Observation and parameterization of bottom shear stress and sediment
resuspension in a large shallow lake
Abstract
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 (1) the sum of surface wave stress and mean
current (quadratic) stress (τb= τw +τc); (2) the log-law (τb= τL); and
(3) the turbulent kinetic energy (τb= τTKE); using two 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 modelled surface wave-induced bottom stress (τw,m).
The performance of different parameterizations for τc,m and τw,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 τc,m, and τw,m were set using a constant canonical
drag coefficient. A RANS-based τL parameterization was developed;
however, the grid-averaged modelled 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.