Residential Impervious Cover Attribute
As described above, research has shown a strong link between impervious coverage and the environmental impacts of stormwater runoff. Aquatic ecosystems can start to experience environmental stress when impervious coverage within a watershed reaches ten percent. Reducing the amount of impervious coverage can reduce quality and volume of runoff that enters surface waters by allowing for greater natural infiltration into the ground (Stormwater).
In addition, a reduction in the surface area of street paving and roofing shingle can help mitigate the urban heat island effect and reduce the use of energy-intensive air conditioning in the summer months (Climate Change). In establishing a performance standard, we selected three targets for different levels of city density: high (more than ten residents per acre), medium (five to ten residents per acre), and low (fewer than five residents per acre).
This classification is used to recognize that, while one would expect denser areas within a city to have a higher percentage of impervious surface area, higher densities in one part of the city can accommodate impervious coverage reductions in other parts of the city.
Impervious coverage was divided into residential and non-residential components. Residential parcels were defined as single family and duplex housing types. We calculated impervious coverage by adding the areas of building “footprints” (the ground surface area occupied by the building), driveways, and sidewalks. Values for the building footprint areas were provided by the City of Madison Engineering Division, and residential driveways and sidewalks were estimated. Driveway areas were estimated by multiplying average driveway width by the distance of the building setback from the road. Similarly, sidewalk areas were estimated by multiplying the width of the lot frontage by the average sidewalk width, which was determined through site surveys. It should be noted that, as residential impervious coverage was not measured directly, our calculations are subject to some degree of error.
Non-residential parcels were defined as commercial, industrial, and multi-family land uses. Non-residential impervious coverage was calculated by adding building footprints, parking lot areas, walkways, and other miscellaneous spaces. Like the residential components, the footprint, parking area, and walkway data were provided by the City of Madison Engineering Division. Sidewalk area estimates were based on a published average for commercial areas and are measured as an additional four percent of impervious area to the total parcel. These estimates are also subject to some degree of error.
Although our goal is to reduce impervious coverage, sidewalks constitute one source of impervious coverage that we are, in fact, recommending. In determining the reductions in impervious coverage needed to meet our standards for this attribute, we adjusted our calculations to compensate for our recommendation of providing sidewalk coverage on 95 percent of residential streets in each district.
- Maximum of 50 percent impervious coverage for high density districts (Isthmus)
- Maximum of 30 percent impervious coverage for medium density districts (Near West)
- Maximum of 20 percent impervious coverage for low density districts (West, South, East, and North).