Quantitative Assessment of Environmental Site Design <scp>vs.</scp> Traditional Storage‐Based Stormwater Management: Impacts on Catchment Hydrology of Minebank Run, Baltimore, <scp>MD</scp>

ABSTRACT Environmental site design (ESD) is a stormwater management approach that prioritises the use of infiltration‐based non‐structural techniques to mimic the natural hydrologic cycle by reducing impervious surfaces, slowing runoff and increasing infiltration. Traditional storage‐based stormwater management is designed for flood control by quickly diverting runoff from developed areas. This study compared the effect of ESD and only storage‐based stormwater management practices on the hydrology of an urban watershed in Baltimore County, Maryland, USA. Minebank Run is an 8.47 km 2 flashy urban stream with a catchment largely developed without stormwater management. A calibrated SWMM model was used to simulate changes in catchment hydrology resulting from ESD and detention basins over a 54‐year period, from the onset of urbanisation in 1948 to the state of urbanisation in 2001. This approach offers a novel, retrospective perspective by simulating how the watershed hydrology might have changed if ESD had been implemented from the beginning of urban development. The model results were analysed by quantifying and comparing different hydrologic metrics to evaluate runoff quantity and flow variability. Results indicated that although storage ponds performed similarly to ESD in reducing annual maximum peak flows (43% vs. 45% reduction, respectively), ESD reduced mean annual runoff coefficients significantly more than ponds (28% vs. 2.7%, p &lt; 0.0001). The Richards–Baker Flashiness Index was reduced from 0.46 to 0.32 with the implementation of ESD, as compared to 0.36 with detention ponds. This study also tested the hypothesis that the impact of urbanisation on the hydrology of the Minebank Run watershed would have been reduced if it had been developed with ESD. The results indicated that the implementation of ESD would have reduced annual maximum peak flows by an average of 46%, annual mean runoff coefficients by 51% and the Richards–Baker Flashiness Index by 37%, as compared to the as‐is condition. The study provides quantitative insights into the performance of traditional and innovative stormwater management techniques at the catchment scale, illustrating the benefits of a combination of both infiltration practices and detention storage in reducing the hydrologic impacts of urbanisation.