The Need to Reduce Impervious
Cover to Prevent Flooding and
Protect Water Quality
Scott Millar
Sustainable Watersheds Office
RIDEM
Impervious Cover
Many Forms of Impervious Cover
in the Urban Landscape
Sidewalks
Roads
Driveways
Parking
Buildings
Center for Watershed Protection
Impervious Cover Impacts
1. Hydrologic Impacts
2. Physical Impacts
3. Biological / Habitat Impacts
4. Water Quality Impacts
5. Health Impacts
Impervious Cover
Hydrologic Impacts
• Increased runoff volume
• Increased wet weather flow
• Decreased dry weather flow
Changes in Hydrology with Increasing
Impervious Cover
US EPA
•
In a forest, rain soaks into the ground
and is either taken up by tree roots or
continues to move down through the soil
and into the groundwater.
•
When rain falls on impervious cover, it
cannot soak into the ground and instead
becomes stormwater runoff.
3
2.5
•
Inches
2
Impervious cover produces 16 times
more stormwater runoff than forest.
1.5
1
0.5
0
Rainfall
Parking Lot
Runoff
Forest Runoff
Center for Watershed Protection
Impervious Cover Influences
Wet Weather Stream Flow
When it rains, a large amount of water . . .
Runs off of
impervious
surfaces
Enters the
stormdrain
system
Is directed straight
to the stream
Center for Watershed Protection
Impervious Cover Influences
Wet Weather Stream Flow
The large amount of stormwater runoff in the stream
system can cause:
More Frequent Flooding
Higher Flood Levels
Center for Watershed Protection
Impervious Cover Influences
Dry Weather Stream Flow
•Many streams draw from
groundwater
•Impervious surfaces can block
water from contributing to
groundwater supply
•This can result in lower
stream flows during dry
weather
Center for Watershed Protection
Hunt River
Ipswich River
Aug. ‘05
Impervious Cover
Physical Impacts
• Channel enlargement
• Riparian cover reduced
• Warmer stream temperatures
In watersheds with
less than 5%
impervious cover,
streams are
typically stable and
pristine,
maintaining a good
pool and riffle
structure.
Photo Copyright 1999, Center for Watershed Protection
At 10% impervious
cover, this stream
has doubled its
original size and
the pool and riffle
structure is lost.
Photo Copyright 1999, Center for Watershed Protection
This stream is
approximately 20%
impervious cover
and shows erosion
due to an absence of
vegetation to
stabilize the bank
Photo Copyright 1999, Center for Watershed Protection
This stream has a 30% impervious cover.
The size of the stream channel has
increased by five to ten times.
Photo Copyright 1999, Center for Watershed Protection
Impervious Cover Impacts
Biological / Habitat
Impervious cover changes the natural stream
environment, resulting in:
Smothering of Habitat by
Sediment Deposits
Loss of Habitat
Variety
Loss of Stream Buffer
Impervious Cover Impacts
Biological / Habitat
Number and Diversity of
Aquatic Life
100
80
60
40
20
0
0
10
20
30
40
50
Percent Impervious Cover in Watershed
As the amount of impervious cover increases,
the number and diversity of aquatic species decreases.
Impervious Cover Impacts
Water Quality
Pollutants build up on impervious surfaces and wash off
into the stream system when it rains.
Center for Watershed Protection
Impervious Cover Impacts
Health
Harmful pollutants in
runoff include:
Bacteria
Nutrients
Pesticides
Oil & Grease
Muddy Water
Heavy Metals (e.g.
Zinc, Copper, Lead)
Center for Watershed Protection
Relationship Between Impervious
Cover and Stream Quality
Impacted
Fair
Impervious Cover Model
Sensitive
Stream Quality
Good
Poor
Damaged
10%
25%
40%
Severely Damaged
60%
100%
Watershed Impervious Cover
Center for Watershed Protection
So what do we do??
Low Impact Development
• Maintain groundwater recharge to
predevelopment levels
• Infiltrate precipitation to mimic
predevelopment hydrology
The Three Objectives of LID
1. Avoid the Impacts
• Protect open space to maintain
hydrology
• Protect natural drainage areas
(streams, surface waters, wetlands)
The Three Objectives of LID
1. Avoid the Impacts, con’t.
• Minimize clearing and grading
• Minimize soil compaction
http://croptechnology.unl.edu/
The Three Objectives of LID
2. Reduce the Impacts
• Minimize impervious cover
• Disconnect the flow of runoff
over impervious surfaces
sustainabilityinmybackyard.ca
The Three Objectives of LID
3. Manage the Impacts
• Use vegetated treatment systems
to infiltrate precipitation
Existing Stormwater Management
http://www.answers.com/topic/curb-road
Photos courtesy of Rich Claytor, Horsley Witten Group
LID Stormwater Management
Photo by Stew Comstock, Maryland Dept. of the Env.
Reducing Impervious Cover
through Better Planning
and Site Design
Percentage of Impervious Cover Associated with
Various Residential Lot Sizes
Minimum Lot Size
Density in Dwelling
Units/Acre
% Impervious Cover
10 Acres
0.1
2.4%
5 Acres
0.2
5%
3 Acres
0.333
8%
2 Acres
0.5
12%
1 Acre
1
20%
.5 Acre
2
25%
.25 Acre
4
38%
1 unit per acre
4 units per acre
8 units per acre
Site: 20%
impervious cover
Site: 38%
impervious cover
Site: 65%
impervious cover
Watershed: 20%
impervious cover
Watershed: 9.5%
impervious cover
Watershed: 8.1%
impervious cover
Protecting Water Resources with Higher-Density Development, US EPA
Community LID Site Planning
and Design Guidance
Site planning and design standards to:
1. Avoid the Impacts
2. Reduce the Impacts
Site Planning and Design Techniques
to Encourage LID
1. Avoid the Impacts
• Preserve open space
• Require vegetated buffers from surface
waters and wetlands
Site Planning and Design Techniques
to Encourage LID
1. Avoid the Impacts, con’t.
• Allow flexible lot configurations and
building envelopes
• Reduce limits of disturbance
Site Planning and Design Techniques
to Encourage LID
2. Reduce the Impacts
• Eliminate curb-and-gutter requirements
• Reduce roadway lengths and widths
Not so good.
Good.
Center for Watershed Protection
Site Planning and Design Techniques
to Encourage LID
2. Reduce the Impacts
• Establish maximum and minimum parking
standards
• Reduce, redesign, or eliminate cul-de-sacs
In Summary…
The key to reducing impervious
cover is better community site
planning and design techniques.
For more information
Scott Millar
RIDEM
(401) 222-4700, x 4419
[email protected]