Coastal Dynamics 2009 Paper No. 146
HYDRODYNAMIC RESPONSE OF A TRANSGRESSIVE SHOAL TO THE PROPOSED MINING FOR RESTORING ADJACENT BEACHES AND BARRIERS: SABINE BANK, OFF LOUISIANA-TEXAS COAST, UNITED STATES Felix Jose 1 , Gregory W. Stone2, Daijiro Kobashi2,3, Seyed M. SiadatMousavi2 and Baozhu Liu1 Abstract Sabine Bank, a transgressive shoal located 30 km off the Louisiana-Texas border, USA, has been considered as one of the plausible resources for re-nourishment of the adjacent barrier islands and beaches. Little has been reported on the bottom boundary layer dynamics and sediment transport from this shallow coastal environment. A comprehensive field investigation, coupled with numerical modeling, has been implemented. Wave and bottom boundary layer interactions were strongly associated with the passage of cold fronts across the region. Strong southerly/southeasterly wind regimes also contributed to the re-suspension and transport of sediments, even during summer season. Modification in bulk wave parameters due to two mining scenarios were computed using modified bathymetries and the result shows minimum impact from the proposed mining from the shoal crest. Sediment re-suspension intensity (RI) was computed and found to be high over the inner shelf and shoal during severe storms. Key words: Sabine Bank, coastal restoration, wave modeling, hydrodynamics, sediment re-suspension, cold fronts, MIKE 21
1. Introduction During the past half century, the Louisiana coast has experienced severe land loss (Penland et al., 2005) and erosion rates considered the highest in the nation. Louisiana’s 3 million acres of wetlands are lost at the rate of approximately 75 square kilometers annually (USGS, 1995). This loss can be attributed to various natural and anthropogenic processes. The former includes land subsidence and deltaic processes of the Mississippi River, eustatic sea-level rise, and the landfall of severe hurricanes and tropical storms. Anthropogenic components are mainly the control of river sediment discharge and interactions with engineering structures (National Research Council (NRC), 2006). Artificial levees and dams prevent natural sediment supply from the rivers, creating disequilibrium in the sediment budget which further exacerbates land loss problems. Dredging of navigation channels across the low lying marshes and the withdrawal of fluids (i.e., oil and gas) from the inner shelf and offshore also contribute to wetland loss along the coast (Chan and Zoback, 2007). The coastal zone encompassing the Louisiana-Texas border has been exposed to extensive erosion due to a myriad of factors, including the landfall of devastating hurricanes, viz., Lili in 2002, Rita in 2005, and Ike in 2008. The entire Holly Beach community (see Figure 1) in southwest Louisiana was devastated during Hurricane Rita’s landfall in September 2005. Later, in September 2008, extensive coastal erosion and infrastructure damage were reported from west of Sabine Pass (Figure 1) after the landfall of Hurricane Ike. Federal and state agencies have embarked on ambitious coastal restoration projects to rebuild and re1
Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803, USA.
[email protected] (Felix Jose),
[email protected] (Baozhu Liu). 2 Coastal Studies Institute and Department of oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA.
[email protected] (Gregory W. Stone),
[email protected] (Seyed M. SiadatMousavi). 3 Present affiliation: Climate Program Office, National Oceanic and Atmospheric Administration, Silver Spring, MD, 20910, USA.
[email protected]
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Coastal Dynamics 2009 Paper No. 146 nourish this rapidly deteriorating stretch of the coast. Given the deltaic nature of the coast, offshore sand bodies are of considerable importance as viable sand resources to implement these ambitious restoration projects. However, before initiating a large scale extraction of sand from these transgressive offshore shoals along the northern Gulf of Mexico, the Minerals Management Service (MMS) has commissioned physical and biological environmental impact studies for the shoals. Previous studies indicated that Sabine Bank has an important affect on shoreward propagating waves; especially those generated during storms (King 2007) and included a shoreline erosion feasibility investigation concentrated along the beach. In the current paper, we summarize physical studies conducted since 2004, including wave bottom interaction and bottom boundary layer dynamics on Sabine Bank (Figure 1), a transgressive sand body located approximately 30 km offshore, encompassing an area of 600 km2. This study also includes output from a third-generation spectral wave model that was implemented to quantify wave transformation over the shoal as well as the modification of the wave field that may occur due to targeted sand mining from the crest of the shoal.
Figure 1. Location map of the study area. Yellow triangles represent the survey deployment sites during 2004, 2006 and 2008. Magenta circles represent the locations where wave and sediment-re-suspension characteristics were computed numerically
2. Methodology 2.1 In situ observations In order to fully understand the wave characteristics and bottom boundary layer dynamics of the bank, three extensive field deployments were conducted during spring 2004, winter 2006, and summer 2008; in situ measurements from these deployments included time series of currents, water level, suspended sediment concentration, bottom elevation change, temperature and salinity. In addition, surface sediments were collected from the shoal during the cruises for deployment and retrieval of the tripods. The computed parameters, based on in situ observations, were bulk wave parameters, wave and current induced shear stress and shear velocity in the bottom boundary layer and re-suspension intensity. Bottom sediment
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Coastal Dynamics 2009 Paper No. 146 characteristics were obtained from basic granulometric analysis of the sediment samples. Three stations were set up along eastern Sabine Bank, across a transect and are identified as STN_1, STN_2 and STN_3 on Figure 1. The middle station was located at the crest of eastern Sabine Bank while both offshore and onshore stations were situated along the lateral edges of the bank. The Spring 2004 deployment consisted of two Acoustic Doppler Velocimeters (ADV’s) and a unique instrumentation array named WADMAS (Stone, 2000), which consisted of three electromagnetic current meters (ECM’s), three Optical Backscatter Sensors (OBS) in addition to a pressure sensor. The WADMAS array was deployed on the crest (STN_2 on Figure 1) while the two ADV’s were deployed at stations STN_1 and STN_3 respectively (see Figure 1). The nearshore ADV array was comprised of a pressure sensor and two OBS sensors while the offshore ADV array utilized a single pressure sensor. For the 2006 and 2008 deployments, the WADMAS array was replaced with a Pulse-Coherent Acoustic Doppler Profiler (PCADP) tripod. During the 2008 deployment, an upward looking ADCP (RDI) was also included in the PCADP tripod. The entire instrument systems measured at variable burst modes to lengthen temporal coverage. The instrument arrays’ sampling frequencies and sensor heights for the tripod deployed at the crest of the shoal are given in Table 1. Details on the instrument array, sampling protocol, sampling duration etc can be obtained from Kobashi et al. (2005, 2006) and Stone et al. (2009). These instrument arrays have been widely used to study bottom boundary layer dynamics elsewhere (Cacchoine et al., 2006). A summary of the data collected during 2004 and 2008 are discussed in this paper. Table 1. Instrument arrays, sampling frequency and sensor height for the tripod deployed at the crest of the shoal, STN_2, during the 2004-2008 study period
Year
2004
2008
System
Instruments
WADMAS
Marsh McBirneyTM Electromagnetic current meters McVanTM Optical Backscatter
PCADP
ParoScientific TM Pressure Sensor SonTekTM Pulse-Coherent Doppler Profiler D&ATM Optical Backscatter DruckTM Pressure Sensor TM RDI ADCP Workhorse 1200 kHz MicroCatTM TS sensors
Sampling Frequency (Hz) 4 4 4 2 2 2 2 -
Sensor height (cm) 29.3, 66.1, 105.5 30.0, 61.5, 103.8 132.8 112 25, 50 112 64 38, 97
2.2 Wave modeling A third-generation wave model, MIKE21 Spectral Wave (hereafter referred to as SW), developed by DHI Water and Environment® was implemented for this study. The SW model has been successfully implemented for the Gulf of Mexico and the Louisiana shelf (Jose and Stone, 2006; Jose et al., 2007) as part of a wave forecasting study. Kobashi et al. (2009c) implemented the model for Ship Shoal, another transgressive shoal located farther to the east of Sabine Bank and Spaziani et al. (2009) implemented it for the Florida Panhandle coast. Detailed model descriptions including the model physics, domain, input parameters, as well as case studies that were implemented are briefly described in the following section. SW is a third-generation spectral wind-wave model based on unstructured meshes. The unstructured mesh approach gives the model a high degree of flexibility. The model solves the wave action balance equation, the spatial discretization of which is performed using an unstructured finite volume method. The integration in time is based on a fractional step approach, where the propagation steps are solved using an explicit method (Sorensen et al., 2004). Description of all the source functions and the numerical methods used in the model are elaborated in Sorensen et al. (2004; 2006).
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Figure 2. Computational mesh used for wave modeling. A high resolution grid for Sabine Bank is meshed with the coastal model The model domain (origin: 95.0º W, 29.0º N) covered Sabine Bank and the entire coastline sheltered by the bank, (see Figure 2). Three bathymetries were used for the computations: one with Sabine Bank and the others with two constructed bathymetries, corresponding to two mining scenarios (Figures 3). The computational grids developed were unstructured triangular mesh grids with an embedded high resolution mesh grid encompassing the shoal boundary. The mesh size was selected based on the degree of grid resolution required to resolve the geomorphologic features, with a maximum size of 1.0 10-5 degree2 over the shoal and 2.5 10-4 degree2 over the surrounding areas. An intermediate mesh with a resolution 5.0 10-5 degree2 was used to connect the two regions. The coastal model was nested with a Gulf of Mexico (GoM) regional wave model (Jose and Stone, 2006) for the model validation. Input parameters were selected from various data sources. For both GoM regional and high resolution coastal models, Re-analyzed North American Regional Reanalysis (NARR) wind data (~ 32 km horizontal resolution), provided by NOAA NCEP, were used (Mesinger et al., 2006). A steady wind friction coefficient of 0.003 was used rather than linearly varying coefficients. For bathymetry, high resolution data (3 arc-second) from the NGDC (National Geophysical Data Center) coastal relief model (Divins and Metzger, 2008) were used. For the GoM model grid development, ETOPO2 bathymetry was also used to cover the southern half of the grid. For the sake of maintaining initial conditions for all model cases consistent, bottom friction for the SW model was estimated from a constant Nikuradse roughness height of 0.04 m, rather than that computed from spatially varying grain size data set. In order to fully comprehend the shoal hydrodynamics associated with various seasons, four case studies were conducted (see Table 2). In this study, based on the annual wave statistics compiled from NDBC buoy 42035, deep water wave conditions were selected as the south-southeast waves (i.e. 160o N). Constant wind fields were included in the domain for varying wind speeds and directions. Four wind directions were selected based on the annual wave climate computed for the region. However, model outputs, corresponding to input wind from the dominant direction (160o N), were provided for further discussion, as listed in Table 2. Deep water wave boundary conditions were applied along the southern boundary and the east and west boundaries were selected as radiative boundaries for the SW model. Wave model results were further analyzed to estimate sediment re-suspension intensity (RI). The model skill assessment is discussed in a later section. Table 2. Case study: Offshore wind and wave boundary conditions T (sec) Wind &Wave Direction Model Wind speed H (m) S P (oN) Case (m/s) A1 15 6 11 160 A2 12 4 9 160 A3 10 3 7 160 A4 5 2 6 160 Note: HS and TP represent significant wave height and peak wave period
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Figure 3. Proposed mining areas designated for Sabine Bank. Yellow polygons over the west and east shoals are designated for a number of beach nourishment projects (cumulative scenario discussed in the paper). The smaller light blue polygons are designated for Holly Beach restoration. The volume and area to be mined are provided by US Minerals Management Service (MMS). 3. Results and discussion 3.1 Bottom boundary layer dynamics and its seasonal varibaility The following section summarizes the meteorological conditions, wave and current fields and the bottom boundary layer parameters during the spring 2004 deployments and it is followed by the data from summer 2008 deployments. Detailed discussion on the impact of cold fronts on the bottom boundary layer dynamics of Sabine Bank were discussed in Kobashi et al., (2005). Figure 4 shows the time series plots of wind speed, wind direction, barometric pressure, and air temperature collected from the nearby SRST2 coastal station, during the spring 2004 deployment period. Winds were usually from the south-east, with the exception of during the cold front passages. Maximum wind speed during the survey was 11.5 m/s on April 21st and at least 10 cold fronts passed over the area during the survey period, according to a criterion set up based on the prevailing wind direction, wind speed and air temperature data. Figure 5 shows time series plots of a stick diagram of current velocity (bottom layer) and significant wave height from the crest of the shoal. Velocities at the middle and bottom layers always were directed north (Figure 5, top). Mean velocity was 7.0 cm/s. Northward dominant peak velocity appeared mostly during the cold fronts and high wind regime. For the onshore station (STN_1), during sustained high wind regimes, the predominant bottom flow was directed offshore. Velocity at the upper layer shows diurnal variations due to tidal influence (data not provided here). Significant wave heights ranged from nearly calm conditions to 2.0 m, in response to either a cold front passage or sustained high wind conditions (see Figure 5, bottom). Mean peak wave period observed during the deployment was 5.2 s. Waves generally propagated from the southeast (150o N). Significant wave
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Coastal Dynamics 2009 Paper No. 146 heights correlate well with the meteorological data from the nearby SRST2 station (see Figure 4). Peak energy spectra due to waves (not shown here) were strongly associated with significant wave height. High frequency waves were detected during the passage of cold fronts and high wind regime, when the wave heights were also naturally high.
Figure 4. Time series of meteorological data from SRST2 coastal station, corresponding to the Spring 2004 deployment at Sabine Bank
Figure 5 Time series stick plots of current velocity and significant wave height at the middle station (Spring 2004). Red triangles indicate the passage of cold fronts. Figure 6 shows the shear velocity and shear stress due to waves computed for the onshore station. The results indicate that many of the spikes of shear stress correspond to high wind speeds. Maximum shear velocity and shear stress due to waves were 2.0 cm/s (mean value = 1.1 cm/s) and 0.45 N/m2 (mean value = 0.12 N/m2), respectively, at the offshore station. Wave induced shear stress at the bottom exceeds the threshold conditions for re-suspension, invariably during every time when a cold front crossed the study area. Similar results were obtained from the other two stations also. The maximum shear velocity and shear stress due to currents was 3.5 cm/s (mean value = 1.9 cm/s) and 1.2 N/m2 (mean value = 0.39 N/m2) on the top of the bank (data not included). This was corroborated using OBS data collected during this period. Time series of OBS data collected from three levels over the shoal crest demonstrated that during the passage of cold fronts and high wind regimes the suspended sediment concentration (SSC) in the water column was higher than during fair weather conditions(Kobashi et al., 2005). Similar findings were reported from Ship Shoal, located farther east (Kobashi et al., 2007a; Kobashi et al., 2009a; Kobashi and Stone, 2009)
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Figure 6. Time series plots of wave induced shear velocity and shear stress at the onshore station. Red triangles indicate the passage of cold fronts. The 2008 deployments were conducted during the summer season (May-July) and were characterized by the calm met-ocean conditions prevailing over the northern Gulf of Mexico. A total of three extratropical fronts passed through the study area during the deployment time. Wind speeds were less than 10 m/s, most of the survey duration, except during early June, when strong winds blew from the southeast due to a high pressure system that prevailed over the U.S. east coast. Figure 7 shows the times series of significant wave height, bottom orbital velocity, OBS and bottom boundary layer parameters during the 2008 survey. In tandem with the occurrence of a high pressure system over the northern Gulf of Mexico, high waves (>1.8 m) were observed during early middle and late June (see Figure 7 top). In early June, wave height exceeded 2 m at the south and crest stations, even though this was not associated with a cold front passage. These high wind and wave conditions were also reported earlier over the northern Gulf (Kobashi et al., 2005; e.g. Pepper, 2000) and appear to be a dominant force during the summer and late spring season.
. Figure 7. Time series of wave orbital velocity (left axis)) and wave height (right axis), (b) shear velocity, (c) turbidity and (d) shear stress, measured at the crest of the shoal during the summer 2008 deployment Waves over the north station were significantly dissipated compared to waves over the south station (data not provided). Waves over the bank crest had little difference or had higher wave height occasionally, with those over the south. This can be attributed to wave shoaling effects. The results also suggest that wave dissipation was high on the lee side of the shoal. Maximal dissipation rates between south and north stations were 73.2%, reported in early June. Similar results have also been reported for Ship Shoal (Kobashi et al., 2007b). During most of the deployment period, low frequency swells, characterized by wave periods higher than 5 seconds, were dominant. During certain time periods, when wave height was low, seas in the higher frequency band were dominant. In early June, 0.7 m/s of near bottom orbital
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Coastal Dynamics 2009 Paper No. 146 velocity was measured, which was associated with high wind speed and high wave height. During this time interval, shear stress due to waves was also high, which was consistent with a sharp increase in lower and upper turbidities (Figure 7c). However, it is interesting to note that the wave and current shear stresses and turbidities were not correlative (Figure 7 lower panels), particularly during mid-June, during which wave height and current speed were both low but turbidity at the upper and lower sensors were both high. For the summer season, since cold fronts do not enter the Gulf of Mexico and therefore, the front passages are infrequent, it has been known that bottom boundary layer dynamics and sediment transport are not significant. However, our results indicate that occasional high winds due to high pressure systems following cold fronts in summer, and significantly influence hydrodynamic and bottom boundary layer dynamics. Also, coastal currents reverse their flow direction in concert with seasonal wind reversal from the southeast to southwest (cf. Cochrane and Kelly, 1986). The dynamics are also affected by the bank bathymetry; the currents are deflected due to the shallow complex bathymetry (Pepper, 2000) and are accelerated over the bank crest due to shallow bathymetry to satisfy continuity (Snedden and Dalrymple, 1999). Results during the 2008 deployments and from Kobashi et al. (2005) suggest that a dominant force that affects bottom sediment suspension are occasional high winds associated with high pressure system following cold fronts. The frequency of this high wind regime is not as frequent as cold fronts, approximately once every two to three weeks; however, hydrodynamic forcing can be as high as winter storms and as a result, influence of this high wind regime on bottom boundary layer is likely significant. Considering these two conditions, waves during fair weather usually cannot re-suspend the sediment, and waves during the cold fronts and high wind regime can sufficiently re-suspend the sediments. Therefore, it is suggested that cold fronts and high wind regimes considerably affect bottom boundary layer dynamics on Sabine Bank. 3.2 Wave transformation over the shoal SW Model validation was conducted using in-situ data collected during the spring 2004 deployments and using archived data from NDBC station 42035. Figure 8 shows comparison between the measured and the simulated significant wave height and peak period at two locations on the shoal. The upper panel corresponds to data from the middle of the shoal whereas the bottom panel in Figure 8 is from the offshore station (see Figure 1 for the locations). In situ wind data are in good agreement with the NARR wind data (data not provided here) except during storm peaks when the NARR wind speed was often lower than the in-situ data, as also reported by Jose et al. (2007). For the spectral wave model, all measured and simulated wave parameters agreed reasonably well. The discrepancy in the model outputs with that of the measured data can be attributed to the resolution and accuracy of the input wind data. It is clear that wave models are extremely sensitive to wind inputs. For a fully developed sea, sensitivity experiments revealed that small errors in the input wind can result in considerable differences in the computation of wave parameters (Sarkar et al., 2000). Shore-advancing deep water waves were significantly transformed as they propagated over complex shoal bathymetry. Six locations, with contrasting bathymetry, were selected from the study area where wave and re-suspension characteristics were computed for comparison (see Figure 1). Station A was located at the extreme western boundary of the shoal and B and D were located at the crests of the western and eastern shoals. Station C was in the Channel, separating the two shoals. Stations E and F were taken as controls, for monitoring the offshore and nearshore conditions. Data from these latter two stations were not presented nor discussed here as the hydrodynamic variability associated with mining scenarios was confined to the vicinity of the shoal. Spatial difference in the wave transformation was similar for all cases although differences in magnitude noticed (see Figure 3 for the bathymetry and orientation of the Sabine Bank). Transformation of significant wave height, peak wave period and mean wave directions for different mining scenarios were plotted for the 4 Case studies (see Table 2). In Figure 9, wave height and wave vector distributions for Case A1 are presented (see Table 2). When the wave height was high, substantial wave refraction on the western shoal was clearly evident compared to that with partial removal of the shoal (Cumulative scenario, Figure 9 middle). As the deep water wave height was low, the difference became less evident. On the eastern shoal the difference in the refraction with partial removal of the shoal was minimal. The alteration in wave conditions associated with the Holly Beach mining scenario was unnoticeable for both the western and eastern shoals (Figure 9 bottom). The wave height on the western shoal was lower than that on the eastern shoal (up to 15 percent difference between the east and west). When the shoal existed, the difference was up to 3 percent higher than the difference with partial removal of the shoal
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Coastal Dynamics 2009 Paper No. 146 (cumulative scenario). The spatial difference in wave height decreased as the deepwater wave height decreased. Wave height between the two shoals, as a general trend, increased from the west toward the east with the percentage of difference decreasing with decreasing offshore wave conditions. The dissipation in the wave height on the western shoal was approximately 12 percent higher than that on the eastern shoal; while, for the model result with partial mining (cumulative scenario), the difference in the height was just 2 percent lower. The dissipation rate progressively decreased for lower energy conditions. The above results indicate that the shoal has significant influence on wave energy dissipation; however, the impacts of restricted mining on wave refraction, will likely be minimal. The computed model results further influence sediment re-suspension on the shoal, which is discussed in the following section.
Figure 8. Simulated wave parameters for Sabine Bank plotted against measured data from the spring 2004 deployments. Top figures correspond to wave observations from the crest of the shoal and the bottom figures correspond to observations from an offshore location. The measurement locations are given in Figure 1 Changes in sediment re-suspension have strong implications for sediment transport and bed characteristics. Sediment re-suspension intensity (RI), defined as wave-induced shear stress minus the critical shear stress for re-suspension, was computed from the simulated bulk wave parameters and bed sediment characteristics, following the procedures discussed in Kobashi et al., 2009c. Due to the scarcity of sediment data from the shoal, a mean grain size of 0.2 mm (Dellapenna et al., 2006) was used for computing the threshold shear stress for the entire shoal. The computed RI values, corresponding to case A1 is provided in Figure 10. The RI is related to wave height, wave period and water depth; the higher the wave height and the shallower the depth, the higher the RI is. When storms were strong (i.e. case A1 and A2, in Table 2), the RI was high across the domain, but was higher at Stn A (Figure 10) and Stn D than on the shallowest station on the western shoal (Stn B), due to wave dissipation over the southward flank of western shoal. As wave energy decreased, a general trend existed in the RI on the shoal which decreased from the west to east following the change in the shoal bathymetry (data not provided). For Case A4 (moderate storms), the RI on the western shoal was twice as high as that on the eastern shoal and approximately 2.5 times as high as that for the western most boundary of Sabine Bank (Stn A). For fair weather conditions, the RI was positive on the western shoal (Stn B) and was negative at Stns (A, C & D). The negative values are characterized by sediment deposition, suggesting, except for the crest of the western shoal, no sediment re-suspension occurs during fair weather conditions. The results were corroborated using in-situ measurements during the summer 2008 deployments as discussed in an earlier section. Results from the RI computations pertaining to two partial mining case studies (Cumulative scenario as
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Figure 9. Significant wave height distribution over Sabine Bank for three different bathymetries (top): present condition, (middle): Cumulative mining scenario, (bottom): Holly Beach restoration
project. The area and volume for targeted mining were provided by U.S. Minerals Management Service (MMS)
Figure 10. Re-suspension intensity (RI) computed for the 4 stations (see Figure 1 for the station reference). The met-ocean boundary conditions corresponding to case A1 have been used for computing the bulk wave parameters.
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Coastal Dynamics 2009 Paper No. 146 well as Holly Beach restoration case) indicate that the variability in wave and coupled sediment transport interactions were insignificant. The reduction in RI values at the western shoal is significant for moderate storm conditions (A4). For the eastern shoal, the Holly Beach restoration scenario did not alter the RI values. It is summarized that the alteration in RI is insignificant for the two mining scenarios. 4. Conclusions Wave bottom interaction and bottom boundary layer dynamics for Sabine Bank were summarized in this study, based on field surveys conducted during 2004 and 2008. MIKE 21 SW model was implemented and validated using different data sets. The possible impacts of targeted mining on the wave field were quantified numerically, based on two hypothetical mining scenarios. The following conclusions were drawn based on our field observations; Waves were weak and did not re-suspend sediment during fair weather conditions. Currents were sufficiently strong to re-suspend sediment during the entire period except for during fair weather conditions. Wave and bottom boundary layer interactions were strongly associated with the passage of cold fronts across the region. Strong southerly/southeasterly wind regimes also affected the wave and the bottom boundary layer interactions during the observation period. During summer 2008, bottom boundary layer dynamics were significantly influenced by the high wind regime associated with high pressure system when it covered the eastern Gulf and U.S. East Coast. The high wind regime, during fair weather, resulted in high wave heights and relatively strong currents over the bank. Frequencies associated with this high wind regime were approximately every 2 to 3 weeks. In summary, it is evident that while waves are not an important factor for sediment transport during fair weather conditions, waves generated during cold fronts and strong southerly/southeasterly wind regimes affect sediment transport significantly. Currents are effectively strong enough to re-suspend sediment for the entire period, except during fair-weather. Therefore, it is evident that the cold fronts and strong wind regimes are important to the bottom boundary layer dynamics of Sabine Bank. Whereas, in summer, the strong wind regime seems to be a dominant force for bottom boundary layer dynamics and sediment transport. The wave modeling studies allow us to make the following conclusions: The spectral wave model (MIKE 21 SW) performed well for the study area. Variations in bulk wave parameters, due to modified bathymetry for the two mining scenarios (cumulative and Holly Beach restoration) were not very significant. Sediment re-suspension intensity (RI) was high over the inner shelf and shoal during severe and strong storms. During severe storm conditions, Higher RI values were reported from the eastern shoal and elsewhere, indicating the level of wave energy dissipation over the western flank. During moderate storm conditions, the RI decreased from the shallowest western flank of the shoal to it’s deeper eastern portion. The RI off the shoal was significantly lower than on the shoal. The RI with partial mining was insignificantly lower than with the shoal present. The data presented here allow us to conclude that targeted sand mining (partial removal of the Sabine Bank) would not significantly alter the hydrodynamics over the shoal, particularly waves and associated wave-induced sediment re-suspension and currents. Therefore, the utilization of material from the studied site has a very high potential for use in restoring the adjacent coast. Acknowledgements The study was supported by U.S. Minerals Management Service, Dept. of the Interior. Authors would like to acknowledge the help from the CSI Field Support Group for extensive field deployments. DHI Water and Environment® is acknowledged for providing the wave and hydrodynamic models. Yuliang Chen, WAVCIS Lab, LSU, assisted with cartography. Comments from Amy Spaziani, Coastal Studies Institute are also appreciated. References Cacchoine, D.A., Sternberg, R.W. and Ogston, A.S., 2006. Bottom instrumented tripods: History, Application and
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Coastal Dynamics 2009 Paper No. 146 Impacts. Continental Shelf Research, 26: 2319-2334. Chan, A.W. and Zoback, M.D., 2007. The role of hydrocarbon production on land subsidence and fault reactivation in the Louisiana coastal zone. Journal of Coastal Research, 23(3): 771-786. Cochrane, J.D. and Kelly, F.J., 1986. Low-Frequency Circulation on the Texas-Louisiana Continental-Shelf. Journal of Geophysical Research, 91(C9): 10645-10659. Dellapenna, T.M., Hiller, S., Fielder, B., Majzlik, E. and Noll, C.J., Jr., 2006. Sabine Bank side scan investigation, 27 pp. Divins, D.L. and Metzger, D., 2008. NGDC Coastal Relief Model. Green, M.O., 1992. Spectral Estimates of Bed Shear-Stress at Subcritical Reynolds-Numbers in a Tidal BoundaryLayer. Journal of Physical Oceanography, 22(8): 903-917. Jose, F. and Stone, G.W., 2006. Forecast of nearshore wave parameters using MIKE 21 spectral wave model. Transactions, Gulf Coast Association of Geological Societies, 56: 323-327. Jose, F., Kobashi, D. and Stone, G.W., 2007. Spectral wave transformation over an elongated sand shoal off south central Louisiana, USA. Journal of Coastal Research, SI 50: 757-761. King, D.B., Jr., 2007. Wave and beach processes modeling for Sabine Pass to Galveston Bay, Texas, Shoreline erosion feasibility study, Engineer Research and Development Center, USACE, ERDC/CHL TR-07-6, 164 pp. Kobashi, D., Jose, F. and Stone, G.W., 2005. Hydrodynamics and sedimentary responses within bottom boundary layer: Sabine Bank, western Louisiana. Transaction, Gulf Coast Association of Geological Societies, 55: 392-399. Kobashi, D., Jose, F. and Stone, G.W., 2007a. Impacts of fluvial fine sediments and winter storms on a transgressive shoal, off south-central Louisiana, U.S.A. Journal of Coastal Research, SI 50: 858-862. Kobashi, D., Jose, F. and Stone, G.W., 2007b. Heterogeneity and dynamics of sediments on a shoal during springwinter storm season, south-central Louisiana, USA. Proceeding of Coastal Sediments ’07, pp 921-934. Kobashi, D., Jose, F., Luo, Y. and Stone, G.W., 2009a. Wind-driven dispersal of fluvial fine sediments for two contrasting storms: extra-tropical and tropical storms, Atchafalaya Bay-Shelf, Louisiana. Marine Geology: In Review. Kobashi, D. and Stone, G.W., 2009. Two contrasting morpho-hydrodynamics over recurring sandy and muddy bottoms of a shore-parallel Holocene transgressive shoal, south-central Louisiana, USA. Continental Shelf Research: In Review. Kobashi, D., Stone, G.W., Khalil, S.M. and Kerper, D.R., 2009b. Impacts of sand removal from a shore-parallel Holocene transgressive shoal on hydrodynamics and sediment transport, south-central Louisiana, USA. In Preparation. Mesinger, F., DiMego, G., Kalnay, E., Mitchell, K., Shafran, P.C., Ebisuzaki, W., Jovic, D., Woollen, J., Rogers, E., Berbery, E.H., Ek, M.B., Fan, Y., Grumbine, R., Higgins, W., Li, H., Lin, Y., Manikin, G., Parrish, D. and Shi, W., 2006. North American regional reanalysis. Bulletin of the American Meteorological Society, 87(3): 343360. National Research Council (NRC), 2006. Drawing Louisiana’s New Map, Addressing Land Loss in Coastal Louisiana. National Academies Press, Washington, DC, 190 pp. Penland, S., Connor, P.F., Beall, A., Fearnley, S. and Williams, S.J., 2005. Changes in Louisiana's shoreline: 1855-2002. Journal of Coastal Research, 44: 7-39. Pepper, D.A., 2000. Hydrodyanmics, Bottom Boundary layer processes and sediment transport on the south-central Louisiana inner shelf: The influence of extra-tropical storms and bathymetric modification. Ph.D. Dissertation Thesis, Louisiana State University, Baton Rouge, LA, 159 pp. Pepper, D.A. and Stone, G.W., 2004. Hydrodynamic and sedimentary responses to two contrasting winter storms on the inner shelf of the northern Gulf of Mexico. Marine Geology, 210(1-4): 43-62. Sarkar, R.K.A., Aggarwal, V.K., Bhatt, V., Bhaskaran, P.K. and Dube, S.K., 2000. Ocean wave model- sensitivity experiments. Proceeding of International Conference PORSEC-2000 NIO, Goa, India. Vol II, pp 621-627. Snedden, J.W. and Dalrymple, R.W., 1999. Modern shelf sand ridges: From historical perspective to unified hydrodynamic and evolutionary model, SEPM Special Publication 64, pp. 13-28. Sorensen, O.R., Kofoed-Hansen, H., Rugbjerg, M. and Sorensen, L.S., 2004. A third-generation spectral wave model using an unstructured finite volume technique. Proceeding of International Conference on Coastal Engineering 29, pp 894-906. Sorensen, O.R., Kofoed-Hansen, H. and Jones, O.P., 2006. Numerical modeling of wave-current interaction in tidal areas using an unstructured finite volume technique. Proceeding of International Conference on Coastal Engineering (ICCE), San Diego, California, USA. Vol 1, pp 653-665. Spaziani, A.L., Jose, F. and Stone, G.W., 2009. Sediment dynamics on an innershelf shoal during storm events in the northeastern Gulf of Mexico. Journal of Coastal Research: Submitted. Stone, G.W., 2000. Wave climate and bottom boundary layer dynamics with implications for offshore sand mining and barrier island replenishment in south-central Louisiana, 90 pp. Stone, G.W., Zhang, X.P., Gibson, W. and Frederichs, R., 2001. A new wave-current online information system for oil spill contigency planning (WAVCIS). Proceeding of 24th Arctic and Marine Oil spill Program Technical Seminar, Edmonton, Alberta, CANADA. pp 401-425. Stone, G.W., Zhang, X., Li, J. and Sheremet, A., 2003. Coastal Observing Systems: Key to the future of coastal dynamics investigation. GCAGS/GCSSEPM Transactions, 53: 783-799.
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Coastal Dynamics 2009 Paper No. 146 Stone, G.W., Kobashi, D., Jose, F., Liu, B., SiadatMousavi, S.M. and Spaziani, A., 2009. Wave-bottom interactions and bottom boundary layer dynamics in evaluating sand mining at Sabine Bank for coastal restoration, southwest Louisiana, U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA, OCS Study MMS 2009, 147 pp. USGS, 1995. Fact sheet.
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