§ 153.06 DETERMINATION OF RUNOFF QUANTITIES.

   (A)   Development sites less than or equal to five acres in size, with a drainage area less than or equal to 50 acres and no depressional storage.

   (B)   (1)   The rational method may be used for sites meeting the stated criteria. In the rational method, the peak rate of runoff, “Q,” in cubic feet per second (cfs) is computed as the following.

 

Q = CIA
C	Runoff coefficient, replacing the characteristics of the drainage area and defined as the ratio of runoff to rainfall
I	Average intensity of rainfall in inches per hour for a duration equal to the time of concentration (tc) for a selected rainfall frequency
A	Tributary drainage area in acres

 

      (2)   Guidance to selection of the runoff coefficient “C” is provided by Table 1 and Table 1A below which show values for different types of surfaces and local soil characteristics. The composite “C” value used for a given drainage area with various surface types shall be the weighted average value for the total area calculated from a breakdown of individual areas having different surface types.

      (3)   Table 2 below provides runoff coefficients and inlet times for different land use classifications. In the instance of undeveloped land situated in an upstream area, a coefficient or coefficients shall be used for this area in its present or existing state of development.

Table 1 Urban Runoff Coefficients
Hard Surfaces
Asphalt	0.82
Concrete	0.85
Roof	0.85
Lawns (Clay)
Flat (0-2% slope)	0.16
Rolling (2-7% slope)	0.21
Steep (greater than 7% slope)	0.30
Lawns (Sandy)
Flat (0-2% slope)	0.07
Rolling (2-7% slope)	0.12
Steep (greater than 7% slope)	0.17
Source: HERPICC Stormwater Drainage Manual, July 1994   The coefficients of this tabulation are applicable to storms of 5- to 10-year frequencies. Coefficients for less frequent higher intensity storms shall be modified as follows:         Return Period (years)            Multiply “C” by       25                        1.1       50                        1.2       100                        1.25

Table 1A Rural Runoff Coefficients
Cultivated (Clay)
Flat (0-5% slope)	0.50
Rolling ( 5-10% slope)	0.60
Steep (greater than 10% slope)	0.72
Cultivated (Sandy)
Flat (0-5% slope)	0.30
Rolling (5-10% slope)	0.40
Steep (greater than 10% slope)	0.52
Pasture (Clay)
Flat (0-5% slope)	0.30
Rolling (5-10% slope)	0.36
Steep (greater than 10% slope)	0.42
Pasture (Sandy)
Flat (0-5% slope)	0.10
Rolling (5-10% slope)	0.16
Steep (greater than 10% slope)	0.22
Woodland (Clay)
Flat (0-5% slope)	0.30
Rolling (5-10% slope)	0.35
Steep (greater than 10% slope)	0.50
Woodland (Sandy)
Flat (0-5% slope)	0.10
Rolling (5-10% slope)	0.25
Steep (greater than 10% slope)	0.30
Source: HERPICC Stormwater Drainage Manual, July 1994. The coefficients of this tabulation are applicable to storms of 5- to 10-year frequencies. Coefficients for less frequent higher intensity storms shall be modified as follows:       Return Period (years)         Multiply “C” by          25                  1.1          50                  1.2          100                  1.25

Table 2 Runoff Coefficients “C” By Land Use and Typical Inlet Times
Land Use	Runoff Coefficients			Inlet Times (Minutes) (4)
	Flat (1)	Rolling (2)	Steep (3)
Churches	0.54	0.60	0.66	5
Commercial (CBD)	0.75	0.83	0.91	5-10
Commercial (neighborhood)	0.54	0.60	0.66
Garden apartments	0.54	0.60	0.66
Industrial	0.63	0.70	0.77
Detached residential	0.40	0.45	0.50	10-15 To be computed
Schools	0.31	0.35	0.39
Semi-detached residential	0.45	0.50	0.55
Half-acre lots	0.31	0.35	0.39
Parkland	0.18	0.20	0.22
Quarter acre lots	0.36	0.40	0.44
(1) Flat terrain involves slopes of 0-2% (2) Rolling terrain involves slopes of 2-7% (3) Steep terrain involves slopes greater than 7% (4) Interpolation, extrapolation and adjustment for local conditions shall be based on engineering experience and judgment (5) The coefficients of this tabulation are applicable to storms of 5- to 10-year frequencies. Coefficients for less frequent higher intensity storms shall be modified as follows:         Return Period (years)      Multiply “C” by          25               1.1          50               1.2          100               1.25   Rainfall intensity shall be determined from the rainfall frequency curves shown in Tables 5A in § 153.06(c)(3)(b) and Table B in this division (B). The time of concentration (tc) to be used shall be the sum of the inlet time and flow time in the drainage facility from the most remote part of the drainage area to the point under consideration. The flow time in the storm sewers may be estimated by the distance in feet divided by velocity of flow in feet per second. The velocity shall be determined by the Manning formula Inlet time is the combined time required for the runoff to reach the inlet of the storm sewer. It includes overland flow time and flow time through established surface drainage channels such as swales, ditches and sheet flow across such areas as lawns, fields and other graded surfaces. It may be computed by using the image below, also known as Figure 1 as shown in this division (B)

 

(Ord. 2011-03, passed 4-4-2011)