§ 18.5-10. Onsite retention.

Onsite retention of stormwater shall be required for all developments unless the site is served by a storm sewer, channel or natural drainageway having an adequate outlet capacity from the development to a major drainage outfall such as but not limited to, a storm drain or natural watercourse approved by the city engineer as a safe place to deposit such waters. It should be kept in mind that the city's master stormwater management plan is based on the requirement that development is designed to retain or detain stormwater onsite from a one hundred-year, two-hour storm. For the purpose of determining the volume required, the project area shall be considered as the gross site area to extend to the centerline of all existing and/or future streets on the exterior project boundaries and to include all interior streets and other rights-of-way within the project. The design engineer shall insure that the retention basin(s) will drain to a fully-improved street in case of overflow. In addition, all retention areas shall be designed to insure that the "post-development" flow from the site will not exceed the flow that would have occurred had no development taken place. It is the intent of the city that retention basins present an aesthetically pleasing appearance. The design engineer shall contour the sides and bottoms of the basins to enhance appearance through varied and naturally appearing slopes. Individual parcels of land having a gross site area of one-half acre or less may be exempted from onsite retention if approved by the city engineer.

(1)

Residential subdivisions:

a.

All stormwater which falls within the gross area of the subdivision from a one-hundred-year storm of two (2) hour duration must be retained within the boundaries of that subdivision. An exception to retention, or a portion of the retention, may be granted by the city engineer if the site is served by a storm sewer, channel or natural drainageway having an adequate outlet capacity from the development to a major drainage outfall such as but not limited to, a storm drain or natural watercourse approved by the city engineer as a safe place to deposit such waters.

b.

Retention basins shall be located such that they can intercept the flows from the entire site. If the basin is located other than at the lowest point of the subdivision, additional basins may be required. In case multiple basins are required, the design engineer shall prepare a drainage map which clearly designates drainage areas for each basin and supporting calculations to insure that basins are properly sized to accommodate their respective drainage areas.

c.

Retention basins shall be protected from further development by a recorded drainage easement and must be fully improved with landscaping, irrigation systems (if required by landscaping), lighting and such other aesthetic and recreational improvements as may be required by the city. Retention areas of five (5) acres net (excluding public rights-of-way) or larger may be dedicated to the city for use as neighborhood parks to be maintained by the city once all improvements have been installed by the developer and accepted by the city. Any basin which is accepted by the city for maintenance shall be deemed public property and shall be dedicated to the city free of charge in fee simple title. Proposed retention areas smaller than five (5) acres net shall not be dedicated to the city as a park unless the planning commission finds in its evaluation of the preliminary subdivision plat that the public benefit is best served by its dedication to the public. In its evaluation of the "public benefit served," the commission shall consider such items as the proximity of existing neighborhood parks, project density, and additional park improvements offered by the developer beyond the minimum retention area improvements; including, but not limited to, playground equipment, benches, barbecues, shade structures, and additional landscaping. The city shall not accept any retention site dedication for any area smaller than three (3) net acres. It shall be the responsibility of the developer to establish a system acceptable to the city to provide perpetual maintenance of all retention areas not dedicated to the city. This maintenance system may be the formation of a homeowners' association or some other type of financial guarantee acceptable to the city.

d.

Subject to city approval, two (2) or more developers may join together to provide a larger retention facility to accommodate the runoff for more than one (1) subdivision if all other criteria are met.

e.

If individual lots are graded for flood irrigation and water users' irrigation is provided and maintained, stormwater which falls on the lots may be considered to be retained thereon. A retention area shall be provided for the street runoff and runoff from any other areas not considered to be part of residential lots. The total subdivision retention must in all cases be equal to or exceed that volume expected from a one hundred-year storm of two (2) hours duration. Such flood irrigation and on-lot retention provisions are subject to review and approval by the city engineer.

f.

Within a subdivision, individual lot retention, except as provided in paragraph (e) above, is specifically prohibited.

(2)

Commercial, industrial and multifamily:

a.

All stormwater from a one hundred-year storm of two-hour duration must be retained on site.

b.

For multifamily developments, all stormwater retention shall be in landscaped areas with no retention allowed on paved parking or driveway areas.

c.

For commercial developments, a maximum of seventy-five (75) percent of the retention volume may be stored on paved areas with the balance being stored in landscaped areas. The maximum water storage depth in paved areas shall not exceed sixty-seven-one-hundredths (0.67) feet. If paved areas are designed to store water at a depth greater than sixty-seven-one-hundredths (0.67) feet, the developer shall post conspicuous warning signs in these areas advising that flooding may occur and vehicles may be subject to inundation.

d.

The property owner(s) or a cooperative owners' association shall be responsible for the maintenance of all retention areas and any associated drywells.

e.

In all developments, provisions shall be made to insure that all fire equipment access routes are outside of all retention areas.

(3)

The requirement for onsite retention may be waived by the city engineer if it is determined that said retention is impractical because of but not limited to steep terrain, poor percolation, or incompatibility with existing or surrounding improvements. The city engineer may require additional drainage studies or reports in such cases to determine if a drainage problem will be created on adjacent or downstream properties.

(Ord. No. 1701, § 1, 10-29-91)