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

Major Design Components Flow Regulation – –Diversion of only WQv to facility Pretreatment – –Trapping of coarse sediments to extend design life Filter Bed and Filter Media – –Primary treatment component of facility Outflow/Overflow – –Safe conveyance of all storms through facility

General Feasibility Residential Subdivisions High Density / Ultra Urban Areas (depending on land area requirements) Not for Regional Stormwater Control

Key Physical Considerations 5 acre maximum – 0.5 to 2 preferred Consumes 5% of impervious area draining to site Minimum 5 feet of head normally necessary 2:1 length to width ratio except residential Bottom of facility 2’ above water table Hotspot concerns Normally off-line – on-line <0.5 acres and stabilize to resist blowouts

Major Components 1. 1.Diversion structure 2. 2.Pre-treatment swale or filter 3. 3.Ponding area – –6” max. depth, 10’x20’ min – –Min. capture the WQ v 4. 4.Mulch layer 5. 5.Planting soil – –2.5 to 4 feet in depth – –Darcy’s law, k=0.5ft/day – –48 hr. drain time, 4’ deep 6. 6.Filter fabric 7. 7.Sand layer (optional) –12-18” –< 15% silt/clay 8. 8.Underdrain system –6” perforated PVC –10% of surface area as rule of thumb 9. 9.Overflow system –If necessary to handle clogging or flow through Vegetation

Diversion Pretreatment Ponding Mulch Layer Soil Bed Filter Fabric Sand Layer Underdrain Overflow Vegetation

Copyright 2000, CWP

Component Functions Diversion – captures design volume Grass strip – reduce velocity, filter larger particles Ponding area – storage, settling Mulch layer – filtration, micro organisms Soil bed – filtration, adsorption sites Plants – biological uptake, stabilization, aesthetics Sand layer - drainage, aerobic conditions Gravel and Drain Pipe – drainage, overflow

Bioretention areas are typically “off-line” On-LineSystem Off-LineSystem Control Control FlowSplitter

Diversion Methods Flow diversion structure Inlet deflector Slotted curb Deflector weir

Planting Bed Soil This is a critical design feature !!! Soil bed should be 2.5 – 4 feet in depth Soils should be sandy loam, loamy sand or loam texture Clay content of 10-25% Organic content of 1.5-3% pH between 5.5 and 6.5 Infiltration rate must be >= 0.5 in/hr Typically “engineered” soils are best Suggested planting bed “recipe” has been updated in CSS! (Section 8.4.3) Suggested planting bed “recipe” has been updated in CSS! (Section 8.4.3)

Design Steps Compute WQ v and if applicable Cp v Screen site Screen local criteria Compute Q wq Size diversion Size filtration area Set elevations Design conveyances Design pretreatment Size underdrain Design overflow Prepare landscape plan

WQ Peak Flow Back out curve number Calculate unit peak discharge using SCS simplified peak figures Calculate peak discharge as: p

Darcy’s Law A f =(WQv) (d f ) / [ (k) (h f + d f ) (t f )] = 975 sq-ft per acre = 975 sq-ft per acre for minimum filter bed and 100% impervious surface for minimum filter bed and 100% impervious surface where: where: A f =surface area of ponding area (ft 2 ) WQ v = water quality volume (or total volume to be captured) d f =filter bed depth (4 feet minimum) k =coefficient of permeability of filter media (ft/day) (use 0.5 ft/day for silt-loam) (use 0.5 ft/day for silt-loam) h f =average height of water above filter bed (ft) (3 inches, which is half of the 6-inch ponding depth) (3 inches, which is half of the 6-inch ponding depth) t f =design filter bed drain time (days) (2.0 days or 48 hours is recommended maximum) (2.0 days or 48 hours is recommended maximum)

An example of bioretention design Taken from Appendix D2

Base Data Location: Atlanta, GA Site Area = 3.0 ac Impervious Area = 1.9 ac; 63.3% R v = (63.3) (0.009) = 0.62 Soils Type “C” Hydrologic Data PrePost CN t c.39.20

Lets skip the rest of the flow volumes since we already know how to do that

Step 2. Determine if the development site and conditions are appropriate for the use of a bioretention area. Step 3. Confirm local design criteria C WQ v C Cp v C Q p-25 C Safe passage of Q p-100

Step 4. Compute WQv peak discharge (if offline facility) See section Step 5. Size flow diversion structure (if needed) See section Not needed for this site – direct runoff sized for 25-year storm of 19 cfs

Step 6. Determine size of bioretention filter area A f = (WQ v ) (d f ) / [ (k) (h f + d f ) (t f )] Where: A f = surface area of filter bed (ft 2 ) d f = filter bed depth (ft) k = coefficient of permeability of filter media (ft/day) h f = average height of water above filter bed (ft) t f = design filter bed drain time (days) (48 hours is recommended) A f = (8,102 ft 3 )(5’) / [(0.5’/day) (0.25’ + 5’) (2 days)] (With k = 0.5'/day, h f = 0.25’, t f = 2 days) A f = 7,716 sq ft

Step 7. Set design elevations and dimensions of facility Step 8. Design conveyance to facility only for off-line facilities

Step 9. Design pretreatment Pretreat with a grass channel. For a 3.0 acre drainage area, 63% imperviousness, and slope less than 2.0%, provide a 90' grass channel at 1.5% slope. The value from Table 2 is 30' for a one acre drainage area.

Step 10. Size underdrain area 10% of the A f Base underdrain area on 10% of the A f or 772 sq ft. Use 6" perforated plastic pipes surrounded by a three-foot-wide gravel bed, 10' on center (o.c.): This is a rule of thumb !

Step 11. Design overflow Size overflow weir to pass the 25-year event with 6" of head, using the weir equation. Q = CLh 3/2 Where C = 2.65 (smooth crested grass weir) Q = 19.0 cfs h = 6“ L = Q / [(C) (h 3/2 )] or (19.0 cfs) / [(2.65) (.5) 1.5 ] = 20.3' (say 20')

Overflow Weir

Overflow Drain

Coastal Challenges… See Handouts for LID Practices… Challenges Associated with Using Bioretention Areas in Coastal GA Site Characteristic How it Influences the Use of Bioretention Areas Potential Solutions Poorly drained soils, such as hydrologic soil group C and D soils Reduces the ability of bioretention areas to reduce stormwater runoff volumes and pollutant loads on development and redevelopment sites.  Use underdrained bioretention areas to manage stormwater runoff in these areas.  Use additional low impact development and stormwater management practices to supplement the stormwater management benefits provided by underdrained bioretention areas.

Coastal Challenges… See Handouts for LID Practices… Challenges Associated with Using Bioretention Areas in Coastal GA Site Characteristic How it Influences the Use of Bioretention Areas Potential Solutions Well drained soils, such as hydrologic soil group A and B soils Enhances the ability of bioretention areas to reduce stormwater runoff rates, volumes and pollutant loads, but may allow stormwater pollutants to reach water supply aquifers with greater ease.  Use liners and underdrains to capture and treat stormwater runoff at stormwater hotspot facilities and in areas with groundwater recharge.  In areas w/o groundwater recharge, use non- underdrained bioretention areas and infiltration practices (Section 8.4.5)

Coastal Challenges… See Handouts for LID Practices… Challenges Associated with Using Bioretention Areas in Coastal GA Site Characteristic How it Influences the Use of Bioretention Areas Potential Solutions Flat terrainMay cause stormwater runoff to pond in the bioretention area for extended periods of time.  Ensure that the underlying native soils will allow area to drain within 48 hours of the end of a rainfall event to prevent the formation of nuisance ponding conditions.

Coastal Challenges… See Handouts for LID Practices… Challenges Associated with Using Bioretention Areas in Coastal GA Site Characteristic How it Influences the Use of Bioretention Areas Potential Solutions Shallow water table May cause stormwater runoff to pond in the bioretention area for extended periods of time.  Ensure distance from the bottom of the bioretention area to the top of the water table is at least 2 feet.  Reduce the depth of the planting bed…  Use stormwater ponds (Section 8.4.1), stormwater wetlands (Section 8.4.2) and wet swales (Section 8.4.6), instead…

Coastal Challenges… See Handouts for LID Practices… Challenges Associated with Using Bioretention Areas in Coastal GA Site Characteristic How it Influences the Use of Bioretention Areas Potential Solutions Tidally- influenced drainage system May prevent stormwater runoff from moving through the bioretention area, 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 ).