Vegetated Filters Dave Briglio, P.E. MACTEC Mike Novotney Center for Watershed Protection.
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Transcript of Vegetated Filters Dave Briglio, P.E. MACTEC Mike Novotney Center for Watershed Protection.
Vegetated Filters
Dave Briglio, P.E.MACTEC
Mike NovotneyCenter for Watershed Protection
An overview of the major components of the enhanced
swale and filter strip sizing and design processes
Enhanced Swales
Description: Description: Vegetated open channels that Vegetated open channels that are explicitly designed and are explicitly designed and constructed to capture and constructed to capture and treat stormwater runoff within treat stormwater runoff within dry or wet cells formed by dry or wet cells formed by check dams or other means.check dams or other means.
2 Design Options
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
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
Major ComponentsDry Swale
1. Inlet and sediment forebay– 0.1” per imp. acre
storage required– 6” drop to pea gravel
diaphragm
2. Soil media – 30” thick, k=1-1.5
ft/day – 2’-8’ bottom width
min.
3. Underdrain – PVC, 6” gravel
around it
4. Check dams– Reduce velocity,
increase contact time– Energy dissipation
below them
5. Side slope– 2:1 max (4:1
preferred)
Dry SwaleDry Swale
Dry SwaleDry Swale
Profile of Dry Swale
Major ComponentsWet Swale
1. Inlet and sediment forebay– 0.1” per imp. Acre
storage required– 6” drop to pea gravel
diaphragm
2. Wetlands plantings – 2’-8’ bottom width
min.– Emergent plantings
3. Water– Standing water or
poorly drained soils– 18” ponding max.
4. Check dams– Reduce velocity,
increase contact time– V notch
5. Side slope– 2:1 max (4:1
preferred)
Wet SwaleWet Swale
Wet SwaleWet Swale
Design StepsLike Flow-Thru Infiltration Trench
1. Compute WQv and if applicable Cpv
2. Screen site3. Screen local criteria4. Size sedimentation
chamber5. Size channel
dimensions (WQ peak flow)
6. Design check dams7. Calculated
drawdown8. Check 2-yr and 25-
yr storms9. Design orifices10. Design inlets,
underdrain11. Prepare vegetation
plan
See design example in Appendix D5 for more information
Engineered Filter Strips
Filter strips are Filter strips are uniformly graded and uniformly graded and densely vegetated densely vegetated sections of land, sections of land, engineered and engineered and designed to treat designed to treat runoff from and runoff from and remove pollutants remove pollutants through vegetative through vegetative filtering and filtering and infiltration.infiltration.
WfMIN
Lf
2%<S<6%
WfMIN
Lf
2%<S<6%
q
Stream Buffer Filter StripStream Buffer Filter Strip
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
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
Design Steps
1. Determine local criteria and site characteristics
2. Calculate allowable loading from Manning
3. Calculate Qwq
4. Calculate WfMIN
5. Calculate length of strip
6. Fit filter strips to site and make adjustments
7. Design flow spreader approach
8. If berm – calculate WQv and determine size of “wedge” of storage
9. Complete design details
2
1
3
500236.0SY
nq
q
QW fMIN
n
SPTL tf 34.3
5.0625.0242
25.1
Parameter Impervious Areas Pervious Areas (Lawns, etc)
Maximum inflow approach length
(feet) 35 75 75 100
Filter strip slope (max = 6%)
< 2% > 2% < 2% > 2% < 2% > 2% < 2% > 2%
Filter strip minimum length (feet)
10 15 20 25 10 12 15 18
Pretreatment Filter Design
An example of enhanced swale design
Taken from Appendix D5
Calculated Volumes….
Step 1. Determine if the site conditions are appropriate
Ground elevation is at 72
High water table is 83…OK
Step 2. Determine Pretreatment volume 0.1” per impervious acre…
1.9 ac x (0.1”) x (1ft/12”) x (43,560 sq. ft/ac) =689.7 cf
We’ll have 2 shallow forebays, each with 345 cf
Step 3. Determine swale dimensions
Maximum ponding depth = 18 inches
1,400 feet of swale available
Minimum slope = 1%...OK
Trapezoidal section: 6-ft wide, 3:1, 9’ ave. depth
= 6.2 sf…x 1400 lf = 8600 cf > WQv (8102 cf)…OK
Step 4. Compute the number of check dams
Max. depth = 18” (1.5’), @ 1% = 150 LF of swale
Northwest fork = 500 LF…4 requiredNortheast fork = 900 LF…6 required Step 5. Compute soil percolation rate (k)Drawdown time = 24 hrs, max. depth = 1.5’
Planting soil selected with k = 1.5’/day
May require gravel/perforated pipe underdrain system
Step 6. Check height of control structure
Need to carry the 25-year flow = 19 cfs
Separate analyses shows that depth of flow = 0.65 feet for 19 cfs
Depth of ponding = 1.5 feet
Freeboard = 0.5 feet
Total height = 1.5 + 0.65 + 0.5 ~ 2.7 feet high
Step 7. Calculate 25-yr weir length
Need to carry the 25-year flow = 19 cfs
Depth of flow = 0.65 feet
Weir equation: Q = CLH 3/2
C = 3.1, Q = 19, H = 0.65
L = 19/(3.1*0.65 1.5) = 11.7 feet, use 12 feet
Coastal Challenges
Challenges Associated with Using Vegetated Filter Strips in Coastal GA
Site Characteristi
c
How it Influences the Use Potential Solutions
Poorly drained soils, such as hydrologic soil group C and D soils
Reduces the ability of vegetated filter strips to reduce stormwater runoff volumes and pollutant loads.
Use soil restoration (Sect. 7.6.1) to improve soil porosity.Place buildings & impervious surfaces on poorly drained soils or preserve as secondary conservation areas (Sect. 7.4.2).Use small stormwater wetlands (Sect. 8.4.2) to capture and treat stormwater.
Coastal Challenges
Challenges Associated with Using Vegetated Filter Strips in Coastal GA
Site Characteristic
How it Influences the Use Potential Solutions
Well drained soils, such as hydrologic soil group A and B soils
Enhances the ability of vegetated filter strips to reduce stormwater runoff volumes and pollutant loads, but may allow stormwater pollutants to reach groundwater aquifers with greater ease.
Avoid the use of infiltration-based stormwater management practices, including vegetated filter strips, at stormwater hotspot facilities and in areas known to provide groundwater recharge to aquifers used as a water supply.
Coastal Challenges
Challenges Associated with Using Vegetated Filter Strips in Coastal GA
Site Characteristi
c
How it Influences the Use Potential Solutions
Flat terrain May be difficult to provide positive drainage and may cause stormwater runoff to pond on the surface of the vegetated filter strip.
Design vegetated filter strips with a slope to promote positive drainage.Where soils are sufficiently permeable, use infiltration practices (Sect. 8.4.5) and non-underdrained bioretention areas (Sect. 8.4.3).Where soils have low permeabilities, use small stormwater wetlands (Sect. 8.4.2)
Coastal Challenges
Challenges Associated with Using Vegetated Filter Strips in Coastal GA
Site Characteristic
How it Influences the Use Potential Solutions
Shallow water table
May cause stormwater runoff to pond on the surface of the vegetated filter strip.
Use small stormwater wetlands (e.g. pocket wetlands) (Sect. 8.4.2) or wet swales (Sect. 8.4.6).
Tidally-influenced drainage system
May prevent stormwater runoff from moving through the vegetated filter strip, particularly during high tide.
Investigate the use of other stormwater management practices to manage stormwater runoff in these areas.
Coastal Challenges…
See Handouts for LID Practices…See Handouts for LID Practices…Challenges Associated with Using Swales in Coastal GA
Site Characterist
ic
How it Influences the Use
of SwalesPotential Solutions
Poorly drained soils, such as hydrologic soil group C and D soils
Does not influence the use of dry swales, but does prevent them from being designed to infiltrate filtered runoff into the underlying native soils.Does not influence the use of wet swales. In fact, poorly drained soils help maintain permanent pools within wet swales.
Use additional low impact development and stormwater management practices in these areas to supplement the stormwater management benefits provided by wet and dry swales.
Coastal Challenges…
See Handouts for LID Practices…See Handouts for LID Practices…Challenges Associated with Using Swales in Coastal GA
Site Characterist
ic
How it Influences the Use
of SwalesPotential Solutions
Well drained soils, such as hydrologic soil group A and B soils
Does not influence the use of dry swales, but does allow them to be designed to infiltrate filtered runoff into the underlying native soils.Makes it difficult to maintain permanent pools within wet swales.May allow stormwater pollutants to reach aquifers easier.
Use dry swales to convey and treat stormwater runoff in these areas.In areas w/o groundwater recharge, design dry swales to infiltrate filtered runoff.Use dry swales with liners and underdrains at hotspots and areas with groundwater recharge.
Coastal Challenges…
See Handouts for LID Practices…See Handouts for LID Practices…Challenges Associated with Using Swales in Coastal GA
Site Characterist
ic
How it Influences the Use
of SwalesPotential Solutions
Flat terrain May be difficult to provide positive drainage and may cause stormwater runoff to pond in the bottom of the swale for long periods of time.
Design swales with a slope > 0.5% to promote positive drainage.Where soils are sufficiently permeable, use non-underdrained bioretention areas (Section 8.4.3) and infiltration practices (Section 8.4.5).Where soils have low permeabilities, use wet swales.
Coastal Challenges…
See Handouts for LID Practices…See Handouts for LID Practices…Challenges Associated with Using Swales in Coastal GA
Site Characterist
ic
How it Influences the Use
of SwalesPotential Solutions
Shallow water table
May cause stormwater runoff to pond in the bottom of a dry swale for extended periods of time.
Ensure distance from bottom of dry swale to top of the water table > 2 ft.Reduce depth of the planting bed…Use wet swales to capture, convey and treat stormwater runoff in these areas.Maximize the use of green infrastructure practices (Section 7.0)
Coastal Challenges…
See Handouts for LID Practices…See Handouts for LID Practices…Challenges Associated with Using Swales in Coastal GA
Site Characterist
ic
How it Influences the Use
of SwalesPotential Solutions
Tidally-influenced drainage system
May prevent stormwater runoff from moving through swales, particularly during high tide.
CSS Design Credits
7.4 Better Site Planning Techniques
7.5 Better Site Design Techniques
7.6 LID Practice
8.4 General Application BMPs
CSS Design CreditsTable 6.5: How Stormwater Management Practices Can Be Used to Help Satisfy the Stormwater Management Criteria
Stormwater Management Practice
Stormwater RunoffReduction
Water Quality Protection
Aquatic Resource Protection
Overbank Flood Protection
Extreme Flood Protection
General Application Practices
Stormwater Ponds
“Credit”:None
“Credit”:Assume that a stormwater pond provides an 80% reduction in TSS loads, a 30% reduction in TN loads and a 70% reduction in bacteria loads.
“Credit”:A stormwater pond can be designed to provide 24-hours of extended detention for the aquatic resource protection volume (ARPv).
“Credit”:A stormwater pond can be designed to attenuate the overbank peak discharge (Qp25) on a development site.
“Credit”:A stormwater pond can be designed to attenuate the extreme peak discharge (Qp100) on a development site.
Stormwater Wetlands
“Credit”:None
“Credit”:Assume that a stormwater wetland provides an 80% reduction in TSS loads, a 30% reduction in TN loads and a 70% reduction in bacteria loads.
“Credit”:A stormwater wetland can be designed to provide 24-hours of extended detention for the aquatic resource protection volume (ARPv).
“Credit”:A stormwater wetland can be designed to attenuate the overbank peak discharge (Qp25) on a development site.
“Credit”:A stormwater wetland can be designed to attenuate the extreme peak discharge (Qp100) on a development site.
Bioretention Areas, No Underdrain
“Credit”:Subtract 100% of the storage volume provided by a non-underdrained bioretention area from the runoff reduction volume (RRv) conveyed through the bioretention area.
“Credit”:Assume that a bioretention area provides an 80% reduction in TSS loads, an 80% reduction in TN loads and a 90% reduction in bacteria loads.
“Credit”:Although uncommon, on some development sites, a bioretention area can be designed to provide 24-hours of extended detention for the aquatic resource protection volume (ARPv).
“Credit”:Although uncommon, on some development sites, a bioretention area can be designed to attenuate the overbank peak discharge (Qp25).
“Credit”:Although uncommon, on some development sites, a bioretention area can be designed to attenuate the extreme peak discharge (Qp100).