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Grad student talk - Numerically studying the influences of extreme events on delta morphology

Thursday, March 05, 2015, 4:30PM - 5:30PM


Fei Xing


RL-1 room 269

Full title

Tropical cyclones (e.g. hurricanes) and winter cold fronts are the two major weather systems influencing morphology of the coastal wetlands along the Gulf of Mexico. Delft3D was applied to the Wax Lake Delta (WLD), Louisiana, to study the impact of coastal storms with different magnitudes and frequencies on wetland morphology. The simulations show that Hurricane Rita, which made landfall 120 km to the west of WLD as a Category 3 hurricane in 2005, has a significant impact on the delta where more than 500,000 m3 of bottom sediment is eroded. While the simulated lartest cold front in 2008–2009 season (mean wind speed of 11.4 m/s) causes net erosion of 100,000 m3. Although the hurricane event causes more sediment transport, many similarities exist between the two events: winds and waves significantly increase the amount of sediment transport; erosion occurs on islands; negative sediment balance for the WLD system (erosion). The evaluation of different factors (winds and waves, hurricane track, aboveground vegetation, roots, and fluvial discharge) on delta morphology during hurricane simulation demonstrates that:

  1. The wind generated flow and waves are the major driving forces of morphological changes, so the northwestern flow pattern causes erosion on the eastern banks of the deltaic islands.
  2. Hurricane tracks are critical in controlling sediment transport through determining the wind fields.
  3. Densely distributed aboveground vegetation slows floodwater propagation and decreases flow velocity on islands, significantly decreasing the amount of erosion on islands.
  4. Roots and fluvial discharge slightly change the sediment balance during hurricane simulation.

Although a single hurricane event causes larger amount of sediment erosion on the WLD system than a cold front, the comparison of yearly accumulative sediment balances show that for an active hurricane year (2008) two hurricanes (Gustav and Ike) cause erosion of 500,000 m3 on the WLD, much less than the total amount of erosion caused by 29 cold front events (1,900,000 m3) with available winds (in total 41 events are recognized), so cold fronts play a larger role than hurricanes in the morphological changes of the WLD system. The simulations of salinity expansion show that saline water intrusion threatens the survival of freshwater species on the WLD during hurricane events but would not influence the species during most cold front events.