Off-stream Detention Design for Storm Water Management

A detention basin is designed for storm water quantity and quality controls. An in-stream basin is often located on a widened floodplain, while an off-stream basin can be part of open space in a parking lot, park, or sport field. The inflow channel associated with an off-stream basin is allowed to carry its base flow not to exceed the downstream capacity. When the inflow channel is full, the excess flood water is diverted into the off-stream basin through the side weir. During the flood recession, the equalizer conduit drains the basin flow back to the channel. In this study, the rational volumetric method was expanded from in-stream to off-stream detention design. The required off-stream detention volume is maximized by the rainfall event longer than the time of concentration of the tributary area. In comparison, the off-stream detention volume can be significantly less than an in-stream, depending on the flow-through capacity in the downstream channel. This paper presents a design procedure to maximize the off-stream detention volume, to size the flow-diversion weir, and to set the flood-gate operation on the equalizer conduit. The simplified derivation on turning moment indicates that the operation of flood gate is dictated by the water depth differential and the weight of the gate.