Influence of Bioroughness Density on Turbulence Characteristics in Open-Channel Flows

Abstract

Bioroughness plays an important role in modifying the velocity and sediment flux near the riverbed. It is therefore pertinent to study the influence of benthic fauna on the bed forms. To this end, large-eddy simulations are performed to investigate the influence of the arrays of mounds and their density on the turbulence characteristics in an open-channel flow. The simulated distributions of the time-averaged streamwise velocity and the turbulence intensity are in good agreement with the experimental data. Four numerical simulations are performed with varying streamwise spacings of mounds. Details of the time-averaged and instantaneous flow velocities are analyzed by multiple visualization methods, and the effects of the bioroughness density on the equivalent roughness height and the Darcy–Weisbach friction factor are quantified. The time-averaged flow in the wake of the mounds is characterized by a symmetric pair of vortices. The mounds behave like bluff bodies, increasing the riverbed roughness and heterogeneity in the flow environment. An increase in mound density is to promote the development of secondary currents and to increase the dispersive stress near the bed. The peaks of the Reynolds shear stress distributions decrease in both the streamwise and vertical directions for the high-density case due to a blockage effect. The instantaneous flow features, in the form of various turbulence structures, are generated near the top edge and the wake zone of mounds. The spacing between low-speed streaks decreases with an increase in equivalent roughness height. Multifrequency behavior that is observed is a result of shear layer roll-up from the edges of mounds and the flapping of wake. Finally, two formulas for equivalent roughness height and Darcy–Weisbach friction factor are proposed involving the bioroughness density and height. The findings demonstrate the effects of the bioroughness on the near-bed turbulence characteristics and sediment stability. Bed forms produced by aquatic benthos are usually of small scale but with a large abundance. We performed a flume experiment as well as large-eddy simulations (a high-fidelity numerical simulation approach) to answer whether these microtopographies are important when studying flow and transport processes near the riverbeds. Four numerical simulations are performed with varying mound density. It was found that the microtopographies created by benthos indeed change the near-bed turbulence characteristics and sediment stability. Specifically at high abundances, benthos-induced bed forms could raise the bed roughness and the friction factor eightfold and threefold, respectively. This suggests the necessity of a modified roughness height formula and a friction coefficient formula based on the benthos abundance, which has been carried out in this study. Thus, engineers who are concerned with the flow and sediment transport in habitats of high-abundance benthos (e.g., wetlands and estuaries) may prefer to use these modified formulas and to develop a more accurate model of sediment transport.