A Method for Calculating the Design Volume of the Initial Rainwater Storage tank

Construction of the initial rainwater storage tank is crucial for managing urban first-flush pollution. Initial rainwater storage tank is designed to maximize pollutant capture efficiency while minimizing storage volume requirement. Considering the random nature of rainfall process, a method is proposed for determining the design volume of initial rainwater storage tank with its specific design return period. First, rainfall events were obtained by dividing the original rainfall series, setting the minimum inter-event time (MIET) as the emptying time of storage facilities. The rainfall events for design purpose were selected from the above rainfall events according to three indicators (the initial rainfall amount, maximum rainfall intensity of the specified period, and antecedent dry periods (ADP)). Then the annual multi-event-maxima (AMEM) method was used for the sampling of initial rainfall period on the basis of the selected rainfall events. Second, samples were arranged in descending order and the empirical frequency of the samples was calculated according to the mathematical expectation formula (Weibull formula). Thus, the theoretical probability distribution function was estimated based on Markov Chain Monte Carlo (MCMC) algorithm and the initial rainfall intensity formula was obtained according to the Horner formula. Finally, for the targeted catchment and the selected return period, the initial rainfall intensity can be calculated setting the initial period equals to overland time of concentration. Afterward, the design rainfall amount (design volume) can be obtained by the initial rainfall intensity, the initial period, initial loss and the catchment area. It was concluded that: (1) A method for the initial rainfall intensity sampling and the initial rainfall intensity formula were proposed for design volume of the initial rainwater storage tank. (2) The return period of the design volume can be taken into consideration in this method. (3) The proposed method is suitable for scenarios of collecting urban first-flush pollutants from small urban catchments.