The land use runoff coefficients for small rainfall depths in the Small Storm Hydrology Method are recommended (runoff coefficients express the fraction of rainfall which is converted into runoff). The project team may choose the runoff volume calculation methodology most appropriate for the project, provided sufficient documentation and justification to demonstrate that the intent of the credit is being met.

The U.S. EPA Stormwater Management Model (SWMM) and National Stormwater Calculator is a general tool that is available for compliance overview; it is not recommended for use in design. It can be used in the project’s conceptual design phase for estimating rainwater and runoff, and choosing initial management strategies. The use of the Cover Complex Method is not recommended at this time for the calculation of runoff volume from small rainfall events (i.e. less than 2.5 in [63.5 mm] of precipitation in 24 hours). Many rainwater software programs include calculation methodologies. Computer modeling programs based on the Small Storm Hydrology Method, such as WinSLAMM, are acceptable tools.

Small Storm Hydrology Method(1)

Large impervious areas– This category describes impervious areas with an average dimension greater than 24 feet (7.3 meters) in any direction. Examples of large impervious areas include parking lots with curbs, roads with curbs, highways, etc.

Small impervious areas – This category describes impervious areas with an average dimension no greater than 24 feet (7.3 meters) in any direction. Examples of small impervious areas include roads without curbs, small parking lots without curbs, and sidewalks.

For each land use type, runoff volume is calculated based on land use area and land use coefficient using the following equation:

IP units

Runoff Volume = P/12 * Rv * A

where:

Runoff Volume is from the percentile rainfall event (ft3)

P = percentile rainfall depth (in)

Rv = Small Storm Hydrology Method runoff coefficient (alternatively, Rv can be calculated using the following equation: Rv= 0.05+0.009(I), where I = percent impervious area expressed as a whole number)

A = area of land use (ft2)

For a site with multiple land uses, Runoff Volume can be calculated as follows:

Runoff Volume=∑_(i=1)^n▒[(P/12*〖Rv〗_i*A_i )+(P/12*〖Rv〗_(i+1)*A_(i+1) )+⋯+(P/12*〖Rv〗_n*A_n )]

SI units

Runoff Volume = P/1000 * Rv * A

where:

Runoff Volume is from the percentile rainfall event (m3)

P = percentile rainfall depth (mm)

Rv = Small Storm Hydrology Method runoff (alternatively, Rv can be calculated using the following equation: Rv= 0.05+0.009(I), where I = percent impervious area expressed as a whole number)

A = area of land use (m2)

For a site with multiple land uses, Runoff Volume can be calculated as follows:

Runoff Volume=∑_(i=1)^n▒[(P/1000*〖Rv〗_i*A_i )+(P/1000*〖Rv〗_(i+1)*A_(i+1) )+⋯+(P/1000*〖Rv〗_n*A_n )]

Footnote (1): Robert Pitt, P.E., Ph.D., DEE, Small Storm Hydrology and Why it is Important for the Design of Stormwater Control Practices In: Advances in Modeling the Management of Stormwater Impacts, Volume 7. (Edited by W. James). Computational Hydraulics International, Guelph, Ontario and Lewis Publishers/CRC Press. 1999.