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, aspazi1@lsu.edu

‡ 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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