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Tom Schueler: "We have met the enemy – imperviousness"

Automobile habitatTom Schueler is the Executive Director of the Center for Watershed Protection, a non-profit organization in Silver Springs, Maryland, devoted to the protection, restoration and stewardship of our nation's watersheds. He also edits the Center's quarterly journal Watershed Protection Techniques, which provides condensed summaries to local planners, engineers and municipal officials on the latest techniques and practices to protect and restore urban watersheds.


Mr. Schueler was formerly Chief of the Anacostia Restoration Team at the Metropolitan Washington Council of Governments, where he directed the restoration of the urban Acacostia watershed. Over the past 15 years, he has pioneered new designs for stormwater ponds, wetlands and filtering systems, developed new methods for urban watershed planning, and demonstrated new techniques for stream restoration, sediment control, riparian reforestation, and stormwater retrofits.

WaterLaws: What is the biggest problem facing urban watersheds and streams?

Mr. Schueler: The high level of impervious coverage. While much attention is focused on the level of pollutants in a stream, the real problem is often the increased volume of a stream which results from high levels of impervious cover. When streams run through watersheds with high levels of impervious coverage, their volume increases dramatically due to increased run-off.

The increased volume of the stream is often the greatest threat to stream and watershed biodiversity and vitality. Phosphorus levels generally do not vary between urban and rural streams, but the increased volume in an urban stream bed results in an increased total phosphorus load. The increase in total amount of phosphorous and other contaminants that accompanies such a load increase is the greatest threat to biodiversity and vitality of streams and watersheds.

WaterLaws: What are the primary sources of impervious coverage?

Mr. Schueler: Car habitat! We average 10 parking spaces in an urban area for every car. Two-thirds of all impervious coverage today is to provide habitat for cars – parking lots, driveways, roads and highways. The remainder of impervious surface is primarily made up of rooftops on residential and commercial buildings.

WaterLaws: How can increased impervious surfaces effect urban streams and watersheds?

Mr. Schueler: Even with a mere 10% of a watershed covered with impervious surfaces, the resulting increase in stream volume from corresponding run-off can have numerous impacts.

  1. Increased floods and flood peaks, leading to stream straightening and streambed erosion;

  2. Increased erosion, leading to loss of trees and vegetation along the banks (at 8% - 10% impervious surface coverage, streams double in the size of the bed due to the increased volume);

  3. Increased pollutant loads;

  4. Increased shell fish diseases and beach closures;

  5. Increase in stream temperature which messes up lots of biological processes;

  6. Increased bacteria, often as a direct of a high density of household pets;

  7. Decreased high weather flow;

  8. Decreased pooling;

  9. Decreased woody debris, a crucial habitat element for aquatic insects;

  10. Decrease in substrate quality;

  11. Decreased fish passage during dry weather flow periods due to the enlarged stream bed; and

  12. Decrease in insect fish and fish diversity. At 12% imperviousness, trout and other sensitive species can no longer survive in the stream.

Imperviousness similarly affects lakes and wetlands.

WaterLaws: What is the level of imperviousness for a typical residential neighborhood?

Mr. Schueler: Typical residential neighborhoods range from 10% - 30% impervious surface coverage. This results in stormwater run off 10 - 20 times greater than your average meadow. The amount of impervious surface covering has increased across the U.S. by over 50% since 1945, with the biggest growth resulting from large increases in roads, road size, and parking areas.

WaterLaws: What are the various effects of increasing levels of imperviousness?

Mr. Schueler: At 10% imperviousness in its watershed, a stream is considered at risk. Between 11% and 25% the stream is considered impacted, unable to sustain its full range of natural biodiversity. At 25% or higher, the stream is unable to support any level of biodiversity. Upwards of 50%, and the stream is unable to support any life form.

The biggest reason for the "death" of urban streams whose watersheds contain over 25% impervious surface is the increased bacterial problems that result from high levels of imperviousness. Often the largest contributor to bacterial problems is pet waste. In some cases, fecal chloroform can be traced to particular neighborhood animals.

WaterLaws: How can a developing community control the amount of impervious surface?

Mr. Schueler: In planning for its future development, communities may take many steps, including:

    1. Narrower residential road widths
    2. Reduced road lengths
    3. Hourglass streets
    4. Cluster development
    5. Shared driveways
    6. Angled parking with one way traffic flow
    7. Smaller parking stalls
    8. Reduced parking space ratios for some uses
    9. Shared parking facilities in commercial areas
    10. Shorter residential driveways
    11. Reduced cul-de-sac radii
    12. Cul-de-sac donuts
    13. Vertical parking structures
    14. Two and three story buildings
    15. Stream buffers
    16. Grass swales rather than curb/gutters
    17. Open space requirements (residential)
    18. Open space landscaping requirements (commercial)
    19. Sidewalks only on one-side of street
    20. Reduced side and rear yard setbacks
    21. Decrease distance between lots (frontage)
    22. Hammerhead-shaped turnarounds
    23. Rear yard grading to buffer
    24. Permeable spillover parking areas

WaterLaws: What about communities that are already fully developed?

Mr. Schueler: In urban watersheds already limited by existing development and impervious surfaces, it is important to evaluate achievable goals prior to commencing efforts to "clean up" or "save" a stream, preserving the original biodiversity of the stream.

If a community believes its impervious level is below the 10% threshold for preserving complete biodiversity, it may choose to use buffer zones to extend the protection for the stream in order to preserve its biodiversity. If the impervious level is well over this threshold. You have a variety of management strategies available, but realistically you will not be able to restore a stream completely.

WaterLaws: What are "buffer zones"?

Mr. Schueler: Buffer zones are stream "right-of-ways" where the expansion of impervious surfaces is limited and the natural landscape is preserved. Buffer zones prevent encroachment and usually include the entire stream floodplain. There is no fixed width-appropriate buffer zone depends on the specific stream conditions.

There are generally three zones to an effective buffer. Twenty-five feet out from the streambank is considered the mandatory zone. The second or middle zone extends from 50 - 100 feet from the streambank, encompassing the entire floodplain. Outer zones of the stream may extend even further as desired or required.

WaterLaws: What can be done to protect the streambed itself from erosion and to improve aquatic habitat?

Mr. Schueler: Generally there are two different types of streambed protection; bioengineering, using natural materials and plants, and bank armoring, using synthetic materials to reinforce streambeds. Similar techniques exist for promoting aquatic habitat. We generally promote bioengineered solutions, but the appropriate techniques are highly dependent on the specific stream and its watershed.

WaterLaws: Are there any methods for controlling stormwater runoff prior to the runoff reaching the stream bed or its banks at its source?

Mr. Schueler: Some states, such as Florida, have used bio-retention in parking lots. Bio-retention requires that medians in parking lots and other similar impervious surfaces be lower than the level of the impervious surface, drawing water into them. These bioretention areas or "pits" contain plants and grit chambers to absorb stormwater runoff. Bioretention is very dependent upon proper drainage for effective operation.

WaterLaws: What effect does street sweeping have on limiting the effects of imperviousness?

Mr. Schueler: Although the new generation of street sweepers is more effective in removing smaller particulate matter from impervious surfaces, thus preventing its affect on streams, the lack of a systematic approach to street sweeping severely limits its effectiveness. With a vigorous, systematic approach, street sweeping may achieve a phosphorus removal rate of about 10-30%.

WaterLaws: What should communities do in determining what strategies to pursue in preserving and protecting urban streams and their watersheds?

Mr. Schueler: Set goals, prioritizing streams within you watershed to one of three categories: best streams; middle quality streams; and non-supporting streams. These streams should be evaluated in terms of the amount of impervious surface within their watershed.

If a community's best streams are below or around 10% imperviousness, the community may wish to set the goal of preserving biodiversity. Important in setting this goal is making protection a public challenge, involving members of government and the public and making them aware of the difficulty, importance, and rewards involved. If these best streams have well in excess of the 10% impervious cover required to maintain biodiversity the community can attempt to preserve the watershed as a living though not biodiverse stream by keeping the level of imperviousness below 25% and using buffer zones to insulate the stream bed from existing impervious surfaces.

In streams that communities identify as middle quality or priority, efforts should be limited to controlling or reversing stream bed degradation, limiting or controlling phosphorus or other pollutants to protect existing fish and to protect downstream rivers and lakes. Communities often find cluster or planned unit developments are good solutions to providing limited amount of stream protection.

Finally, in low priority, or non-supporting streams, communities should focus protection efforts on preserving water quality in order to prevent pollution downstream, and preventing floods. Allowing development in these watersheds may be used as a trade off for protecting streams classified as best or high-quality streams or perhaps even middle quality or priority streams.

WaterLaws: What about stream restoration?

Mr. Schueler: In my experience, successful stream restoration in terms of completely restoring biodiversity is often very difficult. The repairs to the stream itself are often just too costly. Undoing high levels of imperviousness are often impossible, and communities may often use their resources to protect streams.

WaterLaws: What is the best comprehensive approach to protecting streams?

Mr. Schueler: I recommend a six point approach to total stream protection:

  1. Zone consistent with stream quality. Create zones that promote lower levels of impervious surfaces in watersheds which continue to contain biodiversity.

  2. Protect key natural areas. Communities may use stream buffer zones to protect key areas from development.

  3. Establish stream and resource buffer zones. Communities should provide a "right-of-way" for streams. Such buffer zones prevent encroachment of impervious surfaces and sources of pollution introduction. While the national average for effective buffer zones is 100 feet, there is no fixed width. Buffer zone width depends upon the specific stream but should always include the entire flood plain.

  4. Reduce impervious cover in site design using cost-effective methods to reduce impervious coverage. Methods for reducing impervious cover include stream protection clustering, narrower streets, and smaller driveways, sidewalks, cul de sacs, and parking lots.

  5. Limit clearing and erosion during construction. By requiring sequencing of construction which requires developers to clear and build one small portion of a development at a time, and by requiring foot printing, a system of placing buildings within a landscape without landscape, communities can limit the effects of construction on stream quality. Requirements that construction and development be done quickly also positively impact stream quality. The effectiveness of builder and developer regulations are extremely dependent upon frequent inspection and strong civil enforcement.

  6. Treat quantity and quality of stormwater from existing impervious surfaces. The best method for treating stormwater runoff is a system of wetlands and ponds or multiple ponds combined together which receive stormwater and filter the stormwater using grit chambers or similar filters. Generally, the ponds work better than wetlands partly because wetlands are harder to clear out once they become sediment filled, requiring partial or total destruction of the wetland. Some states, including Maryland, have used infiltration trenches instead of sediment ponds or wetlands. Such trenches have had a failure rate in excess of 50% in the state of Maryland, due mostly to problems in engineering, installment, and sediment clogging, which may seal the infiltration trench.

WaterLaws: How do we contact the Center for Watershed Protection?

Mr. Schueler: Center for Watershed Protection

    8737 Colesville Road, Suite L-105
    Silver Spring, MD 20910
    (301) 589-1890 phone
    (301) 589-8745 fax


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