Environmental Engineering Reference
In-Depth Information
Table 6-3 Reduction Factors to Volumetric Runoff Coefficients (
R
v
) for
Disconnected Impervious Surfaces
a
Medium- to
Medium- to
Strip Commercial
High-Density
High-Density
Rainfall
and Shopping
Residential with
Residential
mm
(in.)
Center
Paved Alleys
Without Alleys
1
0.04
0.00
0.00
0.00
3
0.12
0.00
0.08
0.00
5
0.20
0.47
0.11
0.11
10
0.39
0.90
0.16
0.16
15
0.59
0.99
0.20
0.20
20
0.79
0.99
0.29
0.21
25
1.00
0.99
0.38
0.22
30
1.25
0.99
0.46
0.22
38
1.50
0.99
0.59
0.24
50
2.00
0.99
0.81
0.27
80
3.15
0.99
0.99
0.34
125
4.92
0.99
0.99
0.46
Source:
[7].
a
For low-density residential, use connected values for pervious surfaces with clayey soil from Table
330-13 in [7].
Infiltration Models for Runoff Calculations
Several computer packages offer the choice of using soil infiltration models as the
basis of runoff volume and rate calculations. Horton developed perhaps the best-
known infiltration equation—an empirical model that predicts an exponential
decay in the infiltration capacity of soil toward an equilibrium value as a storm
progresses over time [8]. Green and Ampt [9] derived another equation describing
infiltration based on physical soil parameters. As the original model applied only
to infiltration after surface saturation, Mein and Larson [10] expanded it to predict
the infiltration that occurs up until saturation. These infiltration models estimate
the amount of precipitation excess occurring over time. The excess must then
be transformed to runoff with other procedures to predict runoff volumes and
hydrographs.
Urban Runoff Quality Management
Another method for calculating runoff volume is the urban runoff quality man-
agement approach [11]. This approach is suitable for planning-level estimates of
the size of volume-based BMPs. This approach is based on the translation of
rainfall to runoff. The urban runoff quality management approach is based on
two regression equations. The first regression equation relates rainfall to runoff.
The rainfall to runoff regression equation was developed using two years of
data from more than 60 urban watersheds nationwide. The second regression
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