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Borough of Hellertown, PA
Northampton County
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Table of Contents
Table of Contents
(Reserved)
A. 
Rational formula. The rational formula used in calculating runoff is Q = CIA. In this equation, Q is the peak runoff in cubic feet per second, C is the runoff coefficient which depends on the nature of the land cover, I is the intensity of rainfall in inches per hour for a duration equal to the time of concentration for the drainage area involved, and A is the watershed area in acres. The time of concentration is the time required for runoff from the upper reaches of the watershed to reach the point for which runoff rates are being calculated.
B. 
Values of coefficient (C). Runoff coefficients used in the rational formula shall be based on the guidelines in the following table and acceptable to the Borough Engineer:
Runoff Coefficients for the Rational Method
Type of Drainage Area
Coefficient
Commercial
Downtown (also shopping centers)
0.70 to 0.95
Neighborhood (also mixed residential and commercial)
0.50 to 0.70
Residential
Urban
Single-family
0.30 to 0.50
Multi units
0.40 to 0.80
Suburban, single-family
0.25 to 0.40
Garden apartments
0.50 to 0.70
Playgrounds
0.20 to 0.70
Industrial
0.50 to 0.90
Railroad yards
0.20 to 0.35
Parks, cemeteries, golf courses
0.15 to 0.30
Unimproved, with vegetation
0.10 to 0.30
Character of Surface
Coefficient
Pavement
Concrete or bituminous concrete
0.75 to 0.95
Surface treated roadway
0.65 to 0.80
Gravel, earth, other
0.75 to 0.95
Roofs
0.75 to 0.95
Lawns, sandy soil
Flat, 2% or less
0.05 to 0.10
Average, 2% to 7%
0.10 to 0.15
Steep, 7% or more
0.15 to 0.20
Lawns, heavy soil
Flat, 2% or less
0.13 to 0.17
Average, 2% to 7%
0.18 to 0.22
Steep, 7% or more
0.25 to 0.35
Woods, heavy brush
Gravel
0.15 to 0.35
Sandy soil
0.15 to 0.30
Clay soil
0.25 to 0.60
Bare, cultivated or light growth
Gravel
0.20 to 0.40
Sandy soil
0.15 to 0.30
Clay soil
0.35 to 0.75
NOTE: Lower portions of ranges apply to flat slopes and open soils; higher portions apply to steeper slopes and tight or shallow soils.
C. 
Values of rainfall intensity (I). Rainfall intensities to be used in the rational formula are as follows:
Frequency of Storm
(years)
Time Concentration
(minutes)
5
10
25
100
5
5.9
6.5
7.4
9.0
10
4.8
5.4
6.0
7.4
15
4.0
4.6
5.2
6.4
20
3.6
4.0
4.5
5.6
25
3.1
3.6
4.0
5.0
30
2.8
3.2
3.6
4.5
35
2.5
2.9
3.3
4.1
40
2.3
2.6
3.0
3.8
45
2.1
2.4
2.8
3.5
50
2.0
2.2
2.6
3.3
55
1.8
2.1
2.5
3.1
60
1.7
2.0
2.4
2.9
D. 
Average velocities for overland flow. Engineers use several methods to determine the time of concentration for the drainage area in question. Seeley's Nomograph is reproduced here. The average velocities for overland flow presented in PennDOT's Design Manual are also widely used to determine the time of concentration. Any method acceptable to the Borough Engineer may be used.
390 Overland Flow Time.tif
NOTES:
The example shown involves a two-hundred-foot flow length over an average grass surface with a slope of 4%. The time concentration is found to be 15 minutes.
SOURCE: Elywn e. Seeley, Data Book for Civil Engineers, Volume One: Design, 3rd Edition, (New York, John Wiley and Sons Inc., 1960)
E. 
Manning's equation.
(1) 
Manning's equation to determine the velocity of flow in open channels and closed drains not under pressure is listed below. The second equation is used to determine the capacity after the velocity has been determined.
390 Equation.tif
v
=
velocity in feet per second
n
=
coefficient of roughness
a
=
cross-sectional area of flow in square feet
p
=
wetter perimeter, the length of the line of contact between the water and the bottom and sides of the channel of pipe around the cross section in feet
s
=
slope of the channel or pipe in feet per foot
q
=
capacity of the channel or pipe in cubic feet per second
(2) 
The coefficient of roughness used shall be as follows, unless different coefficients are approved by the Borough Engineer.
0.015 for concrete pipes and paved channels
0.021 for corrugated metal pipes
0.035 for earth ditches
0.040 for vegetated channels.
F. 
Maximum stream velocities in open channels.
Material
Maximum Velocity
(feet per second)
Well-established grass on good soil
Short pliant bladed grass
4 to 5
Bunch grass — soil exposed
2 to 4
Stiff stemmed grass
2 to 3
Earth without vegetation
Fine sand or silt
1 to 2
Ordinary firm loam
2 to 3
Stiff clay
3 to 5
Clay and gravel
4 to 5
Course gravel
4 to 5
Soft shale
5 to 6
Other
Bituminous or cement stabilized channels
6
Paved channels
10 to 15