Design of stormwater bioretention systems for improved volume and peak runoff reduction

Stormwater bioretention systems are primarily designed to manage stormwater quality as well as restore more natural hydrology during less intense rainfall events. Thus, design modifications are required to manage intense rain events resulting from climate change. This study examined 54 bioretention system design combinations by varying four design factors: filter media fraction of the total soil depth (i.e. filter media + drainage gravel layer), hydraulic conductivity of the filter media, ponding depth, and storage connection diverting surface water to the storage layer through a pipe. Using a calibrated field-scale Storm Water Management Model (SWMM), the performance of these designs was assessed for volume reduction during common rain events, peak flow reduction during a 10-year return interval rain event (180 mm, 1 h), and the number of overflows in a year. Using a storage connection consistently improved performance, enhancing volume reduction by up to 32% and reducing overflow events (up to 11 of 29), as expected since the storage layer provides additional high-porosity detention. Higher filter media fractions (85%) improved volume reduction during common rain events by approximately 50%. Interestingly, lower fractions (15%) were more beneficial for reducing overflow events (in about 17 out of 29 events annually), suggesting that higher porosity and drainage can sometimes prevent excessive surface ponding. These findings underscore the potential of bioretention system designs to enhance climate resilience and emphasize the need for a balanced approach that improves volume reduction during common rain events while also reducing overflow events during high-intensity rainfall.