Flood control detention facilities are widely used for stormwater control in urban areas. Standard design procedures are in most cases based on the design storm approach: a single flood event at a time is considered, at the beginning of which the facility is assumed completely empty. The possibility of pre-filling from previous events is then neglected and underestimation of storage volume may occur. In this paper an analytical probabilistic approach to estimate the probability of pre-filling is presented and its effects, due to outflow rate and storage volume, are investigated. Two different strategies for the outlet control are analysed. Results are validated on a case study.

The paper proposes a semi-probabilistic approach for the design of rainwater tanks. In particular, the cumulative distribution function of the active storage is derived as a function of rainfall moments. The model is validated through continuous simulation of the hydraulic behaviour of a hypothetical rainwater tank located in Milan (North Italy) using as input a measured series of rainfalls in that area.

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Water Storage tanks have proved to be effective for runoff control in drainage systems. Standard design procedures follow an event-based approach: a single flood at a time is considered and tank is assumed completely empty at the beginning of its filling. The possibility of pre-filling from previous events is then neglected and underestimation of storage volume can occur. In this paper an analytical probabilistic approach to estimate the probability of pre-filling is presented and effects due to outflow rate and storage volume are investigated. Derived formulas are validated by their application to a case study.