§ 151.32 RATIONAL METHOD.

   (A)   In the Rational Method, the peak rate of runoff, Q, in cubic feet per second is computed as:

      Q = CIA

   Where:

   C = runoff coefficient, representing 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. The practical minimum time to be used is ten minutes

   A = tributary drainage area in acres

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

   (C)   (1)   Table 2 provides runoff coefficients for different land use classifications.

      (2)   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.

   (D)   (1)   Rainfall intensity shall be determined from the rainfall frequency table shown in Table 5A below.

      (2)   The time of concentration (tc) to be used shall be the sum of the inlet time and flow time, or travel time, in the drainage facility from the most hydraulically remote part of the drainage area to the point under consideration.

   (E)   Inlet time is the combined time required for the runoff to reach the inlet of a 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.

   (F)   The flow time, or travel 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 using Manning’s Equation.

Table 1: Urban Runoff Coefficients
Type of Surface	Runoff Coefficient “C”
Asphalt	0.82
Compacted gravel/crushed stone	0.85
Concrete	0.85
Lawns (clay)
Flat (0-2%)	0.16
Rolling (2-7%)	0.21
Steep (greater than 7%)	0.30
Lawns (sandy)
Flat (0-2%)	0.07
Rolling (2-7%)	0.12
Steep (greater than 7%)	0.17
Roof	0.85

 

Table 1A: Rural Runoff Coefficients
Type of Surface	Runoff Coefficient “C”
Cultivated (clay)
Flat	0.50
Rolling	0.60
Steep	0.72
Cultivated (sandy)
Flat	0.30
Rolling	0.40
Steep	0.52
Fallow
Farmsteads
Pasture (clay)
Flat	0.30
Rolling	0.36
Steep	0.42
Pasture (sandy)
Flat	0.10
Rolling	0.16
Steep	0.22
Woodlands (clay)
Flat	0.30
Rolling	0.35
Steep	0.50
Woodlands (sandy)
Flat (0-5% slope)	0.10
Rolling (5-10% slope)	0.25
Steep (greater than 10%)	0.30

 

Table 2: Runoff Coefficients by Land Use
Land Use	Runoff Coefficients “C”
	Flat	Rolling	Steep
Churches	0.54	0.60	0.66
Commercial	0.75	0.83	0.91
Commercial (neighborhood)	0.54	0.60	0.66
Detached residential	0.40	0.45	0.50
Garden apartments	0.54	0.60	0.66
Half-acre lots	0.31	0.35	0.39
Industrial	0.63	0.70	0.77
Park land	0.18	0.20	0.22
Quarter-acre lots	0.36	0.40	0.44
Schools	0.31	0.35	0.39
Semi-detached residential	0.45	0.50	0.55
General Notes: —Flat terrain 0-2% slopes. —Rolling terrain 2-7% slopes. —Steep terrain greater than 7% slopes. —Interpolation, extrapolation and adjustment for local conditions shall be based on engineering experience and judgment.

 

(Ord. 8-2014, passed 7-21-2014)