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
flooding under the Full-Tidal Alternative are considered less than significant, and
no mitigation is required.
Conclusion: Less than Significant.
Impact HYD-2: Potential Erosion and Sedimentation
Impacts in the Tidal Basin
Proposed Project
Construction of the new tidal basin in the Project could result in erosion and
sedimentation impacts as tidal flows scoured the perimeter levee. Breach 1
through the perimeter levee would result in an increase in tidal velocity during
ebb (150% of existing) and flood tides (133% of existing) as estimated for a point
approximately 0.38 kilometers west of the connector channel between Petaluma
Navigation Channel and the perimeter marsh, but no change to ebb and flood
tides at a point approximately 2.25 kilometers east of the connector channel.
Implementation of Breach 2 and the perimeter levee lowering would result in an
increase in tidal velocities in downstream Tolay Creek, while midstream areas of
the creek would experience a minor reduction (Moffatt and Nichol 2011).
Increases in tidal velocity in downstream areas of Tolay Creek could generate
erosion along the existing creek channel, and levee breaches along Tolay Creek.
The diked baylands portion of the site (north of the SMART rail) would not be
subject to tidal flows or flooding, though increases in tidal velocities into the
proposed basin could expose the new levee along the SMART rail line and the
lowered perimeter levees to erosion. In addition, perimeter levees surrounding
the restoration area could be subject to erosion from wave forces. Wind waves
generally pose a more significant erosion risk than tidal currents. To address this,
the geotechnical stability berms with erosion/habitat slopes would be constructed
along the outboard side of the new levee parallel to the SMART rail line. These
berms would have a 5:1 slope from the levee top down to an approximate
elevation of +8 NAVD, below which the slope would vary from 10:1 to 20:1 to
protect the raised tracks from erosion and wind-waves. The impact of windwaves
has the potential to be significant, however, flood protection, which is part
of the Project design described in Chapter 2 for the SMART rail line and the new
levee would reduce this impact to a less-than-significant level.
Conclusion: Less than Significant.
Full-Tidal Alternative
Similar to the Project, the proposed breaches would divert tidal flows to the
Sears Point site, thereby increasing tidal velocities both within the basin and
potentially in downstream Tolay Creek. The potential for erosion of the perimeter
Sears Point Wetland and Watershed Restoration
Project Final Environmental Impact
Report/Environmental Impact Statement
3.2-14
April 2012