A semi-analytical approach for flood control reservoir sizing and verification

Flood control reservoirs help manage floods by temporarily storing excess incoming flow and slowly releasing it. Their design requires estimating the total storable water volume or the maximum allowable discharge of predefined return periods. The design storm approach often represents the state-of-the-art design for flood control reservoirs. This approach transforms the design storm into the design hydrograph and routes it into the reservoir to compute the maximum outlet discharge and, thus, the peak reduction effect. The critical part of this approach is that a plethora of shapes and durations of the design storms can be chosen. Moreover, the runoff process of the contributing catchment area is often neglected; thus, the results can be realistically oversized. Previous studies demonstrated that knowing the discharge hydrograph's shape is unnecessary, and what matters are the incoming volumes. Thus, reservoir designing can also employ the frequency law of the average volume over the duration with return period T, known as the Flow Duration Reduction (FDR) curves. This study exploits FDR curves and proposes a semi-analytical approach to find the maximum outlet discharge a detention pond can reach to store a flood event with a specific return period T. The proposed approach allows a rapid evaluation and comparison of the maximum reduction effect offered by different reservoir configurations, modelling the runoff of the contributing catchment.