§ 126-672. Size.

In a subdivision, the sizing of inlets, storm sewers, outfalls, culverts and drainage ditches will be based on the following:

(1)

Design storm. The design storm will be based on the rainfall intensity-frequency data used by the county flood control district. The interior drainage system will be designed for a storm with a frequency of occurrence of once in five years. The intensity of a 60-minute rainfall on this frequency is 2.80 inches. Major drainage ditches and structures will be designed for a storm with a frequency of occurrence of once in 25 years. The intensity of a 60-minute rainfall on this frequency is 3.75 inches.

(2)

Runoff computation. To determine the runoff rates for various areas, the standard rational method will be used, utilizing the following formula:

Q = CIA

Where:

Q	=	Rate of runoff in cubic feet per second
C	=	Runoff coefficient
I	=	Rainfall intensity for the particular duration in inches per hour
A	=	Drainage area in acres

 

Drainage areas will be computed by considering the location of high and low points on street grades, drainage divides in area and the general configuration of existing and finishing grades. Calculations for each major storm sewer line shall be shown on a drainage data sheet and shall be included in the proposed plans. All pertinent information (i.e., drainage areas, time of concentration, rainfall intensity, runoff coefficients, etc.) shall be listed on the data sheet.

(3)

Sizing of storm sewers. Sewers shall be sized to carry the discharge (Q) derived from the formula in subsection (2) of this section. Capacity of storm sewers will be determined by the use of Mannings formula on the basis of hydraulic gradients rather than the physical slope of the pipe.

(4)

Sizing and spacing of inlets. Inlets shall be spaced so that maximum travel distance of water in a gutter will not exceed 600 feet, unless otherwise approved at all low points in the gutter gradient. Inlets will be sized using an allowable capacity of one cubic foot per second per foot of opening for a throat height of five inches. Inlet design shall conform to standards on file with the city engineer.

(Code 1967, § 27-72(a); Ord. No. 3655, § 3, 7-28-83; Ord. No. 4000, § 1, 11-8-84)