(A) The designation of flood hazard areas is set forth on the official flood hazard boundary maps of the city and county. The boundaries of all floodway and flood-fringe areas shall be designated on the preliminary and the final plat of the subdivision or development and shall be clearly marked.
(B) For the areas not otherwise designated in a floodway or a flood-fringe area, the developer shall be responsible for having an engineer, registered in the state, prepare a drainage assessment of all the area of the proposed subdivision or development, and all area 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 herein.
(2) The calculation of stream flow and runoff characteristics of the subdivision shall be carried out in consultation with the City 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
1. Q equals cubic feet per second.
2. A equals 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.
3. I equals percent of imperviousness of the area. This may vary between 40% and 95%, but no value of less than 50% may be used for saturated urban development.
4. R equals rate of rainfall over entire drainage area in inches per hour, based on time of concentration and latest government records for 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 = 1,486r2/3s ½A |
n |
1. Q equals discharge in cubic feet per second.
2. A equals cross sectional area of water in conduit in square feet.
3. r equals hydraulic radius of water in conduit.
4. s equals mean slope of hydraulic gradient, in feet of vertical rise per foot of horizontal distance.
5. n equals roughness coefficient, based on condition and type of material of conduit lining, but not less than 0.013.
(`90 Code, § 16-15) (Ord. 2570, passed - -)