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Vegetated Filters

Vegetated Filters. Dave Briglio, P.E. MACTEC Mike Novotney Center for Watershed Protection. An overview of the major components of the enhanced swale and filter strip sizing and design processes. Enhanced Swales.

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Vegetated Filters

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  1. Vegetated Filters Dave Briglio, P.E. MACTEC Mike Novotney Center for Watershed Protection

  2. An overview of the major components of the enhanced swale and filter strip sizing and design processes

  3. Enhanced Swales

  4. Description: Vegetated open channels that are explicitly designed and constructed to capture and treat stormwater runoff within dry or wet cells formed by check dams or other means.

  5. Dry Swale Linear sand filter Filter bed over underdrain Filtration Residential applications Wet Swale Linear wetland marsh Filtration and biological removal Non-intense non-residential applications 2 Design Options

  6. Key Physical Considerations • 5 acre maximum • Space needed is 10-20% of impervious area draining to site • 2-yr storm non-erosive, 25-year storm within channel floodplain easement • 2’ – 8’ bottom width, flat side slopes (4:1 preferable) • Dry: 24-48 hour drawdown, 30” soil with PVC underdrain, >2’ to water table , 3-5 feet of head dry, < 4% channel slope, drops if > 1-2%, 3”-6” grass • Wet: 18” maximum ponding, 12” avg., V-weirs, positive flow

  7. Inlet and sediment forebay 0.1” per imp. acre storage required 6” drop to pea gravel diaphragm Soil media 30” thick, k=1-1.5 ft/day 2’-8’ bottom width min. Underdrain PVC, 6” gravel around it Check dams Reduce velocity, increase contact time Energy dissipation below them Side slope 2:1 max (4:1 preferred) Major ComponentsDry Swale

  8. Dry Swale

  9. Dry Swale

  10. Profile of Dry Swale

  11. Inlet and sediment forebay 0.1” per imp. Acre storage required 6” drop to pea gravel diaphragm Wetlands plantings 2’-8’ bottom width min. Emergent plantings Water Standing water or poorly drained soils 18” ponding max. Check dams Reduce velocity, increase contact time V notch Side slope 2:1 max (4:1 preferred) Major ComponentsWet Swale

  12. Wet Swale

  13. Wet Swale

  14. Compute WQv and if applicable Cpv Screen site Screen local criteria Size sedimentation chamber Size channel dimensions (WQ peak flow) Design check dams Calculated drawdown Check 2-yr and 25-yr storms Design orifices Design inlets, underdrain Prepare vegetation plan Design StepsLike Flow-Thru Infiltration Trench

  15. See design example in Appendix D5 for more information

  16. Engineered Filter Strips

  17. Filter strips are uniformly graded and densely vegetated sections of land, engineered and designed to treat runoff from and remove pollutants through vegetative filtering and infiltration.

  18. Lf WfMIN 2%<S<6%

  19. Lf WfMIN 2%<S<6% q

  20. Stream Buffer Filter Strip

  21. Plain Filter Strip 5 min contact time minimum 1”-2” flow depth maximum 2%-6% slope so no pooling or concentration of flows Flow spreader at top Dense grass stand Filter Strip With Berm WQv behind berm – can consider spreader 24-hour drawdown Grass withstand inundation Try to mimic Plain Filter Strip for other requirements to gain filtering removal as well Basic Design Considerations

  22. Pollution Removal filtering, infiltration & settling (for berm option) Calculations Balancing width and length of filter to fit site and local criteria Width takes discharge and spreads it out to maintain sheet flow depth Length maintains adequate contact time to allow for removal Filter Width Calculate unit loading (q) to maintain specified depth at given roughness and slope Calculate WQ discharge (Q) Filter width is Q/q Filter Length From kinematic wave solution of sheet flow in TR55 solved for length Considered more accurate than simple Manning – shorter lengths too Basic Design Considerations

  23. Determine local criteria and site characteristics Calculate allowable loading from Manning Calculate Qwq Calculate WfMIN Calculate length of strip Fit filter strips to site and make adjustments Design flow spreader approach If berm – calculate WQv and determine size of “wedge” of storage Complete design details Design Steps

  24. Pretreatment Filter Design

  25. An example of enhanced swale design Taken from Appendix D5

  26. Calculated Volumes….

  27. Step 1. Determine if the site conditions are appropriateGround elevation is at 72High water table is 83…OK Step 2. Determine Pretreatment volume0.1” per impervious acre…1.9 ac x (0.1”) x (1ft/12”) x (43,560 sq. ft/ac) =689.7 cfWe’ll have 2 shallow forebays, each with 345 cf

  28. Step 3. Determine swale dimensionsMaximum ponding depth = 18 inches1,400 feet of swale availableMinimum slope = 1%...OKTrapezoidal section: 6-ft wide, 3:1, 9’ ave. depth= 6.2 sf…x 1400 lf = 8600 cf > WQv (8102 cf)…OK

  29. Step 4. Compute the number of check damsMax. depth = 18” (1.5’), @ 1% = 150 LF of swaleNorthwest fork = 500 LF…4 requiredNortheast fork = 900 LF…6 requiredStep 5. Compute soil percolation rate (k)Drawdown time = 24 hrs, max. depth = 1.5’Planting soil selected with k = 1.5’/dayMay require gravel/perforated pipe underdrain system

  30. Step 6. Check height of control structureNeed to carry the 25-year flow = 19 cfsSeparate analyses shows that depth of flow = 0.65 feet for 19 cfsDepth of ponding = 1.5 feetFreeboard = 0.5 feetTotal height = 1.5 + 0.65 + 0.5 ~ 2.7 feet high

  31. Step 7. Calculate 25-yr weir lengthNeed to carry the 25-year flow = 19 cfsDepth of flow = 0.65 feetWeir equation: Q = CLH 3/2C = 3.1, Q = 19, H = 0.65L = 19/(3.1*0.65 1.5) = 11.7 feet, use 12 feet

  32. Coastal Challenges

  33. Coastal Challenges

  34. Coastal Challenges

  35. Coastal Challenges

  36. Coastal Challenges… See Handouts for LID Practices…

  37. Coastal Challenges… See Handouts for LID Practices…

  38. Coastal Challenges… See Handouts for LID Practices…

  39. Coastal Challenges… See Handouts for LID Practices…

  40. Coastal Challenges… See Handouts for LID Practices…

  41. CSS Design Credits • 7.4 Better Site Planning Techniques • 7.5 Better Site Design Techniques • 7.6 LID Practice • 8.4 General Application BMPs

  42. CSS Design Credits

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