Observation and Parameterization of Bottom Shear Stress and Sediment Resuspension in a Large Shallow Lake.

Parameterizations for bottom shear stress are required to predict sediment resuspension from field observations and within numerical models that do not resolve flow within the viscous sublayer. This study assessed three observation‐based bottom shear stress (τb) parameterizations, including (a) the sum of surface wave stress and mean current (quadratic) stress (τb=τw+τc ${\tau }_{b}={{\tau }_{w}+\tau }_{c}$); (b) the log‐law (τb = τL); and (c) the turbulent kinetic energy (τb = τTKE); using 2 years of observations from a large shallow lake. For this system, the parameterization τb = τw + τc was sufficient to qualitatively predict resuspension, since bottom currents and surface wave orbitals were the two major processes found to resuspend bottom sediments. However, the τL and τTKE parameterizations also captured the development of a nepheloid layer within the hypolimnion associated with high‐frequency internal waves. Reynolds‐averaged Navier‐Stokes (RANS) equation models parameterize τb as the summation of modeled current‐induced bottom stress (τc,m) and modeled surface wave‐induced bottom stress (τw,m). The performance of different parameterizations for τw,m and τc,m in RANS models was assessed against the observations. The optimal parameterizations yielded root‐mean‐square errors of 0.031 and 0.025 Pa, respectively, when τw,m, and τc,m were set using a constant canonical drag coefficient. A RANS‐based τL parameterization was developed; however, the grid‐averaged modeled dissipation did not always match local observations, leading to O(10) errors in prediction of bottom stress. Turbulence‐based parameterizations should be further developed for application to flows with mean shear‐free boundary turbulence. Plain Language Summary: Bottom shear stress is the link between hydrodynamic motions and sediment resuspension, further relating to water quality in the lake. However, it is impractical to directly measure the bottom shear stress in the field. We assessed three observation‐based bottom shear stress parameterizations, using 2 years of observations from a large shallow lake, and found that the parameterization consisting of surface wave‐induced stress and bottom current‐induced stress is sufficient to capture major sediment resuspension events. In the numerical models, which averaged the turbulence dissipation, the parameterization based on modeled surface wave‐induced stress and bottom current‐induced stress was also assessed and compared against the values from observation‐based parameterizations. The usage of a constant, observed or literature‐based parameter in the model parameterization is recommended, and it should be calibrated to account for inaccuracies in modeled hydrodynamic variables (i.e., surface waves and bottom currents). Key Points: Three observation‐based bottom shear stress parameterizations were assessed in a large shallow lakeThe parameterizations of bottom shear stress in Reynolds‐averaged Navier‐Stokes equation models was assessed against the observations [ABSTRACT FROM AUTHOR]