Hydrological Benefits of Greenroofs in New York City

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Hydrological Benefits of Greenroofs in New York
City
Debra Tillinger Department of Environmental
Science Barnard College, Columbia
University Research Mentor Cynthia Rosenzweig,
NASA/GISS Seminar Advisor Martin Stute,
Environmental Science Department Barnard College,
Columbia University
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What is a Greenroof?

• Any roof that has been planted
• Intensive greenroofs / rooftop gardens
• Extensive greenroofs / ecoroofs

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Benefits of Greenroofs

• Reduction of urban heat island effect
• Lowering of local energy costs
• Psychological/Aesthetic
• Biodiversity
• Hydrology water quality and quantity

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Stormwater in NYC

• Combined sewer system
• Maximum sewer capacity twice dry weather
  sewage
• Heavy rain events lead to overflow into rivers
• NYC has an extensive sewer system data
  collection is difficult

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Greenroofs and Stormwater Water Quantity

• Water detention water is temporarily held back,
  increasing runoff time wile keeping runoff
  quantity constant
• Water retention water is held in the soil for
  the duration of the storm, eventually lost
  through evapotranspiration

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Greenroofs and Stormwater Water Quality

• pH, nitrate, phosphate, ammonium
• Problems from both stormwater and sewage
• Previous studies show very little
• Low pollutant levels in rainfall, but large
  amount of runoff

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Current Experiment

• Location Earth Pledge greenroof (midtown
  Manhattan)
• Water quantity Measured every two minutes from
  greenroof and black roof
• Water quality pH measured for greenroof runoff,
  black roof runoff, and rainwater

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Initial Results

• Less runoff from greenroof then from black roof,
  adjusted by size
• Less variability in greenroof runoff
• Intial pH results are not statistically
  significant

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Initial Results
Greenroof runoff, black roof runoff and rainfall
at Earth Pledge, approximately adjusted for roof
size.
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Continued Monitoring

• More storms in November
• Probable lab work
• Data analysis
• Comparison with Dr. Beatties data

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Implications of Research

• Which is more significant detention or
  retention?
• Do greenroofs have the potential to improve
  water quality?
• What percentage of NYC roof area would need to
  be greened to see fewer CSO events?
• How do greenroof retention and detention vary
  with the severity of storms and antecedent
  conditions?