A. Flood Hazard Areas: The designation of flood hazard areas is set forth on the flood hazard boundary map. The boundaries of all floodway and flood fringe areas shall be designated on the preliminary and the final plat and shall be clearly marked.
B. Alternative Methods For Determining Stream Flow And Runoff Characteristics: For all areas not otherwise designated in a floodway or a flood fringe area, the developer shall be responsible for having an engineer, registered in Oklahoma, prepare a drainage assessment of all of the area of the proposed subdivision and all areas affected by runoff resulting from development of proposed subdivision in accordance with the following provisions:
1. The 100-year maximum flood shall be used as the basis for the sizing of all drainage channels, bridges and other structures unless otherwise specified.
2. The calculation of all runoff shall be based on saturated urbanization of the drainage basin for minor tributaries and surface drainage, as reflected in the comprehensive plan for the city and shall be based on the maximum degree of urbanization, as reflected in the comprehensive plan for the drainage basin of a major tributary or river.
3. The calculation of stream flow and runoff characteristics of the subdivision shall be carried out in consultation with the city's consulting engineer, and the methodology and formulas used shall result in quantities which would be not less than those derived from the application of the following formulas and values:
a. Runoff from all drainage areas shall be not less than that determined by rational formula:
Q = AIR
Q - Cubic feet per second.
A - Area to be drained in acres, determined by field surveys for areas less than 640 acres, and by latest government quadrangle maps for larger areas.
I - Percent of imperviousness of the area. This may vary between 40 percent and 95 percent, but no value of less than 50 percent may be used for saturated urban development.
R - Rate of rainfall over entire drainage area in inches per hour, based on time of concentration and latest government records for the area. Note: Do not use for areas above 15,000 square feet.
b. The size of closed storm sewers, open channels, culverts, and bridges shall be not less than that determined by using the Manning formula:
Q + 1,486r2/3s1/2 A |
n |
Q | - | Discharge in cubic feet per second. |
A | - | Cross sectional area of water in conduit in square feet. |
r | - | Hydraulic radius of water in conduit. |
s | - | Mean slope of hydraulic gradient, in feet of vertical rise per foot of horizontal distance. |
n | - | Roughness coefficient, based on condition and type of material of conduit lining, but not less than 0.013. |
(Ord. 1978-10, 10-3-1978)