Abstract:
Two vertical curtains, having depths to cover the epilimnion thickness, were installed across the Terauchi Dam
Reservoir in the western island of Japan to curtail the nutrient supply from nutrient-rich inflows to the downstream
epilimnion of the reservoir. The withdrawal level was also regulated to keep the downstream epilimnion away from
the nutrient supply. This method markedly reduced algal blooming in the reservoir downstream of the curtains during
spring and summer. The physical and biological processes in the reservoir ecosystem were analysed using the 2-D
reservoir model DYRESM and chemical and biological submodels, to predict the water quality and algal species
composition in the reservoir. The horizontal variability was maintained in the model by dividing the horizontal layer
into parcels. Temperature, chlorophyll-a, soluble phosphorus, nitrate, ammonium, dissolved oxygen, biochemical
oxygen demand, internal nitrogen, internal phosphorus were considered as state variables in the model. The simulated
results revealed the mechanism of how algal blooming is reduced, during early spring high algal concentrations
consume large amounts of nutrients, which reduces the nutrient supply to the downstream zone of the reservoir,
whereas during late spring and summer, nutrient dispersion from the upstream epilimnion to the downstream
epilimnion is curtailed by the curtains, markedly reducing algal blooming in the downstream zone.
