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 200 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
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. A = tributary drainage area in acres. Guidance to selection of the runoff coefficient “C” is provided by Table 1 and Table 1A with 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.
Table 2 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 | |
Type of Surface | Runoff Coefficient “C” |
TABLE 1 Urban Runoff Coefficients | |
Type of Surface | Runoff Coefficient “C” |
Asphalt | 0.82 |
Concrete | 0.85 |
Roof | 0.85 |
Lawns (Sandy) | |
Flat (0-2% Slope) | 0.07 |
Rolling (2-7% Slope) | 0.12 |
Steep (greater than 7%) | 0.17 |
Lawns (Clay) | |
Flat (0-2% Slope) | 0.16 |
Rolling (2-7% Slope) | 0.21 |
Steep (greater than 7%) | 0.30 |
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 | |
Type of Surface | Runoff Coefficient “C” |
TABLE 1A Rural Runoff Coefficients | |
Type of Surface | Runoff Coefficient “C” |
Woodland (Sandy) | |
Flat (0-5% Slope) | 0.10 |
Rolling (5-10% Slope) | 0.25 |
Steep (greater than 10%) | 0.30 |
Woodland (Clay) | |
Flat | 0.30 |
Rolling | 0.35 |
Steep | 0.50 |
Pasture (Sandy) | |
Flat | 0.10 |
Rolling | 0.16 |
Steep | 0.22 |
Pasture (Clay) | |
Flat | 0.30 |
Rolling | 0.36 |
Steep | 0.42 |
Cultivated (Sandy) | |
Flat | 0.30 |
Rolling | 0.40 |
Steep | 0.52 |
Cultivated (Clay) | |
Flat | 0.52 |
Rolling | 0.60 |
Steep | 0.72 |
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) | ||
Flat | Rolling | Steep | ||
TABLE 2 Runoff Coefficients “C” By Land Use And Typical Inlet Times | ||||
LAND USE | Runoff Coefficients | Inlet Times (minutes) | ||
Flat | Rolling | Steep | ||
Commercial (CBD) | 0.75 | 0.83 | 0.91 | 5 |
Commercial (Neighborhood) | 0.54 | 0.60 | 0.66 | |
Industrial | 0.63 | 0.70 | 0.77 | 5-10 |
Garden Apartments | 0.54 | 0.60 | 0.66 | 5-10 |
Churches | 0.54 | 0.60 | 0.66 | 5-10 |
Schools | 0.31 | 0.35 | 0.39 | 10-15 |
Semi-Detached Residential | 0.45 | 0.50 | 0.55 | 10-15 |
Detached Residential | 0.40 | 0.45 | 0.50 | 10-15 |
Quarter Acre Lots | 0.36 | 0.45 | 0.44 | 10-15 |
Half Acre Lots | 0.31 | 0.35 | 0.39 | 10-15 |
Parkland | 0.18 | 0.20 | 0.22 | To Be Computed |
(1) Flat terrain 0-2% slopes.
(2) Rolling terrain 2-7% slopes.
(3) Steep terrain greater than 7% slopes.
(4) Interpolation, extrapolation and adjustment for local conditions shall be based on engineering experience and judgement.
(5) The coefficients of this tabulation are applicable to storms of 5 to 20 year frequencies. Coefficients for less frequent higher intensity storms shall be modified as follows:
Return Period Multiply “C” by
25 1.1
50 1.2
100 1.25
Rainfall intensity shall be determined from the rainfall frequency curves shown in Figure 1 or from data shown in Table 5A. The time on 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 Manning’s Equation (see § 55.23(A)).
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 Figure 2.
(B) The runoff rate for areas in excess of 200 acres shall be determined by methods described in § 55.40(F)
(Ord. 16-C-98, passed 6-9-98; Am. Ord. 22-C-04, passed 11-9-04)