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

Imperviousness can have different meanings depending on the perspective.

        A watershed manager will be interested in imperviousness levels by watershed...

       A planner may be more interested in imperviousness levels by housing unit...

The second phase of this project takes a different perspective on imperviousness levels as well as a different approach to measuring imperviousness.

Theorized vs. Measured

In the first phase of this project imperviousness levels were extrapolated from land use information.  Specific land uses were assigned a typical imperviousness number based on national averages.  This approach has many advantages:

    imperviousness levels can be forecasted for planning purposes
    information can be gathered quickly
    processing this information is not costly
                                                                                   see Phase I

There are also many drawbacks - the most significant being accuracy

The second phase of this project attempted to manage for inaccuracies by analyzing high resolution digital orthophotos.

Luck and Simple Methodology

The city's digital orthophotos were assigned lat/long coordinates.  The image could be brought easily into Arc View with other layers matching up with the photos perfectly.

Photos are a half foot pixel resolution - impervious surfaces could be identified with relative ease.  New layers were created using on-screen digitizing.

    Layers created: Rooftops - Roadways - Sidewalks - Driveways/other

Two areas of Ann Arbor were analyzed:

       A new, 2 units per acre development
       and an existing 5 units per acre development

The question this analysis seeks to answer is how much open space can be preserved using cluster developments.

This analysis focused on two small areas in Ann Arbor.   A good comparative analysis was sought.  A 5 unit per acre vs. a 2 unit per acre analysis is a good starting point.  A 1 unit per acre vs. 6 unit or more per acre development would also be desirable.
 

Findings:  Imperviousness Breakdowns & Imperviousness by Housing Unit

 

Total Area (sq.ft.)	Pervious Area (sq. ft.)	Impervious Area (sq. ft.)	Driveways/Other	Roads	Rooftops
1,501,081.19	936,064.48	565,016.72	106,732.77	173,319.24	233,522.63
Total Area (acres)	Pervious Area (acres)	Impervious Area (acres)	Housing Units per Acre	Total Units	Imperviousness per Unit
34.46	21.36	13.10	5.00	172.30	7.60%

Cluster development was able to preserve a significant amount of open space even though the development itself is more impervious.
 

Opportunities

    very accurate
    new data can be created
 

Constraints

    time consuming to perform the analysis
    overall cost involved in the analysis