Table 1
Runoff Factors for the Rational Equation
| ||
---|---|---|
Runoff Factor "C"
| ||
Type of Drainage Area or Surface
|
Minimum
|
Maximum
|
Roofs
|
0.90
|
1.00
|
Pavements, concrete or bituminous concrete
|
0.75
|
0.95
|
Pavements, bituminous macadam or surface-treated gravel
|
0.65
|
0.80
|
Pavements, gravel, macadam, etc.
|
0.25
|
0.60
|
Sandy soil, cultivated or light growth
|
0.10
|
0.30
|
Sandy soil, woods or heavy brush
|
0.10
|
0.30
|
Gravel, bare or light growth
|
0.20
|
0.40
|
Gravel, woods or heavy brush
|
0.10
|
0.35
|
Clay soil, bare or light growth
|
0.25
|
0.75
|
Clay soil, woods or heavy growth
|
0.15
|
0.60
|
Borough business sections
|
0.60
|
0.80
|
Dense residential sections
|
0.50
|
0.70
|
Suburban, normal residential areas
|
0.35
|
0.60
|
Rural areas, parks and golf courses
|
0.10
|
0.30
|
NOTE: In selecting the C factor, high values shall be applied
to denser soils and steep slopes; consideration shall be given to
the future land uses in the drainage area; and a weighted value of
C shall be given to a drainage area which contains several different
types of ground cover.
|
Table 2
Recommended Average Velocities of Runoff Flow for Determining
Time of Concentration
| ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
Description of Course of Runoff Water
|
Velocities in Feet Per Second (slope in percent)
| |||||||||
0.5
|
1.0
|
1.5
|
2.0
|
2.5
|
3.0
|
3.5
|
4.0
|
4.5
|
5.0
| |
Paved
|
.79
|
.93
|
1.00
|
1.05
|
1.10
|
1.13
|
1.14
|
1.19
|
1.22
|
1.26
|
Bare shaley soil
|
.61
|
.70
|
.75
|
.80
|
.83
|
.85
|
.89
|
.90
|
.93
|
.96
|
Bare soil
|
.51
|
.55
|
.58
|
.63
|
.65
|
.66
|
.68
|
.70
|
.72
|
.74
|
Wild grass
|
.34
|
.39
|
.42
|
.46
|
.47
|
.48
|
.49
|
.51
|
.52
|
.54
|
Average grass
|
.26
|
.30
|
.32
|
.34
|
.35
|
.37
|
.38
|
.40
|
.41
|
.42
|
Dense grass
|
.20
|
.23
|
.25
|
.27
|
.28
|
.29
|
.30
|
.30
|
.31
|
.32
|