(A) Flow calculations submitted to the City shall be based on the rational method “Q = CIA” for developments with catchment areas less than ten acres, where Q is the computed runoff in cubic feet per second (cfs), C is the weighted C factor, I is the intensity for a ten-year storm event, and A is the total acreage in the tributary study area. For developments with catchment areas greater than ten acres, the “TR 55” method or other method acceptable to the Director of Public Services will be used for design.
(B) The weighted C value indicates the percent of runoff expected at peak conditions. The weighted coefficient will be computed as the sum of areas multiplied with the associated C value and then divided by the total drainage area. C values for areas of development will be taken from the following table:
Description of Area Coefficients
Business
Downtown areas 0.70 - 0.95
Neighborhood areas 0.50 - 0.70
Residential
Single-family areas 0.30 - 0.50
Multi-unit, detached 0.40 - 0.60
Multi-unit, attached 0.60 - 0.75
Residential (suburban) 0.25 - 0.40
Apartment dwelling areas 0.50 - 0.70
Industrial
Light areas 0.50 - 0.80
Heavy areas 0.60 - 0.90
Parks, cemeteries 0.10 - 0.25
Playgrounds 0.20 - 0.35
Railroad yard areas 0.20 - 0.40
Unimproved areas 0.10 - 0.30
Streets
Asphalt 0.90 - 0.95
Concrete 0.90 - 0.95
Brick 0.80 - 0.90
Drives and walks 0.80 - 0.90
Roofs 0.80 - 0.95
Lawns, sandy soil
Flat, 2% 0.05 - 0.10
Average, 2% - 7% 0.10 - 0.15
Steep, 7% 0.15 - 0.20
Lawns, heavy soil
Flat, 2% 0.13 - 0.17
Average, 2% - 7% 0.18 - 0.22
Steep, 7% 0.25 - 0.35
(C) The value used for the intensity shall be obtained using the ten-year storm event as shown in Appendix A. The time of concentration used will be calculated by a reasonable method and presented to the Director of Public Services for review. Developments located in the following zones will require a minimum time of concentration of 5 minutes: Residential/Commercial, Commercial 1, 2, or 3 and Industrial (RC, C-1, C-2, C-3, and I). Residential developments (R-1, R-2, and R-3) will require a minimum time of concentration of 15 minutes.
(D) The total design acreage should include the entire catchment area, either within or contiguous to the developed limits where water traverses to the proposed drain. The proprietor must provide information to delineate the drainage limits and determine the anticipated catchment area.
(E) The Director of Public Services reserves the right to request a field determination of the catchment boundary to be used for design in order to fulfill the intent and purpose of this chapter.
(Ord. 420-4-97, passed 4-21-97)