While LID techniques are becoming more commonplace across the United States, there is no universally-accepted method for evaluating how well a site adheres to LID principles; namely, how closely the post-development hydrology mimics pre-development (or target) hydrology. As part of the Sustainable Sites Initiative? (www.sustainablesites.org) a method was developed to assess the pre- and post-development hydrology of a site and provide a means for a currently built upon site (a greyfield) to receive credit for partial restoration of hydrology. The Stormwater Management Model (SWMM) was used to model runoff at five locations across the U.S. for 10 years, with varying degrees of urbanization represented by a range of curve numbers. Based on natural soil type and vegetative cover, a target curve number was assigned to each location. A watershed's runoff flow volume and rates were calculated for each curve number and the change in flow volumes were then calculated relative to the target curve number. The percent decrease was then assigned a point value which a developer could potentially use for credit, similar to the U.S. Green Building Council's LEED? program. Graphical representations of hydrologic conditions and corresponding point values were developed. This method provides developers and regulators with a tangible, numeric goal for post-development hydrology when retrofitting developed sites while allowing flexibility in how that goal is achieved. The analysis of additional locations could provide the means for this approach to become the primary method across the country to evaluate a site's adherence to LID hydrologic principles.
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