Utilizing GIS as a Tool in Mapping Impervious Surfaces and Protecting Southeast Michigan's Headwaters

Steve Perry, Amy Hamann

SEMCOG, the Southeast Michigan Council of Governments, conducts a Headwaters Protection Initiative to ensure the long-term coordination and implementation of headwaters protection efforts in Southeast Michigan. Educating local government officials about headwaters protection is an essential part of watershed protection and the sustainability of Southeast Michigan. SEMCOG is assisting local communities in protecting headwaters by providing tools and data to support local efforts. Among the tools are maps identifying current and future imperviousness of specific communities and subwatersheds which allow the comparison between increasing imperviousness and increasing water quality degradation. This process uses both ArcInfo and Arcview to map imperviousness by subwatershed using established impervious percentages and regional land use data developed by SEMCOG.

National Studies show that impervious surfaces are indicators of the quality of water resources as they measure the impacts of land development on aquatic systems¹. This paper examines the methodology for calculating impervious surfaces based on various land uses, provides illustrations and examples of calculating and mapping imperviousness in a specific watershed of Southeast Michigan, and illustrates how this information can be used as a planning tool for better land use decision-making.

Impervious surfaces and water resources
Impervious surfaces are such things as roads, parking lots, rooftops and other impermeable surfaces usually associated with urban landscapes. Increases in imperviousness can have devastating effects on the hydrologic cycle, as a result, on water resources. Such effects include: increased flooding and streambank erosion, degraded aquatic habitat, reduced groundwater recharge and additional pollutants entering the river from storm water runoff.
 

Tom Schueler from the Center for Watershed Protection is a nationally-recognized figure researching imperviousness. Schueler's classification scheme for urban stream quality potential is based on imperviousness. This classification scheme has been adopted for our study and divides urban streams into three categories based on the general relationships between imperviousness and stream quality:
 

1. Stressed streams (1 to 10% impervious surfaces)

2. Impacted streams (11 to 25% impervious surfaces)

3. Degraded streams (26 to 100% impervious surfaces)

Past, current and future land use data
Land use and future land use data are needed to evaluate the change in amounts of impervious surfaces within any given area. This section will provide insight on how land use information is derived and on the procedures used to calculate impervious surfaces for subwatersheds in the Clinton River Watershed (Figure 1).

SEMCOG updates the land use base map for the seven-county region (Figure 2) every five years under a memorandum of understanding with the Michigan Department of Natural Resources, Michigan Resource Information System (MIRIS). The update procedures entail flying the region using black and white panchromatic aerial photography at a scale of 1:24,000. Comparisons are made between the prior years' digital land base and the current air photos with corrections made as necessary. SEMCOG also maintains a Regional Future Land Use coverage (2020) derived from local master plans. This information is digitized and generalized to provide a perspective on growth in the region.

The MIRIS polygon land use base is comprised of 66 different land use categories. Imperviousness values are base on these categories with some exceptions. The MIRIS category "Road Transportation" only refers to limited access highways and the MIRIS category "Water" refers to all areas covered by water such as lakes and reservoirs. Minor streams and water bodies area are included in other MIRIS categories.

Determining imperviousness
Impervious surface values for this project were determined based on previous Southeast Michigan impervious studies of the Rouge and Huron River Watersheds (Figure 3) because they represent the land use characteristics commonly found in Southeast Michigan.

The Rouge River National Wet Weather Demonstration Project (RPO) performed a comprehensive study that addressed impervious area and direct connect impervious area in the Rouge River Watershed. Direct connect impervious areas includes surfaces that flow directly to a storm sewer, drain or channel without flowing over any pervious surface. Examples would be parking lots and roof tops. This information was needed to determine impervious values by subarea; that, in turn, were put into watershed models such as the Watershed Management Model (WMM). The RPO grouped the 66 MIRIS land use categories into 10 similar categories selected based on their consistency with land use categories used in national studies of pollutant loads. The RPO determined the average percent imperviousness for the 10 land use categories within a given subwatershed using SEMCOG's 1990 aerial photographs. These photos, enlarged to

1" = 200', focused on two to three samples for each land use category within each of the subwatersheds in the Rouge Basin. Roughly 300 sample areas were evaluated from the air photos and in the field. Field observations of those sample areas yielded the percent direct connected impervious area along with impervious areas not directly connected.
 
The Huron River Watershed Council (HRWC) - Davis Creek Pilot Study also looked at evaluating the amount of impervious surface within its watershed. RPO's methodologies for determining impervious area values were used to some extent but rather than using only the combined land use categories, the HRWC identified as many of the 66 MIRIS categories as possible. This was done by identifying specific land use categories in the Davis Creek Subwatershed that were similar to those in the subwatersheds in the Rouge.
 
For this project, SEMCOG used the impervious values determined by the Davis Creek Pilot Study. This was based on the fact the study took advantage of the RPO methodology and evaluated more specific land use categories that are found throughout Southeast Michigan.
 
Table 1 illustrates the MIRIS land use types, their descriptions, RPO's grouping and descriptions and the corresponding percent impervious.
 
Creating the Clinton River subwatershed land use coverage
 
The process of creating the subwatershed land use coverage entailed using both ArcInfo^® 7.0 and ArcView^® 3.0 GIS software packages.
 
SEMCOG maintains all GIS base coverages on a county-by-county basis. In order to look at the change in impervious surface in the Clinton River Watershed (Figure 3), the counties within the Clinton Watershed had first to be appended before the Clinton Watershed was used to cut-out (intersect) the desired area. This process was performed on the 1990, 1995 and 2020 land use coverages using ArcInfo^®. The coverages were then brought into ArcView^® and converted to shapefiles. A separate database file (dbf) look-up table was created that contained all the land use codes, categories, descriptions and percent impervious amounts. The look-up table was joined to the 1990, 1995 and 2020 coverage utilizing the land use code as the key field.
 
With the coverages now containing impervious values by land use type, the next step was to determine the percent impervious surface for each subwatershed. Three fields were added to the coverage table file: area in acres, impervious area in acres and average percent impervious. Impervious area in acres is derived by multiplying the acreage of each land use polygon by its corresponding percent impervious value. This determined the area of impervious surface within each polygon.

Calculating and mapping average impervious surface by subwatershed
 
After calculating impervious area for each polygon, the next step is to determine the average percent impervious surface for each subwatershed. The watershed land use coverages contains a field with the names of each subwatershed. This field is queried in ArcView^® to select all polygons within a subwatershed. The fields, area in acres and impervious area in acres, were totaled. By dividing the impervious area in acres by total area of the subwatershed determined the average percent impervious for that subwatershed. This value is entered into the last field: average percent impervious.
 
After all the calculations have been made, the next step is to map the average percent impervious surface by subwatershed for the Clinton River Watershed. In ArcView^® this is done by creating a legend with the three categories mentioned earlier: 0-10 percent.  11-25 percent and greater than 26 percent. The maps in figures 4 - 6 illustrate the Clinton River Watershed's impervious surface changes over time.
 

• to assist in prioritizing areas in which to work and

• be used as a visual tool to illustrate the current and future trends of a watershed, subwatershed or community.

 

Land Use Categories and Percent Imperviousness
 

* For this project, land use categories representing water and/or wetlands were given 0% impervious.