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 (t(b)) parameterizations, including (a) the sum of surface wave stress and mean current (quadratic) stress (tb=t(w)+t(c); (b) the log-law (t(b) = t(L)); and (c) the turbulent kinetic energy (t(b) = t(TKE)); using 2 years of observations from a large shallow lake. For this system, the parameterization t(b) = t(w) + t(c) 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 t(L) and t(TKE) 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 t(b) as the summation of modeled current-induced bottom stress (t(c,m)) and modeled surface wave-induced bottom stress (t(w,m)). The performance of different parameterizations for t(w,m) and t(c,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 t(w,m), and t(c,m) were set using a constant canonical drag coefficient. A RANS-based t(L) 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.