Satellite-Based Assessment of Sediment Transport, Distribution and Resuspension Associated with the Atchafalaya River Discharge Plume

By

WALKER, NAN, ROBERTS, HARRY, HUH, OSCAR, ROUSE, LARRY, INOUE, MASAMICHI, WELSH, SUSAN, BENTLEY, SAMUEL, STONE, GREGORY, SHEREMET, ALEXANDRU, and HSU, S.A.

Coastal Studies Institute and Department of Oceanography and Coastal Sciences, Louisiana State University, Howe-Russell Geoscience Complex, Baton Rouge, LA,

MYINT, SOE

Department of Geography, University of Oklahoma, Norman, OK

The Atchafalaya River is discharging over 80 ´ 10⁶ tons of sediment annually onto the broad shallow continental shelf of central and western Louisiana. Satellite imagery from the NOAA AVHRR, Orbview-2 SeaWiFS and Terra MODIS are used in this paper to quantify suspended sediment concentrations and to assess sediment transport processes along the Louisiana shelf under varying conditions of river discharge and wind forcing. The image data reveal the main sources of sediment, direction of transport and regional extent of wind-wave resuspension. The prevailing easterly winds transport much of the suspended sediments westward towards the Chenier Plain in a well-defined mud stream. Flow rates to the west as high as 50 cm/s (43 km/day) have been measured, yielding a transit time of about 1.5 days from the mouth of Atchafalaya Bay to the Chenier Plain. Progradation rates along the Chenier Plain coast reach 50 m/year. The westward-flowing Atchafalaya “mud stream” is rapidly disrupted by westerly winds and northerly winds, accompanying frequent winter storms and less frequent tropical storms or hurricanes. During these events, the coastal current reverses and sediments are rapidly transported out of Atchafalaya Bay and offshore where substantial sedimentary deposits can also be found. Offshore sediment fluxes during storm events in combination with wind-wave resuspension can result in surface sediment “plumes” extending 70 km offshore and 150 km alongshore.

Field measurements of suspended sediment concentrations, water column transmittance, and optical backscatter are used to assess the vertical distribution of sediments on the shelf. These combined processes are extending the prodelta deposits of the Atchafalaya-Wax Lake delta complex far onto the continental shelf and supplying sediments for a renewal era of progradation along the downdrift Chenier Plain coast.