Low Impact Development Techniques: Background, Current Research, and Site-Specific Design Variations

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Low Impact Development Techniques Background,
Current Research, and Site-Specific Design
Variations

• Michael Dietz, Ph.D.
• DES Drinking Water Source Protection Workshop
• Concord, NH
• May 18, 2006

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The status quo
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A little history

• First scientific documentation of runoff varying
  with impervious in watershed

1889!!!
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Yeah, but they didnt know about water quality
issues

• Law for tanners
• Disposal of wastes to Ilissos river in Greece
  banned

• 440 BC !!

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Urban Runoff

• Contains nitrogen, phosphorus, metals, bacteria
• 32 of impaired estuaries impacted by stormwater
  runoff (USEPA, 2002)

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Why stormwater matters hydrology
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Early attempts at reducing impacts of stormwater
detention basins
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Detention/retention basins

• Designed to reduce PEAK FLOW RATES
• Dry basins typically have poor performance, and
  tend to re-suspend particulates
• Rarely reduce runoff volumes

Source EPA 1999
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LID Site Planning and Design Concepts

• The Goal To preserve pre-development hydrology
• Runoff volume and rate
• Groundwater recharge
• Stream baseflow
• Runoff water quality

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Site Planning and Design Concepts

• Design Developments to Fit the Natural Terrain
• Reduce or Disconnect Impervious Areas
• Preserve and Utilize Natural Drainage Systems
• Limit Land Disturbance
• Provide Setbacks and Vegetated Buffers
• Minimize the Creation of Steep Slopes
• Maintain Pre-Development Vegetation

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Low Impact Development Practices

• Bioretention/Rain Gardens

Vegetated areas designed to infiltrate and
process stormwater
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Low Impact Development Practices

• Bioretention/Rain Gardens

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Bioretention monitoring results

• Maryland (EPA, 2000)
• retention based on
  concentrations
• North Carolina (Hunt, 2003)
• Connecticut (Dietz Clausen, 2006)
• Flow and pollutants reduced by
  99 overall

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Haddam, CT rain garden
Despite measurable frost, no decrease in
infiltration in winter!
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Effect on flow rate
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Important factors with bioretention

• Infiltration capacity of native soils
• P-index of soil mixture
• Underdrain
• Recommended in Bioretention Manual CT SWQM
• May not always be necessary
• Depends on local soil conditions

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Total phosphorus, Haddam CT study rain garden
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Design variation Saturated zone for NO3-N
treatment
Monitoring tank
Underdrain
Also tested in North Carolina and Maryland (see
Kim, et al., 2003 for design)
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Statistical testing ANCOVA results
-TN (-18) and TP (-82) were reduced
significantly due to saturation -ANCOVA not
performed on metals
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?2 AnalysisPercent below detection in saturated
garden
-No significant changes for Cu, Pb -No
significant changes in unsaturated garden
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Design variation high metals loading

• Most settings have low metals loadings
• In high loading areas, provisions may need to be
  made for periodic removal of top mulch/soil layer
• Hyperaccumulating plant/bioretention plant link
  not examined yet

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Design variation high TSS loading

• Heavy sediment loading may clog infiltrating
  surface of bioretention
• A grass filter strip or sediment forebay can be
  installed to reduce this impact

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Design variation hot spots

• Hot spots are areas with high loadings of
  certain pollutants
• Impervious liner can be installed and pipe access
  can be provided (MD Bioretention Manual)

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Design variation low permeability soils

• Extra storage can be gained by installing crushed
  stone below the underdrain (MD Bioretention
  Manual)

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Excavation of bioretention

• Avoid compaction/sealing of bottom with bucket

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Important factors with bioretention

• Seasonal high water table
• Soil compaction before, during construction

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Important factors with bioretention

• Maintenance of flow paths and storage
• Watch for sediment accumulation

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Important factors with bioretention

• Over-Engineering

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Low Impact Development Practices

• Vegetated Roof Covers Intensive

Chicago City Hall
Stamford, CT
Ledyard, CT
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Low Impact Development Practices

• Vegetated Roof Covers Extensive

Ford Motor Company
Assembly Plant, Dearborn, MI
Courtesy of Michigan State University Dept. of
Horticulture
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Low Impact Development Practices

• Vegetated Roof Covers in CT

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Green roof monitoring results

• North Carolina (Moran et al., 2004)
• 63 of precipitation retained (on average)
• Higher TN, TP concentrations, TP export
• Due to media
• Michigan (VanWoert et al.,2005)
• 61 of precipitation retained (on average)
• Thicker media (6 cm) and less slope increased
  retention

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Low Impact Development Practices

• Permeable paving systems

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Pervious pavers monitoring results

• Washington (Brattebo Booth, 2003)
• Grasspave, Gravelpave, Ecostone, Turfstone
• Virtually all rainfall infiltrated
• Percolate water quality generally better than
  runoff from asphalt lot
• North Carolina (Hunt, personal communication,
  2005)
• Zinc, ammonia, TP, and TKN concentrations were
  significantly reduced after infiltrating through
  Ecostone pavers
• Connecticut (Gilbert Clausen, 2005)
• Runoff (average) from Ecostone driveway was 72
  less than asphalt
• Concentrations of all pollutants lower in surface
  runoff from Ecostone driveways, as compared to
  asphalt

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How about clay soils??

• Recent research suggests that significant
  infiltration can still be achieved (Dreelin, et
  al., 2006)
• Grass pave system over 10 inches of gravel, and a
  clay soil subgrade

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Issues with permeable pavers

• Clogging

2005
2002
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Putting it all together...
Glen Brook Green (Jordan Cove)Research/Demo
Project
LID Cluster
Traditional
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Jordan Cove BMP Cluster Subdivision
Bioretention area in turnaround
Pervious driveways
Grass swale
Rain gardens
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So does it work?
Data from J. Clausen, UCONN
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Monitoring resultsBefore vs. after construction
Traditional
LID
LID goal
Flow
Nitrogen
Phosphorus
Metals
TSS
nc
The big picture LID significantly reduced flow
and pollutant export!!
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Conclusions

• LID systems work!!
• A variety of design variations exist to treat
  target pollutants
• LID research still evolving

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Acknowledgements

• Funding for the monitoring of the Haddam rain
  garden was provided by a Long Island Sound
  License Plate Grant
• Additional funding was also provided by the CT
  DEP through a US EPA 319 nonpoint source Clean
  Water Act grant