Hydrologic Model Developed for Stormwater Infiltration Practices

Rain garden (RG) has been developed and applied to low-impact designs for the purpose of water quality enhancement. A RG consists of a surface storage basin and subsurface infiltrating layers. The storage basin is designed to be shallow, flat, and wide with an overtopping weir to pass excessive flows, while the subsurface infiltrating system is composed of two layers of filtering media. The upper layer is filled with sand-mix and the lower layer is formed with gravels. In this study, the storage basin is sized to capture the water quality capture volume (WQCV) that is determined using the log-normal distribution to describe the population of rainfall depths. The subsurface filtering layers are sized to achieve the targeted flow release and drain time. A surface-subsurface hydrologic model is developed in this study to simulate the water loading process in the storage basin, and also the infiltrating process through the sand-mix layer. Based on field tests and data, this hydrologic model is further calibrated to determine the best-fitted values for soil initial moisture content and hydraulic conductivity involved in the flow simulations. This calibrated model provides a useful tool to understand the clogging situation in the RG, and helps the engineer make operational decisions on drain time and flow releases from the RG.