Summary:

This paper deals with one of the aspects important from the point of view of water quality maintenance in storage reservoirs, namely the dynamics of water currents that accompany changes of the water surface elevation in such reservoirs. Numerical simulations were conducted to back observations that high discharges are in the long term beneficial to water quality in the reservoir. Calculations made using the AdH finite elements model confirmed that the water current pattern in shallow regions varies greatly with the total discharge value. The sample analysis shown in this article concerns a mid-size storage reservoir of Tresna located in southern Poland.
It appears that for small discharges the currents that wet some areas of the previously dry reservoir bed are similar to reversed currents that occur when the area is dried. Thus some amounts of water may move from one stagnant area to another forth and back and then the water exchange is limited. It means that it is not enough to lower the water level within the reservoir steering rules and then raise it again to be assured that the water quality improves in the reservoir, especially in the lull parts. But for higher discharges the current pattern is different and the exchange of the reservoir water really does happen. In most cases in order to "flush" a reservoir that way one could just wait for high water to come, but if reservoirs form a cascade creating an artificial controlled freshet that propagates down the cascade may be a considerable means of altering water quality in lower reservoirs.