Journal of Coastal Research, Special Issue 56, 2009
Journal of Coastal Research SI 56 pg - pg ICS2009 (Proceedings) Portugal ISSN
Sediment dynamics on an inner shelf shoal during storm events in the
northeastern Gulf of Mexico, USA
Amy L. Spaziani†, Felix Jose‡, and Gregory W. Stone† †Department of Oceanography and Coastal Sciences and Coastal
Studies Institute, Louisiana State University, Baton Rouge, LA
70803, [email protected]
‡ Coastal Studies Institute, Louisiana State University, Baton
Rouge, LA 70803
ABSTRACT
Spaziani, A.L., Jose, F., and Stone, G.W., 2009. Sediment dynamics on an inner shelf shoal during storm
events in the northeastern Gulf of Mexico. Journal of Coastal Research, SI 56 (Proceedings of the 10th
International Coastal Symposium), pg – pg. Lisbon, Portugal, ISBN
Sediment resuspension and transport during storm events, specifically tropical storms and hurricanes, is poorly
understood on the inner shelf. The increase in frequency and intensity of hurricanes in the northern Gulf of
Mexico, coupled with the increase in development and tourism along the Florida Panhandle, in the southeastern
United States, has increased the interest in sediment transport and erosion during hurricanes. This study utilized
MODIS satellite imagery and a sediment resuspension model for the assessment of inner shelf material
resuspension during hurricanes Ivan and Dennis. True color images and reflectance maps from red channel
satellite imagery were created to corroborate modeled sediment resuspension. The model results indicate that substantial resuspension occurred over several days and down to the 100 m isobath during Hurricane Ivan.
Resuspension intensity for Dennis was nearly as high as Ivan; however, such occurred over a smaller time
interval and depth. Satellite imagery confirms the model’s results with a significant plume that persists for 3 days
after Ivan’s landfall.
ADITIONAL INDEX WORDS: Sediment transport, resuspension, hurricanes, wave model, satellite imagery
INTRODUCTIONRecent increase in the frequency and strength of tropical storms
along the northeastern Gulf of Mexico has raised considerable
concern and interest in sediment transport and beach erosion along
coasts of the United States. These frequent and often destructive
storms coupled with the increase in development and tourism
along the Florida Panhandle, have provoked an increasing interest
in issues involving sediment transport, beach erosion, beach
restoration, and sand mining from offshore shoals.
This study focuses on the continental shelf off Chochtawhatchee
Bay, Florida with particular emphasis on an inner shelf shoal
(Figure 1). Much of the surficial sediments in the region are fine to medium sand, with very little to no clay fractions (Hyne and
Goodell, 1967). These sediments comprise a massive sand sheet
known as the Mississippi Alabama Florida (MAFLA) sheet sand
(Doyle and Sparks, 1980). However, cores and sub-bottom
profiles from the nearshore region and a shoal on the inner shelf
have indicated that finer grain sediments do exist just below and
interbedded in the sub-surface, as well as coarser sand on the shoal
crest (Spaziani, unpublished data).
The region receives little sediment input from riverine sources
during fair-weather conditions, as the majority of sediment is
deposited in bays along the coast that were drowned during rapid
sea level rise in the initial to middle stages of the Holocene (Tanner, 1992). Longshore transport dominates the system and the
dominant net transport is to the west (Stone et al., 1992). Sediment
added to the system is supplied by eroding headlands east of the
study area or material being reworked from the beaches or
offshore (Stone and Stapor, 1996).
The study area is micro-tidal (<2 m tidal range), however, a high-energy wind and wave environment. Cold fronts from the
northwest, frequent the area at a rate of approximately 30 per year,
generally occurring from October to March or early April
(Moeller et al., 1993). The area also lies on the edge of a zone
highly susceptible to hurricane and tropical storm impacts (Muller
and Stone, 2001). Return periods for this area average among the
highest in the Gulf Coast and Atlantic Coast combined, averaging
once every 3 years for tropical cyclones (tropical storms and
hurricanes) and once every 35 years for catastrophic hurricanes
(category 3 or higher) (Keim et al., 2007).
While fair-weather transport mechanics have been well-
documented in this area (e.g. Stone and Stapor, 1996), transport
and resuspension during high-energy storm events are not well understood. This study couples a third-generation spectral wave
model with satellite imagery to assess the resuspension and
transport of sediment during hurricanes Ivan and Dennis.
SEDIMENT RESUSPENSION MODEL
MIKE 21, a spectral wave (SW) model, was implemented in
this study to examine the resuspension of sediments on the inner
shelf during hurricanes Ivan (in 2004) and Dennis (in 2005). The
model has been developed by DHITM Water and Environment,
based on an unstructured mesh. Details on the model physics and
parameterization can be obtained from Sorensen et al., (2004). The
MIKE 21 SW model has been successfully implemented for the Gulf of Mexico and Louisiana coast (Jose et al., 2007). Input for
the model, wind speed, was derived from re-analyzed surface
wind data from NOAA’s Atlantic Oceanographic and
Meteorological Laboratory (AOML). The surface wind analysis is
based on measurements made in-flight (hurricane hunters), as well
as input from any available surface weather observations, such as buoys, coastal platforms, and satellite data. The high resolution
(~6 km) AOML data set was blended with NOAA’s North
Sediment dynamics during hurricanes
American Regional Re-Analyzed (NARR) data set (~32km
resolution) to develop the gridded input wind data.
Hurricane induced wave fields for the entire Gulf of Mexico, during hurricanes Ivan and Dennis, were generated using this
model. Wave boundary conditions for a high resolution coastal
grid were generated for the study area (Figure 1), from the
regional model. This allowed for greater enhancement of the
mesh grid, especially over the shoal. The model data w
validated with in situ data from several National Data Buoy Center (NDBC) buoys. Sediment grain size parameters were compiled
from vibracores samples processed at the Coastal Studies Institute
at Louisiana State University (LSU) and from US
Bathymetry was obtained from NOAA’s National Geoph
Data Center (NGDC). Resuspension intensity (RI), the difference between wave induced shear stress and the critical shear stress,
was estimated from wave shear stress, (Madsen, 1976)
critical shear stress for sand bottoms (Li et al., 1997)
for RI computations were modified from Kobashi et al. (in prep).
MODIS IMAGERY
Few studies have attempted to couple resuspension and satellite
imagery (e.g. Stumpf and Pennock, 1989). A fine
resolution of the sensor and frequent revisit times
are important to capture short-term events such as sediment
resuspension during and post storms. Terra and Aqua (launched in 1999 and 2002, respectively), both carry the Moderate
Imaging Spectroradiometer (MODIS) instrument.
(2005) provide an overview of the advantages of using MODIS
imagery to examine suspended sediments. The spatial
MODIS varies, but at a resolution of 250 m, band 1
channel, and band 2, near infrared (NIR), provide the higher
definition required to observe suspended material along the coast
and inner shelf. A common method for the detection of suspended
sediments is to relate reflectance of sediments in suspension in a
Figure 1. A bathymetric map of the study area. The approximate location of the shoal is outlined in black
Sediment dynamics during hurricanes
Analyzed (NARR) data set (~32km
Hurricane induced wave fields for the entire Gulf of Mexico, during hurricanes Ivan and Dennis, were generated using this
ions for a high resolution coastal
grid were generated for the study area (Figure 1), from the
regional model. This allowed for greater enhancement of the
mesh grid, especially over the shoal. The model data were
ational Data Buoy Center (NDBC) buoys. Sediment grain size parameters were compiled
from vibracores samples processed at the Coastal Studies Institute
at Louisiana State University (LSU) and from USseabed.
Bathymetry was obtained from NOAA’s National Geophysical
esuspension intensity (RI), the difference between wave induced shear stress and the critical shear stress,
(Madsen, 1976), and
(Li et al., 1997). The routines
Kobashi et al. (in prep).
attempted to couple resuspension and satellite
. A fine spatial
of the satellite
term events such as sediment
Terra and Aqua (launched in , both carry the Moderate-resolution
troradiometer (MODIS) instrument. Miller et al.
overview of the advantages of using MODIS
spatial capacity of
t a resolution of 250 m, band 1, the red
provide the higher
definition required to observe suspended material along the coast
A common method for the detection of suspended
of sediments in suspension in a
vertical water column to the reflectance measured in the red
portion (ca. 600-700 nm) of the visible spectrum
Pennock, 1989). In coastal and inland waters scattering from suspended materials frequently dominat
spectrum, compared to pure water and phytoplankton absorption
(Miller et al., 2005).
MODIS images processed to level 1b (Rayleigh scattering
correction applied) were downloaded from NASA Earth
Observing System (EOS) via the Earth Scan Laboratory at Three nearly cloud-free and sunglint-free
during Hurricane Ivan: 17 September 2004 at 18:42 UTC, 18
September 2004 at 19:22 UTC, and 19 September
UTC. The geopositional accuracy of each image was assessed by
overlaying a coastal vector onto each image and adjusting the coordinates manually where needed. An atmospheric correction
reduce scattering from aerosols was applied to the red
images using the clear water pixel assumption.
reflectance of clear water should be zero, or near zero in the
channel due to negligible values of water-leaving radiance and any
reflectance in clear water (black in NIR) will be due
scattering, referred to as the black pixel assumption
2000). Using this theory, the lowest value of reflectance was
obtained from NIR images from each day and subt
entire corresponding red channel image. R
extracted in a 250 m resolution grid over the study arealater eliminated from the coastline to reduce noise from
reflectance over land. The red, blue (channel 3, 500
and green (channel 4, 500 m) channels were also combined
the program TVIS, in the Earth Scan Laboratory, to create a true
color image for each day. True color images can reveal significant
observations of suspended material during or a
et al., 2005), as well as providing a time series of imagery and
suspension events.
Figure 1. A bathymetric map of the study area. The approximate location of the shoal is outlined in black and isobaths are in meters
to the reflectance measured in the red
700 nm) of the visible spectrum (Stumpf and
In coastal and inland waters scattering from suspended materials frequently dominates this reflectance
spectrum, compared to pure water and phytoplankton absorption
MODIS images processed to level 1b (Rayleigh scattering
downloaded from NASA Earth
the Earth Scan Laboratory at LSU. free images were selected
17 September 2004 at 18:42 UTC, 18
September 2004 at 19:22 UTC, and 19 September 2004 at 18:27
UTC. The geopositional accuracy of each image was assessed by
verlaying a coastal vector onto each image and adjusting the An atmospheric correction to
aerosols was applied to the red channel
images using the clear water pixel assumption. Briefly, the
reflectance of clear water should be zero, or near zero in the NIR
leaving radiance and any
reflectance in clear water (black in NIR) will be due to aerosol
referred to as the black pixel assumption (Siegel et al.,
Using this theory, the lowest value of reflectance was
images from each day and subtracted from the
Reflectances were then
250 m resolution grid over the study area. Data was later eliminated from the coastline to reduce noise from
The red, blue (channel 3, 500 m resolution),
were also combined using
in the Earth Scan Laboratory, to create a true
True color images can reveal significant
observations of suspended material during or after storms (Stone
providing a time series of imagery and
and isobaths are in meters.
Journal of Coastal Research, Special Issue 56, 2009
Journal of Coastal Research SI 56 pg - pg ICS2009 (Proceedings) Portugal ISSN
RESULTS AND DISCUSSION
Based on the model results, increasingly negative values of RI
indicate an increasing potential for deposition of sediment, as
shown in the following Figures (2 through 5). Positive RI values
indicate potential for resuspension and therefore erosion. During fair-weather conditions the potential for deposition on the shoal
was very low with RI values of approximately zero to -0.1 N/m2.
During fair-weather conditions and increasingly turbid water, the
seaward side of the shoal consistently showed a sharp increase in
depositional potential in the model results, likely due to the
change in bathymetry.
Ivan
Hurricane Ivan was a devastating tropical cyclone that made
landfall on the northern Gulf Coast (Alabama). Ivan developed off
the west coast of Africa on 31 August, 2004 (Stewart, 2004)
approximately 100 km west of the study area. According to
Hurricane Ivan’s model results (Figure 2), erosional potential began to increase on 12 September at 00:00 UTC, on the highest
portions (shoals and ridges) of the study area. On 14 September at
12:00 UTC, equilibrium was reached as the RI reached 0 N/m2
indicating neither erosion nor deposition. Resuspension occurred
on 15 September at 00:00 UTC, over 24 hours before the closest
pass of Hurricane Ivan to the study area. Long period waves (14 s)
were measured at NDBC 42039 on morning hours of 13
September, which would accelerate the bottom turbulence. RI
continued to increase rapidly, reaching maximum shear stress
values on the shoal on 16 September at 06:00 UTC, approximately
the time of landfall of Hurricane Ivan, and closest pass to the
entire model domain. During this time period RI reached values
of 2 N/m2, occurring on the crest of the shoal, and positive RI
values covered nearly the entire study area, even in the head of the
De Soto Canyon, reaching equilibrium at approximately 100 m
water depth. RI began to decrease by 12:00 UTC, on 16 September.
Deposition was indicated by negative RI values at the head of the
De Soto Canyon in the southwest (Figure 3). By 17 September
00:00 UTC, deposition conditions were manifested by the model
at all depths of 40 m or greater. The lowest values were observed
in the De Soto Canyon, indicating that this area is a potential sediment sink in the region. The area over the shoal maintained
the highest values of RI, indicating sediments were most likely to
be eroded off the shoal and winnowed away. By 18 September, at
06:00 UTC, 48 hours after Ivan passed the study area, no
resuspension was indicated by the model. Pre-hurricane conditions
were reached on 19 September, 06:00 UTC, 72 hours after Ivan’s pass.
While the computed RI decreased substantially, solid material
remained in suspension in the study area, as confirmed by the
satellite imagery (Figure 3). The true color image on 17
September confirms the model’s results that resuspension
occurred at considerable depths. However, in satellite imagery, the
plume persists for several days, as shown in Figure 3, appearing to
move in an easterly direction. In doing so, it appears to move
around the head of the De Soto Canyon, suggesting that some
sediment is lost to deposition. A reflectance value map (Figure 4),
Figure 2. Model simulations of resuspension for time periods discussed in the text during Hurricane Ivan. Hurricane Ivan made landfall
at approximately the same time as maximum resuspension occurred on 16 September at 06:00 UTC. The black outline in each image
denotes the approximate area of the shoal. Note that the color bar values change.
Journal of Coastal Research SI 56 pg
Figure 3. True color satellite imagery. Black rectangle denotes
study area and arrows indicate movement of the plume. A. 17 September 2004 at 18:42 UTC. B. 18 September 2004 at 19:22
UTC. C. 19 September 2004 at 18:27 UTC.
derived from the red channel (band 1) of the imagery shows
reasonably good correlation to the modeled RI for 17 September at approximately 18:00 UTC.
Figure 4. Mapped reflectance values extracted from band 1.
pg - pg ICS2009 (Proceedings) Portugal
Figure 3. True color satellite imagery. Black rectangle denotes
study area and arrows indicate movement of the plume. A. 17 September 2004 at 18:42 UTC. B. 18 September 2004 at 19:22
and 1) of the imagery shows
reasonably good correlation to the modeled RI for 17 September at
Figure 4. Mapped reflectance values extracted from band 1.
Hurricane Ivan made a second pass through the Gulf of Mexico,
and near the study area as a tropical low/depression. As it passed on 23 September, resuspension was initiated on the
ridges and shoal, which continued for over 24 hours.
Dennis
Hurricane Dennis made landfall on Santa Rosa Island, close to
the study area, on 10 July 2005 at approximately
2005). While Dennis was an extremely large andsystem for a July hurricane, it was still much smaller than
Hurricane Ivan, and therefore the impacts on the inner shelf were
shorter in duration, which was observed in both the resuspension
model and satellite imagery. RI on the inner shel
increase over the shoal on 10 July at 00:00 UTC due to the approach of Dennis. The increase in RI was
Ivan’s: the greatest RI change occurred over the shoal,
lesser extent on a ridge in the northwestern portion o
area, and on the smaller shoal directly south of the large shoal.
RI increased very rapidly, reaching values of more than 1.5
N/m2, in 18 hours, just prior to the landfall of Dennis (Figure 5).
This increase in RI by twice that of Ivan (0.04 N
and 0.02 N/m2/hr for Ivan) may be attributed to several factors,
including the forward speed of the hurricane and the size and
intensity of the storm. Moreover, Hurricane Cindy made landfall
over southeastern Louisiana on 6 July as a creating wave heights along the Florida inner shelf large enough to
initiate resuspension along the shoals and ridges.
the sediment resuspension, which might have
Hurricane Cindy, facilitated more resuspension during Dennis
due to already elevated hydrodynamic conditions
During peak RI, depositional potential remained over the head
of the De Soto Canyon, indicating a sink for the sediment in
resuspension. RI rapidly decreased and in less
Dennis’s landfall, deposition was indicated in depths of 40 m or
greater. By 12 July 18:00 UTC, equilibrium was reached, and by
14 July 12:00 UTC, 90 hours after landfall, preconditions were re-established, lasting longer tha
CONCLUSIONS
Results from a sediment resuspension model indicate
significant amounts of sediments were resuspended on the inner shelf, particularly on shoals and ridges during high
events such as tropical cyclones. Resuspension potential was
generated on the inner shelf and extended up
and persisted for several days following landfall of the hurricanes
conspicuously on areas of shallower bathymetry such as shoals.
The shoal in the study area consistently held the highest RI during both hurricanes, while the head of the De Soto Canyon was
consistently the lowest, indicating sediment being winnowed off
the shoal and deposited farther offshore.
These model results were confirmed by true color satellite
images and measured reflectance values from the red channel.
However, satellite imagery indicated that suspended material
plume persisted several days after RI decreased and deposition
began to occur, indicating a slow settling rate post hurricanes,
even though the wave climate rapidly returns to post
conditions. It is proposed that future research should focus on
other storms that frequent the regions, such as cold front storms and other tropical cyclones
ACKNOWLEDGEMENTS
The Department of Environmental Protection of the State of
Florida, Taylor Engineering and Alpine Ocean Seismic Survey, Inc. provided funding for this research. A. Haag, C. Pilley, J.
ISSN
Hurricane Ivan made a second pass through the Gulf of Mexico,
and near the study area as a tropical low/depression. As it passed on 23 September, resuspension was initiated on the crests of the
ridges and shoal, which continued for over 24 hours.
urricane Dennis made landfall on Santa Rosa Island, close to
10 July 2005 at approximately 19:30 (Beven,
. While Dennis was an extremely large and well-developed system for a July hurricane, it was still much smaller than
Hurricane Ivan, and therefore the impacts on the inner shelf were
shorter in duration, which was observed in both the resuspension
model and satellite imagery. RI on the inner shelf began to
increase over the shoal on 10 July at 00:00 UTC due to the approach of Dennis. The increase in RI was similar to that of
nge occurred over the shoal, to a slightly
tern portion of the study
area, and on the smaller shoal directly south of the large shoal.
RI increased very rapidly, reaching values of more than 1.5
, in 18 hours, just prior to the landfall of Dennis (Figure 5).
This increase in RI by twice that of Ivan (0.04 N/m2/hr for Dennis
be attributed to several factors,
speed of the hurricane and the size and
Hurricane Cindy made landfall
over southeastern Louisiana on 6 July as a Category 1 hurricane, creating wave heights along the Florida inner shelf large enough to
initiate resuspension along the shoals and ridges. It is possible that
might have been induced by
pension during Dennis,
hydrodynamic conditions in the region.
During peak RI, depositional potential remained over the head
of the De Soto Canyon, indicating a sink for the sediment in
resuspension. RI rapidly decreased and in less than 24 hours from
Dennis’s landfall, deposition was indicated in depths of 40 m or
greater. By 12 July 18:00 UTC, equilibrium was reached, and by
12:00 UTC, 90 hours after landfall, pre-hurricane established, lasting longer than Ivan (72 hrs).
Results from a sediment resuspension model indicated that
resuspended on the inner shelf, particularly on shoals and ridges during high-energy storm
uspension potential was
and extended up to depths of 100 m
and persisted for several days following landfall of the hurricanes,
conspicuously on areas of shallower bathymetry such as shoals.
ntly held the highest RI during both hurricanes, while the head of the De Soto Canyon was
consistently the lowest, indicating sediment being winnowed off
These model results were confirmed by true color satellite
images and measured reflectance values from the red channel.
However, satellite imagery indicated that suspended material
plume persisted several days after RI decreased and deposition
began to occur, indicating a slow settling rate post hurricanes,
hough the wave climate rapidly returns to post-hurricane
conditions. It is proposed that future research should focus on
other storms that frequent the regions, such as cold front generated
ACKNOWLEDGEMENTS
Department of Environmental Protection of the State of
Engineering and Alpine Ocean Seismic Survey, research. A. Haag, C. Pilley, J.
Journal of Coastal Research, Special Issue 56, 2009
Journal of Coastal Research SI 56 pg - pg ICS2009 (Proceedings) Portugal ISSN
Figure 5. Model results for Hurricane Dennis. Shoal is outlined in black. Note the variation in the scale bar.
\
Calvasina of the Earth Scan Laboratory at LSU assisted in
obtaining the satellite data. DHI Water and Environment provided the wave model. D. Kobashi, WAVCIS Laboratory, LSU,
provided the routine for computing the re-suspension intensity.
Grain size analysis was performed by B. Liu, Y. Luo, Y. Chen,
and D. Kobashi. Y. Chen provided assistance for developing
Figure 1.
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