Vegetated Filters Dave Briglio, P.E. MACTEC Mike Novotney Center for Watershed Protection

An overview of the major components of the enhanced swale and filter strip sizing and design processes

Enhanced Swales

Description: Vegetated open channels that are explicitly designed and constructed to capture and treat stormwater runoff within dry or wet cells formed by 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: hour drawdown, 30” soil with PVC underdrain, >2’ to water table, 3-5 feet of head dry, 1-2%, 3”-6” grass Wet: 18” maximum ponding, 12” avg., V-weirs, positive flow

Major Components Dry Swale 1. 1.Inlet and sediment forebay – –0.1” per imp. acre storage required – –6” drop to pea gravel diaphragm 2. 2.Soil media – –30” thick, k=1-1.5 ft/day – –2’-8’ bottom width min Underdrain –PVC, 6” gravel around it 4. 4.Check dams –Reduce velocity, increase contact time –Energy dissipation below them 5. 5.Side slope –2:1 max (4:1 preferred)

Dry Swale

Profile of Dry Swale

Major Components Wet Swale 1. 1.Inlet and sediment forebay – –0.1” per imp. Acre storage required – –6” drop to pea gravel diaphragm 2. 2.Wetlands plantings – –2’-8’ bottom width min. – –Emergent plantings 3. 3.Water –Standing water or poorly drained soils –18” ponding max Check dams –Reduce velocity, increase contact time –V notch 5. 5.Side slope –2:1 max (4:1 preferred)

Wet Swale

Design Steps Like Flow-Thru Infiltration Trench Compute WQ v and if applicable Cp v Screen site Screen local criteria Size sedimentation chamber Size channel dimensions (WQ peak flow) Design check dams Calculated drawdown Check 2-yr and 25-yr storms Design orifices Design inlets, underdrain Prepare vegetation plan

See design example in Appendix D5 for more information

Engineered Filter Strips

Filter strips are uniformly graded and densely vegetated sections of land, engineered and designed to treat runoff from and remove pollutants through vegetative filtering and infiltration.

W fMIN LfLf 2%<S<6%

W fMIN LfLf 2%<S<6% q

Stream 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 –WQ v 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. 1.Determine local criteria and site characteristics 2. 2.Calculate allowable loading from Manning 3. 3.Calculate Q wq 4. 4.Calculate W fMIN 5. 5.Calculate length of strip 6. 6.Fit filter strips to site and make adjustments 7. 7.Design flow spreader approach 8. 8.If berm – calculate WQ v and determine size of “wedge” of storage 9. 9.Complete design details

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 required Northeast 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 = ~ 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* ) = 11.7 feet, use 12 feet

Coastal Challenges Challenges Associated with Using Vegetated Filter Strips in Coastal GA Site Characteristic 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 ) to improve soil porosity.  Place buildings & impervious surfaces on poorly drained soils or preserve as secondary conservation areas (Sect ).  Use small stormwater wetlands (Sect ) to capture and treat stormwater.

Coastal Challenges Challenges Associated with Using Vegetated Filter Strips in Coastal GA Site Characteristic How it Influences the UsePotential 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 Characteristic How it Influences the Use Potential Solutions Flat terrainMay 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 ) and non-underdrained bioretention areas (Sect ).  Where soils have low permeabilities, use small stormwater wetlands (Sect )

Coastal Challenges Challenges Associated with Using Vegetated Filter Strips in Coastal GA Site Characteristic How it Influences the UsePotential 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 ) or wet swales (Sect ). 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… Challenges Associated with Using Swales in Coastal GA Site Characteristic How it Influences the Use of Swales Potential 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… Challenges Associated with Using Swales in Coastal GA Site Characteristic How it Influences the Use of Swales Potential 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… Challenges Associated with Using Swales in Coastal GA Site Characteristic How it Influences the Use of Swales Potential 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… Challenges Associated with Using Swales in Coastal GA Site Characteristic How it Influences the Use of Swales Potential 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… Challenges Associated with Using Swales in Coastal GA Site Characteristic How it Influences the Use of Swales Potential 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 Credits Table 6.5: How Stormwater Management Practices Can Be Used to Help Satisfy the Stormwater Management Criteria Stormwater Management Practice Stormwater Runoff Reduction 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 (ARP v ). “ Credit ” : A stormwater pond can be designed to attenuate the overbank peak discharge (Q p25 ) on a development site. “ Credit ” : A stormwater pond can be designed to attenuate the extreme peak discharge (Q p100 ) 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 (ARP v ). “ Credit ” : A stormwater wetland can be designed to attenuate the overbank peak discharge (Q p25 ) on a development site. “ Credit ” : A stormwater wetland can be designed to attenuate the extreme peak discharge (Q p100 ) 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 (RR v ) 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 (ARP v ). “ Credit ” : Although uncommon, on some development sites, a bioretention area can be designed to attenuate the overbank peak discharge (Q p25 ). “ Credit ” : Although uncommon, on some development sites, a bioretention area can be designed to attenuate the extreme peak discharge (Q p100 ).