Environmental Impact Statement - Sonoma Land Trust

California Department of Fish and Game
U.S. Fish and Wildlife Service
Chapter 2. Alternatives
Sears Point Wetland and Watershed Restoration
Project Final Environmental Impact
Report/Environmental Impact Statement
The design height selected is equal to the design height of the adjacent Sonoma
Baylands levee, with a more gentlegentler outboard slope to dissipate wave
energy and provide habitat. The dominant soil type, Bay Mud, is a weak and
highly compressible material and varies in depth along the levee alignment.
Levee construction will place a load on the foundation soils causing them to
compress in relation to their depth. For this reason, the new levee would be
constructed to initial elevations at or above the design height, from
approximately +12 to +15.8 NAVD, to account for settlement. The design height
accounts for the current 100-year flood elevation combined with wave run-up
and freeboard. At 50 years the new levee is expected to settle to an elevation
equal to the 100-year flood level adjusted for sea level rise. During this time
sediments would be expected to have accreted throughout the interior of the site
to a height sufficient to support tidal marsh vegetation and limit wave run-up.
Significant sea level rise is expected for the foreseeable future. There are a broad
range of projections for rates of sea level rise in the future and a large degree of
uncertainty surrounding the projections based on the current state of the science
(U.S. National Research Council 2010). In order to address this uncertainty, the
new levee has been designed to facilitate any potential future maintenance
activities to raise the crest elevation.
A stormwater pollution prevention plan (SWPPP) would be prepared by the
general contractor for the project in order to comply with requirements of
applicable permits under the NPDES program. Compliance with permit
conditions is designed to prevent unacceptable erosion during construction and
accelerated erosion following construction.
Core Levee
The proposed core levee would be comprised of bay muds and alluvial soils, and
would be designed to greatly reduce the potential for cracking. It would have 2:1-
3:1side slopes and would serve as the geotechnical base of the levee.primary
impervious layer for seepage control. The top width of this levee would be from
12 to 16feet to allow for construction of a combined Bay Trail/maintenance
roadway.
The construction of the approximately 13,000-foot long core levee would require
placing approximately 320500,000 cubic yards (CY) of approved alluvial/Bay
Mud fill. This fill would primarily be provided for from the upper level
excavation of on-site pilot tidal channels. In addition, the eastern 6,000 feet of
this levee would contain approximately 12,000 CY of contaminated surface soils
from the Black Point Sports Club. This material would be covered by at least 3
feet of clean on-site soils.Adjacent project areas may also serve as borrow areas
to obtain the needed quality and quantity of levee fill and construct other tidal
features detailed below.
Prior to constructing the levee, the footprint would be cleared and grubbed and a
29,000 cubic yard seepage cutoff walltrench approximately four feet wide and
deep would be excavated. The material from this trench would be placed in
adjacent tidal wetland features such as marsh mounds. Approximately 35,000 CY
of select young bay mud would be usedremoved and then recompacted to refill
thisbackfill the trench and build the internal cutoff wall. Some or all of this select
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April 2012