Simulation of Water Temperature, Flow and· Dissolved Oxygen Exchange Processes in Holland Lake

Abstract

As a part. of the study of dissolved oxygen (DO) dynamics in Holland Lake, the underlying flow, temperature and material.transport were simulated. In this report the simulation results are being presented. The simulation was two-dimensional to capture the interactions between the shallow and deep subbasins of the lake in addition to the water/air heat and momentum exchange. The vertical cross section in which flow veloCities, water temperatures and· material concelltrations were simulated extended from the westerly shallow ~ubbasin through Jhe deep basin to the northeasterly shallow subbasin. Flow velocities were induced by wind shear on the lake surface and· by buoyancy forces due to temperature differences. Temperatures were in response to heat exchange through the water surface by radiation, convection and evaporation. The two-dimensional model developed for Holland Lake.computed velocities and water temperatures at 5 minutes intervals. Weather conditions (data) observed during the period from July 1 to August 1, 1999 were imposed as boundary conditions. The flow field calculated for the month of July 1999 was then used to investigate. how a material such as dissolved oxygen or dissolved organic carbon is transported from the shallow subbasin to the deep subbasin. These simulations show the intrusion of bottom waters from the shallow subbasins to the metalimnion of the deep basin. This mechanism can explain the rapid DO. depletion of metalimnetic waters in the summer because the bottom waters in the shallow subbasins are known to be low in DO and rich in detrital carbon. The simulation results also show that the low water temperatures associated with groundwater intrusion are largely responsible for the movement of oxygen-poor water from the shallow subbasins into the deep subbasin. The selection of alternative supplemental aeration techniques will benefit from the simulations results. The depth at which aeration systems should be placed, and their location will be guided by the model results.