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Low Impact Development and Stormwater Management in
Pennsylvania
Catherine Chomat, P.E.F. X. Browne, Inc Brian Oram, PG
Wilkes UniversityDr. Robert Traver, P.E.
Villanova University
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Impacts of Stormwater Runoff on Aquatic
Ecosystems
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Impacts of Stormwater Runoff on Aquatic Systems
• Historical Perspective
• EPA 2000 Water Quality Report
• Water Quality-Ecosystem Impacts
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Wastewater Treatment
• Since 1970…
STP Construction $239 Billion
STP Operations $234 Billion
• EPA Needs Survey
$84 Billion Needed for new STPs for
year 2008
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Wastewater Treatment
• Populations Served by STPs
1972 85 million people
1988 150 million people
• From 1972 to 1988…
4000 new sewer systems
2000 new STPs
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Wastewater Treatment
• 1988:Less than 1% of U.S. Population discharges untreated wastewater
• Meeting Effluent Criteria
Industrial 93%
Municipal 87%
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Summary of Quality of Assessed Rivers, Lakes, and Estuaries
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Leading Causes and Sources* of Impairment in Assessed Rivers,
Lakes and Estuaries
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Development Activities
Site Clearing and Grading
Tree Removal
Loss of Natural Depressions (Storage)
Loss of Topsoil/Humus
Loss of Pervious Area/Percolation
Increase in Impervious Area
Increase in Runoff
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Development Impacts on Water Quality
1. Watershed Hydrology
2. Stream Geometry
3. Degradation of Aquatic Ecosystems
4. Pollutant Export During Construction
5. Pollutant Export After Site Development
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Impervious Cover as an Indicator of Stream Health (Schueler 1995)
Characteristic Sensitive Degrading Non-Supporting
Percent Impervious Cover
0% to 10% 11% to 25% 26% to 100%
Channel Stability Stable Unstable Highly Unstable
Water Quality Good to Excellent Fair to Good Fair to Poor
Stream Biodiversity Good to Excellent Fair to Good Poor
Pollutants of Concern Sediment and temperature only
Also nutrients and metals
Also bacteria
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1. Watershed Hydrology
1. Reduced Infiltration2. Increased Peak Discharges
2-5x Predevelopment3. Increased Volume of Runoff
Moderately Developed – 50% more runoff4. Decreased Time of Concentration
“Time Runoff Reaches Stream” as much as 50% lower5. Increased Flooding
Frequency & SeverityBankfull conditions:
Natural…Once every 2 yearsDeveloped…3-4 times per year
6. Reduced Base Streamflow Due to Loss of Groundwater7. Higher Runoff Velocity
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Changes in Hydrology Due to Development
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Hydrology Changes Due to Development
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Parking Lot vs. Meadow
Parameter Parking Lot Meadow
Runoff Coefficient 0.95 0.06Time of Concentration (minutes) 4.8 14.4Peak Discharge (cfs)
2-Year 4.3 0.4100-Year Storm 12.6 3.1
Runoff Volume (cu. ft.) 3,450 218Runoff Velocity
2-Year Storm (fps) 8 1.8
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COMPARISON OF RUNOFF VOLUME BETWEEN UNDEVELOPED AND DEVELOPED CONDITIONS
STORMFREQUENCY
(years)
24-HOURRAINFALL
(inches)
ESTIMATED RUNOFF
(inches)
Undevel. Devel.
RUNOFF AS PERCENT OF RAINFALL
Undevel. Devel.
2 2.8 0.14 0.60 5% 21%
10 4.0 0.53 1.33 13% 33%
100 5.8 1.40 2.64 24% 46%
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2. STREAM GEOMETRY1. Channel Widening
Developed Area Streams: 2 to 4 times original size
Streambank ErosionIncreased Pollutant Loads
2. Increase in Floodplain
3. Undercut Streambanks
4. Siltation of StreamStream BedloadSubstrate Changes (Biota Changes)
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Physical Impacts of Impervious Cover
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3. DEGRADATION OF AQUATIC ECOSYSTEMS
4. Cumulative Effects
ScouringSedimentationVelocityStreamflowOrganic Matter - Biochemical Oxygen DemandDissolved OxygenNutrientsToxicsChlorides
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3. DEGRADATION OF AQUATIC ECOSYSTEMS
2. Temperature
Increase in % Impervious Increases Temp.Adverse Impact on- Algae- Macroinvertebrates- Fish
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Temperature
% Impervious Area
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3. DEGRADATION OF AQUATIC ECOSYSTEMS
3. Fish and Macroinvertebrates –
Decrease in…Diversity…Populations…Clean Water Species
Increase in Pollution Tolerant Species
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URBAN RUNOFF POLLUTANTS
• Sediments
• Nutrients
• Bacteria
• Organic Matter
• Hydrocarbons
• Metals
• Toxic Chemicals
• Chlorides
3. Degradation of Aquatic Ecosystems
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SEDIMENTS
Produced by: Construction ErosionDeveloped Area ErosionWashoffStreambank Erosion
Problems: TurbidityReduced Light PenetrationClogging of Gills/Filters (Fish & macroinvertebrates)Reduced SpawningHabitat DestructionNutrientsMetalsToxics
Metals and Phosphorus Attach to Sediments
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NUTRIENTS
Nitrogen (TN, NH3, NO3, Organic-N)
Phosphorus (TP, PO4, SRP)
Eutrophication of Lakes, Impoundments & Large Rivers
PROBLEMS:
Excessive Algae and Aquatic Weeds
Dissolved Oxygen Depletion
Taste & Odor Problems
Fish Kills
Loss of Recreation
Toxins
Increased Water Treatment Costs
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Wet vs. Dry Weather
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Stream Hydrograph
Total Phosphorus Concentrations
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Stream Hydrograph
Total Suspended Solids Concentrations
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BACTERIA
Always Found in Urban Runoff
Often Exceed Health Standards
More Developed – More Bacteria
Older Developed – More Bacteria
Special Problem Areas:
Combined Sewer Areas
Sewer Overflow Areas
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ORGANIC MATTER
Biochemical Oxygen Demand (BOD) of Stormwater Runoff is High
Causing:
Decreased Dissolved Oxygen
Stressed Aquatic Life
Fish Kills
Older, highly impervious areas – Highest BOD
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Dissolved Oxygen in Stream0.4 Inches of Rain
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HYDROCARBONS
Oil & Grease – Hydrocarbons
Sources:
Parking Lots
Roads
Gas Stations
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METALS
Toxic Affect on Aquatic Life
Impact on Drinking Water
Common Metals
Lead Copper Zinc
Caveats:
1. Only soluble form is toxic and available for biological uptake
2. Some are attached to sediment
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TOXIC CHEMICALS
NURP Studies: Scan of 120 Pollutants
Small concentrations – no real problem
Great Lakes – Contaminated Sediments
Limited Data Base, Need More Data for
Suburban, and
Urban Ateas
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CHLORIDES
Snowmelt: 2,000-3,000 mg/L
Chlorides Soluble Streams
High Levels: Toxic to organisms
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4. POLLUTANT EXPORT
DURING CONSTRUCTION
Soil Erosion
35-45 Tons/Acre/Year – Uncontrolled
10 Times Agriculture and Developed Land
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5. POLLUTANT EXPORT AFTER DEVELOPMENT
A. Impervious Area WashoffDryfall & WetfallUp to 90% WashoffConstituents
SedimentsMetalsNutrients
SourcesBuildings, Roofs, PipesPet DroppingsVegetative MatterLitter Debris
“THE LAND TAKES A SHOWER”
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5. POLLUTANT EXPORT AFTER DEVELOPMENT
B. Older Community – More Impervious
Household Add-Ons
C. Causes On-Site and Streambank Erosion
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Unit Area Loads
Total Phosphorus
Load Use lb/ac/yr
Forest 0.1 – 0.35
Developed 0.4 – 3.5
Commercial 0.5 – 1.6
Agriculture 0.36 – 1.52
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Ultimate Concern
• Runoff Volume
• Velocity in Stream
• Streambank Erosion
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Our Ultimate Goal