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D ESCJlilYll ON <br />Ext.roded dctrntioo basins are oonnally dry be.twea1 storms. During a storm the bas.in <br />fills. A bottom outlet reka.sc:s tl:K wrucr slowly to proVlde time for sediments to settle. <br />EXPERIENCE IN CALIFORNIA <br />Tbcrc are no k:nawn basins m Calif cruia. Hydraulic de t.entioo basins may function like <br />ext.roded dctaltioo basins if the f on:ncr has been sized to control the i:re-dcvelopmen t 2- <br />ye:ar evenL Mo-re liberal standards do 00( provide sufficient detention time. <br />SELECTJ ON CRITERIA <br />Objective is to remove ool y particulatt JX)ll utan ts <br />Use where lad of water prevents the use of wet ponds., wetlands or biofiJte:rs <br />Use where wet ponds oc wetlands would CaUSe unaccep<able mosquito conditions <br />LIMITATIONS <br />Mliy be less reliable than oilier treatmait control B.MPs. <br />Inability to vegetate banks and bottom may result in erosion and resuspension <br />Llmitatioo of the orifice diameter may preclude use of in small watersheds. <br />Requires diffaentia.l elevatioo betweai inlet an<l outleL <br />DESIGN AND SIZING CONSIDERATIONS <br />Basin volume is sized to capmre a particular fractioo of the runoff <br />Drawoown time of 24 to 40 boon; " <br />Shallow ~in with large surface area pc:rlorms better than ckep basin with same <br />volume <br />P1ace energy dissipatm at lbe c:ntralee to roinimin bottom erosioo and resuspension <br />Vegetiue side ~opes and bottom to the maximum ex~t ~ <br />If side eros.ioo is particularly severe, consider paving or soil stabilization <br />If flOO!ables are a probkm, protect ootlet with trash rack or otber device <br />Provide bypass or ~ through capabilities for 100 year storm. <br />CONSTRUCTION/INSPECTION CONSIDERATIONS <br />Make sure the outlet is installed as pa-design <br />MAINTENANCE REQUIREMENTS <br />Ched outlet regularly for clogging <br />Check banks and bottom of surface basin for erosion and correct as necessary <br />Remove sediment when accumulation reaches 6-i:nc.bes, or if resuspension is observed <br />COST CONSIDERATIONS <br />Generally kss expensive than wet poods and wetlands, but more expensive than <br />hlofiltcrs <br />f.rosion of open space areas in resicbltial devel~ts increases maintenance costs. <br />Co~ns <br />So!J.s <br />~ <br />Slope <br />W st atr A val W>JI Jty <br />~ <br />EnvironmentJIII Sk:::k <br />EMacts <br />Ta~ Colutlt~ • S«J/.mMt <br />~ Nutriont.s <br />~ H-vy Mttt:at. <br />~ T ox Jc Mllli e ria J. <br />~ FJoatsb J& Mat erid 16 <br />~ Oxygen O.m,,nd- <br />Ing Substa~ <br />~ Oil It G,-.e <br />0 &cwrla.& V1ru-. <br />• ~&v. t Jmp;a,ct <br />0 ~L.-or <br />t4,Jcn,c»m lmp4,ct <br />l~tation <br />Roquf~ <br />~ Capbleo.ts <br />~ o&Ueosts <br />~ l,,la/r,t.ru,~ <br />0 Training <br />8 Hlg,h O Low <br />TC5 <br />Municipal Handbook 5 -37 1111m2-D RAFT