Impervious Cover as a Indicator and Tool of Watershed Protection Tom Schueler Director of Watershed Research & Practice Center for Watershed Protection Ellicott City, MD trs@cwp.org
Key Themes A. The strength of the evidence B. Planning implications C. Impervious cover accounting D. Builders for the Bay
A. The Strength of the Evidence More than 200 studies on relationship between IC and stream quality Impervious Cover Model CWP (2003). Impacts of Impervious Cover on Aquatic Systems
Impervious Cover Can Transform a Watershed in Just a Few Years Under Agriculture Construction Undeveloped Developed Existing Development Undeveloped Developed 1950s 1960s 1970s 1990s 1940s Center for Watershed Protection
ICM Disclaimer ICM predictions are general, and may not fully apply to every stream. Factors such as stream gradient, stream order, stream type, age of subwatershed development, prior land use, past management practices can and will make some streams depart from these predictions Must be 18 or older to enter. Not valid in TX, UT and AK. APR of 6.15%. Not everyone qualifies for special financing. Offer may restricted due to Acts of God. You can never win. Center not liable for any damages, we don t have any $ even if we are
Subwatershed -10% IC
Subwatershed - 30% IC
Subwatershed - 60% IC
Subwatershed - 80% IC
Other Watershed Indicators Watershed forest cover (65%) Streamside forest cover (65% of network) Watershed turf cover (?) Road density and vehicle miles travelled (local pollutant emissions)
Physical indicators Loss of headwater streams Floodplain encroachment Loss of intact riparian buffer Stream interruption Increased number of crossings/fish barriers
Northern Virginia Regional Commission
Hydrological Indicators Increased stormwater runoff Increased frequency of flooding Floodplain expansion Diminished baseflow Increased bankfull flooding
Relationship Between Watershed Imperviousness (I) and the Storm Runoff Coefficient (Rv) (Source: Schueler, 1987) 1 0.9 Runoff Coefficient (Rv) 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 10 20 30 40 50 60 70 80 90 100 Watershed Imperviousness (%)
Stream Habitat Indicators Channel enlargement Increased sediment load Declining stream habitat scores Large woody debris Stream warming
IC and Stream Habitat 8-10% < 5% 20% > 65% 30%
Source: Leopold, 1994
10 9 Elevation (ft-msl) 8 7 6 5 4 3 Historic cross-section Current cross-section Historic Section Current Section Bankfull Depth Ultimate Section? 2 1 Ultimate cross-section? 0 0 5 10 15 20 25 30 35 40 45 Cross Section Stations (ft) - Looking Downstream
Channel Enlargement as a Function of Impervious Cover 14.00 Enlargement Ratio as a Function of Impervious Cover Enlargement Ratio (Re)ULT = (ABFL)ULT/(ABFL)PRE 12.00 10.00 8.00 6.00 4.00 2.00 All Other Data Watts Branch Data Regressed Enlargement Curve 0.00 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 Imperviousness (%)
Water quality indicators Violations of Bacteria standards Nutrients and eutrophication Aquatic life toxicity Sediment contamination Trash and debris loads
Aquatic Diversity Indicators Aquatic Insect Diversity Sensitive Insects Fish Diversity Scores Trout and Salmon Wetland and Floodplain Plant Diversity Amphibian Diversity
Fairfax Co, VA Copyright 2000, Center for Watershed Protection
IC also associated with: Urban heat islands vehicle pollutant emissions PAH and metal levels in sediments.forest fragmentation.loss of streamside forest cover.increased risks of spills, leaks, illicit discharges, illegal dumping and sewer overflows bacteria sources.shellfish and beach closure and many other factors
IC: Thresholds and Locations No actual thresholds for individual indicators (variability) Thresholds do make sense for groups of indicators Some individual species have lower thresholds (e.g., brook and brown trout) Location of IC not real important factor (pipes)
IC: The Case for Caps Site IC is not static over time 20% of households add IC each decade (US Census, 2002) Replacement of landscaping every 10 years Turf Cover (lawn compaction and runoff) Watershed IC is most important planning variable Can be measured, managed, tracked and traded Should be used judiciously Conservative: until other tools are tested
B. Planning Implications Stream Classification Watershed Vulnerability Impervious Cover Caps
Sensitive Streams [2 to 10% IC] Key Issue: Predicts that even low intensity residential development will degrade streams w/o watershed treatment Aggressive down-zoning, land conservation, and buffers. No sewers. Other indicators (FC, RFC, TC) may be more predictive
Impacted Streams 10 to 25% IC Show clear signs of declining stream health Stream indicators in the fair to good range Stream corridor may still be intact Available land in subwatershed to install practices Streams have highest restoration potential
Non-Supporting Streams 25 to 60% IC Do not support a full range of designated uses Stream indicators in the fair to poor range Streams in 25 to 40% IC show promise for stream restoration Primary goals are to reduce pollutant loads, improve stream corridor or enhance appearance Allow water contact recreation during dry weather
Urban Drainage 60 to 100% IC Stream corridor eliminated or altered Primarily functions as a conduit for storm water Water quality and aquatic diversity: poor to very poor Channels are highly unstable, where they still exist Limited prospects for stream restoration Still possible to lower downstream pollutant export
The Small Urban Watershed Restoration Manual Series Organizing To Restore Watersheds Alteration of Urban Watersheds Impacts of Urbanization on Streams Range of Subwatershed Restoration Practices Envisioning Restoration Framework for Subwatershed Planning Available for free download at www.cwp.org
Current and future IC real helpful in screening most vulnerable and/or restorable watersheds
IC is but one of eight tools needed to protect local watersheds Center for Watershed Protection
C. Impervious Cover Accounting Total Vs. Effective Impervious Cover Land Use/IC Relationships Effect of BMPs on ICM Stormwater Credits (MD Stormwater Manual) IC: Watershed Planning vs. Site Design
Total Impervious vs. Effective Cover Total best at watershed scale, effective at site scale)
Disconnecting IC is a key site design strategy but not yet a watershed protection strategy Direct Connection Connected; Flowing to Impervious Surface Disconnected; Flowing to Pervious Surface Source: Roger Bannerman Disconnected; Flowing to Disconnected; Flowing to Rain Garden
Center for Watershed Protection Ag = 1.9% 2 Acre Res. = 10.6% Impervious Cover Land Use Relationships 1 Acre Res. = 14.3 % ½ Acre Res. = 21.2 %
Center for Watershed Protection ¼ Acre Residential = 27.8 % 1/8 Acre Residential = 32.6 % Low Variability within Zoning Category Townhome Residential = 40.9 % Multifamily Residential = 44.4 %
Center for Watershed Protection Data from 4 Suburban Counties (CWP, 2001) Light Industrial = 53.4 % Commercial = 72.2 %
Can BMPs make a difference? No BMP can address all IC impacts Designed for and effective in pollutant removal No strong evidence that they protection stream quality (yet) Maintenance and lack of full watershed treatment
Credits promote better site design Next step: Setting forest canopy goals for the site and watershed
D. Reducing IC in Site Design 65% of Impervious Cover is For Car Habitat County codes create needless impervious cover (especially road design) Local roundtables have proven effective in changing codes
An Agreement signed December 3, 2001 by the Center for Watershed Protection, the Alliance for the Chesapeake Bay, and the National Association of Homebuilders Conduct 12 local site planning roundtables A completely voluntary program
Codes & Ordinances Worksheet (COW) Compares local development rules to 67 site planning benchmarks 100 points are possible Overall score indicates how environmentally friendly your codes are Also helps identify specific rules to change Cow
Photo source: Randall Arendt Open space design reduces lawn area, preserves trees, and is more attractive Alternative turnarounds use less pavement Treed lots increase marketability and resale Narrow streets have less environmental impact AND reduce speeds Preserved wooded lots provide recreation