(A) These regulations and standards are designed, intended, and should be administered in a manner to protect the various drainage areas in the city from flooding; to provide clean and sanitary channels for runoff; to prevent pollution of watersheds, streams, and natural drainage channels; to prevent the encroachment of buildings and improvements on natural drainage channels; to equitably apportion the cost of improvements; to protect natural scenic areas; and to provide for the conservation of the natural resources of the area and to fulfill the requirements of the flood insurance administration. The regulations and standards herein contained shall apply to the incorporated area of the city. All subdivisions of land and all improvements of any character which affect drainage in any portion of the city shall be subject to the provisions of these regulations and be subject to approval by the City Public Works Director or his or her designee. Development within the limits of the 100-year flood shall be designed in such a manner that all finished floor elevations shall be at least one-foot above the 100-year floor level; provided, however, that all earth fills within the 100-year flood level shall be approved by the City Engineer; and provided, further, that development within the 100-year flood shall not in any case plan for structures in the floodway as shown on the official flood maps.
(B) (1) The two classes of drainage channels shall be:
(a) Primary drainage channels; and
(b) Secondary drainage channels.
(2) In general, it is intended that channels which are designated as primary channels serve a drainage area of 1,000 acres or more. It is intended by these regulations that the improvement of primary drainage channels shall be primarily the responsibility of the community as a whole, since the whole community is materially benefitted. However, the developer of the land containing a primary channel is expected to lay out, design, and otherwise plan and carry out his or her improvements in a manner that will not interfere with or restrict the natural flow of water or materially change the condition of runoff. Increased runoff and changes in primary channels which are created by improvements of an individual subdivision shall be the responsibility of the developer of the subdivision and shall be made in accordance with the provisions of these regulations. It is intended by these regulations that the improvement of a secondary channel shall be the responsibility of the developer of the area served by the channel, since the primary benefit is to the area served by the secondary channel and not to the community as a whole.
(C) The maximum condition of rainfall for a 100-year interval plus one-foot of freeboard shall be used for the purposes of determining all runoff for the sizing of drainage channels and structures for the city, unless otherwise specified herein. Values used in formulas for runoff and size of drainage structures shall reflect the degree of urbanization set forth in the projected land use pattern contained in the Comprehensive Plan for the city. The following formulas and values shall be used for calculating all stream flow and runoff for the policies and regulations established herein:
(1) Runoff from all drainage areas shall be determined by rational formula:
Q = AIR, in which
Q = Cubic feet per second
A = Area to be drained in acres, determined by field surveys for areas less than 1,000 acres and by latest government quadrangle maps for larger areas
I = Percent of imperviousness of the area may vary between 40% and 95%
R = Rate of rainfall over entire drainage area in inches per hour, based on time of concentration and latest government records for area (using 100-year storm plus one foot for primary and secondary channels)
(2) The size of closed storm sewers, open channels, culverts, and bridges shall be determined using the Manning formula:
Q = 1,486 A r 2/3 s 1/2
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
(D) All primary drainage channels which are located within, or immediately adjacent to an improvement or a subdivision shall be protected and improved by the developer as follows.
(1) All land having an elevation below the 50-year maximum flood elevation and not protected by levies or dikes shall be dedicated to the city for the purpose of providing drainage and for public park and utility easement use. The developer may retain unto himself or herself an easement for landscaping, for installation, and maintenance of ornamental elements and for fencing. Access for proper maintenance to the area of the 100-year flood shall not be obstructed.
(2) The existing channel laying 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 dedicated drainage area.
(3) 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.
(4) 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. Planting of appropriate species for landscape beauty is encouraged.
(5) A drainage channel shall not be located in a street easement unless it is placed in an enclosed storm sewer.
(6) 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.
(E) Surface drainage and all secondary channels within or adjacent to the addition shall meet the following conditions.
(1) (a) When a subdivision is traversed by a secondary drainage channel, watercourse, or stream which drains 160 acres or more of land, there shall be provided a right-of-way for drainage and public parks and public utility purposes adequate to contain all of the floodway of a 100-year maximum flood.
(b) The right-of-way shall include all of the land within the subdivision that is within the floodway as established by the official flood maps.
(2) Secondary drainage channels which have a primary function of collecting surface water from adjacent properties or intercepting and diverting side hill drainage shall be provided with an improved open channel.
(3) (a) Secondary drainage channels which have a primary function of transporting water through the block or collecting water from cross channels and which have a drainage area of less than 40 acres shall be improved with closed storm sewers of an improved open channel when approved by the City Engineer.
(b) Where the secondary drainage channel has a drainage area of greater than 40 acres, an improved open channel or closed storm sewer shall be provided.
(4) A drainage channel shall not be located in a street easement unless it is placed in a closed storm sewer.
(5) Site grading shall be carried out in such a manner that surface water from each lot will flow directly to a storm sewer, improved channel, or paved street without crossing more than two adjacent lots.
(6) (a) Surface water collected on streets shall be diverted to storm drains at satisfactory intervals to prevent overflow of six-inch high curbs during a ten-year frequency rain for the area and grades involved.
(b) However, the minimum finished grade at the building line shall be such that it will not be affected by storms equivalent to a 100-year frequency plus one foot of freeboard.
(7) Drainage area allowed for surface flow on streets at a point of diversion shall not exceed 20 acres, regardless of flow.
(8) Drainage easements of satisfactory width to provide working room for construction and maintenance shall be provided for all storm sewers.
(9) Channel improvements shall be as follows.
(a) Open channels shall be improved by providing a paved section that will carry the runoff from a rain of five-year frequency and a sodded section to carry the runoff from a rain of ten-year frequency. However, the minimum finished grade at the building line shall be such that it will not be affected by storms equivalent to a 100-year frequency plus one-foot of freeboard. The design of the channel improvements shall be in accordance with the standards set forth on the three following figures. Where turbulence is sufficiently limited, where natural beauty is thereby preserved and where erosion will not result, the Planning and Zoning Commission may approve omitting the paved section; and
(b) Whenever an open improved channel is required or authorized for a secondary drainage channel under the provisions of these regulations and the channel crosses large residential lots (area of not less than 10,000 square feet) and the channel improvement is to be designed as an integral part of the landscaping of the area that will be maintained by the property owners of the area, then the Planning Commission may modify the requirements of the first part of this provision to permit a channel improvement design in accordance with the third of the design alternatives shown.
(F) Requirements relating to improvements:
(1) Easements. All storm drainage features shall be placed in proper easements as set out in these regulations.
(2) Bridges and culverts. Bridges and culverts shall be as follows.
(a) All flow of water across continuous streets or alleys shall be through culverts or bridges.
(b) Bridges and culverts on primary drainage channels shall be sized to accommodate a 100-year frequency rain, plus one-foot of freeboard, based on the drainage area involved.
(c) 1. Bridges and culverts on secondary drainage channels which drain 160 acres or more of land shall be sized to accommodate a 100-year frequency rain, plus one-foot of freeboard, based on the drainage area involved.
2. However, a 50-year frequency may be used when overflow provisions are incorporated to accommodate a 100-year frequency rain.
(d) Bridges and culverts on secondary drainage channels which drain less than 160 acres of land shall be sized to accommodate a ten-year frequency rain, based on the drainage area involved; provided, that all building elevations shall have one-foot of freeboard above overflows of the 100-year storm.
(e) Design of bridges and culverts shall be submitted to and have the approval of the City Engineer.
(3) Closed storm sewers. Closed storm sewers shall be constructed of pre-cast or pre-fabricated pipe or built in place of closed box design to serve a ten-year frequency rain for the drainage area involved.
(4) Open paved storm drainage.
(a) Open paved storm drainage channels shall be approved in accordance with specifications approved by the City Engineer.
(b) Side slopes above the paved section shall be shaped and sodded on a slope of four horizontal to one vertical or flatter.
(c) Fences capable of limiting water flow or which would prevent proper maintenance and care of the drainage facility shall not be erected below the shoulder of the sodded section.
(d) Retaining walls or other decorative elements which do not prevent necessary access for maintenance may be erected.
(G) The Planning Commission reserves the right to require improvements, provisions of drainage easements, and for provisions of agreements beyond the boundaries of the addition to facilitate flow of water though the addition and to provide continuous improvement of the overall storm sewer system.
IMPROVED DRAINAGE CHANNEL
General Specifications
1. Use of sloping walls or straight walls is optional.
2. Construction of concrete lining shall conform to city specifications.
3. Straight walls shall be designed to withstand earth pressures.
4. Sloping walls shall have slope ratio of one horizontal to one vertical, or flatter.
5. Sodded slope ratio shall be four horizontal to one vertical, or flatter.
6. Concrete liners shall be structurally designed for calculated hydraulic load to be minimum four inches - maximum six inches of 3,500 lb. concrete.
IMPROVED DRAINAGE CHANNEL
General Specifications
1. Use of sloping walls or straight walls is optional.
2. Construction of concrete lining shall conform to city specifications.
3. Straight walls shall be designed to withstand earth pressures.
4. Sloping walls shall have slope ratio of one horizontal to one vertical, or flatter.
5. Sodded slope ratio shall be four horizontal to one vertical, or flatter.
6. Concrete liners shall be structurally designed for calculated hydraulic load to be minimum four inches - maximum six inches of 3,500 lb. concrete.
IMPROVED DRAINAGE CHANNEL
General Specifications
1. Slopes back of curb shall have a ratio of four horizontal to one vertical, or flatter.
2. Slopes shall be sodded uniformly to permit mowing.
3. Concrete construction shall conform to city specifications.
4. This section shall be constructed only in those locations outlined in general policies and requirements.
5. Concrete liners shall be structurally designed for calculated hydraulic load to be minimum four inches - maximum six inches of 3,500 lb. concrete.
(Prior Code, § 156.067) (Ord. 1041, passed 4-12-1983)