(a) Method. The method outlined herein will provide a general guide as to the criteria and procedures to be used for storm sewer design.
(1) The rational method shall be used for all storm water drainage design.
Q = CIA, in which:
Q = Quantity of storm water runoff in cubic feet per second
C = Coefficient of runoff (0.4 for R-1 Zoning)
I = Average rainfall intensity in inches per hour for the period of concentration to the point under consideration
A = Drainage area in acres tributary to the point of concentration
(2) The following rainfall frequency curves shall be used for minimum design:
Storm sewer systems - 10-year rainfall
Open channel flow - 25-year rainfall
Bridges and culverts - 25-year rainfall without surcharge
Retention basin - 25-year rainfall based on a hydraulic gradient for 50-year rainfall (to be used only in extreme cases).
(3) Storm sewer pipe sizes are to be determined by using Kutters Formula with a minimum coefficient of roughness of 13 (n = . 013) for concrete pipe laid to proper line and grade.
(b) Minimum Velocity. The minimum allowable velocity shall be three feet per second (fps) in pipe.
(c) Maximum Velocity. The maximum allowable velocity shall be twelve fps in pipe and seven fps for open channel flow depending on the characteristics of the channel.
(d) Retention Basin. In event that the existing storm sewer system or natural drainage area is not sufficient to adequately accommodate additional water flow from the planned development, the developer may be required to improve the existing system or if permitted by the City, construct a storm water retention basin. Retention basins shall be of a design proposed by the developer and approved by the City.
(e) Connections to Storm Sewer System. All diverted storm water shall discharge and be connected to the City's storm sewer system, or when permitted by the City, it may outlet into a natural drainage course, i. e., an existing creek. Erosion control measures shall be used when outletting into an existing creek.
(f) Subsurface Drainage and Downspouts. No surface water other than downspouts, or subsurface water shall be piped or channeled onto any street. The developer shall be responsible for piping this water into a piped storm sewer system.
(g) Open Ditches Prohibited; Erosion Control. Open ditches shall not be constructed within a subdivision. Only existing natural drainage ditches may be permitted to remain open, or piping of this by the developer may be required. If a natural drainage area is left open the developer may be required to construct erosion control measures along the natural drainage area. If erosion occurs, the developer shall be required to provide erosion control measures to abate the erosion, after first correcting the damage. Erosion control shall be supplied at all storm system outfalls by the developer.
(h) Overland Erosion Problems. The developer shall take steps necessary to control erosion during and after construction. Diversion, straw dikes, rip-rap, sediment netting and the establishment of proper vegetative cover are among the items that may be required to control erosion problems.
(i) Curbs and Gutters. Reinforced concrete curbs and gutters shall be constructed in all subdivisions along streets in accordance with the Construction Regulations. All curbs and gutters shall connect to an existing storm sewer system.
(j) Culverts. Culverts shall be installed as necessary. Headwalls are required at the entrance to all culverts.
(Ord. 85-11. Passed 4-1-85.)