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.

• 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

• 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

• Phase I

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

• Phase II

                Good second step
                Encompasses two sets of interests
 

• Stream degradation

• Water supplies

• Quality of life

"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)
 

• 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.

• Presently a very "pervious" township
• Forecasted outlook based on zoning map
• With road reductions
• At "build-out"
• At "build-out" with road reductions

• Presently a somewhat "pervious" township
• Forecasted outlook based on zoning map
• With road reductions
• At "build-out"
• At "build-out" with road reductions

• 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

• 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)