Flowchart of water-quality volume computations.

Conventional designs of infiltration swales either neglect infiltration while the swale is filling or approximate the flow in the swale as being normal instead of gradually varied. The adequacy of these approximations are elucidated, and two common design configurations for infiltration swales are considered. For swales designed to store the water-quality volume behind check dams, the retained volume can be on the order of twice the design water-quality volume depending on the magnitude of the inflow rate normalized by the infiltration rate. In a second configuration, the swale is designed to infiltrate the water-quality flow, where the limiting assumption is that the flow is normal along the infiltration length. The actual required infiltration length can be expressed as a function of the normalized bottom width, and the required infiltration length can be up to 30% longer than derived using the conventional design. Graphical relations are developed that can be used to either quantify the factor of safety of conventional designs or provide credit for in the flood-control function of infiltration swales.