GEOG 7946
Field Report from Cape San Blas,
Florida
Rebekah Jones
R. Jones
Field Report from Cape San Blas, Florida
18 October 2012
Introduction and Overview
Barrier islands along the Gulf Coast serve as tourist destinations, resource frontiers and
natural buffers against storm surge and sea level rise. Never in a constant state, the coastlines in
these areas are always struggling for equilibrium, moving on to the next phase at the slightest
disruption. Like a living organism, coastlines respond, adapt and change to their environments,
and understanding how these processes occur has never been more important. This field report
describes a few of the changes observed at Cape San Blas, Florida, located on St. Joseph’s Bay
just east of Mexico Beach, Flo. as observed by Dr. Patrick Hesp’s Coastal Estruine Resources
course at Louisiana State University. These observation were taken between Sept. 24-29, 2012.
The first day of the field trip was riddled with boat and technology malfunctions, stingray
injuries and intense heat. Rather than continue with field surveying or current/wave measurement
data collection, the group opted for a tour of the island, a stunning visualization in rapid coastal
erosion. Losing 13 meters per year of beachfront, Cape San Blas is one of the fastest eroding
beaches in the United States, according to Dr. Hesp. Onward, the class examined several coastal
management strategies being implemented by local governments in an effort to negate erosion,
such as sea walls, retreat and raised homes. The class later measured the salinity, pH,
temperature and conductivity of several near-shore and long-shore locations on day two. The
group also attempted to collect data on currents off the shoreline, but wind-driven waves made
this task especially difficult. The next day was spent conducting land surveys, which were then
compared to previous surveys of the same positions to gage how much has changed in each of
the study areas.
Beach Observations and Land Surveying
The tour of the beach began near a historic lighthouse near the cape of St. Joseph’s
Peninsula (Figure 1). Wave energy appeared to be considerably higher at the lighthouse than that
of the boat launch point, approximately one foot
to the other’s three inches, respectively.
Beaches in this area are typically low-energy, in
a dissipative, transgressive or reflective state.
Near the lighthouse, the area displayed classic
characteristics of a transgressive beach.
The area was littered with downed palm
and pine trees, power lines and rusted fencing.
Dr. Hesp described the beach prior to Hurricane
Dennis, which struck the Florida panhandle on
July 10, 2005, taking 89 lives and causing more
than $4 billion in damage. Dr. Hesp said the
beach had lost more than 20 meters since he last
visited, nine months prior to our field trip. The
large scarps along the beach indicate that a
severe storm had recently eroded much of the
Figure 1: Eroded beach located near a historic
beach, presumably this hurricane was Dennis,
lighthouse in Cape San Blas, Flo.
although hurricanes Katrina, Gustav, and Isaac
may have had impacts on the shoreline, as well.
At the cape, there were fully grown trees near and in some cases along the water line,
indicating a highly eroded coastline. A lack of typical beach vegetation for the location, such as
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R. Jones
Field Report from Cape San Blas, Florida
18 October 2012
sea oats and sea grass, indicates an eroded beach line, as the entire dune structure which would
otherwise form is missing from the beach.
Images 2-5 show the elevation profiles of the beaches at lines 6, 27, 71 and 122, taken
using an all-weather measurement tool during the land surveying process on Sept. 27-28. Line 6
(Figure 2) presented a stable beachfront with a gradual slope and no visible scarps or disruptions
in its accretion process. Line 6 has undergone very little change in terms of its profile since
measurements began in 2006 (Figure 6). Figure 6 lies just inside the bay, protected by the
peninsula and the spit, meaning lower-energy waves.
Line 71(Figure 4) displays an unstable (eroding) dune system.. The vegetation changes
observed at the beach, explained in detail in the next section, accompanied with a narrowing
beach front, also confirm the elevation comparisons of a less than one meter per year erosion rate
(Figure 7). The beach width is largest of the four profiles, with a steep foredune and low
mobility. The arête features of the foredune showed landward translating, and repeated signs of
scarping and then filling as the foredune becomes steeper and lower.
Line 110 (Figure 8) has undergone some of the more drastic changes of all the locations,
with major dune formations quickly moving landward, and an erosion rate of 6-13 meters per
year, significantly higher than at Line 71. The extremely high mobility, or lateral movement of
the foredune, shows extreme erosion and landward movement. Line 110 is located near the Cape
San Blas lighthouse on the far extent of the cape. The depositional patterns which allowed the
island to build at line 110 are no longer present, meaning serious depletion in relief. Combined
with changing tides, currents and surface winds, the beach has become a staple of fast erosion for
the Florida DEP.
Accretion picks up again at Line 122 (Figures 5 and 9), at a rate of about 10 meters per
year. Because of its proximity and orientation to the Apalachicola River, the primary source of
sediment for the barrier island system, and the natural structure of barrier islands, which build
upon themselves toward their relief source. Line 122 is already observing the creation of a new
dune line near the beachfront, which will soon replace the dominant dune as it continues to move
landward.
A fifth data set was collected for Line 32, located at the end of the peninsula where
dramatic changes have been occurring in the last decade. The operators experienced difficulties
in collecting the data due to equipment malfunctions, which is why the graphic is not
accompanying the figures below. Since Line 32 was located at the northern-most point of the
peninsula, this researcher would expect that the area is accreting at a far faster rate than the
others.
In summary, the beach is eroding at its southern-most and western ends, and accreting at
the northern, most western tip of the spit as a result of longshore sediment being deposited from
the Apalachicola River. Human activities could have a profound impact on the natural system,
such as the removal or introduction of vegetation for coastal management purposes, which
should be further investigated by qualified experts.
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R. Jones
Field Report from Cape San Blas, Florida
18 October 2012
3.
2.
4.
5.
Figures 2-5 (above): Elevation profiles of the dune
systems surveyed (in meters) at line 6 (Figure 2), 27
(Figure 3), 71 (Figure 4), and 122 (Figure 5).
A
Left: The locations of each land survey on St. Joseph
Peninsula.
A – Line 32 (Spit); B – Line 33; C – Line 71 (Opposite
boat launch); D – Line 110 (Cape, Lighthouse); E – Line
122; F – Line 27; G – Line 6.
G
B
F
The highest rates of erosion at St. Joseph Peninsula occur
at the Cape (D), and gradually decrease to point C, then
accretion begins near the spit at points A and B.
C
D
E
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R. Jones
Field Report from Cape San Blas, Florida
5.000
6
4.000
3.000
May, 2006
2.000
October, 2007
1.000
August, 2011
0.000
-100
-50
0
50
-1.000
-2.000
10
7
8
6
May, 2006
4
October, 2007
2
August, 2011
0
-150
-100
-50
0
50
-2
-4
3.000
8
2.500
2.000
1.500
-80
-60
-40
1.000
June, 2006
0.500
October, 2007
0.000
August, 2011
-20
0
-0.500
20
40
-1.000
-1.500
-2.000
5
18 October 2012
R. Jones
Field Report from Cape San Blas, Florida
18 October 2012
5.000
9
4.000
3.000
June, 2006
2.000
October, 2007
1.000
-140
-120
-100
-80
-60
-40
0.000
-20
0
-1.000
August, 2011
20
40
-2.000
Vegetation
Each of the locations surveyed were all in varying stages of erosion and accretion. The
most obvious indicator of an erosion/accretion state, having not witnessed the changes in dune
displacement personally, was vegetation type and cover. Dunes form when sand from offshore is
caught and deposited on land by waves or wind. Frontal zone vegetation is heavily beaten with
salt coming from the waves, so the variety and density of vegetation is typically low in this area.
Ipomoea spp, also called Morning Glory , is a short, broad-leafed plant which grows in small
pockets and long single-leaf chains. Panicum amarum begins to form around the vegetation line,
which is a thin, tall grass which forms in dense pockets. The evidence of dune formation is most
pronounced in the areas of heavy Panicum amarum presence. Thorn-covered, short purple floras,
almost resembling a cactus, were present on several of our sites. I am unsure whether this was
Cakile spp or Sesuvium portulacastrum, as they are very similar to one another and both known
to be present in the foredunes of this region.
Uniola paniculata, also called sea oats, trap more sand pushed onshore both because of
the density near the surface and the height – they stop sediment much higher than the berm,
allowing it to fall down beneath it and further build the berm. Windward sand is carried onto the
beach until the velocity drops or until it reaches an obstruction, resulting in a landward
movement of the dune. The backdunes, or the older dunes which have either migrated inward or
whose foredunes have been eroded, were forested. Trees can only form once soil has begun to
retain a higher level of nutrients, is not as easily disturbed, and has very little exposure to salt
water.
On highly eroded beaches, there was little, if any, Uniola paniculata or Panicum amarum
present. These vegetation species are typical during dune construction, as they trigger a cycle of
positive feedbacks that build dunes. The absence of these species indicates an unstable beach
system.
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R. Jones
Field Report from Cape San Blas, Florida
18 October 2012
Water Characteristics
Despite delays caused by mechanical issues for the boat, the group was able to collect
pH, conductivity, temperature and salinity data for all four locations on day two. Overall, there
was very little variation in the four values from each site. Temperature varied by less than .4°C,
at its peak of 27.3°(81.4°F) and bottomed out at 26.9°C (80.42°F). According to NOAA, pH
ranges from 7.5 to 8.5. Our readings averaged near 7.6, so the pH was very low for seawater,
compared to global averages. The salinity of seawater, or concentration of salt in water, averages
35 ppt (3.5 by our scale), but values vary depending on proximity and orientation to estuaries,
tide, currents, etc., declining in spring and rising with higher temperatures. Considering the
warmth of the Gulf, I had expected high salinity levels, as ours averaged only 33.5 ppt. Overall,
the areas near the St. Joseph Peninsula we surveyed were warm, with low acidity and salt
concentration, and average conductivity.
Meanwhile, potassium permanganate was released to observe currents and wave patterns.
The substance dies the seawater purple in concentrated amounts, allowing the water to carry it
while still leaving a clearly visible trail for which to observe the current. Flows reflected the
same trends displayed by the drifters, which is somewhat surprising since we assumed the
drifters were more influenced by the wind than by currents.
RESULTS OF WATER CHARACTERISTIC TESTING
Location:
PH
Conductivity Temperature Salinity
29°57'12.8" N
85°25'26.4" W
10:20 AM
7.56
50.8
27.2°c
3.34
10:21 AM
N/A
51.3
27.2°c
3.37
10:23 AM
7.7
51
27.2°c
3.36
29°52.501 N
85°25.029 W
10:41
7.72
51.5
27.2°c
3.39
10:43
7.54
51.7
27.2°c
3.4
10:45
7.63
51.3
27.3°c
3.38
29°52'22.6" N
85°24'9.6" W
11:10 AM
7.55
51.4
27.3°c
3.39
11:12 AM
7.67
50.7
27.3°c
3.33
11:13 AM
7.63
51.3
27.3°c
3.38
20°51'34.9" N
85°24'46.1"W
11:28 AM
7.63
52
26.9°c
3.43
11:32 AM
7.5
52.3
26.9°c
3.45
11:35 AM
7.54
51.6
26.9°c
3.4
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R. Jones
Field Report from Cape San Blas, Florida
18 October 2012
Drifters
GPS-enabled buoys, or drifters, were released three at a time in four locations to measure
current direction and velocity. The GPS unit inside the drifters recorded the position of each of
the drifters four times a minute, allowing for critical examination of the direction and speed of
the surface currents. Longshore drift is one of the primary sources of deposited sediment on the
shoreline, and the experiment hoped to record the speed of the drift as it came onshore. The
speed would then be able to tell researches what grain size and amount of sediment load was
being carried by the longshore drift.
However, wind-driven surface currents prevailed over longshore drift on the day of our
experiment, pushing the drifters seaward at a rate of .23 m s-1. The drifters moved northwest
(315°) to west-southwest (250°). Wave refraction and reflection from the shore may also have
influenced the results of the experiment. The drifters moved at a velocity of .23 m s-1 on day one.
The median speed was .31 m s-1 to .39 m s-1, meaning other factors were affecting the results,
such as the flag added to one drifter in order to increase visibility for retrieval.
Figure 10: The breakwater has
moved since the satellite image
of the south, southeastern coast of
Cape San Blas, Florida was
taken. There were no waves
breaking in the paths of the buoys
we released during our field
observations. The current is
clearly coming from the south,
southwest, but the buoys were
taking southward by the winddriven currents that day.
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R. Jones
Field Report from Cape San Blas, Florida
18 October 2012
Coastal Management Observations
In response to rapid erosion, local officials and landowners along St. Joseph Peninsula
have employed several techniques to mitigate further damage.
Coastal management strategies observed:




Sea wall construction
Retreat (abandoned military facility)
Raised homes
Restoration (resort town)
Sea walls are in varying stages of construction along the beachfront. Already the erosion
around the wall has become evident, so the wall is succeeding in holding some of the rise in
levels at bay. Sea walls increase reflection, however, intensifying energy at the toe of the
breakwater, causing the bottom of the wall area to erode and increase erosion of the seabed. This
erosion will mean more upkeep for the sea wall.
The last location we surveyed remained under a more environmentally-conscience
approach to regulation. The natural backshore has been largely preserved, without the removal of
dunes for beach of housing construction. Houses are built well into the shoreline, in some cases
as many as five large dunes behind the bay. The preservation of the natural dune structure
provides protection for the community.
Retreat tends to be a last-resort measure for decision makers and land owners as a
strategy to mitigate sea level rise or beach erosion. Several buildings had been destroyed by rapid
erosion, others by storm surge. The remains of an abandoned military facility still jut out of the
low tide, a reminder of how rapidly the beach is disappearing. According to Dr. Hesp, many
houses have been washed away as a result of storm surge resulting from Hurricane Dennis. Many
of the houses along the coastline are empty, although not abandoned. Most of those who have
remained in the area have risen their house on stilts, creating second-floor homes in the hope that
storm surge will not reach the second level during a major hurricane, or erode the beach far
enough inland to make the stilts obsolete.
Human-nature interaction rarely leaves the natural environment undisturbed. So long as
building and owning beachfront property remains the style in these hyper-vulnerable coastal areas,
construction will not cease. Solutions to rising sea levels, coastal erosion and storm surge can
accommodate the demands of residential homes to an extent. Ultimately, the rapid rate of change in
the area, combined with the unknown challenges climate change and rising seas will add to the
equation, beg the question of whether or not people should be building here at all. Allowing the
beach to return to its natural state, which may very well be impossible as the natural system has
already been interrupted, is probably the most-implemented of the practical approaches to mitigation.
Retreat, the least often implements, probably is the most rational.
Insurance laws enable, and I would argue encourage, construction on dangerous coastlines:
by insuring homes which are inevitably going to be destroyed incentivize the well-off to build there
anyways. Changing federal flood insurance laws to only provide relief for one disaster, then
afterward provide incentives for relocation, would discourage the habitual self-destructive nature of
home-buying on a floodplain. State officials could designate quickly eroding beaches as state parks,
as Louisiana and Mississippi have done with several barrier islands to ensure their protection. Local
governments could try to buy the land from home owners and turn the space into public parks, or
zone out residential and commercial operations and leave the areas undeveloped.
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R. Jones
Field Report from Cape San Blas, Florida
18 October 2012
*Josh Gilliland, Ian Floyd, Fan Zhang and Julie Doucet assisted in the collection and processing of data
included in this report. Dr. Hesp and Dr. Li provided equipment and gathered data in the field.
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