LID DESIGN CALCULATIONS AND METHODOLOGY - Low Impact Development and Sustainable Stormwater Management - page 122

6.6 ACCOUNTING FOR THE BENEFITS OF LID:
LINKING VOLUME AND PEAK RATE
The utilization of volume-reduction BMPs and LID practices will obviously
reduce or eliminate the amount of storage required for peak-rate mitigation
because less runoff is discharged. However, quantifying the peak-rate mitigation
benefits of LID can be difficult and cumbersome with common stormwater mod-
els/methodologies. In this section we discuss some available tools for quantifying
the benefits of LID.
In its Surface Water and Storm Water Rules Guidance Manual [23] (available
at http://www.mmsd.com/stormwaterweb/index.htm) the Milwaukee Metropoli-
tan Sewerage District (MMSD) describes five methods of accounting for “dis-
tributed retention” or LID based on the NRCS unit hydrograph method: truncated
hydrograph, scalar multiplication, subtracting retention from rainfall, subtracting
retention from runoff, and adjusting CNs (Appendix A of Appendix L: Low
Impact Development Documentation). Using the method of subtracting reten-
tion from runoff, MMSD developed a spreadsheet model (available from MMSD
for $25) called LID Quicksheet 1.2: “Quicksheet allows the user to quickly
evaluate various LID features on a development site to reduce ... detention
requirements. ... LID features included in the Quicksheet include rain gardens,
rain barrels, green roofs, cisterns, and permeable pavement.” Although Quick-
sheet seems to be a useful tool, the current version does not appear to account
directly for ongoing infiltration during the storm event and therefore may not
fully credit LID practices that achieve significant infiltration (the ongoing infil-
tration volume could be added to the capacity of the LID retention features to
make up for this.
6.7 RECOMMENDED LID STORMWATER CALCULATION
METHODOLOGY
Stormwater management has traditionally focused on peak-rate control for large
storm events (e.g., 10-, 25-, 50-, and 100-year-frequency storms) This low-
impact development stormwater topic recommends that stormwater management
be much more comprehensive, including:
• Total runoff volume
• Water quality
• Peak rate of flow
• Groundwater recharge
• Hydromodification
• Stream channel protection
The site design criteria recommended here are designed to achieve all of
these comprehensive stormwater management objectives. It should be noted that
control of the peak rate of flow of stormwater runoff remains an important part