§ 53.030 DETERMINATION OF RUNOFF QUANTITIES.

Runoff quantities shall be computed for the area of the parcel under development plus the area of the watershed flowing into the parcel under development. The quantity of runoff which is generated as the result of a given rainfall intensity may be calculated as follows:

   (A)   For areas up to and including 100 acres, the Rational Method may be used. In the Rational Method, the peak rate of runoff, Q, in cubic feet per second is computed as:

   Q = CIA

   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.

   A = tributary drainage area in acres.

   (B)   Values for the runoff coefficient "C" are shown in Table I and Table IA 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 value for the total area calculated from a breakdown of individual areas having different surface types.

   (C)   Table 2 provides runoff coefficients 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 assuming ultimate development of the land. The ultimate development of currently undeveloped land shall be determined by using the Comprehensive Land Use Plan for the town, and also for Madison County, for areas outside the limits of the town.

TABLE I
Values Used to Determine a Composite Runoff Coefficient*

Type of Surface	Runoff Coefficient "C"
Asphalt	0.90
Concrete	0.95
Roof	0.95
Lawns
Flat (0-2% Slope)	0.20
Rolling (2-7% Slope)	0.25
Steep (greater than 7%)	0.35

 

*Adopted from HERPICC Stormwater Drainage Manual, July 1994, Table 3.2.3

TABLE IA
Rural Runoff Coefficients**

Type of Surface	Runoff Coefficient "C"
Woodland
Flat (0-5% Slope)	0.40
Rolling (5-10% Slope)	0.50
Steep (greater than 10%)	0.60
Pasture
Flat	0.40
Rolling	0.55
Steep	0.60
Cultivated
Flat	0.60
Rolling	0.70
Steep	0.82

 

**Adopted from HERPICC Stormwater Drainage Manual, July 1994, Table 3.2.1

TABLE 2
Urban Runoff Coefficients***

Land Use	Runoff Coefficients
Business (Downtown)	0.70 to 0.95
Business (Neighborhood)	0.50 to 0.70
Industrial	0.50 to 0.90
Apartment	0.50 to 0.70
Residential - Attached	0.60 to 0.75
Residential - Detached	0.40 to 0.60
Residential - Single Family	0.30 to 0.50
Residential - Suburban	0.25 to 0.40
Parks - Cemeteries	0.10 to 0.25

 

***Adopted from HERPICC. Stormwater Drainage Manual, July 1994, Table 3.2.2

   (D)   Rainfall intensity shall be determined from the Intensity - Duration Frequency Equation 2.2.13 using regional coefficients from Table 2.2.2 for Indianapolis. Equation 2.2.13 and Table 2.2.2 are found in the HERPICC Stormwater Drainage Manual dated July 1994. 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.

   (E)   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.

   (F)   The runoff rate for areas in excess of 100 acres shall be determined by methods described in § 53.085.

(Ord. 1999-05, passed 6-8-99)