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For all areas not otherwise designated in a floodway or floodway-fringe area, the developer shall be responsible for having an engineer, registered in the State, prepare a drainage assessment of all of the area of the proposed subdivision and all areas affected by runoff resulting from development of the proposed subdivision in accordance with the following provisions:
A. The one hundred (100) year maximum flood shall be used as the basis for the sizing of all drainage channels, bridges, and other structures, unless otherwise specified in this section;
B. 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, 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; and
C. 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:
1. Runoff from all drainage areas shall be not less than that determined by the Rational Formula:
Q = AIR
Q Cubic feet per second.
A Area to be drained, in acres, determined by field surveys for areas less than six hundred forty (640) acres and by latest government quadrangle maps for larger areas.
I Percent of imperviousness of the area. This may vary between forty percent (40%) and ninety-five percent (95%), but no value of less than fifty percent (50%) 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.
2. The size of closed storm sewers, open channels, culverts, and bridges shall be not less than that determined by using the Manning Formula:
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.
(Code 1991, § 12-498)
All floodways located within the subdivision shall be designated by the developer as "flowage easements" and shall be maintained as permanent open space for private recreation or agriculture for which no buildings or structures are required or dedicated to the public for drainage, recreation, and utility use.
(Code 1991, § 12-499)
All floodway-fringe areas shall be planned for uses which are permitted in the F2, floodway fringe district of the zoning regulations, and in no case shall the proposed use or construction cause a displacement of floodwater in the floodway fringe that will increase flooding in other areas of the flood way or floodway-fringe.
(Code 1991, § 12-500)
When it is determined by the City Engineer that the development of the subdivision will significantly increase runoff in the flood hazard area or will otherwise adversely affect stormwater runoff, the Planning Commission may require any or all of the following to the extent needed to reduce the adverse effects of the development:
A. The existing floodway lying within or immediately adjacent to the subdivision shall be cleaned to provide for the free flow of water, and the channel shall be straightened, widened, and improved to the extent required to prevent overflow beyond the limits of the floodway;
B. Site improvement shall provide for the grading of all building sites and streets to an elevation where all lots, building areas, and streets will not be subject to overflow and in a manner that will provide for the rapid runoff of all rainfall; however, such improvements shall be carried out in a manner that will preserve and protect large trees and attractive physical features of the area;
C. Whenever channel improvement is carried out, sodding, back sloping, cribbing, and other bank protection shall be designed and constructed to control erosion for all the anticipated conditions of flow for the segment of channel involved;
D. A drainage channel shall not be located in a street easement, unless it is placed in an enclosed storm sewer, or unless a paved street surface is provided on both sides of a paved channel to give access to abutting properties; or
E. Culverts, bridges, and other drainage structures shall be constructed in accordance with the specifications of the City at all locations where drainage channels intersect with continuous streets or alleys.
(Code 1991, § 12-501)
All minor tributary and surface drainage channels located within the subdivision shall be improved in accordance with the standards set forth in the "Improved Drainage Channel" charts in Section 94-290, or other equivalent standards, as determined by the City Engineer. Sections on Chart No. 4 shall be used only for channels of minor tributaries which drain less than eighty (80) acres and which are designed as an integral part of the landscape of the area, so that maintenance of sodded slopes will be the responsibility of the property owners abutting the channel. All channels shall be designed to carry a one hundred (100) year maximum flood, shall be designed for self-cleaning and ease of maintenance, shall have sufficient hard surface along the flow line to prevent ponding of water, and shall have design characteristics of alignment, materials, or construction and cross-sectioned elements that will be hydraulically efficient and visually harmonious with the adjacent landscape.
(Code 1991, § 12-502)
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