Updates to Rainwater Manual Bioretention Guidance

Rainwater and Land Development Manual
Bioretention Design
Guidance Updates
Jay Dorsey & John Mathews
ODNR-DSWR
February 20, 2014
Why Change?
 Improved
Design -> Better Performance,
Fewer Failures; Based on:



Bioretention Practice Inspections/Observations
Research – Scientific Knowledge Base
Ability to Target Specific Pollutants or
Stormwater Management Goals
• Temperature, Nitrogen, Phosphorus
 Facilitate
Design, Review and Inspection
 Runoff Volume and Peak Discharge
Reduction Credits (under Development)
Primary References

Hunt, Davis, and Traver. 2012. Meeting Hydrologic and
Water Quality Goals through Targeted Bioretention
Design. J. Env. Eng. 138(6): 698-707.
 Hunt and Lord. 2005. Bioretention Performance, Design,
Construction and Maintenance. NCSU-CE.
 Brown, Hunt, and Kennedy. 2009. Designing
Bioretention with an Internal Water Storage (IWS) Layer.
NCSU-CE.
 NCDENR Stormwater Manual. 2009.
 Wardynski and Hunt. 2012. Are Bioretention Cells Being
Installed per Design Standards in North Carolina? A
Field Assessment. J. Env. Eng. 138(12): 1210-1217.
 CWP. 2012. West Virginia Stormwater Management and
Design Guidance Manual.
Grassed Bioretention
aka Dry Enhanced Water Quality Swale
Third Federal Bank, North Olmstead
Source: Dan Bogoevski, Ohio EPA
Overhaul or Tweaks?
Updates
 Pretreatment
Requirements
 Planting Soil Media Specifications
 Planting Soil Media Depth
 Filter Layer between Planting Soil and
Gravel Drainage Layer
 Underdrain and Elevated Outlet (Internal
Water Storage)
 Sizing and Drawdown Requirements
 Bioretention Data Submittal/Review Sheet
 Coming Update – Runoff Reduction Credits
Pretreatment Requirements
Clogging of Filter Surface
Clogging of Filter Surface
Source: Bill Hunt, NCSU-BAE
Source: Brad Wardynski, NCSU-BAE
Pretreatment Realities
For the bioretention practice to function:
1. The system must remove most sediment
from runoff before it enters the filter bed
area

2.
The bioretention “system” necessarily includes
pretreatment components
The runoff must be introduced to the filter
bed area with little or no erosive energy

The design must address elevation change
and concentrated flow
Pretreatment Requirements
 Some




form of pretreatment is required
Grass Filter Strip
Gravel Verge plus Grass Filter Strip
Grass Swale
Sediment Forebay
Pretreatment Forebay
Source: Brad Wardynski, NCSU-BAE
Pretreatment
Source: Bill Hunt, NCSU-BAE
Grass Filter Strip
Source: Matt Repasky, ODNR
Grass Filter Strip and Grass Swale
Sterncrest Road, Orange Village
flow too concentrated,
flowpath too short
flow too concentrated,
flowpath too short
too steep?
add grass filter?
Okay
Okay
Much Better
Alternative
Good
Enough?
Education Center, Zanesville
Planting Soil (Filter Bed Media)
PARAMETER
Texture Class
pH Range
Organic Matter
Phosphorus Content
Soil Test Certification
OLD
NEW
Sandy Loam, Loamy Sand
>72% Sand, <10% Clay
Loamy Sand
>80% Sand, <10% Clay
5.2 – 7.0
5.2 – 8.0
5-20% (no specification
whether by weight or volume)
3-5% by Weight
Soil P-Index between 15 and 40 15-60 mg/kg P by Mehlich3
Soil mixes must be certified by
a qualified laboratory
(1 test/100 yd3 soil)
Soil mixes must be certified
by a qualified laboratory
(1 test/100 yd3 soil)
Planting Soil Mix or Recipe
 To
get the appropriate planting soil mix
(loamy sand; >80% sand, <10% clay when
considering only mineral fraction; 3-5%
OM by weight) a good place to start is a
7.5:1.5:1 mix (75% sand, 15% topsoil,
and 10% organic matter by volume).


The sand shall be clean and meet AASHTO
M-6 or ASTM C-33.
Good (lower P) sources of “aged” organic
matter include leaf compost, pine bark fines,
or mulch fines.
Planting Soil Media Depth
30” to 36” bioretention soil (typical)
[24” bioretention soil minimum]
2-3” filter – clean concrete sand
2-3” filter - clean gravel (#8)
12” clean gravel (#57)
Planting Soil Media Depth
removal - minimum 24” filter
media depth provides excellent treatment
for most pollutants
 Pollutant

Exceptions – Temperature, Nitrogen, Phosphorus
 Plant/landscaping
needs - planting soil
depth needs to be adjusted to
accommodate expected rooting depths of
bioretention vegetation – recommend 30”36” for most applications; coordinate with
landscape architect and/or horticulturalist
Filter Layer between Planting Soil
and Gravel Drainage Layer
30” to 36” bioretention soil (typical)
[24” bioretention soil minimum]
2-3” filter – clean concrete sand
2-3” filter - clean gravel (#8)
12” clean gravel (#57)
Filter Layer between Planting Soil
and Gravel Drainage Layer
Geotextile fabric filters no
longer allowed – mounting
evidence that filter fabric
bioretention
clogs causing>24”
failure
of soil
practice
2-3” filter – clean concrete sand
2-3” filter - clean gravel (#8)
12” clean gravel (#57)
Sizing Requirements for WQv
- New Development  From
NPDES Construction Stormwater
Permit
Sizing Requirements for WQv
- New Development  Target
Drawdown Time, Td = 24 hr
Design Drawdown
 Assumption
- Kfs of settled filter bed media
(planting soil) is between 0.5 to 2.0 in/hr
[Maintenance required when Kfs < 0.5/in/hr]
Td = dWQv /Kfs = (12 in)/(0.5 in/hr) = 24 hr
Where:
 Td – drawdown time
 dWQv – equivalent depth of WQv
 Kfs – saturated hydraulic conductivity
Filter Bed Sizing Requirement

If impervious area exceeds 25% of
contributing drainage area, filter bed area
shall be a minimum 5% of contributing
impervious area.
Filter Bed Sizing Requirement

Example 1




Total contributing drainage area = 0.82 Ac
Impervious percent = 45% (>25%)
Contributing impervious area = (0.82 Ac)(0.45)
= 0.37 Ac = 16,073 ft2
Minimum filter bed area = (16,073 ft2)(0.05) = 803 ft2
Filter Bed Sizing Requirement

If impervious area exceeds 25% of
contributing drainage area, filter bed area
shall be a minimum 5% of contributing
impervious area.
 If impervious area makes up less than 25% of
contributing drainage area, filter bed area
shall be at least equal to the WQv divided by
the one foot maximum ponding depth.
Filter Bed Sizing Requirement

Example 2



Total contributing drainage area = 0.82 Ac
Impervious percent = 15% (<25%)
For 15% impervious,
C = (0.858)(0.15)3 – (0.78)(0.15)2 + (0.774)(0.15) + 0.04
= 0.141

WQv = C*P*A = (0.141)(0.75 in)(0.82 Ac)(1 ft/12 in)
= 0.007 Ac-ft = 315 ft3
Minimum filter bed area = (315 ft3)(1 ft) = 315 ft2
Filter Bed Sizing Requirement

If impervious area exceeds 25% of
contributing drainage area, filter bed area
shall be a minimum 5% of contributing
impervious area.
 If impervious area makes up less than 25% of
contributing drainage area, filter bed area
shall be at least equal to the WQv divided by
the one foot maximum ponding depth.
 Assumption - sediment storage requirement
(20% of WQv) will be met with excess bowl
volume
Filter Bed Area
Filter Bed Area
What about Redevelopment?
 For
redevelopment projects, the WQv
must be captured for all new/additional
impervious area, but for impervious area
equal to the existing impervious area the
volume that must be captured is 20% of
the WQv.
WQvTOTAL = WQvNEW + 0.2*WQvEXISTING
What about Redevelopment?

A rule of thumb based on research shows an
optimal 10:1 to 20:1 ratio for contributing
impervious drainage area to bioretention filter
bed area (i.e. hydrologic loading ratio)
 If all best practices are used (pretreatment,
energy dissipation, construction, etc.) a
hydrologic loading ratio of 25:1 is probably
okay for most sites, whereas a loading ratio
up to 40:1 may be okay for “clean” runoff such
as rooftop runoff
What about Redevelopment?
A



couple other options:
For straight redevelopment (no new
impervious), capture and treat the full WQv
from 20% of the site
For mixed redevelopment and new
development, back out the contributing
drainage area that your bioretention area
based on WQv can handle
Build a bioretention practice capable of
capturing the full WQv from the entire site, and
use the rest as credit toward reduction of
stormwater fees or as mitigation
Underdrain & Elevated Outlet
Enhancing Performance through Outlet
Configuration
Source: Bill Hunt, NCSU-BAE
Underdrain & Elevated Outlet
Holden Arboretum, Kirtland
Holden North Cell Drawdown Data
North Cell Well Drawdown Rates
Drawdown Begin Drawdown End
Date/Time
Date/Time
10/7/2013 17:22
10/17/2013 6:42
10/18/2013 2:48
10/20/2013 12:12
10/22/2013 14:16
10/26/2013 18:36
10/27/2013 12:56
11/2/2013 3:48
11/4/2013 1:30
11/9/2013 10:00
11/15/2013 7:16
11/19/2013 4:14
11/23/2013 21:28
10/16/2013 0:30
10/17/2013 15:38
10/19/2013 12:20
10/21/2013 20:30
10/23/2013 7:02
10/26/2013 21:12
10/31/2013 4:00
11/2/2013 9:22
11/6/2013 17:18
11/11/2013 17:46
11/17/2013 18:46
11/21/2013 21:28
12/9/2013 9:06
Beginning
Stage (ft)
2.099
2.085
2.084
2.052
2.07
1.923
1.892
1.883
1.847
1.851
1.794
1.789
1.811
Ending Stage
(ft)
1.17
1.97
1.721
1.624
1.783
1.894
1.352
1.815
1.344
1.355
1.491
1.279
1.165
Avg drawdown rate:
Avg drawdown rate:
Standard Deviation:
Delta Stage
(ft)
0.929
0.115
0.363
0.428
0.287
0.029
0.54
0.068
0.503
0.496
0.303
0.51
0.646
0.125
0.062
0.0507
Delta time Drawdown Rate
(days)
(ft/day)
8.30
0.37
1.40
1.35
0.70
0.11
3.63
0.23
2.66
2.32
2.48
2.72
15.48
ft/day
in/hr
Drawdown
Rate (in/hr)
0.112
0.056
0.309
0.154
0.260
0.130
0.318
0.159
0.411
0.205
0.268
0.134
0.149
0.074
0.293
0.147
0.189
0.095
0.213
0.107
0.122
0.061
0.188
0.094
0.042
0.021
TotalExfiltrated Volume:
Infiltrated
Volume
(ft3)
261
32
102
120
81
8
151
19
141
139
85
143
181
1463
Holden North Cell Drawdown Data
North Cell Well Drawdown Rates
Drawdown Begin Drawdown End
Date/Time
Date/Time
10/7/2013 17:22
10/17/2013 6:42
10/18/2013 2:48
10/20/2013 12:12
10/22/2013 14:16
10/26/2013 18:36
10/27/2013 12:56
11/2/2013 3:48
11/4/2013 1:30
11/9/2013 10:00
11/15/2013 7:16
11/19/2013 4:14
11/23/2013 21:28
10/16/2013 0:30
10/17/2013 15:38
10/19/2013 12:20
10/21/2013 20:30
10/23/2013 7:02
10/26/2013 21:12
10/31/2013 4:00
11/2/2013 9:22
11/6/2013 17:18
11/11/2013 17:46
11/17/2013 18:46
11/21/2013 21:28
12/9/2013 9:06
Beginning
Stage (ft)
2.099
2.085
2.084
2.052
2.07
1.923
1.892
1.883
1.847
1.851
1.794
1.789
1.811
Ending Stage
(ft)
1.17
1.97
1.721
1.624
1.783
1.894
1.352
1.815
1.344
1.355
1.491
1.279
1.165
Avg drawdown rate:
Avg drawdown rate:
Standard Deviation:
Delta Stage
(ft)
0.929
0.115
0.363
0.428
0.287
0.029
0.54
0.068
0.503
0.496
0.303
0.51
0.646
0.125
0.062
0.0507
Delta time Drawdown Rate
(days)
(ft/day)
8.30
0.37
1.40
1.35
0.70
0.11
3.63
0.23
2.66
2.32
2.48
2.72
15.48
ft/day
in/hr
Drawdown
Rate (in/hr)
0.112
0.056
0.309
0.154
0.260
0.130
0.318
0.159
0.411
0.205
0.268
0.134
0.149
0.074
0.293
0.147
0.189
0.095
0.213
0.107
0.122
0.061
0.188
0.094
0.042
0.021
TotalExfiltrated Volume:
Infiltrated
Volume
(ft3)
261
32
102
120
81
8
151
19
141
139
85
143
181
1463
Holden North Cell Drawdown Data
Planning Considerations
 Drainage
Area < 2 Acres
 Existing Infrastructure
 Setbacks from Property Lines, Building
Foundations, Wells, Septic Systems
Planning Considerations
 Drainage
Area < 2 Acres
 Existing Infrastructure
 Setbacks from Property Lines, Building
Foundations, Wells, Septic Systems
 Commitment/Resources
to Maintain Practice
Site Evaluation
 Groundwater

Pollution Concerns
Karst or Shallow Sand/Gravel Aquifer Areas
 Shallow
Depth to Bedrock
 Shallow Depth to Water Table

2 ft separation recommended, 1 ft required
 Soil
Infiltration Capacity
Planning and Design Considerations

HSG Shorthand

HSG-A
• Shallow aquifer?
• Avoid short circuiting from pollutant “hot spots”

HSG-B
• Easy to work with
• Maintain infiltration capacity of soils
• Drainage usually recommended

HSG-C
• Oftentimes in optimal landscape position
• Maintain infiltration capacity of soils
• Drainage required

HSG-D
• Must identify limitations and design accordingly
• Drainage required
BMP Hydrology
P – Precipitation (Rainfall & Snowmelt)
F1 – Infiltration
ET – Evaporation & Transpiration
F2 – Exfiltration
S1 – Temporary Surface Storage
Qin – Runon/Lateral Inflow
S2 – Temporary Subsurface Storage
Qout - Runoff
P
ET
S1
Qoverflow
Qin
F1
Qin-leak
Qtile
Qout-leak
S2
F2
Qout
Estimating Infiltration Rates for
BMPs for Site Planning
Soil Water Characteristics Calculator
Subgrade Kfs Estimates
Subgrade USDA
Soil Texture
Clay
Content
%
Ksat
(in/hr)
Sand
<8
2.8
Loamy Sand
< 15
2.0
Sandy Loam
< 20
0.80
7 – 27
0.16
Silt Loam
< 27
0.05
Silt
< 12
0.05
Sandy Clay Loam
20 – 35
0.07
Clay Loam
27 – 40
0.02
Silty Clay Loam
27 – 40
0.02
Silty Clay
40 – 50
0.01
Sandy Clay
35 – 55
<0.005
> 40
<0.005
Loam
Clay
Infiltration Test for BMP Design?
Bore Hole/
Perc Test (v1)?
3-Dimensional
Flow
Ponded Ring
Infiltrometer Test
~1-Dimensional
Flow
Single Ring Infiltrometer
Single Ring Infiltrometer
Bioretention Cell Components
Bioretention
Decisions
Base Design
30-36” depth;
IWS Layer
HSG A Soils
If Kfs > 1 in/hr,
may not require
underdrain,
aggregate, filter
Temperature
36+” media depth;
IWS Layer (>18”);
48” depth to drain
Nitrogen
Treatment
36” media depth;
IWS layer (>18”),
outlet raised >6”
into planting media
Depth Limitations
(e.g., Shallow
Outlet, High
Water Table)
24” media
depth
HSG D Soils
(depending on
limitations)
Underdrain w/ 3” of
cover & 3” of
bedding
High Water Table,
Karst, Shallow
Bedrock or High
Pollution Loads
Impermeable liner
Base Bioretention Configuration
Base Bioretention Configuration
30”-36” Planting Soil
6” Filter
12” Aggregate
Base Bioretention Configuration
24” Planting Soil above Invert
6” (min) Planting Soil in IWS
Special Designs
 Pollutant



Temperature Mitigation
Nitrogen Removal
Phosphorus Mitigation
 Site




Load Reduction Goals
Conditions or Limitations
High Permeability Soils (> 1 in/hr)
Very Low Permeability Soils (<0.05 in/hr)
Depth Limitations
Groundwater Pollution Potential
Bioretention
Decisions
Base Design
30-36” depth;
IWS Layer
HSG A Soils
If Kfs > 1 in/hr,
may not require
underdrain,
aggregate, filter
Temperature
36+” media depth;
IWS Layer (>18”);
48” depth to drain
Nitrogen
Treatment
36” media depth;
IWS layer (>18”),
outlet raised >6”
into planting media
Depth Limitations
(e.g., Shallow
Outlet, High
Water Table)
24” media
depth
HSG D Soils
(depending on
limitations)
Underdrain w/ 3” of
cover & 3” of
bedding
High Water Table,
Karst, Shallow
Bedrock or High
Pollution Loads
Impermeable liner
Underdrain Configuration
For Basic BRC Installation 30”–36” Media Depth
Elevated outlet recommended for all
HSG-A, B, C soils with Kfs > 0.1 in/hr
- 18”+ for Temp, N & Volume Reduction
D soils – 3” gravel
bedding acts as sump
Why Do Bioretention Practices Fail?
Sediment Clogging of Geotextile Filter
Between Soil and Aggregate Layers
Why Do Bioretention Practices Fail?
Sediment Clogging of Geotextile Filter
Between Soil and Aggregate Layers
Why Do Bioretention Practices Fail?
1.
2.
3.
4.
5.
6.
7.
Sediment Clogging of Filter Bed Surface
Eroding Sideslopes
Undersized Surface Ponding Volume
Construction Issues/Lack of Construction
Oversight
Plant Selection and Management
Lack of Maintenance
Bioretention BMP or Design Poor Fit for
Site
Construction Issues/Lack of
Construction Oversight

Loss of Exfiltration/Infiltration Capacity


smearing or compaction of subgrade soils during
excavation
compaction of filter bed soils during construction
Materials – esp. filter sand and planting media
 Elevations – filter bed surface, overflow
 Existing or Hidden Infrastructure
 Keeping Sediment Out of BRC During
Construction – staging, site drainage and
erosion control during construction, site
stabilization

Bioretention
Design
Checklist and
Review Sheet
Questions:
Jay Dorsey
Water Resources Engineer
ODNR, Soil & Water Resources
(614) 265-6647
[email protected]

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