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

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 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

Changes in Hydrology Due to Development

Hydrology Changes Due to Development

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

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%

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)

Physical Impacts of Impervious Cover

3. DEGRADATION OF AQUATIC ECOSYSTEMS

4. Cumulative Effects

ScouringSedimentationVelocityStreamflowOrganic Matter - Biochemical Oxygen DemandDissolved OxygenNutrientsToxicsChlorides

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

URBAN RUNOFF POLLUTANTS

• Sediments

• Nutrients

• Bacteria

• Organic Matter

• Hydrocarbons

• Metals

• Toxic Chemicals

• Chlorides

3. Degradation of Aquatic Ecosystems

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

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 Stream0.4 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 WashoffDryfall & WetfallUp to 90% WashoffConstituents

SedimentsMetalsNutrients

SourcesBuildings, Roofs, PipesPet DroppingsVegetative MatterLitter 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