Q
|
=
|
CIA
| |
Where
| |||
Q
|
=
|
Peak runoff rate in cubic feet per second (cfs)
| |
C
|
=
|
Runoff coefficient equal to the ratio of the
peak runoff rate to the average rate of rainfall over a time period
equal to the time of concentration.
| |
I
|
=
|
Average rainfall intensity in inches per hour
for a time equal to the time of concentration.
| |
A
|
=
|
Drainage area in acres.
| |
NOTE:
|
Appropriate values for the runoff coefficient and rainfall intensity can be found in the Commonwealth of Pennsylvania Department of Transportation Design Manual, Part 2, Highway Design, Chapter 12.
|
Hydraulic Soil Properties Classified by
Soil Textures*
| |||
---|---|---|---|
Texture Class
|
Minimum Infiltration Rate (f)
(inches per hour)
|
Hydrologic Soil Groupings
| |
Sand
|
8.27
|
A
| |
Loamy sand
|
2.41
|
A
| |
Sandy loam
|
1.02
|
B
| |
Loam
|
.52
|
B
| |
Silt loam
|
.27
|
C
| |
Sandy clay loam
|
.17
|
C
| |
Clay loam
|
.09
|
D
| |
Silty clay loam
|
.06
|
D
| |
Sandy clay
|
.05
|
D
| |
Silty clay
|
.04
|
D
| |
Clay
|
.02
|
D
|
NOTE:
| ||
---|---|---|
*
|
Source: Rawls, W.J. Brakensiek, D.L., and Saxton,
K.E. Estimation of Soil Properties. Transactions of the American Society
of Agricultural Engineers. Volume 25, No. J, pages 1316-1320, 1982.
|
Infiltration rate (inches per hour) = 60 ÷
(percolation rate, minutes per inch)
|