Drain Time for Porous Stormwater Basin

Abstract

Porous basins are popular low-impact-development (LID) facilities for on-site runoff disposal and water quality enhancement. Bioretention basins and rain gardens are recommended to intercept the initial runoff up to the water quality capture volume, which depends on the catchment area and imperviousness. Underneath the basin’s porous bottom exists a top layer of sand filter and a bottom layer of back-filled gravel. The seepage flow through the filtering layers is collected into a perforated subdrain pipe. The challenge facing the design of a porous basin is the trade-off between its residence time and drain time. On the one hand, the longer the drain time, the better the water quality enhancement. On the other hand, the faster the flow is released, the greater is the amount of runoff volume treated. Therefore, the drain time is the key parameter for the design and operation of a porous basin. This paper derived a theoretical solution to estimate the drain time to deplete the seepage flows through filtering layers underneath a porous basin. The discharge coefficient was calibrated to take into consideration the friction losses through the sand and gravel layers. This method provides a direct solution to predict the drain time and significantly improves the simplified formula used in numerical modeling.