1. 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.
2. For the purpose of these regulations, drainage shall be classified as follows:
a. SURFACE DRAINAGE is runoff of such a limited quantity and/or slow rate that it does not cause erosion of a defined channel;
b. A
MINOR TRIBUTARY is any drainage channel having a drainage basin of 640 acres (one square mile) or less in area;
c. A
MAJOR TRIBUTARY is any channel having a drainage basin of not less than one square mile or greater than 25 square miles; and
d. A
RIVER is any channel having a drainage basin of greater than 25 square miles.
3. Responsibility for drainage shall be allocated as follows, unless otherwise designated:
a. The developer of a subdivision is responsible for the following:
(1) All surface drainage in the subdivision;
(2) All increase in surface drainage outside the subdivision which results from the development of the subdivision;
(3) The improvement of all minor tributaries lying within the subdivision;
(4) 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
(5) Provision for the maintenance of all floodway and floodway-fringe areas of major tributaries and rivers which have not been dedicated to the public.
b. The city and other levels of governments will be responsible for the following:
(1) The improvement of floodways of major tributaries;
(2) The improvement of river floodways; and
(3) The maintenance of floodway and floodway-fringe areas dedicated to the public.
4. Flood hazard areas are designated on the adopted, official zoning district map of the city. The boundaries of all floodway and floodway-fringe areas shall be designated on preliminary and final plats, and shall be clearly marked.
5. 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 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.
b. The calculation of all runoff shall be based on saturated development 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 development, as reflected in the comprehensive plan, for the drainage basin of a major tributary or river.
c. 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:
(1) Runoff from all drainage areas shall be not less than that determined by the Rational Formula:
Q = AIR
Where:
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 (USGS) quadrangle maps for larger areas;
I = 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; and
R = Rate of rainfall over the entire drainage area in inches per hour, based on the time of concentration and latest government records for the area.
(2) The size of closed storm sewers, open channels, culverts and bridges shall not be less than that determined by using the Manning Formula:
Where:
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; and
N = Roughness coefficient, based on condition and type of material of conduit lining, but not less than 0.013.
6. 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.
7. 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 flood water in the floodway-fringe that will increase flooding in other areas of the floodway or floodway-fringe.
8. 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 storm water 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, 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.
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.
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.
9. All minor tributary and surface drainage channels located within the subdivision shall be improved in accordance with the standards set forth on the following pages of these regulations, or other equivalent standards, as determined by the City Council. All channels shall be designed to carry a 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, with materials of construction and cross-sectioned elements that will be hydraulically efficient and visually harmonious with the adjacent landscape.
10. Enclosed storm sewers may be required by the City Council where special or unusual conditions make open channels hazardous or otherwise unfeasible.
11. 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 adjacent lots.
12. Surface water collected on streets shall be diverted to storm drains at satisfactory intervals to prevent overflow of six inch-high curbs during a 50-year frequency rain for the area and grades involved; provided, that, in no case shall the drainage area served by any street exceed 20 acres, regardless of the amount of flow.
13. 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 20 feet.
14. Closed storm sewers shall be constructed of pre-cast or pre-fabricated 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.
15. 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.
16. 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, 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 (measured horizontally) to the edge of the paved section. Hand-laid rip-rap may be substituted for sodded shoulders where desired.
Improved Drainage Channels: I and II
General Specifications
1. Use of sloping walls or straight walls is optional.
2. Construction of concrete lining shall conform to specifications.
3. Straight walls shall be designed to withstand earth pressures.
4. Sloping walls shall have slope ratio of 1 horizontal to 1 vertical or flatter.
5. Sodded slope ratio shall be 4 horizontal to 1 vertical or flatter.
Improved Drainage Channels: III and IV
General Specifications
1. Use of sloping walls or straight walls is optional.
2. Construction of concrete lining shall conform to specifications.
3. Straight walls shall be designed to withstand earth pressures.
4. Sloping walls shall have slope ratio of 1 horizontal to 1 vertical or flatter.
5. Sodded slope ratio shall be 4 horizontal to 1 vertical or flatter.
Improved Drainage Channel: V
General Specifications
1. Construction of concrete lining shall conform to specifications.
2. Sloping walls shall have slope ratio of 1 horizontal to 1 vertical or flatter.
3. Sodded slope ratio shall be 4 horizontal to 1 vertical or flatter.
Improved Drainage Channel: VI
General Specifications
1. Slopes back of curb shall have a ratio of 4 horizontal to 1 vertical or flatter.
2. Slopes shall be sodded uniformly to permit mowing.
3. Concrete construction shall conform to specifications.
4. This section shall be constructed only in those locations outlined in general policies and requirements.
Details of Turnouts for Standards Storm Sewer Inlets
17. a. 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.
b. The following kinds of improvements may be required:
(1) Enlargement of undersized drainage structures to provide free flow;
(2) Removal of obstructions;
(3) Straightening of channels;
(4) Widening or deepening of channels;
(5) Construction of erosion control structures;
(6) Backsloping, sodding and/or rip-rapping 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.
18. Property owner agreements, where required, shall be designed to protect the city from law suits for damage caused by changed runoff conditions.
19. 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 Council. The construction and maintenance of retention reservoirs shall be the responsibility of the developer.