Transcript Open Channel System Four Design Variations

Design of
Vegetative
Filtering
Systems-Open
Channels and
Filter Strips
Prepared by the Center for
Watershed Protection
Vegetative Filtering Systems
Four Design Variations
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Grass Channels
Dry Swales
Wet Swales
Filter Strips
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Conventional Drainage channel
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Vegetative Filtering Selection Guide
Channel
type
Grass Channel
Dry Swale
Wet Swale
Filter Strip
Ultraurban
No
No
No
No
Parking
lots
No
Maybe
Maybe
Maybe
Roads
Residential
Pervious Rooftops
Maybe
Ideal
No
Yes
Maybe
Ideal
No
Yes
Yes
Yes
Maybe
Ideal
Maybe
Maybe
Maybe
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
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Vegetative Filtering Selection Guide
Key Feasibility Factors
Channel Type
Grass Channel
Dry Swale
Wet Swale
Filter Strip
Space
required
6.5%
10-20%
10-20%
100%
Minimum
head
2 feet
2-6 feet
2-6 feet
2 feet
Maintenance
burden
mowing
mowing
wetland
edge scraping
Cost
low
moderate
low
low
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Open Channel Systems
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Vegetative Filtering Systems
Grass Channels
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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)
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Vegetative Filtering Systems
Dry Swales
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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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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
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Dry and Wet Swales
Design Criteria
Parameter
Design Criteria
Channel shape
Trapezoidal or parabolic
Bottom width
2-8 feet wide
Side slopes
2:1 maximum, 3:1 preferred
Longitudinal slope
1-2%
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Dry and Wet Swales
Sizing 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 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
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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
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Dry and Wet Swales
Flow 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 Channels
Design Criteria
Parameter
Design Criteria
Channel shape
Trapezoidal or parabolic
Bottom width
2-6 feet wide
Side slopes
< 3:1
Longitudinal slope
1% minimum, 4% maximum
Flow depth
4” for water quality treatment
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Grass
Channel
Schematic
with Key
Design
Criteria
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Grass Channels
Design Criteria
Parameter
Manning’s n
Flow Velocity
Length
Design Criteria
0.15 for water quality treatment
0.15-0.03 for depths 4-12”
0.03 minimum for depths greater than 12”
1.0 fps for water quality treatment
4.0-5.0 fps for 2-year storm
7.0 fps for 10-year storm
length required for minimum 10
minute residence time
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Source: CWP, 1996
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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
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Grass Channels
Flow 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 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
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Filter Strip
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Filter Strips
Design Criteria
Parameter
Sizing
length, depth, slope
width
min. length
Design Criteria
WQv
width of area draining to it
25 feet
Slope
minimum
maximum
2%
6%
Treatment DA
maximum overland flow length
150ft (pervious)
75 ft (impervious)
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Filter Strip
Schematic
with Key
Design
Criteria
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Filter Strips
Pretreatment
• Pea gravel diaphragm
– located along top of slope
• Uphill area, above shallow ponding limit,
provides additional pretreatment
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Filter Strips
Flow Regulation and Overflow
• On-line volume based on WQv
• Divert higher flows to by-pass the practice
• Provide overflow spillway to ensure nonerosive condition for storms greater than
WQv
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Vegetative Filtering Systems
Construction Specifications
Parameter
Dry Swale Soil
Specification
sand-soil mix
sand: ASTM C-33 fine agg.
concrete sand
soil: USCS ML, SM, or SC
Check Dam
pressure-treated, rot
resistant wood
Filter Strip
Sand/Gravel
Pervious Berm
sand/gravel mix
sand: ASTM C-33 fine
agg. concrete sand
gravel: AASHTO M-43
Size
sand: 0.02-0.04”
6” by 6”
sand: 0.02-0.04”
gravel: 1/2”-1”
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Vegetative Filtering Systems
Construction Specifications (con’t)
Parameter
Pea Gravel Diaphragm
and curtain drain
Specification
ATSM D-448 size no. 6
Size
1/8”-3/8”
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
Diaphragm:
Side slopes:
Inflow forebay:
Sand/soil bed:
Bed surface:
Pre-treatment
inspect annually, remove sediment as needed,
replace when voids filled
inspect for erosion rills and gullies, seed/sod bare
areas as needed
inspect annually for sediment buildup, remove
excessive sediment
Dry Swale Bed
periodically inspect and correct eroded areas
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)
Mowing:
Vegetation
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
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