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).