A Milwaukee Model for LID Hydrologic Analysis

In 2002, the Milwaukee Metropolitan Sewerage District (MMSD) began to explore the potential for Low-Impact Development (LID) techniques to help development projects meet the District's storm water management regulations. One general concern associated with using LID was the apparent complexity of determining the cumulative hydrologic effect of relatively small, interconnected LID components such as green roofs, bioretention cells, permeable pavements, etc. An adaptation of the Natural Resources Conservation Service (NRCS) unit hydrograph method was developed to provide an approach to the LID hydrologic analysis that remained relatively simple. This adaptation aggregates the LID retention volume across the site and treats that volume explicitly as a depth of runoff (i.e., excess rainfall) that must be exceeded before the convolution calculations generate a positive value for the runoff hydrograph. For practical purposes, this approach compared favorably against other alternatives, such as adjusting the composite NRCS curve number for the site, or treating all the retention volume as if it were an in-line storage volume situated immediately above the drainage area outlet. Calculations for the preferred approach have been formulated in a spreadsheet that allows the user to input the amount of retention provided by each of several kinds of LID components, and then see immediately a recalculated hydrograph that reflects the predicted effect of the aggregate retention volume. The spreadsheet, which has been dubbed the "LID Quicksheet", is expected to be incorporated along with its documentation into the MMSD stormwater management guidance.