Modeling and Sizing Bioretention Using Flow Duration Control

Low-impact development (LID), such as bioretention, is increasingly used as a best management practice (BMP) to manage storm-water runoff. With LID becoming an integral part of storm-water management plans, it is critical to understand the hydrologic performance of these devices and their positive impacts on downstream hydrology, geomorphology, and ecology. This paper focuses on presenting a model evaluation of bioretention using calibrated and verified algorithms and describes the application of flow duration control (FDC) as a design strategy. The goal of bioretention, like many LID-based controls, is to replicate natural hydrologic processes. Storm-water controls designed to match preproject flow duration characteristics demonstrate that they come close to mimicking the natural hydrologic cycle, thereby protecting beneficial uses and promoting long-term sustainable solutions. This work presents the FDC approach for sizing bioretention and other similar flow control BMPs. This paper concludes with a summary of the sizing requirements to meet the flow duration criteria. Results from this evaluation suggest that typical water quality design criteria can be effective for individual sites with 20–30% imperviousness. For areas draining 100% imperviousness, typical of LID strategies, storage requirements should be 5–10 cm (2–4 in.) with areas of 12–25% the catchment draining to the bioretention facility. However, storage and land area requirements should be sized based on watershed and site-specific characteristics.