Assessing calculations of vertical eddy diffusivities in nearshore lake environments
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This study aims at assessing calculations of vertical mixing coefficients, or diffusivities, in the nearshore environment of large lakes, for use in simulating interactions among suspended and particulate phosphorus, benthic algae, and mussels at the bed. These interactions are of particular interest in understanding current nearshore water quality problems in the Great Lakes. Diffusivities in many ocean circulation models are often based on turbulence closure schemes, but a common problem associated with modeling diffusivities using this approach is an underestimation of vertical mixing--a major reason for which has been cited as the exclusion of wave-induced mixing. Recent attempts to improve the specification of vertical diffusivities have used models that take the sum of wave-induced diffusivities and diffusivities from ocean circulation models (such as the Princeton Ocean Model [POM]). However, these models were generally developed for deep-water oceanographic applications and need to be verified and/or modified for finite water depths. This study aims to evaluate model approaches describing vertical diffusivities in shallow water regions by evaluating the additive models for shallow water depths, modifying deep water models for finite depth, and verifying the model approach most suitable for shallow water regions by comparing model-calculated diffusivities with field eddy diffusivities. Results from the testing of several additive model approaches indicate that all models are in relative agreement with field conditions near the bottom of the water column, and deviate from each other in the upper half of the water column. A conclusion of this study is that the wind-wave induced mixing model by Hu and Wang (2010), both in its original form (for low wind speeds) and when modified to account for finite depth for higher wind speeds is the preferred method to calculate diffusivities in shallow water regions. It is anticipated that the results from this study will be useful for future modeling of nearshore regions of large water bodies such as the Great Lakes.