Impervious surface presentation

Here are the notes for today's presentation to the San Francisquito Creek Watershed Council on the cumulative impacts of impervious surfaces in Santa Clara County, a project we are researching with great help of a grant from the Santa Clara Valley Water District.

-Brian

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Impervious Surfaces, Cumulative Impacts, and Divergence Between NPDES and CEQA Requirements

The Problem and Two-staged Solution

The Problem: CEQA requires more extensive consideration of cumulative impacts than NPDES C.3 provisions. There are (almost) no exceptions to CEQA requirements to consider cumulative impacts from impervious surfaces, while NPDES C.3 requirements focus on larger projects, and don’t consider smaller projects.

Why Now? State-issued CEQA Guidelines were changed last fall in two ways: first, they increased the emphasis and the possibility that small impacts should be considered cumulatively significant. Second, they removed the possibility that simple compliance with other regulatory standards like NPDES would, without more, constitute CEQA compliance. These Guidelines were changed in reaction to successful lawsuits by environmental groups two years ago. Local agencies may not have changed their CEQA compliance to reflect the new guidelines.

Specific Area of Divergence – the HMP. The Hydromodification Management Plan examines how large impervious surface projects (one acre or more) could alter stream flows so as to increase erosion, and prohibits them from increasing erosion for less-than 10 year storm events. A complete “no impact” requirement would eliminate the divergence, but the HMP’s failure to cover small projects and 10-year-or-greater flood events leave open potential un-analyzed and unmitigated cumulative impacts.

Specific Area of Divergence – Water Quality. To treat water quality problems from impervious surfaces, NPDES C.3 provisions require medium and large projects (10,000 square feet or more of impervious surface to capture 80% of the annual runoff, or to treat 10% of the 50-year peak flow rate. This has a similar problem as the HMP – smaller projects are not mitigated, nor are all the impacts from the larger projects mitigated.

Specific Area of Divergence – Other Environmental Impacts. Increased impervious surface eliminates habitat and biomass, and increases the urban heat island effect, and none of these impacts are analyzed, and may not be mitigated by the NPDES provisions. Difficult to analyze.

Where Divergence May Occur – Map. Watersheds draining to hardened channels and tidal areas only have no HMP divergence (but possible water quality impacts). All other areas have at least the potential for unalyzed, cumulative impacts. Areas with the highest percentage of existing impervious surface and the highest percentage of existing buildout are the least likely to have cumulative impacts.

Summary of the Problem – NPDES analyzes and eliminates some but not all cumulative impacts. Local agencies must analyze the remaining impacts, and where feasible, mitigate them.

Two-Stage Solution – First Stage, Complete the Missing Analysis. Examining the “trend line” in impervious surface coverage will indicate potential cumulative effects from impervious surfaces. An upward trend line suggests a potential problem. The data is readily available from compliance with C.3 provisions, from EIRs, Negative Declarations, and from building permits.

Analysis Gap – Trend Line Suggests But Does Not Completely Prove a Cumulative Impact. If the project in question does not have an EIR, however, the reasonable inference of a potential impact requires preparation of an EIR.

Second Stage of the Solution – Assume a Cumulative Impact and Fully Mitigate It.
1. Reduce the Impervious Coverage
2. Change from Impervious to Permeable Cement/Pavement/Concrete
3. Pay Into a Fund for Offsite Mitigation

Problems with our approach:
1. Incorporation of available science.
2. Nuts and bolts of tracking the data.