LID DESIGN CALCULATIONS AND METHODOLOGY - Low Impact Development and Sustainable Stormwater Management - page 116

Table 6-2 Small Storm Volumetric Coefficients ( R v ) for Urban Runoff
Flat Pitched Roofs Small Pervious Pervious

Roofs and and Large Impervious Areas, Areas,

Large Impervious Areas and Sandy Clayey Soils

Rainfall

Unpaved Areas (Large Narrow Paved Soils Groups

mm in. Parking Lots Parking Lots) Streets Streets Group A C and D

1 0.04 0.00 0.25 0.93 0.26 0.00 0.00

3 0.12 0.30 0.75 0.96 0.49 0.00 0.00

5 0.20 0.54 0.85 0.97 0.55 0.00 0.10

10 0.39 0.72 0.93 0.97 0.60 0.01 0.15

15 0.59 0.79 0.95 0.97 0.64 0.02 0.19

20 0.79 0.83 0.96 0.67 — 0.02 0.20

25 1.00 0.84 0.97 0.70 — 0.02 0.21

30 1.25 0.86 0.98 0.74 — 0.03 0.22

38 1.50 0.88 0.99 0.77 — 0.05 0.24

50 2.00 0.90 0.99 0.99 0.84 0.07 0.26

80 3.15 0.94 0.99 0.99 0.90 0.15 0.33

125 4.92 0.96 0.99 0.99 0.93 0.25 0.45

Source: [7].

The SSHM is a straightforward procedure in which runoff is calculated using
volumetric runoff coefficients. The runoff coefficients, R v , are based on extensive
field research in the United States and Canada over a wide range of land uses and
storm events. The coefficients have also been tested and verified for numerous
other U.S. locations. Runoff coefficients for individual land uses generally vary
with the rainfall amount—larger storms have higher coefficients. Table 6-2 lists
SSHM runoff coefficients for six land use scenarios for storms ranging from 0.04
to 4.92 in. Table 6-3 presents runoff coefficient reduction factors for disconnected
impervious surfaces associated with three types of development: strip commercial
and shopping center, medium- to high-density residential with paved alleys, and
medium- to high-density residential without alleys.
Runoff is simply calculated by multiplying the rainfall amount by the appro-
priate runoff coefficient (it is important to note that these volumetric runoff
coefficients are not equivalent to the peak rate runoff coefficient used in the
rational method, discussed below). Since the runoff relationship is linear for a
given storm (unlike the curve number method), a single weighted runoff coeffi-
cient can be used for an area consisting of multiple land uses. Therefore, runoff
is given by

Q v = P × R v

where

Q v = runoff (in.)

P = rainfall (in.)

R v = area-weighted volumetric runoff coefficient