Exploring the Effect of Reservoir Storage on Peak Discharge Frequency

In this paper, a simple hydrologic example is employed to illustrate the important features of reservoir regulated flood frequency. Despite its practical significance, the estimation of reservoir regulated flood frequency is largely dominated by empirical methodologies containing assumptions that could lead to incorrect results. The goal of this paper is to show by means of a continuous rainfall-runoff simulation how several reservoir variables, including the reservoir storage capacity, the size of release structures, operation rules, and the statistical variability of inflows to the reservoir, quantitatively control the regulated flood frequency. Although the example presented in this paper does not encompass the full complexity of the problem, it reveals important features of the regulated flood frequency. The study also highlights how specific assumptions in the traditional and widely used inflow volume-duration-frequency (VDF)-based methodology could lead to underestimation of flood risk for locations downstream from reservoirs. This paper, therefore, provides insight into the steps necessary to move away from the VDF-based empirical analysis towards a more realistic framework for estimating regulated flood frequencies. Simultaneously, the difficulties in fully addressing this problem are acknowledged and, consequently, continued examination of this important subject by the hydrological-science and engineering community is advocated.