SIMULATION OF WATER EXCHANGE TIMES FOR CONTAMINANT RISK ASSESSMENT IN AN URBAN LAKE USING A DEPTH-AVERAGED 2D MODEL

This paper describes the numerical modeling of a shallow urban lake using a depth-averaged 2D hydrodynamic model to evaluate potential retention times of contaminants. The bottom friction coefficient was calibrated using data of 15 years of measured chloride concentrations and comparing them with concentrations of a simulated tracer. The calibrated hydrodynamic model was used to evaluate the dynamics of three scenarios all focusing on the management of a lake pipeline: (a) a reference scenario, (b) a scenario in which the main managed inflow had decreased discharges, and (c) a scenario in which the main managed inflow received increased discharges. Each scenario was simulated for one year with a heavy rainfall event happening for 15 days at day 180 of the year which was represented in the simulation by increased wind velocities and discharges. We evaluated three characteristics for each scenario: (a) the hydraulic residence times in the main basin, (b) the influence times in the total model domain, and (c) the integrated hydraulic residence time in the main basin during the heavy rainfall event. The median hydraulic residence times of the main basin ranged from 16 to 22 days. An increased discharge at a managed inflow decreased the influence time of Lake Tegel to two months compared to a reference influence time of approx. 3 months. Therefore, the management of the lake pipeline can be an effective tool to control the dynamics of a hazardous contaminant.