Detecting Localized Variation in the Three-Dimensional Flow Structure of a Confluence Using the Velocity Mapping Toolbox and ADCP Technology

Abstract

Studies on the complex flow dynamics of fluvial systems have benefited from recent advancements in hydroacoustic technology and post-processing and visualization software. The Velocity Mapping Toolbox (VMT) is one such software that allows users to rapidly process flow-velocity and channel bathymetry data collected along multiple transects into a single-averaged cross section across a river channel. In this study, I aimed to use Acoustic Doppler Current Profiler (ADCP) technology and the VMT, as tools to detect localized variations in three-dimensional flow structure of an asymmetrical river confluence during varying flow conditions. The study site was a reach of the lower Navasota River, which is an anastomosing system located on the border of the Brazos and Grimes counties in Texas. Flow velocity and channel bathymetry data were collected using a Sontek M9 ADCP from January 2019 to March 2019 and processed using the VMT to create three-dimensional flow velocity profiles. Results indicate that only during the highest mean discharge, and when all subchannels were active, was a clockwise helical cell present. During lower flow conditions, a combination of high surface winds, topographic steering, and data quality, lead to difficulties in the interpretation of flow patterns. By further analyzing the complex relationships between three-dimensional flow structure and discharge, this study builds upon the existing knowledge of asymmetrical confluence hydrodynamics and the processing of hydroacoustic river data.