A. Compliance With Design And Improvement Requirements: All subdivisions of land shall comply with the design and improvement requirements herein established for the protection of flood hazard areas and the prevention of erosion.
B. Drainage Classifications: For the purpose of this title, drainage shall be classified as follows:
1. Surface drainage is runoff of such a limited quantity and/or slow rate that it does not cause erosion of a defined channel;
2. A minor tributary is any drainage channel having a drainage basin of six hundred forty (640) acres (1 square mile) or less in area;
3. A major tributary is any channel having a drainage basin of not less than one square mile or greater than twenty five (25) square miles; and
4. A river is any channel having a drainage basin of greater than twenty five (25) square miles.
C. Allocation Of Responsibility: Responsibility for drainage shall be allocated as follows, unless otherwise specifically designated:
1. The developer of a subdivision is responsible for the following:
a. All surface drainage on the subdivision;
b. All increase in surface drainage outside the subdivision which results from the development of the subdivision;
c. The improvement of all minor tributaries lying within the subdivision;
d. Any significant increase in the rate or quantity of runoff in any minor or major tributary, or river, which results from the development of the subdivision; and
e. Provision for the maintenance of all floodway and floodway fringe areas of major tributaries and rivers which have not been dedicated to the public.
2. The city and other levels of governments will be responsible for the following:
a. The improvement of floodways of major tributaries;
b. The improvement of river floodways; and
c. The maintenance of floodway and floodway fringe areas dedicated to the public.
D. Designation On Maps And Plats: Flood hazard areas are designated on the official zoning district map of the city. The boundaries of all floodway and floodway fringe areas shall be designated on the preliminary and final plats and shall be clearly marked.
E. Preparation Of Drainage Assessment: For all areas not otherwise designated in a floodway or floodway 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 the 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 all runoff shall be based on saturated urbanization of the drainage basin for minor tributaries and surface drainage, as reflected in the community plan, and shall be based on the maximum degree of urbanization, as reflected in the community plan, for the drainage basin of a major tributary or river; and
3. The calculation of stream flow and runoff characteristics of the subdivision shall be carried out in consultation with the planning commission and the city council, 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 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 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.
b. The size of closed storm sewers, open channels, culverts and bridges shall not be less than that determined by using the Manning formula:
Q = 1,486r 2/3 s1/2 A
n
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.
F. Flowage Easements: 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.
G. Uses Restricted: All floodway fringe areas shall be planned for uses which are permitted in the appropriate district of the zoning ordinance, 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 floodway or floodway fringe.
H. Reduction Of Adverse Effects: When it is determined by the planning commission 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:
1. 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;
2. 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;
3. Whenever channel improvement is carried out, sodding, backsloping, 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;
4. 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
5. 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.
I. Minor Tributary And Surface Drainage Channels: All minor tributary and surface drainage channels located within the subdivision shall be improved in accordance with the standards set forth in subsections G through O of this section, and the diagram headed "Improved Drainage Channel, Width For 100-Year Frequency Rain, 5 - 25 Year Frequency Rain", or other equivalent standards, as determined by the city engineer. The diagram headed "Improved Drainage Channel, 100-Year Frequency Rain, 6'-6" Min." 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 100-year maximum flood, shall be designed for self-cleaning and care or maintenance, shall have sufficient hard surface along the flow line to prevent ponding of water, and shall have design characteristics of alignment, materials of construction and cross sectioned elements that will be hydraulically efficient and visually harmonious with the adjacent landscape.
J. Enclosed Storm Sewers: Enclosed storm sewers may be required by the city council where special or unusual conditions make open channels hazardous or otherwise unfeasible.
K. Site Grading: Site grading shall be carried out in such a manner that surface water from each lot shall flow directly to a storm sewer, improved channel or paved street without crossing more than two (2) adjacent lots.
L. Surface Water Diverted To Storm Drains: Surface water collected on streets shall be diverted to storm drains at satisfactory intervals to prevent overflow of six inch (6") high curbs during a 50-year frequency rain for the area and grades involved; provided, however, that in no case shall the drainage area served by one street exceed twenty (20) acres, regardless of the amount of flow.
M. Drainage Easements: Drainage easements of satisfactory width to provide working room for construction and maintenance shall be provided for all storm sewers. In no case shall the total easement be less than twenty feet (20').
N. Construction Of Closed Storm Sewers: Closed storm sewers shall be constructed of precast or prefabricated pipe or built in place of closed box design to conform with municipal construction specifications to serve a 100-year frequency rain for the drainage area involved.
O. Bridges Or Culverts: Bridges or culverts shall be provided where watercourses cross continuous streets or alleys, shall be sized to accommodate a 100-year frequency rain (based on the drainage area involved), and shall be designed in conformance with municipal construction specifications.
P. Open, Paved Storm Drainage Channels; Fences: Open, paved storm drainage channels shall be constructed in accordance with municipal construction specifications. Side slopes above the paved section shall be shaped and sodded on a slope of four horizontal to one vertical (4:1), or flatter. Fences shall be outside of the 100-year frequency flooding line, shall not be erected below the shoulder of the sodded section and in no case shall fences be closer than six feet (6') (measured horizontally) to the edge of the paved section. Hand laid riprap may be substituted for sodded shoulders where desired.
Q. City Right To Require Certain Improvements: The city reserves the right to require: 1) improvements; 2) the provision of drainage easements; and 3) the provision of agreements beyond the boundaries of the subdivision, to facilitate flow of water through the addition, to avoid probability of lawsuits, based on damage from changed runoff in the subdivision and to provide continuous improvements of the overall storm drainage system. The following kinds of improvements may be required:
1. Enlargement or replacement of undersized drainage structures to provide free flow;
2. Removal of obstructions;
3. Straightening of channel;
4. Widening or deepening of the channel;
5. Construction of erosion control structures;
6. Backsloping, sodding and/or riprapping of bank; or
7. Construction of closed or open, paved storm sewers for the purposes of closing gaps or continuation of the overall storm sewer system.
R. Property Owner Agreements: Property owner agreements, where required, shall be designed to protect the city from probable lawsuits for damage caused by changed runoff conditions.
S. Retention Reservoirs: When subdivision development will result in increased runoff beyond the boundaries of the subdivision, which cannot be accommodated through channel improvements without causing downstream flooding, the planning commission may require the construction of one or more retention reservoirs on the subdivision which will temporarily impound and discharge water from the subdivision site at the rate and volume equivalent to the discharge from the undeveloped subdivision site. The design shall be for a 100-year frequency flood. Plans shall be approved by the city's engineer. The construction and maintenance of retention reservoirs shall be the responsibility of the developer. (Ord. 1-10-85, 1-10-1985, eff. 2-10-1985)
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