Figure 1. Rational Method of Design
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Q = C/A
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Where:
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Q
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=
|
Rate of runoff in cubic feet per second.
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C
|
=
|
Weighted runoff coefficient (average of the coefficients assigned
to the different types of contributing areas).
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I
|
=
|
Average rainfall intensity, inches per hour, for the selected
frequency and for duration equal to the time of concentration.
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A
|
=
|
Drainage area, in acres, tributary to the point under design.
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Values of runoff coefficient (C) for various types of contributing
areas, indicated in Table 1 and Table 2, shall be utilized in the
solution of this method of design.
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Table 1. Runoff Coefficient (C) for Use in Rational Method
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---|---|---|---|
Type of Surface
|
Runoff Coefficient (C)
|
Notes
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Rural areas
|
0.15 to 0.3
|
1. For flat slopes or permeable soil, use the lower values.
For steep slopes or impermeable soil, use the higher values.
2. Contributing drainage areas shall be evaluated based on the
fully developed land in accordance with the existing or proposed zoning.
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Concrete or sheet asphalt pavement
|
0.8 to 0.9
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Asphalt macadam pavement
|
0.6 to 0.8
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Gravel roadways or shoulders
|
0.4 to 0.6
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Bare earth
|
0.2 to 0.9
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Steep grassed areas (2:1)
|
0.5 to 0.7
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Turf meadows
|
0.1 to 0.4
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Forested areas
|
0.1 to 0.3
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Cultivated fields
|
0.2 to 0.4
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Table 2. Runoff Coefficient (C) for Different Types of
Surfaces
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---|---|---|
Type of Surface
|
Runoff Coefficient (C)
| |
Flat residential, with about 30% of area impervious
|
0.40
| |
Flat residential, with about 60% of area impervious
|
0.55
| |
Moderately steep residential, with about 50% of area impervious
|
0.65
| |
Moderately steep built-up area, with about 70% of area impervious
|
0.80
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Flat commercial, with about 90% of area impervious
|
0.80
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Figure 2. Manning's Equation
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V = 1.49 R2/3 S1/2/n
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Where:
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V
|
=
|
Velocity in feet per second.
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n
|
=
|
Manning's coefficient of channel roughness.
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R
|
=
|
Hydraulic radius in feet.
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S
|
=
|
Slope, in feet per foot.
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Values of Manning's coefficients (n) for various types
of channel linings shall be used as indicated in Table 3.
|
Table 3. Manning's Roughness Coefficients (n)
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---|---|---|---|
Type of Conduit
|
Roughness Coefficient (n)
| ||
Closed Conduits
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Reinforced concrete pipe (RCP)
|
0.012
| ||
Corrugated metal pipe (CMP) or pipe arch plain or fully coated
(unpaved)
|
0.024
| ||
High-density polyethylene (HDPE)
|
0.011
| ||
25% of circumference paved
|
0.021
| ||
Fully paved
|
0.012
| ||
Lined Open Channels
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Concrete (float finish)
|
0.014
| ||
Plain riprap
|
0.040
| ||
Grouted riprap
|
0.035
| ||
Swales And Channels With Maintained Vegetation
| |||
Grass (mowed to 2 inches)
|
0.045
| ||
Grass (good stand: 12-inch height)
|
0.09
| ||
Street Gutters
| |||
Concrete
|
0.012
| ||
Asphalt
|
0.013
|