AN ANALYSIS OF HYDRODYNAMICS OF RIVER INFLOW AND ITS EFFECT ON WATER QUALITY DURING SUMMER IN A SHALLOW RESERVOIR

Abstract

The concentration of nutrients such as phosphorus and nitrogen increases in July to August in Watarase reservoir. This increase is caused by the release of nutrients from bottom sediments. Physical disturbance on bottom sediments is seemed to have significant role in the nutrient release. Physical disturbance on bottom sediments is mainly driven by wind waves in shallow lakes and reservoirs. Since such events occur intermittently, it is necessary to monitor continuously to detect the events. We have conducted several monitoring including current, temperature profiles, meteorology and water quality measurement in Watarase reservoir to detect the record of physical disturbances on bottom sediments and its driving force. Due to its shallowness, the reservoir is warmed to the bottom sediments in summer. The temperature of surface layer of bottom sediments exceeded 30°C. Density current penetrated to the bottom layer during river inflow in summer since the temperature of river water is cooler than that of reservoir water. Monitored records showed that this density current seems to have great disturbance on bottom sediments. Cooler and denser river inflow to such shallow reservoir should be paid more attention in analysis of material flow between water column and bottom sediments. Based on field survey and numerical simulation results, we discussed such event and its effect on water quality of a shallow reservoir.