Environmental Impact Statement - Sonoma Land Trust

California Department of Fish and Game
U.S. Fish and Wildlife Service
Section 3.2. Surface-Water Hydrology,
Tidal Hydraulics, and Sedimentation
Full-Tidal Alternative
As described above under the Project, the Full-Tidal Alternative is unlikely to
result in lower local sediment concentrations in San Pablo Bay. Therefore, the
impact of the Full-Tidal Alternative on overall San Pablo Bay sedimentation
processes is considered to be less than significant, and no mitigation is required.
Conclusion: Less than Significant.
Impact HYD-8: Impact of Sea Level Rise on Marsh
Formation and on Levee Protection
Proposed Project
A variety of estimates quantify the range of potential sea level rise, report
observed trends and offer predictions of global warming and the potential
impacts (Watson et al. 2001, California Climate Change Center 2006). The
Intergovernmental Panel on Climate Change reports that over the last 100 years
the eustatic (globally averaged) sea level rise was 1 - 2 mm/year (0.3 - 0.6
ft/century). In 2001, the IPCC projected rates of sea level rise to increase over the
next century, with projected increases ranging from 0.4 - 2.9 ft by 2100 (Watson
et al. 2001). Note that the IPCC estimate conservatively assumed no
“speculative” critical threshold changes in Greenland ice sheet wasting, a process
that would substantially accelerate and amplify rise in sea level (Overpeck et al.
2006). More recent estimates by the California Climate Change Center report sea
level rise in California projects increases of 39 to 55 inches (3.2 to 4.6 feet) by
2100 under medium emissions scenarios (California Climate Change Center
2009). The projected increase in sea-level will alter historical storm frequency
predictions by decreasing recurrence intervals and increasing vulnerability of
coastal regions to flooding (California Climate Change Center 2006). To provide
context with a generalized scenario, an increase in sea-level of 1 foot means that
storm-surge induced flood events that formerly occurred as 100-year events
would more likely occur at 10-year intervals (California Climate Change Center
2006). Local sea level rise depends upon a number of physical factors including
local land vertical movement (uplift/subsidence) and hydrodynamic responses.
The ecological consequences of accelerated sea level rise are a concern for all
tidal marsh restoration projects. At low rates of sea level rise, such as those that
have prevailed in the last 3,000 years, tidal marshes generally keep pace with
rising sea level by compensatory increases in either mineral sedimentation,
primary production (organic peat accumulation), or both (Morris et al. 2002,
Reed 1995). Tidal marsh succession may proceed from low intertidal marsh to
high intertidal marsh through the same processes. In contrast, during coastal
submergence at rates that exceed the ability of marshes to equilibrate, higher
marsh zones may submerge and convert to low marsh (often cordgrass), intertidal
unvegetated flats, or shallow open water (Reed and Cahoon 1992).
Sears Point Wetland and Watershed Restoration
Project Final Environmental Impact
Report/Environmental Impact Statement
3.2-20
April 2012