Bioretention Design Specification Updates

Rainwater and Land Development Manual
Bioretention Design
Specification Updates
Jay Dorsey & John Mathews
ODNR-DSWR
April 16, 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.

Third Federal Bank, North Olmstead
Source: Dan Bogoevski, Ohio EPA
Grassed Bioretention
aka Dry Enhanced Water Quality Swale

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

Source: Bill Hunt, NCSU-BAE
Clogging of Filter Surface

Source: Brad Wardynski, NCSU-BAE

Pretreatment Requirements
 Pretreatment is required
 Grass Filter Strip
 Gravel Verge plus Grass Filter Strip
 Grass Swale
 Sediment Forebay

Planting Soil (Filter Bed Media)
PARAMETER OLD NEW
Texture Class
Sandy Loam, Loamy Sand
>72% Sand, <10% Clay
Loamy Sand
>80% Sand, <10% Clay
pH Range 5.2 – 7.0 5.2 – 8.0
Organic Matter
5-20% (no specification
whether by weight or volume)
3-5% by Weight
Phosphorus Content Soil P-Index between 15 and 40 15-60 mg/kg P by Mehlich3
Soil Test Certification
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
5:1:1 mix (70% sand, 15% topsoil, and
15% 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.

12” clean gravel (#57)
2-3” filter - clean gravel (#8)
2-3” filter – clean concrete sand
30” to 36” bioretention soil (typical)
[24” bioretention soil minimum]
Planting Soil Media Depth

Planting Soil Media Depth
 Pollutant removal - minimum 24” filter
media depth provides excellent treatment
for most pollutants
 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

30” to 36” bioretention soil (typical)
[24” bioretention soil minimum]
Filter Layer between Planting Soil
and Gravel Drainage Layer

>24” bioretention soil
Filter Layer between Planting Soil
and Gravel Drainage Layer
Geotextile fabric filters no
longer allowed – mounting
evidence that filter fabric
clogs causing failure of
practice

Enhancing Performance through Outlet
Configuration
Source: Bill Hunt, NCSU-BAE

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

Special Designs
Temperature Mitigation
Nitrogen Removal
Phosphorus Mitigation

Temperature Mitigation
 Planting soil media depth - minimum 36”,
48” preferred
 Underdrain/outlet configuration - upturned
elbow with internal water storage (IWS)
layer, minimum 18” sump

Nitrogen Removal
 Planting soil media depth - minimum 36”
 Underdrain/outlet configuration
 upturned elbow with internal water storage
(IWS) layer, minimum 18” sump
 if necessary, orifice on drain outlet to control
discharge rate

Phosphorus Removal
 Planting soil media depth - minimum 36”
 Planting soil phosphorus content – 15-40
mg/kg P by Mehlich3
 Recommend adding water treatment
residuals (WTR) or other iron or aluminum
rich amendment

Sizing and Drawdown Requirements
 Assumptions
 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]
 20% of WQv sediment storage requirement
will be met with excess bowl volume

Design Drawdown
 Td – drawdown time
 dWQv – equivalent depth of WQv
 Kfs – saturated hydraulic conductivity
Td = dWQv /Kfs = (12 in)/(0.5 in/hr) = 24 hr

Bioretention
Design
Checklist and
Review Sheet

Questions:
Jay Dorsey
Water Resources Engineer
ODNR, Soil & Water Resources
(614) 265-6647
jay.dorsey@dnr.state.oh.us

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