V. STORM DRAINAGE RUNOFF CALCULATIONS

      A.   Rational Formula.

         1.   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 a 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 increase. 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".

         1.   Runoff coefficients used in the rational formula shall be based on the guidelines in the following table and acceptable to the Township Engineer.

RUNOFF COEFFICIENTS FOR RATIONAL METHOD
Type of Drainage Area	Coefficient	Source
Commercial	0.70 - 0.95	1
Downtown (also shopping centers)
Neighborhood (also mixed residential and commercial)	0.50 - 0.70	1
Residential
Urban
Single-Family	0.30 - 0.50	1
Multi-Units	0.40 - 0.80	1
Suburban, Single Family	0.25 - 0.40	1
Garden Apartments	0.50 - 0.70	1
Playgrounds	0.20 - 0.70	1
Industrial	0.50 - 0.90	1
Railroad Yards	0.20 - 0.35	1
Parks, Cemeteries, Golf Courses	0.15 - 0.30	2
Unimproved, with Vegetation	0.10 - 0.30	1

 

 

RUNOFF COEFFICIENTS FOR RATIONAL METHOD
Character of Surface	Coefficient	Source
Pavement
Concrete or Bituminous Concrete	0.75 - 0.95	2
Surface Treated Roadway	0.65 - 0.80	2
Gravel, earth, other	0.25 - 0.60	2

 

Type of Drainage Area	Coefficient	Source
Roofs	0.75 - 0.95	1
Lawns, Sandy Soil
Flat, 2% or less	0.05 - 0.10	1
Average, 2% to 7%	0.18 - 0.22	1
Steep, 7% or more	0.25 - 0.35	1
Woods, Heavy Brush
Gravel	0.15 - 0.35	2
Sandy Soil	0.15 - 0.30	2
Clay Soil	0.25 - 0.60	2
Bare, Cultivated, or Light Growth
Gravel	0.20 - 0.40	2
Sandy Soil	0.15 - 0.30	2
Clay Soil	0.35 - 0.75	2

NOTE: Lower portions of ranges apply to flat slopes and open soils; higher portions apply to steeper slopes and tight or shallow soils.

         1.   Design and Construction of Sanitary and Storm Sewers - prepared by a Joint Committee of American Society of Civil Engineers and the Water Pollution Control Federation.

         2.   Design Manual, Part 2, Chapter 12, Pennsylvania Department of Transportation.

      C.   Values of Rainfall Intensity "I".

         1.   Rainfall intensities to be used in the rational formula are as follows:

Frequency of Storm (Years)
Time of Concentration minutes	5	10	25	100
5	4.90	5.40	6.0	7.30
10	3.80	4.30	4.8	6.00
15	3.30	3.70	4.1	5.00
20	2.80	3.20	3.6	4.50
25	2.53	2.88	3.23	4.03
30	2.25	2.55	2.85	3.55

 

 

Frequency of Storm (Years)
Time of Concentration minutes	5	10	25	100
40	1.92	2.18	2.45	3.12
50	1.65	1.90	2.12	2.77
60	1.45	1.70	1.85	2.50
120	.92	1.10	1.30	1.60

 

      D.   Average Velocities for Overland Flow.

         1.   The time of concentration can be determined from the overland flow velocities listed in the following table.

 

Description of Course or Runoff Water	Percent Slope vs. Velocities (ft. per second)
	0-2%	2-4%	4-7%	7-10%	15%	20%
Woodland or Dense Grass	0.5	1.0	1.5	1.7	2.0	2.7
Pasture of Average Grass	0.8	1.5	2.2	2.6	3.0	4.1
Row Crop	1.0	2.0	3.0	3.5	4.0	4.5
Pavement	2.0	5.0	8.0	12.0	15.0	18.0
Natural Draw	0.8	2.5	4.0	6.0

 

      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 = wetted perimeter, the length of the line of contact between the water and the bottom and sides of the channel or 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 Township 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	4-5
Bunch grass-soil exposed	2-4
Stiff stemmed grass	2-3
Earth without vegetation
Fine sand or silt	1-2
Ordinary firm loam	2-3
Stiff clay	3-5
Clay and gravel	4-5
Course gravel	4-5
Soft shale	5-6
Other
Bituminous or cement stabilized channels	6
Paved channels	10-15

 

(as amended by Res. 1992-29, 8/19/1992)