Reservoir Effects on Flood Peak Discharge at the Catchment Scale

This paper proposes a method to easily quantify the attenuation due to a reservoir on downstream flood peak discharges, that is, to the downstream flood frequency curve. Using a parsimonious Instantaneous Unit Hydrograph-based model, we show that the flood peak attenuation is mainly controlled by three system characteristics: (1) the reservoir position along the river channel, (2) the spillway dimensions, quantified by the reservoir storage coefficient; and (3) the storage capacity. These three system characteristics are quantified by three dimensionless numbers, which are derived analytically for an idealized catchment. The degree of flood peak attenuation increases for increasing storage capacity and spillway dimensions, in different ways depending on the reservoir position along the river channel. An optimal position exists, which maximizes the degree of flood peak attenuation, and is in general different from the outlet of the catchment. Interestingly, for large reservoirs with relatively small spillways, a range of quasi-optimal positions exists. With the Instantaneous Unit Hydrograph-based model, we also investigate how the duration of extreme rainfall relevant for determining the maximum flood peaks at the catchment outlet changes depending on the three system characteristics. Some of the assumptions of the method (i.e., catchment simple morphology and linearity of reservoir response) are relaxed in a real-world example, which demonstrates that the synthetic results approximate well what would be obtained by a more realistic model.