4.3 Water Resources
continuous turbidity plumes associated with use of a cutterhead dredge would be highly localized. Because
silty sands occur in a shallow and non-uniform overburden at the site, there is the potential for more
extensive turbidity plumes to develop, however, they should be of short duration if they occurred. There
would be no significant turbidity impacts at any other borrow sites.
Water quality monitoring will be required as part of the RWQCB 401 Certification Order which also sets
conditions on proposed operations. If monitoring indicates that RWQCB permit specifications are
exceeded, the dredging operation will be modified/suspended and appropriate measures taken to ensure
compliance. Modification of dredge and/or disposal operations, as necessary, to adhere to the RWQCB
conditions will help ensure no significant impacts to water quality resulting from the alternative.
Combined Turbidity (Borrow and Receiver Sites)
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Simultaneous plumes at the borrow site and receiver sites only would be generated if a cutterhead dredge
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is used. That dredge involves pumping of sands directly from the borrow site to the receiver site. The
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hopper dredge moves between the borrow site and receiver site. Because the borrow sediments are
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predominantly sands, turbidity plumes generated at the borrow site with the hopper dredge would be
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expected to dissipate during the transit and offloading time required to deliver sands to the receiver site.
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The hopper dredge operation involves surface overflows (with suspended particulates) at the borrow site,
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therefore, turbidity plumes generated during dredging are greater with this method than with the cutterhead
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dredge, which lacks surface overflows. Due to overflow while the hopper dredge is in transit to the
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receiver site, there also will be some tailwater discharge behind the vessel that should quickly dissipate.
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Because the hopper dredge operation involves these overflows, the sediment conveyed to the receiver site
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generally will have less fines than the material conveyed by a cutterhead dredge. The turbidity estimates
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of about 1,000 to 3,000 feet identified for receiver sites in prior text represent worst case estimates based
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on a maximum current speed, the greater plume generated by the hopper dredge at the borrow site, and
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potentially by the cutterhead offshore the receiver site if training dikes were not used and material were
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placed directly in the nearshore. Turbidity plumes under more typical current speeds would range from less
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than 100 to about 1,000 feet at the borrow site with the hopper dredge and less 500 feet with the
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cutterhead dredge. Plumes offshore the receiver sites would be expected to be less than 1,000 ft under
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typical current speeds, and training dikes would be used to further control turbidity. Additionally, the
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turbidity plume calculations assume placement in the nearshore which would not occur with this project.
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All material would be placed on the beach as far landward as possible whichwould further reduce turbidity.
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Regional Beach Sand Project EIR/EA
Page 4.3-7
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