Effect of Channelized and Unchannelized Lateral Outflow on Three-Dimensional Flow Structure and Sediment Transport Mechanisms in a River Delta

Spatial and temporal patterns in three-dimensional flow structure have been linked to channel morphology and processes in many environments, including river meander bends, confluences-diffluences, and bedrock canyons. However, there is not yet an understanding of how channelized and gradual, distributed lateral outflows that are often prevalent in deltaic distributary systems influence three-dimensional flow structure and sediment transport mechanisms. This thesis presents an analysis of 3D flow structure data collected from Wax Lake Delta, a naturally developing river-dominated delta in the northern Gulf of Mexico. Three hydrographic surveys were conducted using boat-mounted acoustic Doppler current profiler (ADCP) at two sites: an asymmetrical bifurcation and a distributary channel experiencing distributed lateral outflow. Flow structure data from these surveys were investigated to identify secondary circulation cells induced by lateral outflow, which may influence the sediment transport to the islands. Spatial patterns in flow structure were also compared to previous numerical modeling and experimental studies on open channel diversions and compound channels to define the preconditions behind the formation of these secondary cells. The results are then used to develop a conceptual model linking the formation of secondary circulation cells and suspended sediment transport from the distributary channels to interdistributary islands in a delta. The results suggest the mechanisms of sediment transport into the islands from the channels may depend on a threshold momentum flux ratio per unit length of outflow value of which lies in between 0:211km-1 and 0:344km-1. This study provides the first detailed quantification of flow structure in an actively prograding river delta and offers important implications for coastal restoration by linking coastal sediment transport mechanism to patterns in flow structure