4.1 Geology and Soils
Alternative 1
4.1.2
Impacts of Alternative 1a
Receiver Sites
For all receiver sites, sediment deposited on the beach would be spread alongshore and cross-shore
through natural littoral transport. Shoreline positions were modeled based on the anticipated sediment
movement and were predicted for periods of 1, 2, 3, 4, and 5 years after sand placement.
Borrow materials would be similar to the receiving beach because the fill material has been the subject of
grain-size analyses and was found to be compatible with the receiver sites' existing sediments (Sea
Surveyor 1999). Temporary changes in the functional properties (e.g., permeability) of the existing beach
are expected until the replenishment material is mixed with existing sand and dispersed by coastal
processes, and no long-term impact would occur in terms of grain-size distribution.
For all receiver sites, seismic activity associated with the Rose Canyon fault and other nearby faults may
lead to liquefaction, ground failure, sand volcanoes, and seaward slumping of beach material. The impact
of beach replenishment would be of no greater significance than conditions expected in the absence of
additional sediment.
Coastal Geology
Similar coastal geology processes for each receiver site would occur regardless of the season the
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replenishment activity occurs. After placement of sand onto a receiver site, the existing beach area north
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and south of the receiver site would widen as a result of longshore and cross-shore spreading. The results
ofthe modeling indicating the length of time that the beach fill would return to its pre-fill condition are shown
in Table 4.1-1. Seasonal cross-shore movement would transport the fill material offshore in the winter and
back onto the beach in the summer, repeating this trend over subsequent seasons. Also, the longshore
transport changes direction seasonally, moving the sand north in the summer and south in the winter.
Seasonal loss of the beach would occur from natural littoral processes. Placing the material on the beach
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in spring instead of summer would increase the chance that more material would be available on the shore
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during the peak recreation period. Placing it in late summer/fall increases the opportunity for winter storms
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to remove the material prior to heavy summer usage the following year.
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Page 4.1-4
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