Cumulative impacts: when the whole is greater than the sum of its parts

CGF receives grant to study impervious surfaces
by Kathy Switky

Some of the most challenging environmental problems are known as "cumulative impacts," whose effects creep up on us through the accumulation of small, often relatively insignificant impacts. CGF has recently received funding to help manage some of these problems locally by tracking and managing the accumulated impacts of impervious surfaces in Santa Clara County.

Waterways particularly vulnerable to cumulative impacts
Cumulative impacts are among the most significant environmental problems for watersheds, which by definition collect water - as well as pollutants, erosion, physical barriers and other problems - from wide areas.

The accumulation of impervious surfaces is one of the cumulative impacts most threatening to watersheds. Pavement, buildings and other changes to the landscape that prevent water from entering the ground can lead to dramatic changes in water flow patterns both above and below ground. The challenge is that the effects of cumulative impacts are not immediately apparent to decision-makers reviewing individual projects, each of which constitute only a small part of a larger problem.

For example, when a small house is replaced with a much larger home, its driveway is typically expanded to accommodate firefighter access. The permitting process might limit the increase in impervious surfaces only to keep that individual project from having a significant impact on its own, while ignoring the cumulative impacts created by the sum of many projects that add pavement to the watershed.

Paving paradise?
Scientific evidence indicates that high levels of impervious surfaces in a watershed can have serious consequences, including accelerated erosion, accentuated flooding and damage to biological systems and physical habitat.

Given the pattern of increasing development in Santa Clara County, we can expect that most or all watersheds in the county are affected by increasing amounts of impervious surfaces.

But according to CGF's research, local governments do not currently track the net change in impervious surfaces caused by development, even though the data are readily available. Not a single jurisdiction tracks countywide changes in impervious surfaces to see if, when combined with changes from other projects, the cumulative impact of paving over county watersheds is significant.

CGF to develop methods for tracking cumulative impacts
In September, Committee for Green Foothills received funding from the Santa Clara Valley Water District under its Watershed Stewardship Grant Program, which awarded nearly $400,000 in grant funds to 19 local organizations to improve ecosystem health, water supply and water quality.

The $7,600 grant will allow CGF to study the usefulness of tracking cumulative changes in impervious surfaces and develop methods for managing them. With a policy report and follow-up workshops, we expect to help local agencies develop planning processes that account for and - when possible - mitigate the cumulative impacts from changes in impervious surfaces.