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
The ecological expression of accelerated sea level rise in approaching decades at
the Sears Point Restoration Project may include: (a) more gradual than expected
emergence of low marsh from intertidal mudflat, with prolonged persistence of
mudflats, though the high expected sedimentation rates may partially compensate
for this effect; (b) delayed or arrested emergence of pickleweed-dominated midmarsh
zones (low end of estimated sea level rise rates), or stabilization of the
marsh at the transitional cordgrass-alkali bulrush-mudflat stage (high end of
estimated sea level rise rates); (c) compression of tidal marsh zonation along
steeper slopes of the new levee; (d) reduced or delayed development of speciesrich
high marsh, including critical sub-habitats for marsh wildlife. These
potential alternative outcomes affect all tidal marsh restoration projects in the
San Francisco Estuary. The Sears Point vicinity marshes, however, are relatively
well-buffered against the impacts of accelerated rates of sea level rise because of
their proximity to exceptionally large sources of mobile fine sediment at the
mouth of the Petaluma River. Impacts related to systemic sediment deficits, or
submergence due to sea level rise, may be less readily compensated if rates of sea
level rise exceed local sediment budgets.
A number of features have been incorporated into Project design that are likely to
minimize the impact of sea-level rise on marsh restoration and its physical
evolution. These features include:
• Construction of a gradual habitat slope surface (i.e., the terrestrial ecotone)
that provides an elevation gradient over which elevation zones of tidal marsh
may shift upslope as sea level rises;
• Early initiation of marsh vegetation to maximize sediment-trapping, marsh
elevation gain, and biomass accumulation before excessive sea level rise
acceleration may occur;
• Levee design height will account for the moderate predictions of increased
sea level rise.
Thus, the project is designed to accommodate a moderate amount of sea level rise
in terms of wetland formation likely success. However, more accelerated sea
level rise at the higher end of predicted ranges could result in sea level rise
exceeding local sediment budgets.
The project has also been designed to minimize the effects of sea level rise
(potential tidal flooding) on the SMART Rail Line and agricultural area north of
the SMART line. The new levee has been designed to account for anticipated sea
level rise through approximately 2050. Initially, the new levee would provide a
substantially higher level of protection against coastal flooding than the perimeter
levee currently provides. The actual degree of sea level rise after 2050 is difficult
to ascertain (estimates vary widely, and depend greatly on the success of carbon
emission reduction efforts), and attempting to mitigate for sea level rise beyond
this date would be speculative. Nonetheless, the new levee would be built to
better standards than the informally constructed perimeter levee, and would be
more resistant to damage from storms and high tides than the existing levee. It is
also likely that sea level rise would result in levee overtopping/failure on
surrounding properties with older levees before affecting Sears Point. Thus, this
potential impact is less than significant.
Sears Point Wetland and Watershed Restoration
Project Final Environmental Impact
Report/Environmental Impact Statement
3.2-21
April 2012