Bioretention Overview, Research & Common Design...
Transcript of Bioretention Overview, Research & Common Design...
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Bioretention Overview, Research & Common Design Elements
Bill Hunt, PE, PhD
Associate Professor & Extension Specialist
NC State University
www.bae.ncsu.edu/stormwater
Bioretention: One Size Doesn’t Fit All
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Presentation Outline
• Summary of what BRC is & looks like
• Pollutant Removal & Hydrologic Mechanisms
• Hydrologic “Improvement” Mechanisms
• Designing BRC Components
– What the State Mandates
– What the Research Suggests
• Where do we go from here?
www.bae.ncsu.edu/stormwater
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Where can you find Bioretention/ Rain Gardens?
www.bae.ncsu.edu/stormwater
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Louisburg, NC - Joyner Park
www.bae.ncsu.edu/stormwater
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“We Bring Engineering to Life”
Glencoe School Parking Lot retrofit - before
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“We Bring Engineering to Life”
Glencoe Parking Lot Infiltration
landscape
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“Skinny” Street – Seattle, Wash.
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“We Bring Engineering to Life”
Rain Gardens Integrated throughout - Seattle
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In a Round-a-bout, Kapiti Coast, NZ
www.bae.ncsu.edu/stormwater
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“We Bring Engineering to Life”
Huntersville, NC - Residential
www.bae.ncsu.edu/stormwater
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Integrating into “High $” Landscapes – Seattle, Washington
“We Bring Engineering to Life”
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Rooftop Treatment… (Albany, NZ @ Mitre 10)
www.bae.ncsu.edu/stormwater
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Bioretention Schematic
Permeable Media Fill
Often Sandy
Underdrain System
Vegetation on Surface Runoff
www.bae.ncsu.edu/stormwater
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Bioretention Water Table
2 ft
Draw Water Table Down 2
ft below surface in
maximum of 2 days
www.bae.ncsu.edu/stormwater
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www.bae.ncsu.edu/stormwater
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Q: How do Rain Gardens work: Hydrologic (Flow) Control
• Temporary surface storage
• Slow flow through porous media (peak flow control).
• Media with good field capacity means volume control, whether or not exfiltration is possible.
• Especially effective for small(ish), frequently occurring storm events typically little to no system discharge!
www.bae.ncsu.edu/stormwater
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www.bae.ncsu.edu/stormwater
+ 4
Hours
+14
Hours
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Q: How does Bioretention work: Pollutant Removal Mechanisms
• Sedimentation (temporary) – Trash, TSS, Phosphorus
• Microbial Processes – Nitrogen
• Chemical Processes & Media Filtration – Metals, Phosphorus
• Exposure to Sunlight & Dryness – Pathogens, Oil & Grease
• Infiltration
www.bae.ncsu.edu/stormwater
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“We Bring Engineering to Life”
Chapel Hill Cell, C1
STP/WS = 0.14
Conventional Drainage
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Annual Loadings (2002-03)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
TN NO3-N TKN TPMass (Kg)
In
Out
Chapel Hill Cell C1 - Hunt et al., 2006
www.bae.ncsu.edu/stormwater
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Mass Loads (kg/ha/yr)
CP SS
In Out In Out
TSS 1190 37 570 38
Chromium 0.09 0.015 0.02 ~0.007
Copper 0.26 0.073 0.12 0.045
Lead 0.09 0.013 0.03 ~0.005
Zinc 1.0 0.063 0.36 0.017
Chloride 6800 458 320 25
TN 27 7.2 9.6 3.6
Nitrate 12 2.5 3.7 ~0.19
TKN 15 4.1 6.0 3.6
TP 3.6 0.72 0.9 0.38
TOC 44 154 43 78
Li & Davis, J. Env. Eng. 2009
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Hal Marshal (Bioretention)
www.bae.ncsu.edu/stormwater
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Cumulative Probability Plots – Bioretention
0.00
0.20
0.40
0.60
0.80
1.00
1 10 100 1,000 10,000
E. Coli (MPN/100 ml)
Cum
ula
tive P
robabili
ty
Inflow
Outflow
126
www.bae.ncsu.edu/stormwater
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Mass Loads (kg/ha/yr)
CP SS
In Out In Out
TSS 1190 37 570 38
Chromium 0.09 0.015 0.02 ~0.007
Copper 0.26 0.073 0.12 0.045
Lead 0.09 0.013 0.03 ~0.005
Zinc 1.0 0.063 0.36 0.017
Chloride 6800 458 320 25
TN 27 7.2 9.6 3.6
Nitrate 12 2.5 3.7 ~0.19
TKN 15 4.1 6.0 3.6
TP 3.6 0.72 0.9 0.38
TOC 44 154 43 78 Li & Davis, J. Env. Eng. 2009
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Where are pollutants removed?
TSS
TP
Temp
TN
Pathogens
Metals
Oil & Grease
www.bae.ncsu.edu/stormwater
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Literature/Research Justification for Minimum Media Depths: WQ
Pollutant Depth (ft) Studies
TSS 1 Diblasi et al. 2009, Li et al. 2008
Metals 1 Li and Davis 2008, Hatt et al. 2009
O&G 1 Diblasi et al. 2009,
Phosphorus 2 (min); 3 (conservative)
Hatt et al. 2009, Hsieh and Davis 2007, Passeport et al. 2009
Nitrogen 3 Passeport et al. 2009
Temperature 3 (min); 4 (optim) Jones and Hunt 2009
DON’T FORGET HYDROLOGY… deeper cells = greater potential for volume control
www.bae.ncsu.edu/stormwater
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Design Features
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Things We Design For:
• Surface Area / Depth of Water
– (In NC) per WQ Volume
• Vegetation Selection
• Media Type & Media Depth
• Underdrain Configuration
– Internal Water Storage
• Underlying Soil Type (account for)
• Location of Seasonally High Water Table
www.bae.ncsu.edu/stormwater
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Seasonally High Water Table Depths & Bioretention
• Seasonally High Water Tables. A Problem?
• Depends on Depth of Bioretention area
• Recommend: No W.T. within 1-2 ft of bottom
B-R Area
www.bae.ncsu.edu/stormwater
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Question: Ponding Depths
What is this depth?
www.bae.ncsu.edu/stormwater
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Bowl Depth Particularly Important. Determines S.A. of BRC
www.bae.ncsu.edu/stormwater
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Calculate Surface Area
• S/A = Surface Area Required
• Volume = Total Volume Captured (Water Quality Volume in NC)
• Depth = Average Depth of water over normal pool (Depth of Storage Volume)
S/A = Volume ÷ Depth
www.bae.ncsu.edu/stormwater
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www.bae.ncsu.edu/stormwater
Ponding?
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“We Bring Engineering to Life”
Maximum Ponding Depths
NC DOT rest stop
BR area, near
Hickory
www.bae.ncsu.edu/stormwater
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Recommended Ponding Depths
– PG Co, Maryland specifies 0.15 m
– Dr. Hunt (NCDENR) suggests 0.3 m reasonable for most applications WITH maintenance
– 0.45 m only if VERY SANDY application (e.g. on coastal dunes?)
– TP10 - NZ (2003): average 0.22 m
– Waitakere City LID CoP - NZ (2010): 0.3 m
www.bae.ncsu.edu/stormwater
Though Deeper Depths are a not too distant possibility.
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Brink Storage Capacity Must Capture the WQ Volume (assuming no intra-event
Infiltration)
www.bae.ncsu.edu/stormwater
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“We Bring Engineering to Life”
What’s the First Flush?
Selecting Capture Volume
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First Flush = Most Polluted Water
• Get Cleaner Quicker @ Beginning of Shower
www.bae.ncsu.edu/stormwater
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80% & 90% storms for NC cities
1.55
1.28
1.60 1.56 1.44
1.55
1.85
1.59
2.24
1.08 0.83 1.06 1.02 0.97 1.03 1.15 1.0
1.40
0.0
0.5
1.0
1.5
2.0
2.5
De
pth
(in
)
90% 80%
www.bae.ncsu.edu/stormwater
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Determining Volume: a pair of options (NC DENR)
• NRCS Curve Number Method (LID only)
– Discrete
– Composite
– Q = (P – 0.2 S)2 ÷ (P +0.8 S)
– V = Q * A
• Simple Method
V = ((Imp%*0.9) + 0.05) * P * A
www.bae.ncsu.edu/stormwater
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• Ability of bioretention to exfiltrate water leads to
reductions in thermal load
• Effluent reductions were greatest for bioretention
media volumes larger with respect to their
watershed Percentage of
Watershed Area
Events with
Outflow
Asheville 16% 12%
Lenoir 4% 79%
Brevard East 7% 76%
Brevard West 11% 27%
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BRC Question: Does Turf covered
bioretention function well?
• Could save money on construction and
maintenance
www.bae.ncsu.edu/stormwat
er
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Graham High School (2006-2007)
• Watershed area = 0.69 ha
• Bioretention Cells Area =
204m2
• Fill Media/ Soil
– 90% Expanded Slate Byproduct
– 10% Top Soil
– P-Index: Low
– 0.6 m & 0.9 m depth
• Both Cells Covered in Turf
(Hybrid Bermuda)
www.bae.ncsu.edu/stormwat
er
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“We Bring Engineering
to Life”
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“We Bring Engineering
to Life”
TN concentrations: Grassed
Graham HS Bioretention (2006)
0
0.5
1
1.5
2
2.5
3
3.5
4
1 2 3 4 5 6 7 8 9 10 11 12 13Event
TN
Co
nc
en
tra
tio
n (
mg
/L) N S IN
www.bae.ncsu.edu/stormwat
er
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Inflow & Outflow Pollutant
Concentrations: Grassed Cell
Pollutant Inflow Outflow – S Outflow - N
TP 0.14 0.051 0.058
Ortho-P 0.057 0.013 0.015
TN 1.66 0.76 0.76
TKN 1.11 0.45 0.57
NO2-3 0.42 0.38 0.28
Passeport et al. (2009)
All Units in mg/L
www.bae.ncsu.edu/stormwat
er
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Grassed Bioretention in Eastern
Wake County
www.bae.ncsu.edu/stormwater
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Grassed Bioretention Cell EMC Reductions
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
TKN NO2,3-N TN NH4-N TP
Co
ncen
trati
on
(m
g/L
)
Bioretention Inlet
Small Cell Outlet
Large Cell Outlet
Target TN Conc.
Target TP Conc.
Average influent and effluent nutrient concentrations
P
P
www.bae.ncsu.edu/stormwater
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I’m not saying shrubs & trees aren’t
a better option (than grass)… • Australian Research
– Griffith
– Monash
• Improve infiltration
rates
• Prevent compaction
• Increased uptake
• Shading…
www.bae.ncsu.edu/stormwat
er
…But both are
acceptable
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“We Bring Engineering to Life”
Fill Media (“Soil”)
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“We Bring Engineering to Life”
Q:How much Fill
Soil Media
Needed?
www.bae.ncsu.edu/stormwater
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Media Depth
• Major Cost Consideration
What is this depth?
www.bae.ncsu.edu/stormwater
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Fill Media Depth predicated upon 3 factors
• Vegetation Health
• Hydrologic Goals
• Water quality needs
• Perhaps the most restrictive goal dictates design
www.bae.ncsu.edu/stormwater
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Bioretention Soil Depth: Vegetation Health
VVVeeegggeeetttaaatttiiiooonnn DDDeeepppttthhh (((mmm)))
Grass 0.3-0.45
Shrubs 0.45-0.75
Shrubs/Trees 0.75-1.0
A.
Some native grasses may require deeper rooting depths.
Some thoughts… 1. Deeper cells may provide moisture reserves for extended dry
periods. 2. Deeper cells provide runoff VOLUME reduction, regardless of
in-situ soil condition or lining.
www.bae.ncsu.edu/stormwater
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Hydro & WQ Design Goals
“We Bring Engineering to Life”
1510203040506070809095990.001
0.01
0.1
1
10
Exceedance Probability
Vo
lum
e D
isch
arg
e R
ati
o,
f v o
r f v
24
CP SS G1 G2 LB1 LB2
Proportionally Larger Cells
Proportionally Smaller Cells
www.bae.ncsu.edu/stormwater
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Media Selection
www.bae.ncsu.edu/stormwater
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Initial NCSU Research
• Relationship between P-Index (Soil Test P) and TP outflow load.
Greensboro Chapel Hill
TP +240% - 65%
P-Index 85-100 4-12 (Hunt 2003)
P-Index 50-100: High P-Index 0-25: Low
www.bae.ncsu.edu/stormwater
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Blame it on the Media…
Phosphorus Index (P-Index) is a measure of how much phosphorus is already in the soil media.
Low P-Index: Can capture more phosphorus
High P-Index: Soil is “saturated” with phosphorus
Very High: > 100 High: 50-100
Medium 25-50 Low: 0-25
www.bae.ncsu.edu/stormwater
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Where does the P go in BR?
• Is it plant uptake?
• Leaching?
• On the clay fraction?
– P strongly held by Al & Fe in acid soils
– History of ‘liming’ with excessive P in NC
Why the “red” in red clay is good!
www.bae.ncsu.edu/stormwater
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Mecklenburg Co. Hal Marshall Bioretention Cell (2004-2006)
Fill Soil/ Media
– 80% Mason Sand
– 20% Fines + Compost
– P-Index = 6
– 4 ft (1.2 m) Depth
www.bae.ncsu.edu/stormwater
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TP Concentrations: Hal Marshall Bioretention (2004-2006)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1/1/04 7/1/04 12/30/04 6/30/05 12/29/05
[TP
] in
mg
/L
Date
TP-In
TP-Out
Concentration Reduction = 31% Load Reduction ≈ 50%
www.bae.ncsu.edu/stormwater
Hunt et al. 2008
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Why Can’t We Just Use Sand?
www.bae.ncsu.edu/stormwater
Can Fill Media Perc Rate be
too fast?
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What happens if High Fill K meets TN removal?
Fill Media Type TKN NO2-3-N TN
Sand 0.82 0.49 1.31
Sandy Loam 0.19 0.21 0.40
Effluent Concentrations in mg/L
Brown and Hunt, 2011b
2 (nearly) side-by-side cells in Rocky Mount, NC
www.bae.ncsu.edu/stormwater
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In NC, N & P are typical design considerations
• So, err on side of lower Infiltration rates
• 1 -2 inches per hour optimal
• 8-12% fines recommended
• 8% fines = 2 inch per hour rate
• 12% fines = 1 inch per hour
• Remember, infiltration rates may slow over time
www.bae.ncsu.edu/stormwater
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Summary: The “ideal” bioretention soil
• 85% coarse sand, ~10% fines, 2-5% OM
• Low Phosphorus index (10-30)
• Low but measurable organic matter (denitrification)
• Low cost
• Locally available
• Easily spec’ed
www.bae.ncsu.edu/stormwater
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Underdrain Configuration – Including an Upturn
www.bae.ncsu.edu/stormwater
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New Design Guidance: Discussed Later
www.bae.ncsu.edu/stormwater
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While you can’t “design” it… Underlying Type Counts
www.bae.ncsu.edu/stormwater
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State of North Carolina Does Differentiate Nutrient Removal
Among Underlying Soil Type • Use IWS Layer in Coastal
Plain & Sandhills with A/B HSG soil
– 60% TN and 60% TP Removal
• Use IWS Layer in Piedmont & Mountains with B/C soil
– 40% TN and 45% TP Removal
www.bae.ncsu.edu/stormwater
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Excavation Technique to Enhance Infiltration – Read Bulletin
• Scoop vs. Rake
– For final 0.25m of excavation, depth most affected by compaction
www.bae.ncsu.edu/stormwater
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Average Infiltration Sand Site
Soil Type Moist.
Cond.
Excav.
Tech.
Ave. Infil.
(in/hr)
Std Dev
(in/hr)
Sand Wet Rake 24.4 10.4
Sand Wet Scoop 17.2 6.6
Typical Sand 8 --
Loamy Sand Dry Rake 2.6 2.0
Loamy Sand Dry Scoop 1.2 0.6
Typical Loamy Sand 2.5 --
www.bae.ncsu.edu/stormwater
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Average Infiltration Clay Site
Soil
Type
Moist.
Cond.
Excav.
Tech.
Ave. Infil.
(in/hr)
Std Dev
(in/hr)
Clay Wet Rake 0.47 0.11
Clay Wet Scoop 0.09 0.02
Clay Dry Scoop 0.17 0.20
Typical Clay 0.20 --
www.bae.ncsu.edu/stormwater
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Excavation Summary
• Use Rake method of excavation to promote higher exfiltration rates
• In (loamy sand clay) soils:
– Excavate in dry soil moisture conditions
– Avoid:
• Excavation immediately after a storm
• Water ponded in bottom of cut
• Excavation if a storm will hit before the cell can be refilled
www.bae.ncsu.edu/stormwater
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So we’re done, right?
• Are our Designs on Paper being Realized in the Field?
• Can we adjust our base design & still get “good” performance?
• How can we credit that?
– What Factor of Safety is needed?
Some Remaining Workshop Items
www.bae.ncsu.edu/stormwater