Laserfiche WebLink
P6- rGAlT/d Al �� Se/ S/F//i& <br />a <br />DESCRI nON <br />Extended dt=bon basins are normally dry between storms. During a storm the basin <br />M. A bottom outlet rekascs the water slowly to provide time for sediments to settle. <br />EXPERIENCE IN CAL FORMA <br />Tbcre arc no known basins in California Hydraulic detention basins may function like <br />extended detention basins if the former has been sized to control the pre -development 2 - <br />yew event. More liberal standards do not provide sufficient detention time. <br />SELECTION CRTIT—RIA <br />• Objective is to remove only particulate pollutants <br />• Use where lack of water prevents the use of wet ponds, wetlands or biofiiters <br />• Use when wet ponds or wetlands would cause unacceptable mosquito conditions <br />LIMITATIONS <br />• May be less reliable than other treatment control BMN. <br />• Inability to vegetate banks and bottom may result in erosion and resuspension <br />• limitation of the orifice diamescr may preclude use of in small watersheds. <br />• Requires differential elevation between inlet and outlet_ <br />DESIGN AND SIZING CONSIDERATIONS <br />• Basin volume is sized to cgpture a particular fraction of the runoff <br />• Drawdown time of 24 to 40 hours <br />• Shallow basin with large surfacc arca performs better than deep basin with same <br />volume <br />Place energy dissipates at the entrance to minim= bottom erosion and resuspension <br />• Vegetate side slopes and bottom to the maximum extent practical <br />• If side erosion is parbculariy severe, consider paving or soil stabilization <br />• If floatables are a problem, protect outlet with trash rack or other device <br />• Provide bypass or pass through capabilities for 100 year storm <br />CONSTRUCTION/INSPECTION CONSIDERATIONS <br />• Make stm the outlet is installed as per design <br />i <br />MAP47ENANCE REQ S <br />• Check outlet regularly for clogging <br />• Check banks and bottom of surface basin for erosion and correct as Wcessary <br />• Remove sediment when accumulanoe reaches 6-incbes, or if resuspension is observed <br />COST CONSIDERATIONS <br />• Generally less expensive than wet ponds and wetlands, but [Hort expensive than <br />bwfiitets <br />• Erosion of open space areas in residential developments increases maintenance costs. <br />Water Avallabliity <br />CA"Meicx <br />Ac Hood <br />Envimnm&ntal Sloe <br />Effects <br />TargeW Constituents <br />O// i Grasse <br />Sedinient <br />Bacteria i Wn sae <br />Nutrients <br />® <br />NORVy mahvis <br />Toxic Materiait <br />Floatabie Materiels <br />Oxygen Demand- <br />ing Substances <br />Q <br />O// i Grasse <br />Q <br />Bacteria i Wn sae <br />LYMY b ttay. <br />tlmpaat <br />Q <br />Low ortAib�own <br />in+pact <br />--] <br />I 1 0 High O Low 11 <br />Municipal Handbook 5- 37 1 V13N2 ..D R A <br />a► <br />• <br />I 1 0 High O Low 11 <br />Municipal Handbook 5- 37 1 V13N2 ..D R A <br />