Historic Shoreline Changes in Southwest Florida
Emmett R. Foster, P.E. and Rebecca J. Savage
------------------------------------------------------------Notes Added To The WEB Copy , 3/04/97, by E.F. :
1)
This
paper
Proceedings
of
was
published
the
Sixth
Management, 1989,
Vol. 5,
American Society of Civil
to place this copy on
in
Coastal
Symposium
on
pp. 4420-4433,
Zone
'89,
the
Coastal and Ocean
published by
Engineers, N.Y., USA.
the
Permission
the FDEP/BBCS WEB site was
granted by
ASCE in January 1997.
2)
The
agency
Environmental
Systems.
name
is
Protection,
now
Office
Florida
of
Department
Beaches
and
of
Coastal
The author's e-mail address is as follows :
[email protected]
3) The
the
opinions expressed
(as of jan. 2000).
in this
paper are
those of
the
authors and do not necessarily reflect those of the FDEP.
4) The conditions in this area have not remained static.
particular, there have
been major renourishment
In
projects on
Anna Maria Key, Longboat Key, Venice, and possibly Lido
Key.
A smaller renourishment project occurred on Knight Island
Charlotte County, in
Pass ebb shoals.
the erosion pocket
Several
south of the
Stump
groinfields on Anna Maria Key
Longboat Key were removed prior to the filling.
in
Erosion
and
has
spread southward in the pocket beach south of Point of Rocks,
in Sarasota County.
4) The figures for the ASCII text version have been enclosed in
the zipped total file as separate bitmap files, which can
viewed
and
Wordperfect,
printed
via
Adobe,
etc.
many
software
The
intent
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-------------------------------------------------------------
Abstract
An
overview
of
the
results
of
an
analysis
of historic
shoreline changes in three adjacent southwest Florida coastal
counties
coastal
is
presented.
survey
maps
and
The
data
Florida
set
includes historic
Department
of
Natural
Resources beach profile surveys.
findings is that the
part of this region
and underlying
One of the more interesting
shoreline configuration in the
is strongly influenced by
limestone topography.
The
central
the nearshore
areas of concern
from an erosion viewpoint are those locations where there
a deficit of sand supply
sand transport.
too
close
to
in a wave climate of
net southerly
In many areas, structures were simply
the
beach
to
allow
will depend
upon a
built
for normal beach width
fluctuations or gradual shoreline evolution.
management plans
is
Effective beach
basic understanding of
the coastal processes as developed in these studies.
Introduction
The subject area encompasses three adjacent southwest Florida
counties bordering the
Gulf of Mexico.
Figure 1
shows the
coast of Manatee County, consisting of Anna Maria Key and the
north portion of Longboat Key.
Figure 2 shows the
coast of
Sarasota County, which includes the southern part of Longboat
Key, all of Lido Key, Siesta Key, Casey Key, the Venice area,
and most of Manasota Key.
Coastal Charlotte County is
shown
in Figure 3 and consists of the southern end of Manasota Key,
Knight Island, and the northern portion of Gasparilla Island.
The sands along this semi-tropical coast generally consist of
varying mixtures of quartz and shell.
In an analysis of historic shoreline changes, it is generally
useful to
have a
both the wave
large reliable
data base,
climate and geology
understanding
of
the
and to examine
of the subject
coastal
features
area.
which
characteristic of the area can then be developed and
An
are
applied
both in an overall and site by site review.
Data Set and Methods of Analysis
These
items
article
in
are
discussed
this
However, a brief
in
detail
publication:
in
Foster
review is warranted
an accompanying
and Savage (1989).
to help visualize
how
the results were obtained.
In essence, all the historic coastal topographic survey
of the area
National
(USGS),
by the U.S.
Ocean
have
Survey
been
standardized scale
done
by
contract
the
(NOS),
carefully
at
U.S.
digitized
Florida
Florida
Geodetic Survey (USC&GS),
and
and coordinate
researchers
with
Coast &
maps
Geologic Survey
and
systems.
State
compared
on
This work
was
University
under
Department of Natural Resources
(DNR).
The DNR coastal survey monument and beach profile set
was used
as a
shoreline sampling
1000 feet (300 meter) spacing.
pattern with
DNR beach profile survey data
from 1972 to 1988 were added to the data sets.
a set
of computer-generated
approximate
composite maps
The result is
showing all the
digitized shorelines, and tabulated data sets of quantitative
changes along profile bearings from the 1850's to 1988.
shoreline changes
are all
relative to
the location
The
of the
approximate Mean High Water (MHW) contour.
In the subject
reaches
area, the data
according
to
the
were divided into
location
significant littoral boundaries.
longshore
sequences
changes were
of
not at
of
inlets
geographic
or
other
It was readily observed
distance/time
all constant
in
plots that shoreline
over time
in many areas.
With the large amount of data, it became possible to identify
basic long-term coastal processes at work within the
of shoreline changes.
pattern
Historic and recent aerial photography
sets were used for qualitative verification of the timing and
conditions
shown
vegetation,
dune
patterns,
as
nearshore rock
in
the
ridge,
well
as
survey
overwash,
for
features.
maps,
and
for
studying
wave
the
refraction
locating coastal structures and
Field observations
present beach conditions and processes.
were made
of
Historic bathymetric
surveys
were
qualitative
offshore
also
digitized,
comparison
profile
at
surveys
shapes and bottom features
the beach.
but
this
were
have been used only for
time.
Soundings from DNR
used to investigate profile
within 3000 feet (900
A literature review
meters) of
of previous studies
of the
areas was also conducted in order to compare the results
conclusions.
Previous
studies
include
and
those by the U.S.
Army Corps of Engineers (1977, 1979, and 1984).
Weather and Wave Climate
The shoreline changes and evolving shoreline orientations
the
subject
area
over
the
last
100
years are extremely
consistent with net
southerly littoral transport.
wave data
U.S.
from the
Army Corps
Experiment Station (1988) for
as the personal observations
since
1972,
confirm
that
Hindcast
of Engineers Waterway
the period 1956-1975, as
of one of the
higher
energy
the southeasterly passage
cold fronts across the southern states.
well
authors (Foster)
waves
generally
approach from the northwest during the winter season.
waves result from
in
The NW
of a series
A discussion of
of
the
cold front pattern has been given by Rosendal (1965).
Summer
waves are typically lower energy, high frequency, wind
waves
and lower frequency swell from the southwest.
wave energy coast
in general, with
height of 0.8
meters, and there
coastal shelf.
Nevertheless,
higher
energy
NW
geology and the
wave
a mean significant
is a very
in this
pattern,
This is a
as
low
wave
wide and shallow
area the
prevailing
modified by the local
occasional tropical storm,
was found to
be
the primary force controlling shoreline changes over the last
100 years.
It is noted that there
are indications in the data
littoral drift
reversals other
occurred,
the
but
randomly and
surveys
and
unevenly spaced
future, more frequent
than seasonal
that net
have probably
aerial photography are too
to prove
this point.
monitoring of beach
In the
conditions should
help to verify how and when this process occurs.
The
tropical
storm
season
extends
from June to November.
These storms can affect the shoreline in many ways
on the path, speed,
Lists of
intensity, and frequency of
historic tropical
storms have
depending
occurrence.
been generated from
Neumann, et.al (1981), the Mariner's Weather Log of the
Dept.
of
studies.
past
are
Commerce
(1981-1988),
The effects
difficult
as
of any storm of
to
determine
well
U.S.
as from previous
set of storms in
with accuracy.
the
However,
there
is
occasionally
shoreline change
should
be
with a
noted.
photographic
a
strong
storm or
There
records
lower
Florida
transport of
are
linking
break-throughs or washouts
that a tropical
correspondence
set of
sand to
storms, and
major
changes
to certain storms.
coast
major
this
also occasional written and
storm following a
Gulf
of
It is
noted
northward path along
could
the north,
such as inlet
the
theoretically cause a net
due to
the more organized
wind field in the northeast quadrant.
Discussion of Geology
It
is
well
founded
on
known
that
limestone,
the
Florida
including
the
peninsula is largely
coastal shelf on the
Gulf, as discussed by White (1970) and Hine, et.al.
(1985).
It is
coastal
perhaps not
so well
island group is strongly
known that
the subject
influenced and possibly founded
in
part on limestone and beachrock outcroppings.
This
coast
pocket
contains
beaches,
as
a
can
sequence
be
of inlets, headlands, and
seen
in Figures 1 thru 3. The
coastal islands extend northwest and southeast of a
area which directly fronts the Gulf.
mainland
This is the Venice area
of Sarasota County, which is a geologically complex
headland
feature
and peat
having
stratifications,
several
readily
limestone
offshore bathymetry, and
quartz,
shell,
heavy
observable
outcrops,
extremely
dark colored beach
minerals,
sand
and
irregular
sand containing
fossils
(e.g.
shark
teeth).
In
addition
to
the
there are sequences
somewhat
centralized Venice headland,
of major and
minor headlands along
the
coastal islands to either side which are apparently based
on
high spots in the topography of the underlying limestone
and
superficial beachrock formations.
Beachrock is defined
here
as a composite of cemented layers of sand and/or shell.
The
limestone varies in composition
mixture of mud, sand, and
but is generally a
hardened
shell, at least in nearshore
grab
samples obtained by the authors and as described by Gould and
Stewart
(1956).
The
formations is unknown.
thickness
and
true
extent
of the
However, the report of the U.S.
Corps of Engineers for Sarasota County (1984) contains a
of minimum
area
sediment depth
(Plate
1
of
that
extrapolated landward,
and
Manasota
formations
Key
may
be
contours in
report).
These
intersect with
major
headlands.
related
as
a
the offshore
This
group
map
county
contours,
the Longboat,
Army
when
Casey,
suggests
the
and that there is
considerable
regional
extent.
In
Manatee
County to the
north, there are also three known nearshore rock exposures of
significant size along the
Further
north,
formation
in
southern half of Anna
there
is
roughly
the
a
massive
same
Maria Key.
offshore
alignment
limestone
off the central
Pinellas County coast, related to the Indian Rocks
as discussed by
Evans, et.al.
headland,
(1985), and Winston,
et.al.
(1968).
The
few
available
geologic
opinions
between Miocene and Pleistocene
the limestone, e.g.
as the age(s) of
(1985),
The Pinellas County feature is reported
to be
Miocene per
Evans, et.al.
which
appears
cover
probably
origin for
Gould and Stewart (1956), Bland
and Campbell (1985).
to
appear to be divided
Pleistocene,
the
(1985).
limestone
particularly
at
The
in
beachrock,
some areas, is
Point
of
Rocks on
Siesta Key, per Puri and Vernon (1964).
It is observed that several known past and present inlets are
located
in
close
suggested that
proximity
the inlets
to
may have
areas of the limestone topography.
hydraulically stable
as a
natural jetty effects.
natural
headlands.
formed in
It is
weak or
low
Such inlets could be more
result of
less resistance and/or
These findings suggest the
A
systematic
seismic
need for more geologic
and
core
beach, islands, and offshore
research.
sampling program along the
is suggested to accurately
map
the underlying formations.
Characteristic Coastal Features
It is important
headlands are
to understand that
subject to
level storm events, and
the upland areas
severe erosion
during high
most have consequently been
armored by property owners with seawalls, revetments,
groinfields to protect
protection offered by
on the
the upland property.
the underlying rock
water
heavily
and/or
The amount
of
depends primarily
on the elevation of the rock layer and the longshore/offshore
extent
of
the
formation.
The post-storm recovery of the
beach in these areas depends a great deal on the availability
of sand
to resupply
that moved
off the
site.
Very little
sand may be available from offshore due to a hard bottom, and
the longshore
sufficient,
sand supply
depending
on
may or
site
may not
specific
be available
conditions.
or
The
heavily armored limestone associated headlands in the subject
area are the following, from north to south:
three locations
along central and southern
Anna Maria Key; the
Longboat Key
headland; Point of Rocks; the Casey Key headland; the
headland; the Manasota Key major headland; and the
County Line
minor headland.
The
headlands are
Venice
Charlotte
labeled in
Figures 1 thru 3.
It has
also been
observed that
sand often
moves in
masses or "slugs", up to 10000 feet (3000 meters) in
within
between
the
headlands
apparently
aerial
larger
in
pocket
and
inlets.
response
photographs
to
suggest
fluid traveling back
beaches
between
The
similarity
in beach width, on
the order of 150
energy.
The
to a highly viscous
a bowl.
result in significant
headlands, and
wave
also occasionally move across the headlands.
in general can
length,
movement is slow but
directional
and forth in
large
The sand masses
This
phenomena
short-term variability
feet (50 meters).
The
large pockets which exhibit this phenomena are the following:
between the Casey Key headland and Venice inlet; between
Venice headland
and the
Manasota Key
the
headland; between the
latter and the Charlotte County Line minor headland; and from
the latter to Stump Pass.
The
remaining
bulbous-shaped
major
features
to
be
accretion/fluctuation
discussed
areas
are
located
the
on the
south
sides
of
the
unstabilized
inlets.
These
areas
developed for several reasons:
the large ebb tidal shoals on
the south
shelter the
side of
the inlets
energy wave activity except
the shoals
areas from
during high water level
occasionally merge
onto the
high
storms;
shoreline; and
shoals can refract or bend waves such that sand transport
generally directed into
seasons.
the area from
the south during
the
is
all
Frequent and random shoal re-orientations result in
extremely unstable beaches in these areas.
Anna Maria Key,
Island, and
Longboat Key, Lido
Gasparilla Island
The north ends of
Key, Siesta Key,
are all
Knight
typical cases.
north sides of the inlets are equally or more unstable
The
since
they are protected by lesser shoals.
A Brief Tour
Some of the more
will now be
interesting site specific beach
described, working from
conditions
north to south
through
the area.
Anna Maria Key has a highly fluctuating, north bulbous shaped
area.
Due
to
the
potentially tremendous hurricane surge
volume which can pass to and from Tampa Bay, the north end of
Anna Maria
away, as
Key is
particularly vulnerable
happened during
a hurricane
to being
in 1921.
washed
The
north
central zone appears to be somewhat protected and supplied by
the
massive
offshore
shoals,
and
the
beach
width
has
fluctuated depending on the shoal and storm conditions.
There are
three nearshore
limestone features
and southern Anna Maria Key.
south of a
the 1920's
ft/yr (less than 1 m/yr).
been
This section of the
refraction nodal zone
retreating since
obscured
by
the
along central
island is
and has historically
at an
approximate rate
been
of 1-3
An estimate of just how fast
construction
of
a
has
near continuous
mixture of seawalls, revetments, and groinfields starting
the 1950's with
major development of
the
the
same
time
southernmost
stabilized with a terminal groin
the island.
end
of
the
At about
island
and a groinfield.
If
was
left
alone, the shoreline would have been expected to continue
evolve in a curved
orientation to face the
in
to
northwest waves,
with some anchoring by the rock outcrops.
Ground
level
photographs
from
the
1950's and 1960's show
essentially the same conditions as the present in that
a mix
of coastal
pockets.
structures with
It appears
no beach
that buildings
area:
except in minor
and roads
were simply
built on
lots too
close to
and perhaps
water from the very beginning.
remain:
continuing
storm
even out
into the
The costs of the construction
damage,
perpetual
structural
repairs, and no recreational beach.
At
Longboat
Pass,
dredging
for
rearranged the shoals and added
navigation
purposes
sand to the north and
has
south
shores.
This pass was also washed out by storms in the early
1960's.
The inlet has migrated northward over time,
structures
built
on
the
previous
leaving
bulbous area exposed to
major erosion and storm damage.
Continuing with Longboat Key, there is a small, high
erosion
pocket immediately south of
the armored area, followed
longer,
to
The
relatively
Longboat
approximately
stable
Key
at
major
the
mildly eroding pocket beach.
headland
Manatee/Sarasota
appears
to
begin
County
Line.
This
shoreline contains many groinfields and seawalls
during and since the 1960's.
area
was
heavily
eroding
by a
constructed
However, it does not appear the
before
or
after
the armoring.
Erosion rates have been on the order of 0-1 ft/yr (0-0.5
m).
South of the headland there is a continuing shoreline retreat
towards
New
Pass,
at
approximately
2-3
ft/yr (0.5-1 m),
indicating a sand deficit exists across the headland.
It
is
noted that Longboat Key is actually composed of two
islands which were once separated
previous
by an inlet (prior to
the
1883 survey) adjacent to the most prominent rock outcroppings
on the headland.
Lido
Key
is
unique
in
that
shoreline which was constructed
between
a
set
of
small
it
has an entirely man-made
in the 1920's by
islands.
There
filling in
have
been six
renourishment projects to maintain the beaches on the heavily
developed island
from 1964
to 1987,
totaling approximately
1.4 M cubic yards (1.1 M cubic meters).
building
at
both
ends
over
time,
The shoals have been
modifying
the
wave
refraction pattern and resulting in accretion at both ends of
the shoreline.
There is
a historic erosion "hot spot"
south of the northern shoals.
the refracted
wave pattern
Aerial photography shows
tends to
remove sand
just
that
from this
area without replacement.
Continuing
southward,
it
is
of
interest
to
note
that
pre-development 1948 aerial photographs of Siesta Key show
pattern of seaward tending accretionary dune ridges,
on this coast
system.
Key by
of the south
side of an
on the
shoals, and
typical
extremely large pass
Big Sarasota Pass has been pressured against
wave action
a
Siesta
the shoal-protected
north point is not a stable landform.
continuous along
the channel
Armoring is heavy
and northernmost
point.
large pocket beach between the armored north point and
of
Rocks
has
historically
stable in recent years.
been
tends
to
align
itself
to
The
Point
accreting, and relatively
The only obvious source of
supply is the ebb shoal system.
and
present
The beach within this pocket
face
directional
wave energy,
seasonal or otherwise.
From
Point
of
County,
there
similar
and
Rocks
is
essentially
relatively
headlands.
The same
each cell.
The problem
is generally true
For
southward
example,
stable
a
repetitive series of very
pocket
beaches and armored
basic processes occur along
road
and within
of structures built too far
all along the
the
to Stump Pass in Charlotte
along
major and minor
the
Casey
seaward
headlands.
Key headland has
repeatedly washed out since the 1930's.
It is noted that Midnight Pass has existed within the
between Point of
Rocks and the
caused highly fluctuating beach
the inlet.
and has
widths in areas adjacent
to
The channel extended as far northward as Point of
Rocks in 1883.
relocate
Casey Key headland,
pocket
the
The inlet closed in 1983 during an attempt to
unstable
channel
away
from
some threatened
residential structures.
A more
complex headland
exists
at
(0.5-1 m)
Venice,
range.
jetties have
very
with
little
significant erosion
rates
At Venice
been in
have acted as
and a
generally
Inlet on
place and
in the 1-3 ft/yr
the north
repaired since
effective littoral barriers.
maintenance
developed to the
dredging,
south along the
a
problem
side, the
1937-8, and
There has
been
net sand deficit has
Venice headland, and
sand
has accreted against the north jetty.
The previously described
moving sand mass
phenomena appears
to be the dominant cause of major beach width changes in
sequence of
three large
pocket beaches
the
extending southward
from Venice into Charlotte County.
It
is
noted
that
an
existed prior to 1883
inlet
known
as Old Blind Pass once
immediately south of the
Manasota Key
major headland.
In the southern end of our subject area, in Charlotte
County
south of Stump Pass, there is an unusual island which will be
referred
to
here
as
Knight
Island.
composite of three recent islands
:
It
is
actually a
Knight, Don Pedro,
and
Little
Gasparilla
Islands.
This
island
and
the
immediately north of and including Stump Pass are
area
distinctly
different from the coastal islands to the north in that there
are no apparent stabilizing
many
major
multiple
shoreline
inlets
and
rock features.
changes
in
by
more
landward,
storms,
such
as
This area is located at the end of a
long southward littoral transport system.
of
have been
this vicinity, including
rearrangements
occurred in the 1950's.
There
older
islands.
There are portions
It
is
also
transitional zone between two distinct shoreline
in
a
alignments.
To the south, Gasparilla and a successive chain of islands in
Lee
County
orientation.
have
a
significantly
closer
to
north-south
Gasparilla Island also has extensive
nearshore
beachrock outcroppings along its' central and southern
in
apparent
alignment
with
the
new
half,
general island chain
orientation.
Knight Island has a bulbous-shaped, highly fluctuating
north
end adjacent to Stump Pass, followed by a refraction-
caused
erosion
pocket.
The
remainder
retreating toward the south
Bulges in
the shoreline
being smoothed out.
beaches
on
both
of
the
and the Gasparilla Pass
due to
prior inlets
There are fluctuating,
sides
of
island is slowly
Gasparilla
shoals.
are gradually
shoal-influenced
Pass.
A
large,
bulbous-shaped beach
exists on
the north
tip of Gasparilla
Island, followed by a relatively stable, shoal- supplied zone
extending into Lee County.
The processes and
Gasparilla Island, although the
presently incomplete,
conditions on
analysis for that island
appear very
similar to
is
those of Anna
Maria Key at the very opposite end of the subject area.
It can be said that, in the entire subject area,
erosion occurs in
general at any
net sand supply deficit either
littoral barrier
cases are
or a
location where there
due to a natural or
wave refraction
the following:
significant
the
pattern.
southern half
man-made
The worst
of Anna Maria
Key; the armored area just south of Longboat Pass; the
half
of
Longboat
immediately
south
Key;
of
the
Point
middle
of
of
is a
south
Lido Key; the area
Rocks; and the Venice area
south of Venice Inlet.
Conclusions
The Gulf coast of
in
southwest
Manatee, Sarasota, and Charlotte
Florida
beaches, and headlands.
be a direct result
contains
a
Counties
series of inlets, pocket
Historic shoreline changes appear to
of long-term net southerly
transport, as
modified by geologic features and tropical storms.
There are
erosion problems in areas lacking adequate sand supply due to
littoral
barriers
refraction effects
(man-made
around shoal
or
natural)
systems.
and/or
In many
wave
problem
areas, residential structures were simply built too close
the
active
beach
fluctuations or the
The
armored
to
allow
for
natural
gradual pattern of
natural
headlands
will
beach
to
width
shoreline evolution.
continue
to
be very
vulnerable to storm damage, although the mean high water line
tends to recover in the long term if there is available
supply.
Areas adjacent to unstabilized inlets will
to experience
large scale
beach width
sand
continue
fluctuations due
random changes in the offshore shoals and storm attack.
larger pocket beaches
beach width
can experience significant
variability due
to the
movement of
to
The
short-term
large sand
masses within the bowl-shaped pockets and occasionally across
the boundary headlands.
With the large and
growing amount of coastal
for this
is now
area, it
coastal processes
possible to
occurring at
a local
data available
recognize the
major
and regional level,
and to use this information for making better management
regulatory decisions.
and
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