Author(s): Anil Caliskan; Sebnem Elci
Linked Author(s):
Keywords: Stratification; Hydrodynamics; EFDC; Tahtali
Abstract: This study investigates the hydrodynamics of a 15 m deep (average), 20 km2 water supply reservoir for different flow and weather conditions. The study site was selected as the main pool of Tahtali Reservoir since 40% of the water used in the city of Izmir (Population: 3. 4 million by 2000) is provided from Tahtali. The influence of stratification on water quality and hydrodynamics was investigated in this study due to the fact that the lake experienced dense stratification during summer months. The degree of stratification in Lake Tahtali was assessed through non-dimensional parameter analysis. Together with lack of water inflow during these months, the habitants are under the risk of bad drinking water quality. The threat was investigated by numerical modeling and monthly observations of water velocity and temperature profiles. Field measurements were performed from July to December 2006 in the main lake and in two rivers feeding the lake (Sasal and Tahtali Stream). The data collection in Lake Tahtali was achieved from a moving boat. The water velocity measurements in the lake were made using a 1. 5-MHz mini down-looking acoustic Doppler current profiler with fiberglass pontoons. For water temperature and quality measurements, a hand-held water quality meter with a depth sensor were used for both lake and river field measurements. The instrument is capable of measuring water quality parameters including pH, conductivity, salinity, dissolved oxygen, turbidity, and temperature. Furthermore, a weather station was set up by the lake in order to collect accurate data for wind conditions in the study site. A three-dimensional numerical model, Environmental Fluid Dynamics Code (EFDC) has been selected in the present study to model the hydrodynamics in the lake using wind forcing, inflows, outflows, atmospheric temperatures, solar radiation, evaporation and rain as input data. The initial conditions of the model were defined due to the measurements implemented in the area, and the bathymetry of the model was also adapted to the current conditions at the time of the comparison. The results of the numerical model were compared with the measured data collected in the study site to provide a check on the accuracy of the model simulations. As a result, the numerical model predicted the observed velocity profiles and temperature time series satisfactorily. Possible reasons for discrepancies were investigated. Since the wind forcing has considerable effect on hydrodynamics of the lakes, the wind speed collected by the weather station was compared to the measured wind speed on the lake surface to get the most reliable wind data to be used in the numerical model. Numerical model results indicated that water velocities were strongly dominated by the wind data and correct measurement of wind stress on lake surface was necessary for accurate prediction of velocities in the water column. This study investigated hydrodynamics of a stratified lake through field measurements and numerical modeling. The numerical model was validated with the measurements satisfactorily. The results of this study can be used to guide the further investigations in the lake including modeling of water quality, investigation of the withdrawal and climate change effects to the hydrodynamics for better management practices.
Year: 2007