Stormwater Quality Design Manual (May 2007) - City of Sacramento ...

Runoff Treatment Control Measure Fact SheetConstructed Wetland BasinStep 8 – Design basin shapeWhenever possible, shape the basin with a gradual expansion from the inlet and a gradual contractiontoward the outlet. The recommended length to width ratio is between 2:1 to 4:1, with 3:1 optimal. Internalbaffling with berms or modification of inlet and outlet points may be necessary to achieve this ratio.Step 9 – Design basin side slopesSide slopes should be stable and sufficiently gentle to limit rill erosion and to facilitate maintenance.Internal side slopes should be no steeper than 4:1; external side slopes should be no steeper than 3:1.Step 10 – Design Maintenance AccessProvide for all-weather access for maintenance vehicles to the bottom and outlet works. Maximum gradesof access ramps should be 10 percent and minimum width will vary according to local permitting agencyrequirements, but usually between 15-20 feet. Pave ramps with concrete or porous pavement, subject tothe approval of the permitting agency.Step 11 – Design Security FencingTo protect habitat and for safety reasons, provide aesthetic security fencing approved by the permittingagency around the basin, except when specifically waived by the permitting agency.Step 12 – Select VegetationSelect wetland vegetation appropriate for planting in the wetland bottom. Consider the water fluctuationsthat are likely to occur. Consult a qualified wetland specialist regarding selection and establishment ofplants. The shallow littoral bench should have a 4- to 6-inch layer of organic topsoil. Berms and sideslopingareas should be planted with native or irrigated turf grasses.Construction Considerations• If possible, stabilize the entire tributary area to the basin before construction begins. If this is notpossible, divert flow around the basin to protect it from sediment loads during construction. Ifsediment does enter the facility during construction, the contractor will be required to remove soilfrom the basin floor after the entire site has been stabilized, to the satisfaction of the permittingagency inspector.• Prevent construction traffic from entering basin.• Ensure that final grading produces a level basin bottom without low spots or depressions.• Install seepage collars on outlet piping to prevent seepage through embankments.Maintenance During Vegetation Establishment• Control the permanent pool water levels as necessary to allow establishment of wetland plants(typically up to 5 years); raise it to the final operating level after plants are established.• Inspect frequently during vegetation establishment, and identify and re-plant areas immediately asneeded.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsCWB-6 May 2007