bioretention design specification updates
TRANSCRIPT
Rainwater and Land Development Manual
Bioretention Design Specification Updates
Jay Dorsey & John MathewsODNR-DSWRApril 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 InspectionRunoff Volume and Peak Discharge
Reduction Credits (under Development)
Primary References Hunt, Davis, and Traver. 2012. Meeting Hydrologic and
Water Quality Goals through Targeted BioretentionDesign. 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 OlmsteadSource: Dan Bogoevski, Ohio EPA
Grassed Bioretentionaka Dry Enhanced Water Quality Swale
Overhaul or Tweaks?
Updates Pretreatment Requirements Planting Soil Media Specifications Planting Soil Media Depth Filter Layer between Planting Soil and
Gravel Drainage LayerUnderdrain and Elevated Outlet (Internal
Water Storage) Sizing and Drawdown Requirements Bioretention Data Submittal/Review SheetComing Update – Runoff Reduction Credits
Pretreatment Requirements
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 CertificationSoil 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
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]
Filter Layer between Planting Soil and Gravel Drainage Layer
12” clean gravel (#57)
2-3” filter - clean gravel (#8)
2-3” filter – clean concrete sand
>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
Underdrain & Elevated Outlet
Enhancing Performance through Outlet Configuration
Source: Bill Hunt, NCSU-BAE
Underdrain & Elevated Outlet
Holden Arboretum, Kirtland
Underdrain ConfigurationFor 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 MitigationNitrogen RemovalPhosphorus 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 Mehlich3Recommend adding water treatment
residuals (WTR) or other iron or aluminum rich amendment
Source: Bill Hunt, NCSU-BAE
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
Filter Bed Area(%
)
Filter Bed Area
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
BioretentionDesign
Checklist and Review Sheet
