Reservoir operation rules for controlling algal blooms in a tributary to the impoundment of three Gorges Dam

Abstract

Since the first impoundment of Three Gorges Dam in 2003, algal blooms occurfrequently in the near-dam tributaries. It is widely recognized that the impoundment-inducedchange in hydrodynamic condition with the lower current velocity will make theeutrophication problem even more severe when an excessive amount of nutrients is alreadyloaded into a reservoir and/or its tributaries. Operation tests carried out by Three GorgesCorporation in 2010 point to some feasible reservoir operation schemes that may havepositive impacts on reducing the algal bloom level. In our study, an attempt is made toobtain, through a numerical hydrodynamic and water quality modeling and analysis, thereservoir operation rules that would reduce the level of algal blooms in the Xiangxi River(XXR), a near-dam tributary. Water movements and algal blooms in XXR are simulatedand analyzed under different scenarios of one-day water discharge fluctuation or two-weekwater level variation. The model results demonstrate that the reservoir operations canfurther increase the water exchange between the mainstream of the Three Gorges Reservoir(TGR) and the XXR tributary and thus move a larger amount of algae into the deep waterwhere it will die. Analysis of the model results indicate that the water discharge fluctuationconstituted of a lower valley-load flow and a larger flow difference for the short-termoperation (within a day), the rise in water level for the medium-term operation (e.g., over weeks), and the combination of the above two for the long-term operation (e.g., overmonths) can be the feasible reservoir operation rules in the non-flood season for TGR.