LID DESIGN CALCULATIONS AND METHODOLOGY - Low Impact Development and Sustainable Stormwater Management - page 115

pervious and directly connected impervious areas should be estimated separately
for storms less than approximately 4 in. [3, 4]. When impervious areas are dis-
connected effectively from the drainage system, some runoff can be absorbed by
pervious surfaces. To account for this, the worksheets in Appendix A include
credits for disconnection.
To account for the land development process, all disturbed pervious areas that
are not restored using one of the techniques described earilier should be assigned
a curve number that reflects a “fair” hydrologic condition as opposed to a “good”
condition for post-development volume calculations. For example, lawns should
be assigned curve numbers of 49, 69, 79, and 84 for soil groups A, B, C, and D,
respectively.
The curve number method is less accurate for storms that generate less than
0.5 in. of runoff, and the SCS recommends using another procedure as a check
for these situations. For example, the storm depth that results in 0.5 in. of runoff
varies according to the CN; for impervious areas (CN of 98) it is a 0.7-in. storm,
for “open space” in good condition on C soils (CN of 74) it is 2.3 in., and
for woods in good condition on B soils (CN of 55) it is over 3.9 in. The CN
methodology can significantly underestimate the runoff generated from smaller
storm events [5]. An alternative method for calculating runoff from small storms
is described below.
Recently, some researchers have suggested that the assumption that I a = 0 . 2 S
does not fit the observed rainfall-runoff data nearly as well as I a = 0 . 05 S .
The incorporation of this assumption into the curve number method results
in a new runoff equation and a new curve number. Woodward [6] describes
the new runoff equation and a procedure to convert traditional CNs to new
values based on I a = 0 . 05 S . They also describe a plan to implement these
changes into all appropriate NRCS documents and computer programs. The most
notable differences in runoff modeling with these changes occur at lower curve
numbers and lower rainfalls (using the traditional curve number assumption of
I a = 0 . 2 S results in higher initial abstractions and lower runoff volumes under

these conditions). When utilized to predict runoff from developed sites in southern
California during typical design storms, the difference is likely to be insignifi-
cant. Therefore, the standard assumption is used in this chapter. The curve number
method, applied with appropriate CNs and the foregoing considerations in mind,
is recommended for typical runoff volume calculations and is included in the
worksheets.

Small Storm Hydrology Method

The small storm hydrology method (SSHM) was developed to estimate the runoff
volume from urban and suburban land uses for relatively small storm events.
Other common procedures, such as the runoff curve number method, are less
accurate for small storms as described previously. Specifically, the SSHM should
be used when the land cover with the lowest curve number for a particular project
produces 0.5 inch or less of runoff using the curve number method.