LID DESIGN CALCULATIONS AND METHODOLOGY - Low Impact Development and Sustainable Stormwater Management - page 118

equation relates mean annual runoff-producing rainfall depths to the maximized
water quality capture volume , which corresponds to the “knee” of the cumulative
probability curve. This second regression was based on analysis of long-term
rainfall data from seven rain gages representing climatic zones across the coun-
try. The maximized water quality capture volume corresponds to approximately
the 85th percentile runoff event, and ranges from 82 to 88%.

6.3 EXISTING METHODOLOGIES FOR PEAK-RATE/HYDROGRAPH
ESTIMATES

The Rational Method

The rational method has been used for over 100 years to estimate peak runoff
rates from relatively small, highly developed drainage areas. Both the standard
and modified rational methods may be the most commonly used runoff methods
in many states. The peak runoff rate from a given drainage area is given by

Q p = C × I × A

where

Q p = peak runoff rate [cubic feet per second (ft 3 /sec)]
C = runoff coefficient of the area (assumed to be dimensionless)
I = average rainfall intensity (in./hr) for a storm with a duration equal
to the time of concentration of the area
A = size of the drainage area (acres)

The runoff coefficient is usually assumed to be dimensionless because 1 acre-
inch per hour is very close to 1 cubic foot per second (1acre-in./hr = 1 . 008
ft 3 /sec). Although it is a simple and straightforward method, estimating both the
time of concentration and the runoff coefficient introduces considerable uncer-
tainty in the peak runoff rate calculated. In addition, the method was developed
for relatively frequent events, so the peak rate as calculated above should be
increased for more extreme events. Because of these and other serious deficien-
cies, the rational method should be used to predict the peak runoff rate only for
very small, highly impervious areas [12].
Although this method has been adapted to include estimations of runoff hydro-
graphs and volumes through the modified rational method, the universal rational
hydrograph, the DeKalb rational hydrograph, and so on, these are further compro-
mised by assumptions about the total storm duration and therefore should not be
used to calculate volumes related to water quality, infiltration, or capture-reuse.

The NRCS (SCS) Unit Hydrograph Method

In combination with the curve number method for calculating runoff depth, the
U.S. Soil Conservation Service (now the NRCS) also developed a system to