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
Environmental Consequences and Mitigation
Measures
Approach and Methods
Impact mechanisms for tidal hydraulic changes were based on baseline
hydrologic and hydraulic information (described above), and on an understanding
of how tidal wetland restoration generally affects localized tidal hydraulics. The
potential tidal hydraulic changes and environmental consequences related to
sedimentation and flooding of the restoration alternatives were identified based
on these impact mechanisms and additional technical analysis was conducted
where required to quantify or mitigate impacts associated with the alternatives.
To assess the impacts of the tidal wetland restoration on the hydrology of the site,
Moffatt and Nichol (2007, 2008, 2011) collaborated with Wetlands and Water
Resources (WWR) to develop restoration scenarios. They developed a numerical
model to evaluate likely tidal hydraulics within Tolay Creek and the tidal
wetlands, and to evaluate likely effects on sedimentation patterns under each of
seven scenarios. Existing and restored land features such as the drainage ditch
network, internal dikes, and future constructed channels, were included in the
model bathymetry. Calibration and validation of the model were performed using
water level and current data provided by WWR.
Impact Mechanisms
The Project is based on the concept that reintroducing tidal waters and capturing
suspended sediment in the water column would raise the subsided site to an
elevation that would facilitate marsh growth (above MTL). Continued tidal
action would develop tidal channels, and maintain an active exchange of water,
sediment, and nutrients between the marsh habitat and the San Pablo Bay.
The following types of activities and processes associated with implementation
of the Project could result in changes in surface-water hydrology, tidal
hydraulics, and sedimentation in the vicinity of the project area:
• Removal of existing storm water pumps;
• Installation of 2 new storm water pump stations and conveyance
infrastructure at culverts landward of the SMART rail line;
• Excavation of levee breaches between the restoration site, Tolay Creek, and
San Pablo Bay, which opens formerly diked areas to tidal flow;
• Excavation of approximately 0.86 acres in the floodplain near the northern
project boundary;
• Lowering of the existing perimeter levees;
Sears Point Wetland and Watershed Restoration
Project Final Environmental Impact
Report/Environmental Impact Statement
3.2-10
April 2012