Appendix 307. Detention Basin Types.

The applicant may provide detention storage in any of a variety of detention basin types. These detention ponds may be considered retention ponds provided that the standards for applying an infiltration credit are met and infiltration releases are used in place of releases to the downstream system. The standards applied to particular detention facilities will vary based on the type of facility as follows.

A.

Dry Detention Basins: Dry detention basins shall be designed to be normally dry over the majority of its bottom area, except that sediment basins and wetland features may be incorporated into the detention basin design.

1.

Low Flow Channels: Concrete low-flow channels should be avoided wherever possible. When necessary, concrete low-flow channels may be provided with a grade of not less than one-half percent.

2.

Ground Slope: The ground slope of a dry bottom detention basin shall not be less than two percent nor steeper than 4:1 (horizontal to vertical).

3.

Freeboard: The detention pond design shall provide freeboard of at least one foot above the one hundred-year high water level in the pond and at least six inches above the design high water level of the emergency overflow structure.

4.

Trash Racks: Trash racks will be required at the pond inlet to any outlet control devices. Trash racks will be designed to assume that the rack is at least fifty percent plugged during the design event and to be easily accessible for cleaning under normal dry conditions.

B.

Wetland Detention Basins: A wetland basin is a dry detention with all or a portion of its bottom area used as a wetland.

1.

Identification of Wetland Basins: Grading plans should clearly identify wetland basins and should clearly distinguish between the wetland portions of the basin from the dry portions.

2.

Residence Time: Wetland basins should be designed so that the portion of their bottom area that is intended to be dry shall have standing water for no longer than forty-eight hours for any runoff event less than the one hundred-year event.

3.

Ground Slope: The ground slope of the dry portion of the detention basin shall not be less than two percent in grade. The ground slope of any portion of the detention basin shall not be steeper than 4:1 in grade.

4.

Freeboard: The detention pond design shall provide freeboard of at least one foot above the one hundred-year high water level in the pond and at least six inches above the design high water level of the emergency overflow structure.

5.

Trash Racks: Trash racks will be required at the pond inlet to any outlet control devices. Trash racks will be designed to assume that the rack is at least fifty percent plugged during the design event and to be easily accessible for cleaning under normal dry conditions.

C.

Wet Detention Basins: Wet detention basins will be designed to maintain a permanent pool of water with the following characteristics:

1.

Permanent Pool Volume: The permanent pool volume in the basin shall be equal to or greater than the runoff volume from its watershed for a two-year event.

2.

Permanent Pool Surface Area: The surface area of the permanent pool shall be no greater than ten percent of the total surface area draining to the pond.

3.

Permanent Pool Depth: Wet basins shall be at least three feet deep, excluding near shore banks and safety ledges. If fish habitat is to be provided, the permanent pool should be at least ten feet deep over a bottom area equal to at least twenty-five percent of the normal water surface area.

4.

Safety Ledge: All permanent pools shall have a safety ledge at least ten feet wide and between two and one-half to three feet below the normal water elevation.

5.

Side Slopes: The side slopes above the safety ledge and extending to the top of the detention pond berm shall have a slope of no steeper than 5:1 (horizontal to vertical). Side slopes below the safety ledge shall have slopes of no steeper than 3:1 (horizontal to vertical).

6.

Freeboard: The detention pond design shall provide freeboard of at least one foot above the one hundred-year high water level in the pond and at least six inches above the design high water level of the emergency overflow structure.

7.

Trash Racks: Trash racks will be required at the pond inlet to any outlet control devices. Trash racks will be designed to assume that the rack is at least fifty percent plugged during the design event and to be easily accessible for cleaning under normal conditions.

D.

Underground Detention Facilities: Underground detention will be allowed in pipes and vaults that meet the following standards.

1.

Storage Requirement: The detention storage requirement shall be increased by 1.10 times the standard storage requirement in order to account for freeboard and for the difficulty in developing an effective emergency overflow route.

2.

Ease of Maintenance Access: The detention facility shall be accessible for routine maintenance and inspection.

3.

Effective Hydraulic Access: The stormwater conveyance system serving the detention facility shall be shown to effectively carry the one hundred-year storm event to the detention facility with less than six inches of ponding at any inlet.

4.

Overland Emergency Overflow Route: An overland emergency overflow route shall be developed that assumes that the detention facility is flooded and there is at least six inches of ponding at the low inlet to the facility.

E.

Parking Lot Storage: Parking lot detention that meets the following standards will be allowed.

1.

Maximum Ponding Depth: The maximum ponding in the parking lot shall not exceed twelve inches for the one hundred-year design event. The maximum ponding depth shall not exceed six inches for more than four hours after peak high water.

2.

Freeboard: The design shall provide freeboard of at least one foot above the one hundred-year high water level to the lowest floor elevation in the vicinity of the ponding. The design shall provide at least six inches of freeboard above the design high water level of the emergency overflow route.

3.

Dry Access: A dry access route shall be provided from a site driveway to any structures on the site. The parking lot storage shall not be designed to flood any handicap accessible parking stalls.

F.

Other Types of Storage Facilities: Other types of storage facilities may be allowed at the discretion of the city engineer. In these cases, the city engineer may apply whatever standards he deems necessary for public benefit and to meet the purposes of the Surface Water Management Ordinance.

Utilization of Retaining Walls: Retaining walls may be used with any relevant type of detention facility and will not be subject to the normal slope requirements affecting detention facilities as long as the following conditions are met.

1.

The foot of the retaining wall is at least one foot above the design high water level in the pond or the retaining wall is designed to account for saturated conditions to at least one foot above the design high water level in the pond.

2.

Normal maintenance access is provided to the detention facility.

3.

Appropriate safety features precluding unsafe pedestrian access to the facility are incorporated.