A. Policy: It is the policy of the city to require storm drainage basins for all development.
B. Basin Size And Location: All new development and redevelopment sites are required to have a stormwater detention system designed as provided in this chapter. It is critical for developments that are in close proximity to a stream or river, or within "drinking water source protection zones" 1 or 2 as defined by the "state division of drinking water rules". All detention basin designs and calculations shall be reviewed by the city engineer for approval.
C. Certain Development Requirement: On site detention is required for all commercial, manufacturing, industrial, or similar development in the city.
D. Ownership: The ownership of stormwater basins is specified as follows:
1. Private Basins: Where the development will have a homeowners' association (HOA) or in commercial applications, local detention basins shall be owned and maintained by the owner or HOA.
2. Local Public Or Private Basins: In subdivisions, local detention basins shall be constructed by the developer. Following acceptance of the construction, the ownership, operation, and maintenance may either be conveyed and maintained by the city or HOA under an appropriate maintenance agreement.
3. Regional Detention Basins: Regional basins shall be owned and maintained by the city and constructed according to the criteria given herein and approved by the city engineer. Actual ownership and responsibility shall be specifically defined in the owner dedication certificates, development agreements, or deed.
E. Basin Access And Setback: Basin access shall be as follows:
1. Public Basins: The developer shall provide the city ingress and egress easements for construction, operation, service, maintenance, and repair for any public basin.
2. Private Basin: The owner or owning association shall be provided ingress and egress easements for construction, operation, service, maintenance, and repair for any private basin. The city shall be provided an emergency easement for emergency access, service, and repair for a private basin.
3. Setback: Each basin shall be constructed with a flat rim circling the basin which shall be set back from a property line, adjoining property, and from any structure an appropriate distance determined by the city engineer to prevent erosion, and to allow for a backhoe and dump truck to circle the rim for maintenance and repair.
F. Percolation: No reduction due to percolation for detention basins volumes shall be permitted in design, due to the nature of basins silting in over time and also possible frost conditions during a storm.
G. Basin Construction: Basins shall be constructed as follows:
1. Policy: Basins must be constructed to enhance safety, health and aesthetics of the area.
2. Engineering: Basins, whether detention or retention, must be designed and stamped by a licensed civil engineer.
3. Location: Detention basins shall be located with convenient access for maintenance and repair by maintenance personnel. This generally means that the basin property has frontage along a public roadway. Volume in ditches or roadside swales shall not be considered in the volume calculation.
4. Fencing: If unfenced and open to general public, the maximum depth of water should not exceed three feet (3'). If a fence is required, six foot (6') chainlink fencing is to be installed in accordance with the city's public works standards and technical specifications, or as may more specifically be stated in any land use regulations.
5. Side Slopes: Side slopes should not exceed 3:1 (horizontal to vertical) (4.5:1 is desirable) for ease of mowing and access.
6. Bottom Slope: The basin floor shall be designed so as to prevent the permanent ponding of water. The slope of the floor of the basin shall not be less than one percent (1%) to provide drainage of water to the outlet grate and prevent prolonged wet, soggy or unstable soil conditions. The preferred minimum slope is two percent (2%).
7. Freeboard: There should be at least one foot (1') of freeboard (berm above the high water mark).
8. Spillways: Spillways must be included and a path with a maintained swale and drainage easement to a safe location. Attention should be given to the design of the spillway to avoid erosion. Overflow spillways are intended to introduce flows back into the main pipe and are typically downstream of the outlet control. Emergency spillways are intended to carry flows beyond the capacity of the overflow spillway to a safe downstream location. All spillways shall be designed to protect adjacent embankments, nearby structures, and surrounding properties.
9. Outlet Control: Small, local, private detention basins may be allowed to have calculated fixed orifice plates mounted on the outlet of the basin. Large, regional, public detention basins shall have movable screw type head gates (Waterman C-10 OAE) set at a calculated opening height for the discharge and with a chain to fix the position.
10. Grates: All grates on inlets and outlets must be hot dipped galvanized (not painted) with bars at spacing to prohibit feet from falling in and yet avoid clogging with debris. Generally bar spacing should never exceed three inch (3") spacing.
11. Low Flow Piping: The inlet and outlet structures may be located in different areas of the basin, requiring a buried pipe to convey any base flows that enter and exit the basin, rather than a cross gutter or surface flow. The minimum pipe size and material for the low flow shall be twelve inch (12") diameter or as approved by the city engineer.
12. Ground Covers: The surface area of the basin may either be seeded, sodded, or covered with fabric and cobbles, as specified by the city. If seeded, measures shall be taken to eliminate erosion until grasses are established. A minimum of four inches (4") of topsoil must be installed prior to sod or seed placement. Cobble sizes shall be four inches (4") or greater in size overlying a city approved weed barrier. Cobble rock may be permitted if the basin is fenced and no other alternative purpose is proposed for the land. Grass or hydroseeding on all basins shall be installed in accordance with the city's public work standards and technical specifications. The basin shall be provided with an automated sprinkler system previously approved by the city engineer. Landscaping shall be installed in accordance with the applicable landscaping regulations.
13. Embankment (Fill) Construction: If a raised embankment is constructed for a basin (constructed with granular materials), it shall be provided with a minimum of six inches (6") of clay cover on the inside of the berm to prevent water passage through the soil.
14. Excavation (Cut) Construction: If the basin is constructed primarily by excavation, then it may be necessary to provide an impermeable liner and land drain system when constructed in the proximity of basements or other below grade structures as determined by a geotechnical evaluation.
15. Multiuse Basins: Basins may be designed as multiuse facilities when appropriate precautions are incorporated into the design. If amenities such as pavilions, playground equipment, volleyball courts, or other amenity are to be constructed within the water detention area of a basin they shall be designed appropriately. Structures shall be designed for saturated soil conditions and bearing capacities are to be reduced accordingly. Restrooms shall not be located in areas of inundation. Inlet and outlet structures should be located as far as possible from all facilities. No woodchips or floatable objects may be used in the area that will be inundated.
H. Detention Volume: Detention basins shall be sized based upon the criteria set forth in section 15.25.020 of this chapter. Detention basins are designed to allow a predetermined amount of flow to discharge during and after a storm event as discussed above. Detention basins are preferred over retention basins. No groundwater injection is allowed. Above grade detention basins are preferred over below grade basins, yet both are allowable. Discharge rates shall comply with section 15.25.050 of this chapter.
I. Retention Basins: The following applies to retention basins:
1. Policy: Regional retention basins shall not be permitted by the city. It is the policy of the city to prohibit local or private retention basins for developments, unless certain criteria are met.
2. Retention Discouraged: Any retention basin must be specifically approved by the city engineer. Retention basins shall not be permitted within zones 1, 2, or 3 of any "drinking water source protection zone" of any drinking water source. Due to the silting in potential, no percolation rate may be used in the calculation of volume unless an approved oil/sand separator is installed upstream. The volume must be based upon the 100-year, 3-hour storm. Local storm retention basins shall be designed for the 100-year, 3-hour storm, based upon the intensity-duration-frequency (IDF) curve for the area as approved by the city engineer. Local basins are typically private ownership and maintenance, and serve only one or two (2) specified subdivisions or sites as may be approved.
3. Retention Basin Criteria: Retention basins are basins which hold and store water until it is evaporated or percolated, and shall not be permitted for developments unless all the following conditions are met:
a. The basin is greater than five hundred feet (500') or fifty feet (50') times the number of lots in the entire development (whichever is greater) from the city storm drain system or waterway, and is topographically capable of draining to the city system.
b. The basin is not located within a hazardous area such as a steep slope, floodplain, high groundwater area, "drinking water source protection zone", or some other hazard area.
4. Percolation For Basins: The percolation rate of the ambient soils may be permitted in the calculations only if an oil/sand separator is installed upstream and only ten percent (10%) of the percolation rate may be used due to eventual silting in of the basin.
J. Standing Water: The following regulations apply to standing water:
1. It is the policy of the city to eliminate standing water wherever possible as an effort to minimize a mosquito problem and associated viruses, except where wetland protection is enforced.
2. Basins should completely drain within forty eight (48) hours of the primary storm event.
3. Low flow bypass pipes may be required.
K. Ground Surface Improvements: It is the city policy that the finished surface of the basin shall be improved to eliminate erosion and dust and to enhance the aesthetics of the area by being appropriately landscaped with approved plant material, landscape material, and approved trees. All basins shall be fully landscaped or constructed in accordance with any applicable landscaping regulations. (Ord. 2012-05)