(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.
|
|
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.
 |
|
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
| ||
