Effect of Lateral Outflow on Three-Dimensional Flow Structure in a River Delta

Abstract

Spatial and temporal patterns in three-dimensional flow structure have been linked to channel processes and morphology in many environments, including river meander bends, confluences-diffluences, and bedrock canyons. However, there is not yet an understanding of how channelized and gradually distributed lateral outflows that are often prevalent in river deltas influence three-dimensional flow structure and sediment transport mechanisms. This study 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 a boat-mounted acoustic Doppler current profiler (ADCP) at two sites: a channelized outflow zone and a distributary channel experiencing distributed lateral outflow. The flow structure was analyzed 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. A conceptual model is developed linking the formation of secondary circulation cells and suspended sediment transport from the distributary channels to interdistributary islands in a delta. The results suggest that a transition from advective to turbulent diffusion transport mechanism may occur depending upon a threshold outflow momentum flux ratio which lies in between 0.211 km-1 and 0.375 km-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.

From Grains to Plastics: Modeling Nourishment Patterns and Hydraulic Sorting of Fluvially Transported Materials in Deltas

“…Regarding the latter, because the numerical solution from ANUGA is 2D, the model is unable to resolve the vertical component of hydrodynamics. This may be important near the channel‐island interface, where recirculations can lead to an ostensibly 3D flow structure that could impact the flux of materials leaving the channels (Chowdhury et al., 2022; Proust & Nikora, 2020). While this is a limitation of our approach, fully 3D models are very computationally expensive and challenging to implement and calibrate, particularly in a tidal system with regular wetting and drying, and usually these efforts do not lead to noteworthy improvements in model accuracy (Bates, 2022).…”

Effect of Lateral Outflow on Three‐Dimensional Flow Structure in a River Delta

“…The discharge and waterlevel data can be found at https://waterdata.usgs.gov/ and https://tidesandcurrents.noaa.gov/, respectively. The ADCP data used in this article are available via figshare repository https://doi.org/10.6084/m9.figshare.15094149.v1 in ASCII text and .mat file format for using in Velocity Mapping Toolbox (VMT) with Attribution 4.0 International (CC BY 4.0) license (Chowdhury et al., 2021).…”