Design of Vegetative Filtering Systems-Open Channels and Filter Strips Prepared by the Center for Watershed 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 ChannelUltra-ParkingRoadsResidential PerviousRooftops type urban lots Grass ChannelNoNoMaybeMaybeYesMaybe Dry SwaleNoMaybeIdealIdealYesMaybe Wet SwaleNoMaybeNoNoMaybeMaybe Filter StripNoMaybeYesYesIdealYes 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 Factors Channel TypeSpaceMinimumMaintenanceCost requiredheadburden Grass Channel6.5%2 feetmowinglow Dry Swale10-20%2-6 feetmowingmoderate Wet Swale10-20%2-6 feetwetlandlow Filter Strip100%2 feetedge scrapinglow

Open Channel Systems 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

Vegetative Filtering Systems Dry 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

Copyright 2000, CWP

Vegetative Filtering Systems Wet 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

Copyright 2000, CWP

Dry and Wet Swales Design 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%

Copyright 2000, CWP Dry and Wet Swales Sizing Criteria Length, width, depth, and slope required to accommodate the WQ V Outlet structures sized to release WQ V over 24 hours Maximum depth of 18” for water quality volume, 12” average depth

Copyright 2000, CWP Dry Swale Schematic with Key Design Criteria

Copyright 2000, CWP Wet Swale Schematic with Key Design Criteria

Copyright 2000, CWP Dry and Wet Swales Pretreatment 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

Copyright 2000, CWP Dry and Wet Swales Design 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

Copyright 2000, CWP Dry and Wet Swales Flow Regulation and Overflow Primarily on-line, but consider off-line when inflow is piped Sized for WQ V 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

Copyright 2000, CWP Grass Channels Design 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

Copyright 2000, CWP Grass Channel Schematic with Key Design Criteria

Copyright 2000, CWP Grass Channels Design Criteria ParameterDesign Criteria Manning’s n0.15 for water quality treatment for depths 4-12” 0.03 minimum for depths greater than 12” Flow Velocity1.0 fps for water quality treatment fps for 2-year storm 7.0 fps for 10-year storm Lengthlength required for minimum 10 minute residence time

Copyright 2000, CWP Source: CWP, 1996

Copyright 2000, CWP Grass Channels Pretreatment 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

Copyright 2000, CWP Grass Channels Flow Regulation and Overflow On-line, but consider off-line when inflow is piped Rate-based on peak flow Sized for WQ V 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

Copyright 2000, CWP Vegetative Filtering Systems Filter 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 Copyright 2000, CWP

Filter Strips Design Criteria ParameterDesign Criteria Sizing length, depth, slopeWQv widthwidth of area draining to it min. length25 feet Slope minimum2% maximum6% Treatment DA maximum overland flow length150ft (pervious) 75 ft (impervious)

Copyright 2000, CWP Filter Strip Schematic with Key Design Criteria

Copyright 2000, CWP Filter Strips Pretreatment Pea gravel diaphragm –located along top of slope Uphill area, above shallow ponding limit, provides additional pretreatment

Copyright 2000, CWP Filter Strips Flow Regulation and Overflow On-line volume based on WQ v Divert higher flows to by-pass the practice Provide overflow spillway to ensure non- erosive condition for storms greater than WQ v

Copyright 2000, CWP Vegetative Filtering Systems Construction Specifications Parameter Dry Swale Soil Check Dam Filter Strip Sand/Gravel Pervious Berm Specification sand-soil mix sand: ASTM C-33 fine agg. concrete sand soil: USCS ML, SM, or SC pressure-treated, rot resistant wood sand/gravel mix sand: ASTM C-33 fine agg. concrete sand gravel: AASHTO M-43 Size sand: ” 6” by 6” sand: ” gravel: 1/2”-1”

Copyright 2000, CWP Vegetative Filtering Systems Construction Specifications (con’t) ParameterSpecificationSize Pea Gravel Diaphragm ATSM D-448 size no. 6 1/8”-3/8” and curtain drain Underdrain gravel AASHTO M-431/2” - 1” PVC Piping AASHTO M-2786” rigid schedule 40

Copyright 2000, CWP 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

Copyright 2000, CWP 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