Transcript Low Impact Development and Stormwater

Low Impact Development and
Stormwater Management in
Pennsylvania
Catherine Chomat, P.E.
F. X. Browne, Inc
Brian Oram, PG
Wilkes University
Dr. Robert Traver, P.E.
Villanova University
Impacts of Stormwater
Runoff on Aquatic
Ecosystems
Impacts of Stormwater Runoff on
Aquatic Systems
• Historical Perspective
• EPA 2000 Water Quality Report
• Water Quality-Ecosystem
Impacts
Wastewater Treatment
•
•
Since 1970…
STP Construction
$239 Billion
STP Operations
$234 Billion
EPA Needs Survey
$84 Billion Needed for new STPs for
year 2008
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
Wastewater Treatment
•
1988:Less than 1% of U.S. Population
discharges untreated wastewater
•
Meeting Effluent Criteria
Industrial
93%
Municipal
87%
Summary of Quality of Assessed
Rivers, Lakes, and Estuaries
Leading Causes and Sources* of
Impairment in Assessed Rivers,
Lakes and Estuaries
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
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
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
1. Watershed Hydrology
1. Reduced Infiltration
2. Increased Peak Discharges
2-5x Predevelopment
3. Increased Volume of Runoff
Moderately Developed – 50% more runoff
4. Decreased Time of Concentration
“Time Runoff Reaches Stream” as much as 50% lower
5. Increased Flooding
Frequency & Severity
Bankfull conditions:
Natural…Once every 2 years
Developed…3-4 times per year
6. Reduced Base Streamflow Due to Loss of Groundwater
7. Higher Runoff Velocity
Changes in Hydrology Due to Development
Hydrology Changes Due to
Development
Parking Lot vs. Meadow
Parameter
Runoff Coefficient
Time of Concentration (minutes)
Peak Discharge (cfs)
2-Year
100-Year Storm
Runoff Volume (cu. ft.)
Runoff Velocity
2-Year Storm (fps)
Parking Lot
0.95
4.8
Meadow
0.06
14.4
4.3
12.6
3,450
0.4
3.1
218
8
1.8
COMPARISON OF RUNOFF VOLUME BETWEEN
UNDEVELOPED AND DEVELOPED CONDITIONS
STORM
24-HOUR
ESTIMATED RUNOFF
(inches)
Undevel.
Devel.
RUNOFF AS PERCENT
OF RAINFALL
Undevel.
Devel.
FREQUENCY
RAINFALL
(years)
(inches)
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%
2. STREAM GEOMETRY
1. Channel Widening
Developed Area Streams: 2 to 4 times original size
Streambank Erosion
Increased Pollutant Loads
2. Increase in Floodplain
3. Undercut Streambanks
4. Siltation of Stream
Stream Bedload
Substrate Changes (Biota Changes)
Physical Impacts of Impervious Cover
3. DEGRADATION OF AQUATIC
ECOSYSTEMS
1. Cumulative Effects
Scouring
Sedimentation
Velocity
Streamflow
Organic Matter - Biochemical Oxygen Demand
Dissolved Oxygen
Nutrients
Toxics
Chlorides
3. DEGRADATION OF AQUATIC
ECOSYSTEMS
2. Temperature
Increase in % Impervious  Increases Temp.
Adverse Impact on
- Algae
- Macroinvertebrates
- Fish
Temperature
% Impervious Area
3. DEGRADATION OF AQUATIC
ECOSYSTEMS
3. Fish and Macroinvertebrates –
Decrease in
…Diversity
…Populations
…Clean Water Species
Increase in Pollution Tolerant Species
3. Degradation of Aquatic Ecosystems
URBAN RUNOFF POLLUTANTS
• Sediments
• Nutrients
• Bacteria
• Organic Matter
• Hydrocarbons
• Metals
• Toxic Chemicals
• Chlorides
SEDIMENTS
Produced by:
Construction Erosion
Developed Area Erosion
Washoff
Streambank Erosion
Problems:
Turbidity
Reduced Light Penetration
Clogging of Gills/Filters
(Fish & macroinvertebrates)
Reduced Spawning
Habitat Destruction
Nutrients
Metals
Toxics
Metals and Phosphorus Attach to Sediments
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
Wet vs. Dry Weather
Stream Hydrograph
Total Phosphorus Concentrations
Stream Hydrograph
Total Suspended Solids Concentrations
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
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
Dissolved Oxygen in Stream
0.4 Inches of Rain
HYDROCARBONS
Oil & Grease – Hydrocarbons
Sources:
Parking Lots
Roads
Gas Stations
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
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
CHLORIDES
Snowmelt: 2,000-3,000 mg/L
Chlorides  Soluble  Streams
High Levels: Toxic to organisms
4. POLLUTANT EXPORT
DURING CONSTRUCTION
Soil Erosion
35-45 Tons/Acre/Year – Uncontrolled
10 Times Agriculture and Developed
Land
5. POLLUTANT EXPORT AFTER
DEVELOPMENT
A. Impervious Area Washoff
Dryfall & Wetfall
Up to 90% Washoff
Constituents
Sediments
Metals
Nutrients
Sources
Buildings, Roofs, Pipes
Pet Droppings
Vegetative Matter
Litter Debris
“THE LAND TAKES A SHOWER”
5. POLLUTANT EXPORT AFTER
DEVELOPMENT
B. Older Community – More Impervious
Household Add-Ons
C. Causes On-Site and Streambank Erosion
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
Ultimate Concern
• Runoff Volume
• Velocity in Stream
• Streambank Erosion
Our Ultimate Goal