4.1 Geology and Soils
initial beach fill length is approximately 2,310 feet, and at post-project equilibrium the length of beach
widening increases to 15,000 feet. At the end of the first year the length of coast affected increases to
20,000 feet based on an analytical modeling method and remains constant to the fifth year (Moffatt &
Nichol 2000a). No significant increases are expected as a result of implementing this alternative.
The net longshore sediment transport at this receiver site is to the north, away from the river inlet. This
effect is caused by wave refraction over the relic Tijuana River delta offshore and to the north of the river
and south of Imperial Beach, driving wave-induced currents northward at the project site (USACOE
1991). Based on wave refraction effects occurring at Imperial Beach and longshore transport estimates
provided by the USACOE (1991), the fill will likely disperse northward. A sand berm would be expected
to form in the shallow subtidal areas as a result of sediment transported into these zones, but no significant
increases are expected as a result of implementation of Alternative 1.
Borrow Sites
For all six offshore borrow sites, dredge site deepening would alter local bathymetry; however, the
proposed dredging action has been determined to be in accordance with recommendations based on
geotechnical investigations of the proposed borrow sites and accepted engineering practice.
As described in Section 1.1, the proposed dredging activities would take sand from borrow sites outside
(deeper than) the depth of closure and place sand within the three littoral cells. New sand would be
introduced to the system. As such, the borrow sites would not intercept sand that typically rebuild beaches
in the summer. No significant geology and soils impacts are anticipated to occur to the dredge borrow sites
with implementation of Alternative 1.
Mitigation Measures for Alternative 1a
As no significant impacts have been identified, no mitigation measures would be necessary.
Impacts/Mitigation Measures for Alternative 1b
This alternative would not change the impact analysis described above for Alternative 1a. The only
differences under this alternative with respect to geology and soils is that less sand would be dredged and
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