Master of Science (MS)
Geology and Geophysics
The Mississippi River has undergone at least seven cyclic avulsions during the Holocene epoch. The latest avulsion, down the Atchafalaya River into the Gulf of Mexico, has produced two bayhead deltas prograding into Atchafalaya Bay. The Wax Lake Delta, typical of other Mississippi sub-deltas, has a natural anastomosing channel pattern. In contrast, the Atchafalaya Delta, situated in the eastern side of the Bay, has experienced sporadic and limited growth due to the dredging of a navigation channel below natural depth. Channel bifurcation, and sediment transport processes and responses, were investigated in the Wax Lake Delta, using channel flow velocities, suspended sediment concentrations, cross-channel bottom profiles, and short push-core stratigraphy during flood and non-flood conditions. Center channel flow velocities averaged 2 to 2 1/2 times higher during flood conditions than during non-flood conditions. Velocities maintained near constant values from proximal to distal, then decreased near distributary channel mouths. Cross-channel flow velocities reached a maximum above the thalweg. During non-flood conditions, flow velocities, inversely proportional to tidal fluctuations, were greatly reduced during strong southerly winds; however, tidal and wind influences were negated by flood condition flow velocities. Homogeneous suspended sediment concentrations of coarse silt to very fine sand (mean grain size) were found throughout the system, indicating well-mixed, turbulent flow. Suspended sediment concentrations were up to 20 times higher during flood than during non-flood conditions. Most calculated boundary shear stresses were greater than critical boundary shear stresses, indicating little deposition was occurring in distributary channels during sample collection. Bedload sediment size remained near constant throughout the system in all samples from proximal to distal end, indicate sediment moves efficiently through the deltaic system with very little grain size fractionation in suspended or bedload sediments. Downstream sediment fluxes vary directly with velocity. Thus, the thalweg transports the highest volume of sediment per unit time even though the sediment concentrations per unit volume are homogeneous. Sediment deposition per unit time is greatest at the distributary mouth channel thalweg, where velocities slow, creating a distributary mouth bar and subsequent channel bifurcation. This process has been termed sediment flux controlled deposition.
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DuMars, Anton J., "Distributary mouth bar formation and channel bifurcation in the Wax Lake Delta, Atchafalaya Bay, Louisiana" (2002). LSU Master's Theses. 1856.
Arnold H. Bouma