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

Runoff Treatment Control Measure Fact SheetStormwater PlanterStep 3 – Calculate planter surface area (A S )The design surface area of the planter is determined from the design WQV and d s as follows:A S = WQV/d s (see Figures SP-1 and SP-2)Step 4 – Design base coursesBottom Gravel layer – Provide a 9-inch gravel layer (ASSHTO #8 Coarse Aggregate)Sand/Peat layer – Provide an 18-inch (minimum) sand and peat layer over the gravel layer as shown inFigures SP-1 and SP-2. Place filter fabric between sand/peat mixture and gravel layer.Topsoil layer – Provide a sandy loam topsoil layer above the sand/peat mix layer. This layer should be aminimum of six (6) inches deep, but a deeper layer is recommended to promote healthy vegetation.Step 5 – Select subbase linerIf expansive soils or rocks are a concern, chemical or petroleum products are handled or stored within thetributary catchment, or infiltration is not desired for any reason, use a flow-through stormwater planterwith an impermeable liner (see Figure SP-2). Otherwise, use an infiltration stormwater planter and installa non-woven geotextile membrane below the gravel layer to allow infiltration.Step 6 – Provide underdrain if neededProvide a perforated underdrain pipe if the planter has an impermeable bottom liner or if the underlyingsoils are less permeable than the sand/peat layer. Size the underdrain to ensure a 12-hour drawdown andconnect it to the downstream storm drain system.Step 7 – Select vegetationSelect vegetation that:• Is suited to well-drained soil but will withstand being inundated for periods of time;• Will be dense and strong enough to stay upright, even in flowing water;• Has minimum need for fertilizers;• Is not prone to pests and is consistent with integrated pest management practices which promoteless use of chemical pesticides; and• Is consistent with local water conservation requirements.Avoid the use of bark or similar buoyant material that will tend to float when the vegetated area isinundated. Check with the local agency’s Ordinance if the planter will be used to satisfy landscapingrequirements.Step 8 – Design overflow deviceProvide an overflow device with an inlet to the storm drain system. Set the overflow inlet elevation abovethe WQV surcharge water level. A drop inlet or an overflow standpipe with an inverted opening areappropriate overflow devices (see Figures SP-1 and SP-2).Construction Considerations• Divert runoff (other than necessary irrigation) during the period of vegetation establishment.Where runoff diversion is not possible, cover graded, seeded, and/or planted areas with suitableerosion control materials.• For planters flush with the surrounding landscape, install sediment controls (e.g., staked strawwattles) around the planter to prevent high sediment loads from entering the planter duringconstruction activities.• Repair, seed, or re-plant damaged areas immediately.Stormwater Quality Design Manual for the Sacramento and South Placer RegionsSP-4 May 2007