ENCOURAGING LOW-IMPACT DEVELOPMENT BY KEEPING THE HYDROLOGIC ANALYSIS SIMPLE

In 2002 the Milwaukee Metropolitan Sewerage District (MMSD) began to examine the potential for Low-Impact Development (LID) techniques to help developments meet regulatory peak runoff targets and to increase the number of options suitable for compliance with the District's storm water management regulations. Various stakeholders were known to have misgivings about the technical effectiveness of LID components, and it was apparent that a major disincentive to the use of LID was the complexity of modeling the flow and storage of runoff among interconnected LID components (such as green roofs, bioretention cells, permeable pavement areas, infiltration trenches, etc.). Consequently, MMSD sought an approach to LID hydrologic analysis that was technically reasonable, not difficult to execute by development engineers, and relatively easy to check by permitting personnel.An approach that came to the forefront for serious consideration consisted of a straightforward adaptation of the TR-20 unit hydrograph method. The adaptation takes the sum of all the LID retention volumes and treats it as a depth of retention storage across the site. The depth of runoff is calculated using conventional TR-20 formulas, but only after the runoff depth exceeds the retention depth is a component of the runoff hydrograph calculated for each subsequent time step. In other words, rather than calculating hydrograph components on the basis of excess rainfall, the components are calculated on the basis of excess runoff, i.e., the runoff that exceeds the available retention capacity.A technical review of this approach has led to favorable comparisons against other alternatives for accounting for retention storage. These alternatives consisted of a curve number adjustment, truncating the runoff hydrograph, treating the retention volume as an initial abstraction, and taking the runoff hydrograph representing the developed conditions and multiplying each ordinate by the ratio of runoff volumes with retention and without it.However, because the most desirable approach could not be readily incorporated into existing software, the need to develop a customized computational tool became apparent. Ultimately, a spreadsheet was created with a simple interface that allows the user to input the amount of retention provided by each of several LID components, and observe immediately the predicted impact of that retention on the runoff hydrograph and the amount of detention storage that might otherwise be required at the drainage area outlet. Stakeholder involvement before and during the development and testing of the tool has been a key aspect of this project. Stakeholder groups have included representatives from the development community, design engineers, and various government agencies. Their input helped identify the initial needs to be met, and their response to a draft version of the spreadsheet has spurred the effort on to testing the method on actual design projects.