Modeling the Effectiveness of Maryland's Environmental Site Design Criteria

The Maryland Stormwater Management Act of 2007 mandated a change in the state's approach to stormwater management for new development, called Environmental Site Design (ESD). In 2008, The Maryland Department of the Environment (MDE) developed new design guidance to reflect this approach, with the following goals: (1) Improve the use of environmentally sensitive development and other site planning techniques listed as Stormwater Credits in the 2000 Manual. (2) Integrate site planning and structural controls in one procedure. (3) Encourage the use of onsite or micro-scale structural controls for both water quality and channel protection treatment, and discourage the use of end-of-pipe treatment. As part of the effort, MDE contracted with KCI to conduct a pilot test of the revised design guidelines. The first task was to identify recently completed site development plans designed according to the current manual to use as scenarios for the pilot study. Three were selected: one commercial site and two residential sites with different densities. The core of the pilot project was to test the proposed sizing criteria and SWM alternatives in the draft manual. KCI developed a complete redesign of the stormwater management facilities for all three sites, focusing on non- structural and micro-scale practices described in the draft manual, including runoff disconnection, sheetflow to buffers, infiltration, swales, filters, and bioretention. Feasibility of the draft procedures was verified with TR-20 modeling, which provided hydrograph analysis to compare the ESD approach with the conventional design in the original development plans. The results showed that the ESD approach to stormwater management was feasible for a wide range of sites and that the designs using the new criteria could meet the same goals for stormwater control as the conventional designs.