Direct Numerical Simulations of Miniature Along‐Shelf Current‐Supported Turbidity Currents: Conceptual Investigation of Velocity Structure and Drag Coefficient

Abstract

Alongshore current‐supported turbidity currents (ACSTCs) are a subclass of wave‐ and current‐supported turbidity currents. They are one of the agents responsible for the dispersal of the river‐borne sediments on the continental shelf, which constitutes a major phenomenon controlling the geomorphic evolution of ocean‐basin margins over geological time. Therefore, parameterization of the sediment flux associated with ACSTCs will help its implementation in operational models and quantify the sediment flux budgets on the continental shelf. The velocity structure of ACSTCs and the amount of sediments suspended by them are crucial to determine the suspended sediment flux. This study investigates the velocity structure of a simplified miniature ACSTC over an erodible bed composed of fine sediments. Direct numerical simulations are conducted for various bed erosion parameters and sediment settling velocity. The role of sediment‐induced stable density stratification on the velocity structure of ACSTCs is analyzed. The simulation results indicate that density stratification and the drag coefficient are functions of the product of sediment settling velocity and sediment concentration. The velocity profile was found to deviate toward the alongshore direction with strengthening density stratification, which enhances the drag coefficient. By using the Monin‐Obukhov theory, the drag coefficient associated with the cross‐shelf propagation of ACSTCs is formulated as a function of the Reynolds number, sediment concentration, and sediment settling velocity.

A conceptual Investigation of Turbidity Current Trigger from Alongshelf Current-supported Turbidity Currents

“…The disagreement in bulk Richardson numbers reported in the literature can perhaps be explained by the role of sediment entrainment parameters on the total suspended sediment load. Recent field observations close to the Rhine River mouth (Flores et al, 2018), numerical simulations for wave-supported turbidity currents (Yue et al, 2020), wave boundary layers (Cheng et al, 2015), alongshelf current supported turbidity currents (Haddadian et al, 2021), and laboratory experiments in oscillating water tunnel (Hooshmand et al, 2015) reported Ri b as low as 0.01. To explain the reported discrepancy in Ri b different arguments can be made.…”

“…For field and laboratory studies, win-nowing can perhaps be a plausible but unproven explanation. However, numerical experiments (Yue et al, 2020;Cheng et al, 2015;Haddadian et al, 2021) were conducted for uniform sediment size; therefore, winnowing is not a valid explanation for low Ri b in numerical simulations.…”

“…The bottom boundary condition for sediment entrainment is specified following the previous studies (Cheng et al, 2015;Yue et al, 2020;Haddadian et al, 2021):…”

“…On the other hand, simulating ACSTCs at a realistic scale by DNS is almost impossible due to computational requirements. To this end, a downscaled ACSTC, which will be referred to as miniature ACSTC (Haddadian et al, 2021), will be used. For the miniature ACSTC, the flow depth is selected as h = 0.10 m, the initial bed shear stress is τ bo = 0.013 Pa, the critical shear stress is selected as τ c = 0.01 Pa.…”

“…For developed turbulence to occur, the Reynolds number must be sufficiently high so that scale change will minimally impact the turbulent features. With the imposed friction velocity, one can obtain the Reynolds number as Re = 7 × 10 4 (see Haddadian et al, 2021), where the Reynolds number is defined as…”