Sand settling through bedform‐generated turbulence in rivers

Abstract

Fluvial bedforms generate a turbulent wake that can impact suspended-sediment settling in the passing flow. This impact has implications for local suspended-sediment transport, bedform stability, and channel evolution; however, it is typically not well-considered in geomorphologic models. Our study uses a three-dimensional OpenFOAM hydrodynamic and particle-tracking model to investigate how turbulence generated from bedforms and the channel bed influences medium sand-sized particle settling, in terms of the distribution of suspended particles within the flow field and particle-settling velocities. The model resolved the effect of an engineered bedform, which altered the flow field in a manner similar to a natural dune. The modelling scenarios alternated bed morphology and the simulation of turbulence, using detached eddy simulation (DES), to differentiate the influence of bedform-generated turbulence relative to that of turbulence generated from the channel bed. The bedform generated a turbulent wake that was composed of eddies with significant anisotropic properties. The eddies and, to a lesser degree, turbulence arising from velocity shear at the bed substantially reduced settling velocities relative to the settling velocities predicted in the absence of turbulence. The eddies tended to advect sediment particles in their primary direction, diffuse particles throughout the flow column, and reduced settling likely due to production of a positively skewed vertical-velocity fluctuation distribution. Study results suggest that the bedform wake has a significant impact on particle-settling behaviour (up to a 50% reduction in settling velocity) at a scale capable of modulating local suspended transport rates and bedform dynamics.

An Explicit Solution for Characterizing Non-Fickian Solute Transport in Natural Streams

“…Furthermore, as shown in Figure 3, the riverbed mainly consists of sand and has many complex braided flow sections, as shown in Figure 3c,d. Notably, sand dunes are formed in the entire section, which can increase the residence time of the solute owing to the dynamic interaction between the hyporheic zone and the main flow area [36][37][38]. The geomorphological features of Gam Creek indicate the existence of flow stagnation zones and the storage effect; additionally, this river has been used as an appropriate study site to verify the storage effect reproduced by the model, as detailed by Kim et al [7].…”

Special issue: Marine and river bedforms dynamics, MARID VI

“…Because of the interaction between flow and a complex bed shape, the movement of sand grains above a bedform differs from that of sand grains above a flat bed. Through numerical modelling, Yuill et al (2020) showed that settling velocities were reduced by up to 50% above bedforms compared with over a flat bed. This is due to the eddies generated in the bedform wake and, to a lesser extent, to turbulence arising from velocity shear at the bed which maintain sediment in suspension over the bedform lee side and trough.…”