Appendix 306. Detention Basin Outlet Design.

Every detention facility will be fitted with an appropriately designed outlet control structure. To the extent that the storage facility is a retention pond, stages in the outlet control may be omitted. However, every facility must include an emergency overflow route.

Minimum Detention Outlet Size: Where an orifice plate is used to control discharge, it shall have a minimum diameter of three inches. If this minimum orifice size permits release rates greater than those specified in these regulations, and if regional detention is not a practical alternative, then alternative designs that incorporate self-cleaning flow restrictors shall be investigated. If a practical alternative is not identified, then the city engineer may allow a variance. Should a variance be granted, the detention basin shall still provide the storage that would have been required if the standard release-rate requirement had been met.

Hydraulic Considerations: Backwater on the structure from the downstream drainage system shall be evaluated when designing the outlet.

Maintenance Access: Outlet structures shall be designed and located to be easily accessible for maintenance and cleaning.

Inlet and Outlet Orientation: To the extent feasible, the distance between the detention inlets and outlet shall be maximized. If possible, they should be at opposite ends of the basin.

Velocity Dissipation and Erosion Control: Where applicable, velocity dissipation and erosion control methods shall be incorporated at all detention basin inlets and outlets.

Overflow Structures: All stormwater storage basins shall be provided with an overflow structure capable of passing excess flows at a stage at least one foot below the lowest foundation grade in the vicinity of the detention basin. The design flow rate of the overflow structure shall be equivalent to the one hundred-year inflow rate from the site and all tributary areas. The overflow structure does not necessarily have to release to the same downstream system as the standard basin outlet, but shall release to the most practical downstream route capable of receiving these flows.