River/Atchafalaya, Morganza, Bonnet Carre, and Bohemia), are designed
to protect New Orleans, much of which is below sea level.
Flow through the Atchafalaya Spillway is maintained at 30% of
Mississippi River discharge at the Old River Control Structure (Kazmann
and Johnson 1980), The Bonnet Carre Spillway, 15 km upriver from New
Orleans, can divert 7 million liters/second into Lake Pontchartrain, an
embayment of the Gulf of Mexico. The structure's 350 moveable gates,
completed in 1931 (USACE n.d.), have been opened only 7 times--the last
in May 1983).
In addition to spillways, the United States Army Corps of Engineers constructed a freshwater siphon at Violet. It is designed to
stop the advance of saltwater into fresh or brackish areas and preserve
the marshes southeast of New Orleans. The project's success has led
the Corps to investigate the feasibility of diverting water and
sediments into the Barataria estuary. Because the Mississippi's east
bank south of Bohemia is unleveed, it functions as an additional
overflow spillway.
Within the "birdsfoot" portion of the delta, 2 of the 3 major
deltaic distributaries are presently jettied. The earliest jetties
were constructed in the 1850s, but it was not until the 1870s that the
631
A
Mandeville
L,:"1,twrg
U1/..e Pomd,artra;,,
2 km
0
.........
cri,icol
critical
~os,on
0
(,,.1,.14 11,11111
·-..
KE\'TOrt.t.CfNA,l,\ES
6 a.-..-. Col'W!Odo
7 G,ond ....
9f~SIOte,.o,lt
10
,.........0,...,..
... ..,
)
Fig. 1. The Louisiana coastal zone and selected locations illustrati ng
a vari ety of artificial structures.
632
prototype of the modern jetties (the Eads jetties at South Pass) was
built (Elliott 1932). Southwest Pass, the major navigation channel
today, has been modified by the construction of jetties, bulkheads, and
channel- training spur dykes since 1900 (Fig. 1, inset B).
WETLANDS DRAINAGE AND RECLAMATION
When New Orleans was first surveyed in 1792, each residential block was
encircled with canals, and the city's dependence upon a drainage
network was established. By 1743, an ordinance required property
owners to complete their levees or forfeit their lands (Samuel 1959 ) ,
As the city expanded into adjacent wetlands, it became even more
dependent on levees and artificial drainage systems. One result of
these modifications has been subsidence; portions of the city are now
at least 6 m below sea level (Davis and Detro 1980). Presently, New
Orleans operates a total of 140 km of canals and 92 km of large
pipelines. These conduits, along with 21 pumping stations, can remove
more than 85 million liters of water/minute from the city streets
through at least 50,000 curbside catch basins and 2,024 km of subsurface drainlines (Wagner and Durabb 1976).
Drainage and reclamation endeavors are not restricted to New
Orleans. Since the colonial period, privately funded levees have been
built to expand cropland. This reclamation effort was especially
intensified during the period between 1880 and 1930, when Louisiana's
"alluvial empire" was perceived as a great agricultural frontier. Over
50 projects, which ranged in sizes from 260 to over 14,000 ha, were
started. Between 2% and 5% of each project's area was allocated to
ditches and canals designed to transfer water from a surface that had
subsided to 3 m below sea level to surrounding surface levels of one m
above sea level (Okey 1914), All but 4 of these projects failed; 2 of
them are now residential and industrial centers near New Orleans. Many
of these projects failed because of levee breaks and subsequent
inundation. Reminders of these ill-fated ventures appear as rectangular waterbodies on maps and aerial images. Impoundment of wetlands
for purposes of marsh management (mainly for the maintenance of desired
salinities) has been popular since the 1930s, particularly in the
chenier plain marshes (Gosselink et al. 1979), Present-day reclamation
activities are primarily related to the construction of elaborate
hurricane-protection levees around several of the more seaward dis tributary ridges such as Bayous Lafourche and Terrebonne (USACE 1982a).
Drainage of the wetlands has left an extensive canal network that,
when combined with other canal types such as those made for trapping,
logging, transportation, and hydrocarbon- extraction (Fig. 2), form the
most visible structural modification within the coastal zone (Davis
1973). The canalization process has resulted in the loss of at least
520 kma of surface area. For example, in one oil and gas field there
are 68 km of petroleum-related canals, representing the removal of at
least 3.2 million m3 of soil (Davis and Place 1983). Canal construction which is usually accompanied by the breation of spoilbanks on
633
adjacent lfetlands, modifies the hydro logic regime, accelerates saltwater intrusion, and impedes over land flow .
THE SHORELINE
Structural modification of the Louisiana shoreline has been conducted
for 2 basic purposes: 1) to maintain entrances to navigation channels,
and 2) to control erosion for protection of development sites, The
1 ost couon structures utilized for the first purpose are jetties,
although offshore break,.aters can be used in conjunction with them. A
greater variety of structures are employed for erosion control;
exaaples include seawalls, revetments, groynes, T-groynes ( causeway-connected breakwaters), bulkheads, sand fences, nourished beaches, and
ve1etatively-stabilized shorelines.
The mouths of 10 navigation channels in coastal Louisiana (including Lake Pontchartrain) are jettied; at all but one mouth they are in
sets of 2, These structures confine water flo", minimize channel-mouth
shoaling, and prevent sedimentary infilling because of longsbore
drift. Holfever, accelerated shoreline erosion is occurring downdrift
of 4 of the jettied channel mouths (Fig. 1, inset D). The earliest
Jetties 11ere constructed at the Mississippi River mouths in the 1850s,
Jetties built at Sabine Pass in 1920 are now 18 km long. The remainder
were constructed between 1940 and 197 5 ( Table 1). Several of the
jetties have been extended seaward since initial construction, reflecting channel-deepening activities. The only solitary jetty is found at
the east end of Grand Isle, where reduction in shoaling and trapping of
sand have been realized. The 700 m jetty (initially 300 min 1959)
trapped one million m, of sand and extended the shorelines 370 m
seaward 11i thin 4 years of construction ( Conaster 1969). This accreted
sand has subsequently served as convenient borrow material for local
beach nourishment and dune construction projects, notably following
hurricanes in 1965 and 1974 (USACE 1980).
Over 61.5 km of seawalls and shoreline revetments are located in
Louisiana, primarily along Lake Pontchartrian and East Timbalier
Island. The first comprebensi ve protection system for the New Orleans'
lakeshore was not implemented until the 1920s ( Lewis 1976). At that
ti1e, 8.3 km 2 of land was reclaimed from Lake Pontchartrain (between
W
est End and Lakefront Airport--Fig. 3) and a 9. 2 km concrete seawall
ns constructed to protect this "artificial" land. Levees, totalling
36 . 7 km in length, were built along the entire lake shore of Jefferson
and Orleans Parishes, At the North Shore resort community of Man1devi lle, a 2,5 km concrete seawall, with 31 groynes 60 m long and
spaced 77 m apart, has been constructed (deLawreal and Moses 1960-Fig, 1, inset A). East Timbalier Island, a low-elevation barrier
island over a major oil and gas field, has been heavily modified by
Gulf Oil Corporation since 1966 (Anon 1982). Riprap revetments protect
'.6 ka of the gulf shore, and a dyke of at least equal length meanders
along the backshore area. On nearby Timbalier Island, an experimental
600 • riprap revetment with 5 short groynes was constructed by the
l
1
..
~
TABLE l.
STRUCTURAL DEVELOPMENTS ALONG THE LOUISIANA SHORELINE (Incl. Lake Pontchartrain)
Jetties
1850
Seaw~ls/Revetments
Groynes/T-Groynes
Beach Nourishment
Vegetative Stabilization
Southwest
Pass,
Pass a
Lout re
1875
1900
1910
1920
Eads jetties,
South Pass
Southwest
Pass(improved)
Sabine Pass
N.O. Lakefront seawall
(9 . 2 km)
N.O. Levee/revetment
1930
(36. 7 km)
1940
Belle Pass
Calcasieu
Mandeville Seawall
Fountainbleu (0. 3 km)
Pass
1950
Grand Isle
( 14 groynes)
Grand Isle
(I g royne)
F.mpire-Gulf
Waterway
Barataria
Grand Isle
Pass (Grand
1960
1970
1975
Isle)
Bayou Cast1ne(Handeville)
Hississipoi
RiverGulf Outlet
Belle Pass
(enlargement)
HermentauGulf
Navigation
Channel
1980
Gr~nd Isle
East Timbalier revetment (7 . 6 km)•
Grand Isle
Peveto Beach revetment
(4.8 km)
Grand Isle
Cypremont Pt. Beach
(0.6 km)
Timbalier revetment
(0. 6 km)
Fountainbleu revetment
(0.2 km)
Grand Isle 2
Fountainbleu (0 . 6 km)
Fountainbleu
(I T-groyne)
East Timbalier
(5 T-groynes)
lCulf front•ge only
Grand Isle (in
progress) (12 km)
Isle Dernieres (in progress)
( l. 8 km)
Grand Isle (in progress)
(12 km)
1984
TOTAL 10 jetties
T1mbalier (0 . 3 km)
61. 5 km
2
21 groynes/T-groynes
1n conjunction with aandfilled longard tubes
635
N
t
RED
""'PASS
PASS
.-TANTE
PH/NE
1958
O""""-==:::::ilMI.
RED
PASS
-PASS
TAN TE
PH/NE
1974
Fig, 2, Coastal marsh modification due to initial canalization and
subsequent land loss. The round canal network, near Venice, Louisiana,
Is the surface expression of activities connected with the extraction
of hydrocarbons from the perimeter of a subsurface salt dome . Most oi l
~d gas fields in coastal Louisiana are assoc iated with salt diapirs
which penetrated through the sedimentary deposits and trapped large
quantities of hydrocarbons on their flanks (After Adams et al. 1976 ).
NIWORLEANS LAKEFRONT RECLAMATION
••••••••••
pre-1926 shoreline
Sourte: 1953 I :62.SOO USGS topographic
quadrangle (Spanish Fon . La.)
0
1000
2000
ig. 3. New Orleans Lake front reclamation .
The ex pansion of New
into Lake Pontchartrain entailed the reclamation of 8 . 3 km 2 o f
and from the lake, the construc tion of a 9. 2 km long seawall , and the
levation of the fill area to 3 m above sea level , Begun in 1927 , i t
as co1pleted with the construction of Lakefront Airport in 1934 ( See
"is 1976 ).
~UM
637
636
Sta te Off ice o f Public Works . in 1975. Unfortunately, downdri ft beachu
experienced a ccelerat ed erosion as a consequence.
A 4 . 8 km stretc h of highway, between Holly Beach and Constance
Beach i n s outhwe s t Louisiana, has been protected by a gobiblock revete n t since 1970 ( Deme nt 1977), but several severe storms have reaoved
: os t of t he r e ve tment mats and undermined the highway, The State pla:is
to reconstruc t t he r evetment, testing a wide variety of revetment
types . It is a lso c onsidering the con~truction of 5 T- groynes along
this sho re. Several other short experimental revetments were laid at
Foun t a i nb l e u Stat e Park on Lake Pontchartrain in 1979 (Fig . 1, inset
A); thus far, t he y have functioned satisfactorily (Moffatt and Nichol
1981).
Apar t fro m t he groyne fields assoc iated with the seawall/revetments, 21 groynes and T-groynes have been constructed in Louisiana witl
public o r industrial mone y . In addition, hundreds of private piers an~
catwalks associat e d wi th r ec reational camps have been bui lt, especiallr
in the Lake Pontc hartrain area . Three-quarters of the groynes,
intended to tra p l ongsho re moving sand and minimize shoreline retreat,
a r e found on Gr a nd Isle, a 12 km long barrier island that evol ved into
the major coa stal r ec r e ational beach c ommunity in Louisiana once it m
c onn ected to t he mainland b y highway in the 1930s (Fig. 1, i nset C),
Two groyne f i eld s , one wi th 4 groynes near the western end and one will
10 groynes n ear t he c e n ter o f the island, were construc ted in 1951
(USACE 1972). One a dditional groyne was built by Humble Oil Co . in
1956. Th e groynes have n ot been suc cessful, as downdr ift erosion (on
t he east s i de) h a s l eft several c amps standing in the surf zone, and
hurricanes have int e n si fi ed s cour a c tivity at the base of the groynes
and removed the sand bet ween them ( USACE 1980). Also, 5 experimental
T- gr oynes were construc ted by Gulf Oi 1 Corporation on Eas t Timbalier
Island in 1980 (F i g. 1, ins e t E) .
Small scal e bea c h n ouri s hment projec ts have been undertaken since
t he 1940s, although stabili zat i on by vegetation was not attempted until
the last several years . Most o f the nourishment has been conducted at
Grand Isle, esp ecia lly fo llowing hurricanes ( Table 1) . At present
( 1983), a $13 million Corps of Engineers dune c onstruction/vegetative
stabilization pro j ect is underway at Grand Isle, the sand being dredged
from offshor e bo rrow areas ( USACE 1980) . A short ( 300 m) shoreline
reach t hat was nour i s hed a t Fountainbleu State Park in 1942 has all but
eroded away · I t has been proposed that 1. 8 km of this shoreline be
nouris~ed (USACE 197 6 ) , Experimental Spartina plantings occupy a 600•
reach in t h e park's east e rn portion, and have successfully stabilized
the ~rea since i ntr oduc t ion in 1979. A 300 m e xperiment al san~
fe ncin~/dun~-building / vege tati ve-stabilization projec t was constructed
0
? a Timba her I s l and wash over in 1980 ; it too has so fa r been rela·
tively successful. I n Vermilion Bay , a 600 m man - made beach (Cypreaort
Be~ch) was.created wi t h sand truc ked in from a Mississippi R~H
P~int-bar in 1966, but erosion has narrowed the beac h to onl y a fewI
wide.
In a dd 1' t'ion to t he s e lar ge l y public -sponsored project s , a to tlof
a
43 2
· km of sh?relin e have b ee n mod i fi e d b y private methods , mainly
al ong the residential canals and s horeline s of Lake Pontchartrian, but
t Constance Beach , Cypremort Point , Cheniere Caminada , and t he
I
~:s~ore of Grand Isle . These modifications are primaril y in the f o rm
rt aber and concrete bulkheads and rubble ( c oncrete fragments , r ocks,
G ed\ires et c ,) placed along the shoreline.
The success of thes e
; it• aetbods has been mixed. The non- comprehensive nature o f sho re
\ ction in many of the water-edge c ommunities has left a j agged,
'.~e:ted shoreline , best exampled along a part of Lake Pontc ha rtrain ' s
lcrlb Shore,
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~~~-•
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. . . to a oint alled Chef Menteur . ,
Samuel, R., 1959 ·
story of the property known as New O leans East , Inc. New Orleans , La,
Photo 1.
Shore forms on the Louisiana coast.
640
j
)
Photo 2,
Highway erosion on the Louisiana coast (1985),
)
)
Photo 3. Artificial levee along the Mississippi River in south
Louisiana.