Impervious Surfaces

Impervious surfaces are roads, parking lots, rooftops, and other hard surfaces characteristic of urban areas that prevent rainwater from infiltrating into the ground, delivering it instead as stormwater to rivers, lakes and estuaries. As a result, rivers in developed landscapes tend to have higher maximum flows and lower minimum flows than those in more pristine settings. This “flashy” flow pattern degrades riverine habitat and causes property flooding.

Runoff from impervious surfaces bypasses natural pollutant removal processes in soil, carrying sediment and pollution into streams, lakes, and coastal waters, causing many of the water quality impacts documented elsewhere in this report. The changed hydrology raises water temperature, reducing habitat value. Many studies have shown that rivers, lakes and coastal waters surrounded by watersheds with a high percentage of impervious surface tend to have poor habitat and reduced biodiversity. Storm water from impervious surfaces is also a major source of bacteria, leading to the closure of swimming areas and shellfish beds.

Status & Trends:

Recent GIS analysis of data from aerial photographs indicates that about 14 percent of the land area in the Narragansett Bay watershed (R.I. and Mass.) is covered by impervious surfaces. These surfaces are distributed unevenly within watersheds; developed areas tend to be near watercourses, where the impact of runoff is greater.

The analysis shows that more than half of of all subwatersheds in the Bay watershed have greater than 10 percent impervious surface cover.

In 2004, 10 percent of Rhode Island was covered by impervious surfaces, with coverage greater than 30 percent in major population centers. Only 17 of Rhode Island’s municipalities have less than ten percent impervious surface area, with coastal towns averaging 14 percent.
These analyses provide baseline data regarding impervious areas, in order to begin making definitive statements regarding trends. It is clear, however, that, there is a trend toward increasing impervious surface coverage throughout the Narragansett Bay Region. It is also clear that, in many areas, impervious surface coverage has already reached the point at which significant ecological impacts can be expected. Studies have shown that this threshold ranges from four to 20 percent, depending upon how directly the surfaces are connected to water bodies and other local factors.

Information:

Impervious surface is currently measured using a combination of data sources including aerial photographs and satellite imagery. A new consortium of researchers and information-users intends to use NASA satellite data to develop land use and impervious surface coverages at five-year intervals. Impervious surface data for Rhode Island was developed from 2004 aerial photography, processed by computers; the results were then checked by technicians on the ground.

Improvements in technical hardware and software should provide increasingly finer resolution data for this data from aerial photography and satellite imagery. Finer resolutions will allow improved assessments at the subwatershed and even down to the neighborhood level, increasing watershed managers’ ability to develop better management tools. This is, however, dependent on the availability of sufficient resources to fund satellite and aerial photography as well as the analytic capacity needed to interpret this information.

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