Surcharge runoff over the capacity of drainage pipes is one of main flood breaking factors in urban area. Change into larger pipes is costly and time consuming solution. Rainwater tanks can be a sustainable solution for the control of heavy runoff.

The Rainfall-Storage-Drain(RSD) model is developed for the design of rainwater tanks of flood control. Design rainfall of Huff's distribution and runoff analysis on building rooftop makes inflow curves into the tanks. Through a water balance equation, tank volume versus peak flow curves can determine the minimum tank volume needed for controlled peak flow value of a design period. The case study in Seoul City shows a tank of 29 L/m2 can control the runoff of 30-years with the drainage pipes of 10-years design period. The RSD model can give simple and easy curves to understand for tank capacity determination in a local flood condition.

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