EFFECT OF LATERAL OUTFLOW ON THREE-DIMENSIONAL FLOW STRUCTURE IN A RIVER DELTA

27 Spatial and temporal patterns in three-dimensional flow structure have been linked to 28 channel processes and morphology in many environments, including river meander bends, 29 confluences-diffluences, and bedrock canyons. However, there is not yet an understand30 ing of how channelized and gradually distributed lateral outflows that are often preva31 lent in river deltas influence three-dimensional flow structure and sediment transport mech32 anisms. This study presents an analysis of 3D flow structure data collected from Wax 33 Lake Delta, a naturally developing river-dominated delta in the northern Gulf of Mex34 ico. Three hydrographic surveys were conducted using a boat-mounted acoustic Doppler 35 current profiler (ADCP) at two sites: a channelized outflow zone and a distributary chan36 nel experiencing distributed lateral outflow. The flow structure was analyzed to iden37 tify secondary circulation cells induced by lateral outflow, which may influence the sed38 iment transport to the islands. Spatial patterns in flow structure were also compared to 39 previous numerical modeling and experimental studies on open channel diversions and 40 compound channels. A conceptual model is developed linking the formation of secondary 41 circulation cells and suspended sediment transport from the distributary channels to in42 terdistributary islands in a delta. The results suggest that a transition from advective 43 to turbulent diffusion transport mechanism may occur depending upon a threshold out44 flow momentum flux ratio which lies in between 0.211 km−1 and 0.375 km−1. This study 45 provides the first detailed quantification of flow structure in an actively prograding river 46 delta and offers important implications for coastal restoration by linking coastal sedi47 ment transport mechanism to patterns in flow structure. 48 Plain Language Summary 49 In a river delta, channels lose a significant amount of water to the islands because 50 of lateral discharge through small crevasse and also as overbank. With the water, sed51 iment and nutrients also get carried from the channels into the delta islands. This pro52 cess supports the deltaic ecosystem and influences the evolution of the delta. Although, 53 it is yet to be shown how such lateral discharge may affect the three dimensional flow 54 field in the delta channels and sediment transport mechanisms. Here we use high res55 olution three-dimensional velocity data from a river delta to determine the influence of 56 two different outflow processes on the nearby flow field. We observe strong coherent he57 lical circulations in the channels when the lateral outflow is concentrated in a small area 58 (i.e., a side channel) whereas weak transient structures when the lateral discharge gets 59 distributed over a large area (i.e., overbank flow). The results suggest the existence of 60 a threshold outflow momentum that triggers the formation of such coherent helical cir61 culations and induces changes to the sediment transport mechanism. The results of this 62 study have implications for better understanding of delta hydrodynamics and morpho63 logical evolution. 64