Appendix 303. Detention Design Criteria.

Detention facilities shall incorporate water quality features and multiple uses where practicable. Uses considered compatible with storm water management include open space, aesthetics, aquatic habitat, recreation, wetlands, and water quality mitigation. The drainage system should be designed to minimize adverse water quality impacts downstream and on the property itself. Detention basins shall incorporate design features to capture storm water runoff pollutants. In particular, designers should give preference to wet bottom and wetland designs.

All detention facilities will incorporate the following volume and peak flow rate controls.

Release Rates. The drainage system for a property shall be designed to control the peak rate of discharge from the property for the two-year events of up to twenty-four hours, and the one hundred-year events of up to twenty-four hours. The peak discharge from events less than or equal to the two-year event shall not be greater than 0.04 cfs per acre of property drained. The peak discharge from events less than or equal to the one hundred-year event shall not be greater than 0.20 cfs per acre of property drained.

Storage Requirements. The maximum storage to be provided in a detention basin shall be based on routing the design runoff hydrograph through the basin to the standards described in Part A of this section. Where the maximum storage required is found to be less than 0.25 acre-feet, the developer may ask for an exemption from the detention requirement. This variance must be requested in accordance with Chapter 16.85 of this code.

Credit for Infiltration. An applicant may seek credit for infiltration in his detention facilities which would be applied by accounting for the infiltration discharges as an unregulated release for the purpose calculating the maximum storage to be provided. Such a credit will be allowed if the following conditions are met.

1.

The normally dry facility is located in soils in hydrologic soils groups A or B as designated by the natural resource conservation service.

2.

The bottom of the detention facility is at least four feet above the seasonal high groundwater and bedrock.

3.

Upstream measures are implemented to remove coarse sediment from the storm water flows before they reach the detention facility.

4.

The system can be shown to safely route the one hundred-year storm through the downstream system even if the infiltration system were to fail.

Tributary Bypass Flows. Whenever practicable, flows from upstream areas that are not to be detained should be routed around the basin. Exceptions to this must be specifically approved by the city engineer.

1.

Tributary Bypass Flows Routed Around the Detention Basin. Tributary flows that are routed around the detention facilities shall not be subject to the volume control and release rate requirements of this ordinance. However, these flows shall be contained in a one hundred-year flood route as indicated in the storm water conveyance system regulations.

2.

Tributary Bypass Flows Routed Through the Detention Basin. Flows from tributary areas upstream of the applicant's property may be routed through the detention basin when this is the only practicable alternative.

• The final releases shall be based on the combined total of the applicant's property plus tributary bypass areas. It must be shown that the routed runoff passed on to the downstream system does not exceed the combined site and tributary runoff that would have been passed on to the downstream system was tributary runoff routed around the detention basin.

• If the tributary properties have already been developed to the standards of this ordinance, or if the tributary properties have not yet been developed, then the tributary bypass flows will be calculated as the worst case of either runoff from the tributary property in its existing condition or runoff from the tributary property were the property developed to the standards of these regulations.

• If the tributary properties have already been developed to standards that do not meet the requirements of these regulations, the city engineer will require the applicant to explore regional detention on the applicant's property.

3.

Regional Detention on the Applicant's Property. When dictated by this regulation, at the applicant's discretion, or at the discretion of the city engineer, the applicant may be required to explore regional detention on the applicant's property. The following steps shall be followed:

• The applicant shall compute the storage volume required for his site exclusive of the tributary areas.

• The applicant shall compute the total storage needed for the combination of the applicant's property and all tributary areas that do not already meet the standards of this ordinance. If the tributary property has not yet been developed, a reasonable estimate of fully developed land cover, based on the comprehensive land use plan, shall be assumed for the purposes of computing storage.

• The city may choose to pay for sizing the detention basin to accommodate the regional flows and a regional detention facility will be implemented. The applicant's responsibility will be limited to the storage required for his property exclusive of the tributary areas.

4.

Storage of Tributary Runoff in Exchange for Uncontrolled Release of Site Runoff. In cases where runoff from a portion of the applicant's site is particularly difficult to control in a site detention basin, and where the applicant opts not to pay a fee in lieu of detention, the applicant may provide regional storage upstream of an uncontrolled area as compensation for an uncontrolled release. In these cases:

• The facility shall be designed as a regional detention facility. The total storage requirement will be calculated in accordance with the second bulleted item in Part 3 above.

• The net releases from the watershed, including those areas not route through the basin, shall be shown to be in keeping with the net requirements of this regulation, including those that regulate the protection of sensitive downstream systems. The applicant shall still be required to provide adequate construction site erosion and sediment control for the entire site.

• To the maximum extent practicable, only runoff from green space shall be released uncontrolled.

• The applicant shall bear the full cost of the regional detention facility.