Vegetated Filters

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

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Enhanced Swales
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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.
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2 Design Options

• Wet Swale
• Linear wetland marsh
• Filtration and biological removal
• Non-intense non-residential applications

• Dry Swale
• Linear sand filter
• Filter bed over underdrain
• Filtration
• Residential applications

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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 (41
  preferable)
• Dry 24-48 hour drawdown, 30 soil with PVC
  underdrain, gt2 to water table , 3-5 feet of head
  dry, lt 4 channel slope, drops if gt 1-2, 3-6
  grass
• Wet 18 maximum ponding, 12 avg., V-weirs,
  positive flow

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Major ComponentsDry Swale

• Inlet and sediment forebay
• 0.1 per imp. acre storage required
• 6 drop to pea gravel diaphragm
• Soil media
• 30 thick, k1-1.5 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
• 21 max (41 preferred)

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Dry Swale
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Dry Swale
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Profile of Dry Swale
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Major ComponentsWet Swale

• 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
• 21 max (41 preferred)

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Wet Swale
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Wet Swale
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Design StepsLike Flow-Thru Infiltration Trench

• 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

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See design example in Appendix D5 for more
information
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Engineered Filter Strips
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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.
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Stream Buffer Filter Strip
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Basic Design Considerations

• 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

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Basic Design Considerations

• 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

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Design Steps

• 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

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Pretreatment Filter Design
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• An example of enhanced swale design
• Taken from Appendix D5

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Calculated Volumes.
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Step 1. Determine if the site conditions are
appropriateGround elevation is at 72High
water table is 83OK Step 2. Determine
Pretreatment volume 0.1 per impervious
acre1.9 ac x (0.1) x (1ft/12) x (43,560 sq.
ft/ac) 689.7 cfWell have 2 shallow forebays,
each with 345 cf
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Step 3. Determine swale dimensionsMaximum
ponding depth 18 inches1,400 feet of swale
availableMinimum slope 1...OK Trapezoidal
section 6-ft wide, 31, 9 ave. depth 6.2
sfx 1400 lf 8600 cf gt WQv (8102 cf)OK
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Step 4. Compute the number of check damsMax.
depth 18 (1.5), _at_ 1 150 LF of
swaleNorthwest fork 500 LF4
requiredNortheast fork 900 LF6 required
Step 5. Compute soil percolation rate
(k)Drawdown time 24 hrs, max. depth
1.5Planting soil selected with k
1.5/dayMay require gravel/perforated pipe
underdrain system
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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
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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/(3.10.65 1.5) 11.7
feet, use 12 feet
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Coastal Challenges
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Coastal Challenges
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Coastal Challenges
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Coastal Challenges
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Coastal Challenges
See Handouts for LID Practices
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Coastal Challenges
See Handouts for LID Practices
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Coastal Challenges
See Handouts for LID Practices
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Coastal Challenges
See Handouts for LID Practices
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Coastal Challenges
See Handouts for LID Practices
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CSS Design Credits

• 7.4 Better Site Planning Techniques
• 7.5 Better Site Design Techniques
• 7.6 LID Practice
• 8.4 General Application BMPs

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CSS Design Credits