Modelling effects of curtain method on algal blooming in reservoirs

Abstract

The Terauchi Dam Reservoir, one of largest reservoirs in Kyushu, Japan has been supplying water for paddy field irrigation and domestic consumption. During recent years it has shown serious deterioration of water quality, especially during the summer season. Destruction of the thermocline has been prohibited in order to maintain desirable temperature downstream of the reservoir for paddy field irrigation, thus, most of the existing control devices cannot be applied to the reservoir. As one of the means of reducing algal blooming in the reservoir, two vertical curtains, having sufficient depth to cover the epilimnion thickness, were installed across the reservoir in order to curtail the nutrient supply from nutrient-rich inflow to the downstream epilimnion of the reservoir. Withdrawal level was also regulated to keep the downstream epilimnion off the nutrient supply. The physical and biological process in the reservoir ecosystem were modelled, by assuming a stratified layered structure at each zone of the reservoir, to predict the water quality and algal species composition in the reservoir. The state variables, temperature, four types of phytoplankton as chlorophyll- a, soluble phosphorus, nitrate, ammonium, dissolved oxygen, biochemical oxygen demand, internal nitrogen, internal phosphorus, and when diatoms are modelled explicitly silica, are considered in the model. The model is calibrated with the measurements taken in the field. The effects of different curtain depths on the algal concentration at downstream zones were also studied.