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The Effects of Imperviousness on Watersheds: Scio and Superior Townships by Mark Brush

First Steps Find out from city and organizations what types of analysis they would like to see. City: City of Ann Arbor

Also interested in percentage of imperviousness cover in a typical development.
What type of reductions in imperviousness can we expect with a cluster development.
Further analysis on a planned development in the North-east.

Organization: Huron Watershed Council

Work on forecasted impervious cover based on zoning maps
See what type of impervious cover could be expected at “build-out”
See what types of reductions would be evident with road reductions
Percentage of imperviousness cover in a typical development

What path do I choose?

Step 1 - get a grasp for how impervious data is calculated and used
Step 2 - get to know how to manipulate and work with the imperviousness data using GIS
Step 3 - formulate a plan

The Plan

Phase I

Work with the Huron Watershed Council in determining future levels of
imperviousness

                Good "introduction" method
                Plenty of data to work with
                Accessible resource

Phase II

Work with the City of Ann Arbor and the HWC in coming up with imperviousness
percentages & with investigating cluster development

Good second step
Encompasses two sets of interests

Why is imperviousness important?

Stream degradation

        Biodiversity
        Water Quality
            Heavy metals, salts, oil, nutrient loading, temperature pollution
            CSO vs. SWO
        Erosion
            High runoff coefficient, high velocity and volume, flashing creeks

Water supplies

Well water
Surface water

Quality of life

Living next to a clean stable stream

"Stressed streams" - 1 to 10% impervious cover

"Impacted streams" - 11 to 25% impervious cover

"Degraded streams" - 26 to 100% impervious cover
Watershed Protection Techniques Vol. 1, No.3, Fall 1994 pp.100-111)

Methodology

Join imperviousness by land-use table to land-use minus water theme and calculate impervious area.
Intersect sub-basin theme with land-use minus water theme to create new shape file.
Sum new theme by sub-basin, including area and impervious area. Calculate impervious percentage.
Road reductions required joining "built" land-use with "unbuilt" land-use.
"Build-out" required changing the Agricultural land-uses into Single Family Residential land-uses.

Land-Use / Imperviousness conversion table

Road reductions

Based on research done by the American Society of Civil Engineers.
Reduced road widths by aprrox. 1/3 (essentially 32 ft wide to 22 ft wide).
Didn't change the widths in "built" areas.
Changed 80% of the widths in "unbuilt" areas as 20% are considered built.

Outlook of Imperviousness: Superior Township

calculations based on zoning map Outlook of Imperviousness: Scio Township

calculations based on zoning map Implications

Graphically represents the kinds of pressures their watersheds may expect to see
Shows what they could expect to see with typical road reductions
Indicates that road reductions may not be the only answer

                    may want to look at creative more solutions
                                    cluster development
                                    shared driveways
                                   retrofits / curb - cuts
                                    stream buffers -etc.

Drawbacks

Imperviousness based on land use can be highly variable - not standardized
Looking at the issue from a single perspective
More studies need to be done in different regions (many studies done out west)

See Phase Two - Measured Study