Low Impact Development and Bioretention: Lessons Learned in North Carolina Jon Hathaway, PE Bill Hunt, PE, PhD www.bae.ncsu.edu/stormwater
Pamlico Sound www.switchstudio.com
NC Stormwater Applied Research / Extension Land Grant University North Carolina Cooperative Extension Service Close interaction with the State Studies designed to address state needs Regulatory changes based on research
Experiential Learning
Experiential Learning
Experiential Learning
Experiential Learning
What other practices can we use besides the standards?
Stormwater Engineering Group
Photo: Courtesy of Ed Snodgrass
Photo: Courtesy We of Bring Ed Engineering Snodgrass to Life
Cemetery Branch Watershed
Cemetery Branch Watershed
Hydrographs Less than 1-year storm: 5.2 mm fell within 7 hours Art museum developed to 36% impervious Art museum in natural state (5% impervious) Art museum with wet pond at outlet
2: Post Development Traditional 3. Post Development LID 1: Pre Development Low Impact Development Hydrologic Analysis (Source: Prince George s County, Maryland)
Stormwater BMPs Low impact development Numerous benefits to water quality and restoration of hydrology North Carolina coastline
Stormwater BMPs Bioretention Permeable Pavement Water Harvesting Level Spreader Filter Strips Snippet of lessons learned
Bioretention Performance
What is Good General TN Removal Rate? 25% (assigned rate in 2001) was extra conservative. Due to research by NC State, removal rate increased to 35% by NC DENR Additional research may lead to another rate increase to 40%
What is Good General TP Removal Rate? 45% phosphorus removal rate. However, the fill media must have low P Index, with suggested range from 10 to 30. Incorporated as part of the State of NC Stormwater Guidance
Bioretention Phosphorus removal?
Greensboro Outflow [TP] from 2 Cells (2002 2004) 14 mg-p/liter 12 10 8 6 High P Index Fill Soil Medium P Index Soil 4 2 0 S O N D J F M A M J J A S O N D J F M A M J J A S O N D G2-Conventional G1-Internal Storage
Blame it on the Media Phosphorus Index (P Index) A measure of how much phosphorus already in 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
Concentration vs. P Index Site P-Index Depth (in) Outflow (mg/l) C-1 6 48 0.13 L-1 1-2 30 0.16 L-2 1-2 30 0.18 G-1 35-50 48 0.57 G-2 85-100 48 1.85
Mecklenburg Co. Hal Marshall Bioretention Cell (2004 2006) Soil 80% Mason Sand 20% Fines + Compost P Index = 6 4 ft (1.2 m) Depth
[TP] in mg/l 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1/1/04 7/1/04 12/30/04 6/30/05 12/29/05 TP-In TP-Out Date Concentration Reduction = 31% Load Reduction 50%
Bioretention How well does bioretention perform for indicator bacteria?
Inlet and Outlet Concentrations (# / 100 ml) Location Fecal coliform E. coli Enterococci inlet outlet inlet outlet inlet outlet Charlotte 1 2420 258 241 20 Graham N 2 4172 125 Graham S 2 4172 646 Wilmington S 1 130 284 375 378 Wilmington D 1 130 39 375 39
Wilmington Bioretention
Performance Differences? Favorable environment Temperature Soil moisture Soil chemistry Hydrology / contact time Direct connections (worm holes) Physical properties Soil depth
Worm Smokin (Thanks Garey Fox)
Results Bioretention S on average: Slightly more wet Slightly warmer ph, CEC similar Soil bacteria concentrations Highly variable No substantial differences identifiable More research needed in this area
Hydrology
Why does hydrology matter? Poorer adherence to sand Soil water flux Easier to pull bacteria out of soil column Expected higher in shallow system Contact time
Take Home Points Soil depth matters Cost vs. performance balance Bioretention design can be manipulated for indicator bacteria removal Bioretention shows promise There are minimum bioretention depths 60 cm or greater seems safe
Bioretention Is tree/shrub/mulch the only choice?
Graham High School (2006 2007) Fill Media/ Soil 90% Expanded Slate Byproduct,10% Top Soil P Index: Low 0.6 m & 0.9 m (2 & 3 feet) depth Both Cells Covered in Turf (Hybrid Bermuda)
TN concentrations: Grassed Graham HS Bioretention (2006)
Grassed GHS Bioretention North Cell Concentrations: 60% TN reduction 8% TP reduction North Cell Load (Estimated) 80% TN reduction 40% TP reduction Data from only 1 site. Show me more!
Bioretention What is internal water storage?
What is an Internal Water Storage Zone? Internal Water Storage = IWS Originally developed to improve nitrate reduction in N sensitive watersheds Bottom of cell remains saturated anaerobic conditions created to reduce nitrate TN
Bioretention Cell Schematic
Rocky Mount Imperial Centre: Museum & Performing Arts Grassed Cell (sandy loam underlying soil) Mulched/Shrub Cell (sand underlying soil)
Typical Cross Section
Water Balance
Take Home Points Internal water storage (IWS) Important design feature in sandy soil environments for reducing outflow Main benefit Infiltration Enhancer Events w/ drainage (out of 151): 21 Sandy Loam Cell 6 Sand Cell Deeper IWS depth reduces drainage volume & frequency
Bioretention What about undersized systems?
Bioretention Cells Contributing drainage area: ~1.3 acres Standard Cell SA: 2000 ft 2 Captures 0.7 inch storm in bowl Undersized Cell SA: 1000 ft 2 Captures 0.4 inch storm in bowl Internal storage zone
Water Quality Sampling Points Sampling point Sampling point Sampling point
Bioretention Cells Hydrology November 2009 May 201 Storms between 0.2 1 14 storms
Average influent and effluent nutrient concentrations in the bioretention cells
Bioretention Cells Mean Load Reductions Storms weighted equally in mean load reduction calculation
Take Home Points Standard cell vs. undersized cell Lower TSS and nutrient effluent concentrations Greater volume reduction Undersized cell did provide pollutant removal and flow mitigation benefits Suggested as retrofit in locations with limited space
Bioretention How do construction errors influence performance?
Research Site Map Nashville
Crusher Run Base
Sedimentation & Clogging Layer
Drawdown NC recommended bowl drawdown rate 12 hours (1 2 in/hr) Actual drawdown rate 48+ hours (0.1 0.5 in/hr) What if it rains now? Right after rainfall ~ 24 hours later
Fixing Bioretention Remove clogging layer & top 2 3 inches to increase surface ponding volume
Repair Characteristics Improvements nearly doubled surface storage volume 2 ft Media Depth storage vs. required: Was 28%, now 53% 3 ft Media Depth storage vs. required: Was 35%, now 66% Faster drawdown rate Was 0.1 0.5 in/hr, now 0.5 4.5 in/hr
Nashville: Fate of Runoff
Take Home Points Properly sized bowl key to treating runoff Undersized bioretention cells can generate: Substantial overflow volume Frequent overflow Longer durations of high outflow rates
Take Home Points Clogged surface reduced drawdown rate, resulting in more overflow (limited intra event drawdown) Drawdown time 6 hours vs. 48+ hours During construction: Essential to protect surface of media Provide adequate construction oversight
Long term Model Need Quantify treatment for various designs Get away from one size fits all approach Different physiographic regions Undersized systems Oversized systems Performance in wet weather or droughts Level of restoring LID hydrology Groundwater recharge requirement
Comparison to Bioretention Concepts of water movement in BRCs are very similar to Ag. fields with drain pipes Many bioretention design specifications correspond directly to DRAINMOD inputs
Bioretention Design: 2003 The Way We Were Media Depth: 4 ft Media Composition: 30% Compost Remainder, Sandy Loam Standard Drainage Configuration Trees & Shrubs with Mulch TSS, TN, TP E.R s
Bioretention Design: 2003 The Way We Were Media Depth: Variable Media Composition: Low P Index 85% Sand, 10 12% Fines IWS Drainage Configuration Grass OK, too TN, TP E.R s updated & now Thermal + Pathogen
RECAP Bioretention Media characteristics matter The power of the upturned elbow (internal water storage) Influence of media depth Undersized systems can provide benefits Will not perform as well as appropriately sized systems Importance of correct construction Benefit of maintenance
Where are we going? Bioretention Design specifications dependent on target pollutants Long term models
Where are we going? Temperature remediation? Pathogens? Concentration based management? Irreducible concentration How to we evaluate performance?
Acknowledgements Stormwater Research Group Rob Brown Stacy Luell Shawn Kennedy
Acknowledgements NC Department of Environment and Natural Resources US EPA Nonpoint Source 319(h) NC Department of Transportation CICEET (The Cooperative Institute for Coastal and Estuarine Environmental Technology) WRRI Urban Water Consortium Stormwater Group City of Charlotte
NCSU Resources www.bae.ncsu.edu/stormwater Workshops Design and maintenance Publications Journal citations Extension publications Additional websites Specific to each research area