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Design of Vegetative Filtering
Systems-Open Channels and Filter Strips
Prepared by the Center for Prepared by the Center for Watershed ProtectionWatershed Protection
Copyright 2000, CWP
Vegetative Filtering Systems
Four Design Variations
• Grass Channels
• Dry Swales
• Wet Swales
• Filter Strips
Copyright 2000, CWP
Conventional Drainage channel
Copyright 2000, CWP
Vegetative Filtering Selection Guide
Channel Ultra- Parking Roads Residential PerviousRooftops
type urban lots
Grass Channel No No Maybe Maybe Yes Maybe
Dry Swale No Maybe Ideal Ideal Yes Maybe
Wet Swale No Maybe No No Maybe Maybe
Filter Strip No Maybe Yes Yes Ideal Yes
Ideal: physically, economically best alternative for site
Yes: generally suitable
Maybe: depends upon space available, soils, water table, etc.
No: seldom or never suitable
Copyright 2000, CWP
Vegetative Filtering Selection Guide
Key Feasibility FactorsChannel Type Space Minimum Maintenance Cost
required head burden
Grass Channel 6.5% 2 feet mowing low
Dry Swale 10-20% 2-6 feet mowing moderate
Wet Swale 10-20% 2-6 feet wetland low
Filter Strip 100% 2 feet edge scraping low
Open Channel Systems
Copyright 2000, CWP
Copyright 2000, CWP
Vegetative Filtering Systems
Grass Channels• Broad, mildly sloped vegetative filtering• Grass cover crop• Checkdam (optional)• Filter bed confined to top inch of soil and thatch• Pea gravel diaphragm• Provides water quality treatment for smaller, more
frequent storms• Flow velocity is principle design criteria variable
(rate based design)
Copyright 2000, CWP
Copyright 2000, CWP
Copyright 2000, CWP
Copyright 2000, CWP
Vegetative Filtering SystemsDry Swales
• Broad, mildly sloped vegetative filtering• Grass cover crop• Checkdams (may be necessary)• 30” soil bed with underdrain system• Pea gravel diaphragm• Temporarily stores water quality volume• Drains between storm events within one day• Volume based design
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Copyright 2000, CWPCopyright 2000, CWP
Copyright 2000, CWPCopyright 2000, CWP
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Vegetative Filtering SystemsWet Swales
• Broad vegetative filtering• Checkdam (optional)• Grass cover crop or wetland vegetation acts as filter
bed• Swale soils are fully saturated or have standing water• May intercept water table• Pea gravel diaphragm• 24-hour storage of water quality volume• Volume based design
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Dry and Wet SwalesDesign Criteria
Parameter
Channel shape
Bottom width
Side slopes
Longitudinal slope
Design Criteria
Trapezoidal or parabolic
2-8 feet wide
2:1 maximum, 3:1 preferred
1-2%
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Dry and Wet SwalesSizing Criteria
• Length, width, depth, and slope required to accommodate the WQV
• Outlet structures sized to release WQV over 24 hours
• Maximum depth of 18” for water quality volume, 12” average depth
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Dry Swale Schematic with Key Design Criteria
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Wet Swale Schematic with Key Design Criteria
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Dry and Wet SwalesPretreatment
• Vegetated filter strip• Shallow forebay
– located at inflow point– forebay volume = (0.5”)x(impervious acres of
drainage)
• Pea gravel diaphragm– located along the top of the channel– provides treatment for lateral flows
• Mild slopes (< 3:1) provide treatment for lateral flows
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Dry and Wet SwalesDesign Criteria: Soil Bed
• Dry Swale– moderately permeable soils (USCS ML, SM, or
SC)– 30” deep with gravel/pipe underdrain system
• Wet Swale– undisturbed soils– no underdrain
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Dry and Wet SwalesFlow Regulation and Overflow
• Primarily on-line, but consider off-line when inflow is piped
• Sized for WQV treatment, larger storms bypassed
• Swales can receive runoff as concentrated or sheetflow
• 2-year non-erosive velocities• Adequate capacity for 10-year storm with 6”
freeboard
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Grass ChannelsDesign Criteria
Parameter
Channel shape
Bottom width
Side slopes
Longitudinal slope
Flow depth
Design Criteria
Trapezoidal or parabolic
2-6 feet wide
< 3:1
1% minimum, 4% maximum
4” for water quality treatment
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Grass Channel Schematic with Key Design Criteria
Copyright 2000, CWP
Grass ChannelsDesign Criteria
Parameter Design CriteriaManning’s n 0.15 for water quality treatment
0.15-0.03 for depths 4-12”0.03 minimum for depths greater than 12”
Flow Velocity 1.0 fps for water quality treatment4.0-5.0 fps for 2-year storm7.0 fps for 10-year storm
Length length required for minimum 10minute residence time
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Source: CWP, 1996
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Grass ChannelsPretreatment
• Shallow forebay– located at inflow point
– forebay volume = (0.5”)(impervious acres of drainage)
• Pea gravel diaphragm– located along top of channel
• Vegetated filter strip• No minimum volume• Mild slopes (< 3:1) provide pretreatment for lateral flows
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Grass ChannelsFlow Regulation and Overflow
• On-line, but consider off-line when inflow is piped• Rate-based on peak flow
• Sized for WQV treatment, larger storms are bypassed
• Can receive runoff as sheetflow or concentrated flow• 2-year non-erosive velocities• Adequate capacity for 10-year storm with 6” freeboard
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Vegetative Filtering SystemsFilter Strips
• Uses vegetation to slow runoff velocities and filter sediment and pollutants from stormwater
• composed of stone trench, grass strip, and wooded strip• must have presence of sheetflow
– over 150 feet of pervious areas, or
– over 75 feet of impervious areas
• typically on-line• non-erosive for 100-year design storm• grass portion is pretreatment for wooded portion
Filter Strip
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Copyright 2000, CWP
Copyright 2000, CWP
Filter StripsDesign Criteria
Parameter Design CriteriaSizing
length, depth, slope WQvwidth width of area draining to itmin. length 25 feet
Slopeminimum 2%maximum 6%
Treatment DAmaximum overland flow length 150ft (pervious)
75 ft (impervious)
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Filter Strip Schematic with Key Design Criteria
Copyright 2000, CWP
Filter StripsPretreatment
• Pea gravel diaphragm– located along top of slope
• Uphill area, above shallow ponding limit, provides additional pretreatment
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Filter StripsFlow Regulation and Overflow
• On-line volume based on WQv
• Divert higher flows to by-pass the practice
• Provide overflow spillway to ensure non-erosive condition for storms greater than WQv
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Vegetative Filtering Systems
Construction Specifications ParameterDry Swale Soil
Check Dam
Filter Strip Sand/Gravel Pervious Berm
Specificationsand-soil mixsand: ASTM C-33 fine agg. concrete sandsoil: USCS ML, SM, or SC
pressure-treated, rot resistant wood
sand/gravel mixsand: ASTM C-33 fineagg. concrete sandgravel: AASHTO M-43
Sizesand: 0.02-0.04”
6” by 6”
sand: 0.02-0.04”gravel: 1/2”-1”
Copyright 2000, CWP
Vegetative Filtering Systems
Construction Specifications (con’t)
Parameter Specification Size
Pea Gravel Diaphragm ATSM D-448 size no. 6 1/8”-3/8”
and curtain drain
Underdrain gravel AASHTO M-43 1/2” - 1”
PVC Piping AASHTO M-278 6” rigid
schedule 40
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Vegetative Filtering System
Maintenance Guidelines Pre-treatment
Diaphragm: inspect annually, remove sediment as needed, replace when voids filled
Side slopes: inspect for erosion rills and gullies, seed/sod bare areas as needed
Inflow forebay: inspect annually for sediment buildup, remove excessive sediment
Dry Swale Bed
Sand/soil bed: periodically inspect and correct eroded areas
Bed surface: roto-till or cultivate the top of the bed as needed to ensure filtration
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Vegetative Filtering Systems
Maintenance Guidelines (con’t)
Vegetation
Mowing: maintain grass levels between 3-4” (except wet swales)
Grass species: replace with alternative species if the grass fails to establish
Sediment buildup: remove sediment when accumulation exceeds 25% of the design volume
