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SENSITIVITY TESTS ON SEABED SEDIMENT ERODIBILITY OF THE TEXAS- LOUISIANA CONTINENTAL SHELF Rangley Mickey 1 ([email protected]), Kehui Xu 1 , Courtney Harris 2 , Robert Hetland 3 , James Kaihatu 3 1 Coastal Carolina University; 2 Virginia Institute of Marine Science; 3 Texas A&M University 2. Background 7. Preliminary Findings and Future Work 6. Sediment Erodibility Measured Using Gust Microcosm System 5. Maximum Erosional Depth During the Storm in March 1993 Reference Dickhudt, P.J., Friedrichs, C.T., Schaffner, L.C., Sanford, L.P., 2009. Spatial and temporal variation in cohesive sediment erodibility in the York River estuary, eastern USA: a biologically influenced equilibrium modified by seasonal deposition. Marine Geology 267, 128–140. Harris, C.K., Xu, K., Sherwood, C., Fennel, K., Hetland, R., 2010. Coupling sediment dynamics and biogeochemical models within ROMS with application to the Louisiana – Texas shelf. Poster presented at the Community Surface Dynamics Modeling System (CSDMS) All-Hands Meeting, San Antonio, TX, USA. Rinehimer, J.P., Harris, C.K., Sherwood, C.R., and Sanford, L.P., 2008. Estimating cohesive sediment erosion and consolidation in a muddy, tidally-dominated environment: model behavior and sensitivity. Estuarine and Coastal Modeling, Proceedings of the Tenth Conference, November 5-7, Newport, RI. Warner, J.C., Sherwood, C., Signell, R.P., Harris, C.K. and Arango, H.G., 2008. Development of a three-dimensional, regional, coupled wave, current, and sediment-transport model. Comput. Geosci. 34, 1284– 1306. Xu, K.H., Harris, C.K., Hetland, R.D., Kaihatu, J. M., 2011a. Dispersal of Mississippi and Atchafalaya Sediment on the Texas-Louisiana Shelf: Model Estimates for the Year 1993, Continental Shelf Research, 31, 1558–1575. doi:10.1016/j.csr.2011.05.008. Xu, K.H., Briggs, K.B., Cartwright, G.M., Friedrichs, C.T., Harris, C.K., 2011b. Spatial and Temporal Variations of Sea Bed Sediment Erodibility on the Texas-Louisiana Shelf and Their Implications to the Formation of Hypoxic Water, 21st Biennial Conference of the Coastal and Estuarine Research Federation Societies, Daytona Beach, FL. Acknowledgement This work was funded by the National Science Foundation and NOAA Center for Sponsored Coastal Ocean Research (NA03NOS4780039 and NA06NOS4780198), with additional support provided by the US Department of the Interior, Minerals Management Service under Cooperative Agreement No. M07AC12922. We thank the model development by Community Sediment Transport Modeling System (CSTMS), and discussions with John Warner and Chris Sherwood (USGS). In addition, we thank Drs. Jeffress Williams (USGS) and Chris Jenkins (INSTAAR, University of Colorado) for sharing sea bed grain size data, as well as Drs. Charles Demas and Robert Meade (USGS) for providing the water and sediment discharge data. 3. ROMS Model Setup and Shear Stresses (AGU Ocean Science, Salt Lake City, UT, February, 2012, CMWS, CCU) Fig. 1 (left) SeaWiFS satellite image of the Gulf coast, specifically around Louisiana where the Atchafalaya and Mississippi rivers enter the Gulf. This image was taken on 12/14/1998 and downloaded from NASA visible Earth website. Fig. 2 (Top) Model grid for Northern Gulf of Mexico used in the ROMS; 3 sites to be analyzed: Atchafalaya Bay, Mississippi River mouth, and Mid Hypoxic Zone (Station 10B) . Fig. 3 (Top) Initial sediment compositions across Northern Gulf of Mexico New Orleans Atchafalaya River Mississippi River 4. Time Series Seabed Elevation Changes in the Year 1993 1. Abstract Sediment re-suspension and transport are controlled by hydrodynamic conditions and seabed erodibility, and have important implications to coastal processes and benthic ecosystems. A sediment transport model for the Texas-Louisiana continental shelf was developed to test the sensitivity of seabed sediment erodibility under various oceanographic conditions. The Regional Ocean Modeling System (ROMS) model includes winds, river discharge, waves derived from a Simulating WAves Nearshore (SWAN) model, and spatially-variable sea bed conditions. Freshwater and sediment discharge measurements from the Mississippi and Atchafalaya rivers are incorporated in the model. Six sediment tracers are used to perform the sensitivity tests of settling velocities and critical shear stress of sediment on the Texas-Louisiana shelf during fair-weather and storm conditions. Sea floor sediment erosion/deposition are calculated during multiple events. Measured field sediment erodibility data from a Gust Erosion Microcosm System are being applied into the sea bed model to represent more realistic sediment dynamics and to reveal the possible sediment impact on the formation of hypoxic events in the northern Gulf of Mexico. The SeaWiFS satellite image (Fig. 1) shows the scale of the Mississippi River sediment dispersal system, and its impact along the coast. Implemented in Regional Ocean Model System (ROMS; Warner et al., 2008), the model domain covered an area of 800 km x 300 km. Fig. 2 shows the model grid domain and indicates three sites that will be analyzed for seabed sensitivity tests. The blue circle indicates the Atchafalaya Bay region, the magenta circle indicates the area in close proximity to the Mississippi river mouth, and the green circle represents an area that is located in the middle of the observed hypoxic zone during summer seasons. The green circle actually represents one of the 24-hour stations (10B) that is analyzed by the NOAA–funded Mechanisms Controlling Hypoxia (MCH) process cruises. Fig. 3 represents the initial sediment types for each model run. 94 93 92 91 90 89 88 28 29 30 Longitude (degree) Latitude (degree) Tau=0.025 Pa Atchafalaya River Mississippi River 10m 20m 50m 100m 300m 94 93 92 91 90 89 88 28 29 30 Longitude (degree) Latitude (degree) Tau=0.05 Pa Atchafalaya River Mississippi River 10m 20m 50m 100m 300m 94 93 92 91 90 89 88 28 29 30 Longitude (degree) Latitude (degree) Tau=0.075 Pa Atchafalaya River Mississippi River 10m 20m 50m 100m 300m 94 93 92 91 90 89 88 28 29 30 Longitude (degree) Latitude (degree) Tau=0.10 Pa Atchafalaya River Mississippi River 10m 20m 50m 100m 300m 94 93 92 91 90 89 88 28 29 30 Longitude (degree) Latitude (degree) Tau=0.15 Pa Atchafalaya River Mississippi River 10m 20m 50m 100m 300m 94 93 92 91 90 89 88 28 29 30 Longitude (degree) Latitude (degree) Tau=0.20 Pa Atchafalaya River Mississippi River 10m 20m 50m 100m 300m Pa -7 -6 -5 -4 Log 10 (m) For the model runs of 1993, the changes in seabed elevation were analyzed for the three sites plotted in Fig. 2. The time series plots to the right were generated to analyze how sensitive the seabed sediment is to varying levels of critical shear stress. Fig. 6 illustrates the changes in seabed thickness at three sites that occur for each different critical shear stress level. There is more deposition occurring at the areas close to the two river mouths. At the mouth of the Mississippi River the amount of deposition was greatest due to proximity to discharge source. The black box that runs through Fig. 6 represents the energetic storm period observed in March 1993. Fig. 7 is a zoom-in illustration of the changes to seabed elevation during that storm period from March 12 through March 17, 1993. The maximum change in seabed elevation occurs during this period, and is most pronounced for the critical shear stress 0.025 Pa for each area. This observation was expected due to the very low shear stress levels needed to re-suspend sediment material. Near the Mississippi river mouth, about 2 cm of seabed erosion occurred during the peak of the storm. Fig 6. Time series of seabed elevation changes for the entire year 1993 using 6 different critical shear stress levels. The box going through all 3 time-series indicates storm period in March 1993. Sediment Type τcr (Pa) Ws (mm/s) Fraction Mississippi Small flocs Same critical shear stress for each sediment type Models ran at: 0.025 Pa, 0.05 Pa, 0.075 Pa, 0.10 Pa, 0.15 Pa, and 0.20 Pa 0.1 50% Large flocs 1 50% Atchafalaya Small flocs 0.1 50% Large flocs 1 50% Sea bed Sand 10 Spatially Variable Mud 1 1993 1994 0.4 0.45 Station 10b 1993 1994 0.4 0.45 Seabed Thickness (cm) Atchafalaya Bay 1993 1994 0.4 0.45 Time Miss. River Mouth 03/12 03/13 03/14 03/15 03/16 03/17 0.38 0.39 0.4 Station 10b 03/12 03/13 03/14 03/15 03/16 03/17 0.38 0.39 0.4 Seabed Thickness (cm) Atchafalaya Bay 03/12 03/13 03/14 03/15 03/16 03/17 0.38 0.39 0.4 Time Miss. River Mouth 94 93 92 91 90 89 88 28 29 30 Atchafalaya River Mississippi River 10m 20m 50m 100m 300m Longitude (degree) Latitude (degree) Fig. 2 Atch. Bay Mid Hypoxic Zone (Station 10B) Miss. River Mouth 0 20 40 60 80 100 -94 -93 -92 -91 -90 -89 -88 27.5 28 28.5 29 29.5 30 30.5 Sediment type, mud% longitude latitude 20m 50m 100m 300m Sandy Muddy 03/16 03 0.25Pa .05Pa .075Pa .10Pa .15Pa .20Pa Fig. 7. Time series (Month/Day) of seabed elevation changes for the storm period in March 1993 The ROMS model was set up using the initial seabed and sediment discharge conditions that are presented in Table 1. There were 6 model runs for 1993 that were based on differing critical shear stresses for re-suspension that ranged from 0.025 Pa to 0.20 Pa; all other conditions were held constant for each model run. Fig. 4 shows the wind speed, wave height, and river water and sediment discharges in the year 1993. In Fig. 5, the left panels show the ranges for shear stress generated in the model throughout the entire year 1993 at the three sites. The right panels of Fig. 5 represent the frequency of those shear stresses observed throughout the year. These conditions were constant throughout all model runs. Table 1. Initial seabed and river discharge conditions for all model runs; Critical shear stress differs for each model run. Fig.4 (Left) (A) Wind speed (B) Wave height (C) River Water Discharge and (D) River Sediment Discharge in the year 1993. From Xu et al. (2011a). Fig. 5 (Below) A) Observed critical shear stress throughout 1993 at each site. B) Modeled frequency of critical shear stress at different ranges (0-0.04 Pa, 0.04-0.08Pa, and so on). Gust Erosion Microcosm System were used to measure the profiles of eroded mass vs. shear stress in the northern Gulf of Mexico (Figs. 9 and 10). The photograph in Fig. 9 below illustrates the entire Gust System setup and the filtration device used to filter solid particles from the water samples produced from the Gust chamber. This experimental setup allows for shear stress manipulation from a laptop to be directly applied to the rotating heads that spin the water above the sediment and cause re-suspension. The re-suspended material is then transferred by hoses through a turbidimeter, for turbidity measurements, and then into bottles. The water samples are filtered through pre-weighed filters and dried for weight measurements. The figures below compare Gust experimental results from the Gulf of Mexico (left) and Chesapeake Bay (right). It seems that the sediment on the northern Gulf of Mexico is less erodible than that in the Cheaspeake Bay, especially much lower than that in the turbidity maxima of the York River, VA (Fig. 11; Dickhudt et al., 2009) Fig. 9 (Above) Photograph of Gust Erosion Microcosm System on R/V Pelican Fig. 10 Profiles of eroded mass and shear stress collected in Northern Gulf of Mexico (Xu et al., 2011b). Fig. 11 Profiles of eroded mass and shear stress collected in Chesapeake Bay (Dickhudt et al., 2009). A B Fig. 5 Table 1 The maximum erosional depth over the entire model grid for the March 1993 storm event is illustrated below in Fig. 8 for six critical shear stress levels. The erosional depths (m) are in the log scale and are represented by different colors. Referring to the color bar indicates that areas that are shaded red have a larger erosional depth during the storm event than areas that are shaded in blue. Using the log scale allows us to see that areas across the Texas-Louisiana shelf have different sensitivity levels to re-suspension by multiple orders of magnitude, even when the critical shear stress is the same for all sediment material. At first glance there does not seem to be much difference in the erosional depths for each of the critical shear stresses modeled; however, it can be observed that there is a more pronounced color difference in areas of high erosion when comparing the lowest (0.025Pa) and highest (0.2 Pa) critical shear stresses. While the erosional depth changes due to shear stress are relatively small (0.1-0.001mm), the areal extent of these changes is fairly large. As all the figures indicate, areas along the 20 m isobath are where the most erosion and re-suspension of material occurred during this major storm event. Shear stress frequency histograms (Fig. 5) indicate >90% of time the combined wave-current shear stress is less than 0.2 Pa. The site in the middle of hypoxic area (station 10b, 20m deep) has the lowest shear stress whereas the Atchafalaya site (5m deep) has relatively higher stresses. The time series for 3 different sites showed that major changes in the seabed elevation only occur at areas closer to river sources and during major storm events. The maximum erosional depth during this storm event seems not to be very sensitive to different shear stresses (Fig. 8), but did vary dramatically across the Texas-Louisiana shelf. Future work for improving the ROMS modeling system will be to analyze the sediment composition at different sites across the Texas-Louisiana shelf to formulate more accurate seabed conditions; also by incorporating the seabed consolidation model (Rinehimer et al., 2010) and the biogeochemical model being developed by (Harris et al., 2010). There will also be a more focused attempt at quantifying the percent of sediment and organic matter that accumulates into a ‘fluff’ layer along water-sediment interface. Fig. 8 Maximum erosional depth for six critical shear stress levels during March 1993 storm event. Fig. 3 0 0.2 0.4 0.6 0.8 1 0 20 40 60 80 100 Shear Stress Range (pa) Frequency (%) 01/01 01/01 0 0.5 1 1.5 2 2.5 3 3.5 4 Shear Stress (Pa) Middle Hypoxic Region Middle Hypoxic Region 01/01 01/01 0 0.5 1 1.5 2 2.5 3 3.5 4 Shear Stress (Pa) 0 0.2 0.4 0.6 0.8 1 0 20 40 60 80 100 Shear Stress Range (pa) Frequency (%) Atchafalaya Bay Atchafalaya Bay 0 0.2 0.4 0.6 0.8 1 0 20 40 60 80 100 Shear Stress Range (pa) Frequency (%) 01/01 01/01 0 0.5 1 1.5 2 2.5 3 3.5 4 Month/Day of the year 1993 Shear Stress (Pa) Mississippi River Mouth Mississippi River Mouth Abstract ID: 11965 Fig. 1 Fig. 4 .025Pa Seabed Thickness (m) Seabed Thickness (m) Fig. 9 Fig. 11 Fig. 10 Aug 10 Apr 11 Aug 11
