Drainage Design Manual - the City of San Luis Obispo

9.1.1 Detention BasinDetention basins may be classified as:Dry basins. The basins are designed to store water for only a short time during periods ofhigh stormwater runoff. A drainage control structure, usually consisting of a pipe whichcontrols the rate of outflow from the basin, is set in the bottom of the basin, thus providingfor nearly complete emptying of the pond when inflow ceases. A sump may be providedbelow the invert to allow for sediment settling.Wet basins. These basins are designed to maintain a permanent pool of water. The outletsare located above the permanent water surface, so the basin does not drain completely. Wetbasins may be used for water quality improvement and also to provide wildlife enhancement.Because of higher, long-term maintenance requirements necessary to preserve that properfunctioning, these facilities are allowed only with specific approval of the City or County.9.1.2 Retention BasinsA retention structure stores all or a portion of the inflow for a prolonged time period. Theseresemble detention basins; yet have no outlet (other than emergency outlets). Outflow is viainfiltration or evaporation. In general, the design requirements for a retention structure areconsistent with those for a detention structure, unless noted otherwise herein.Because of the general lack of permeable soils in the San Luis Obispo watershed, retentionbasins are allowed only with specific approval of the City or County. Also, as with a wetdetention basin, a retention basin requires long-term maintenance to insure proper performance.Therefore, any proposal for a retention basin will require submittal of an acceptable long-termmaintenance plan.9.1.3 Special FacilitiesParking lots or athletic fields may be used to provide additional storage of stormwater runofffrom less-frequent, higher-intensity storms when used in conjunction with another storagefacility. Parking-lot storage may be used for storms greater than the 10-year design storm,provided that the following conditions are met:• The depth of water detained does not exceed one foot at any location in the parking lotarea for the 100-year design storm; and• The minimum gradient of the parking lot area subject to ponding is 1%; and• The emergency overflow path meets the requirements for pond systems; and• Ponding is restricted to areas that will cause the least inconvenience to parking area users.SLO Creek Drainage Design Manual 100 February 2003