855773644-LEED-v5-BD-C-Reference-Guide-Launch-Edition - Page 135
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Using the data from these resources for the project’s location, the selected timeline, and other relevant information on the rainfall event from the selected resource, teams must calculate the percentile of the rainfall event by using the USGBC-approved calculator or provide an additional calculator including the necessary information for credit compliance. [57]
Runoff calculations Teams must calculate the runoff volume using the modified rational method, the Technical Release 55 (TR-55), Natural Resources Conservation Service method, the U.S. EPA Rainwater Management Model (SWMM), or other runoff methodologies most appropriate for the project. [58][,][59][,][60][,][61] Equation 1 shows an example of a simplified version of the runoff calculation. The runoff calculations must include the watershed boundary, such as buildings, parking lots, landscaping, pervious surfaces, and all other impervious surfaces. This approach ensures a comprehensive method to managing stormwater.
Equation 1. Runoff volume calculation
𝑅𝑢𝑛𝑜𝑓𝑓 v𝑜𝑙𝑢𝑚𝑒 = 𝑅𝑎𝑖𝑛𝑓𝑎𝑙𝑙 d𝑒𝑝𝑡ℎ × 𝐴𝑟𝑒𝑎 × 𝑅𝑢𝑛𝑜𝑓𝑓 𝐶𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡
where:
Rainfall depth = Measure the rainfall in inches (or meters) as determined by the percentile rainfall calculator.
Area = Measure the area of both impervious and pervious surfaces within the watershed boundary in square feet (or square meters).
Runoff coefficient = This is an aggregated factor that represents the percentage of rainfall that becomes runoff. For impervious surfaces like concrete or asphalt, this is typically close to 1 (e.g., 0.95).
Rainwater harvesting Rainwater harvesting contributes to rainwater management. Rainwater harvesting is a sustainable practice that captures and stores rainwater to allow for water reuse within the project site. Projects that collect and reuse rainwater from the chosen percentile event volume for one of the eligible end-uses can earn one additional point.
57 “LEED v4.1 Rainfall Events Calculator”, U.S. Green Building Council, updated April 10, 2020, https://www.usgbc.org/resources/leed-v41-rainfall-events-calculator. 58 “Modified Rational Method”, Bentley SewerCAD SS5, Bentley Systems, (n.d.), https://docs.bentley.com/LiveContent/web/Bentley%20SewerCAD%20SS5-v1/en/GUID- 85A442CDB33D4B1684EE9E795BA6BABE.html. 59 “WinTR-55 Small Watershed Hydrology”, United States Department of Agriculture, Agricultural Research Service, (n.d.), https://www.ars.usda.gov/research/software/download/?softwareid=8&modecode=80-42-05-10. 60 “Conservation Planning”, Natural Resources Conservation Service, United States Department of Agriculture, (n.d.), https://www.nrcs.usda.gov/getting-assistance/conservation-technical-assistance/conservation-planning. 61 “Storm Water Management Model (SWMM)”, United States Environmental Protection Agency, (n.d.), https://www.epa.gov/water- research/storm-water-management-model-swmm.
U.S. Green Building Council LEED v5 Reference Guide for Building Design and Construction, April 2025 Launch Edition 125