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

Runoff Treatment Control Measure Fact SheetWater Quality Detention BasinsStep 6 – Design for SafetyIncorporate features for safety• Consider fencing the facility with post and cable (6” x 6” post minimum) or other approvedfencing material to discourage access.• Hinge and lock gates on structures.• Provide gates or removable bollards across access roads.If applicable, design the dam embankment for safety• Obtain approval from State Division of Safety of Dams, if required based on the size of the dam/storage volume. If that is not required, nonetheless design the embankment-spillway-outletsystem to prevent catastrophic failure.• Design the embankment not to fail during 100-year and larger storms.• Create embankment slopes to be 3:1 or flatter for outside slopes and 4:1 or flatter for insideslopes and plant them with turf forming grasses.• Compact embankment soils in accordance to geotechnical engineer’s specifications.• Design spillway structures and overflows in accordance with local drainage criteria.Step 7- Incorporate Treatment for Dry-Weather FlowsFor wet and combination basins, the permanent pond provides treatment for dry-weather flows. For drybasins, it is necessary to provide dry-weather flow treatment such as a vegetated submerged gravel bed orother equal treatment that is approved by the local permitting agency. See Figure DB-4.Dry weather flows vary by land use, drainage basin size, soil types and other factors. Determine dryweather design flows, Q, using the values in Table DB-2 or other criteria acceptable to the local agency.Vegetated Submerged Gravel BedsVegetated submerged gravel beds can be used to reduce contaminants in dry weather flows within oroutside of a dry basin (See Figure DB-4). Design vegetated submerged gravel beds so that:• Anticipated dry weather flows pass through the gravel bed without overland flow or flooding.• Anticipated dry weather flows pass through the gravel bed without dry out (excessive dryheadspace) at the inlet zone of the bed.• The bed remains functional in the likely event of changing hydraulic conductivity (As the bedclogs with roots and sediment, it should not flood.)• Water levels are fully controllable through the use of inlet and outlet structures.• The system achieves desired removal of contaminants.• The gravel bed shall be planted with emergent plants (See Table DB-3).• The top 3” of the gravel bed shall be above the outlet flow line.• Gravel shall be held to 2” below the outfall flow line within a 4 foot radius of the outfall pipe.Basin geometry: Choose a length-to-width ratio that results in a sufficient hydraulic gradient to push thewater through the gravel bed. A length-to-width ratio of 5 to 10 is common, but other length-to widthratios can be used provided the hydraulic gradient is adequate. (As the length-to-width ratio is increased,the linear velocity of the water passing through the gravel bed increases, the pressure drop increases, andthe hydraulic gradient decreases. At some point, the hydraulic gradient is not sufficient to push the waterthrough the gravel bed, resulting in overland flow.)Stormwater Quality Design Manual for the Sacramento and South Placer RegionsMay 2007DB-7