4.3 Water Resources
longer, while larger sand materials (greater than 63 m in size) would settle very quickly to the bottom in
minutes.
Increased turbidity would occur along the shore as a result of sediment disposal. The action involves
pumping a sand/sea water mixture directly onto the beach from a dredged location offshore. Increased
turbidity would be caused by return water from pumping operations, as suspended sediments in the
sand/sea mixture would flow into the surf zone subsequent to pumping. As described in Section 2.4,
training dikes would be constructed during discharge to direct the flowofdischarge and slow the water flow
thereby allowing more sediment to settle onto the beach instead of washing back into the surf zone. This
would reduce the potential water quality impact. The calculated turbidity plumes in this document are
R
worst-case and do not factor in the reductions associated with training dikes or sand placement onshore
R
instead of nearshore.
R
Sediment pumping operations would vary depending on the type of dredge. With the cutterhead dredge,
turbidity would be nearly continuous. Turbidity would occur in pulses with the hopper dredge and the time
intervals between pump cycles would provide for greater settling of the dredged sands.
As described in Appendix D, there have been several nearby beach replenishment projects in the recent
past where turbidity plumes have been monitored. The extent and concentration of turbidity plumes are
directly proportional to the silt/clay content of the replenishment materials. When the silt/sand content
ranges from 2.5 to 10.5 percent, visible turbidity plumes range from approximately 50 to 2,000 feet down
current. However, the concentration of particulates generally was not noticeable above background levels
in the surf zone when the silt/clay content was less than 5 percent. The offshore extent of turbidity is much
more restricted but dependent upon wave conditions, rip currents, and speed and direction of longshore
currents. Measured turbidity plumes did not extend much beyond the surf zone when the silt/clay
concentrations were very low, and generally were limited to water depths less than 10 feet above MLLW
when silt/clay concentrations were 10 percent or less. Even when the silt/clay content ranged from 11 to
18 percent, turbidity plumes were not measured beyond 1,000 feet offshore (about -15 to -20 feet
MLLW).
South Oceanside and North Carlsbad
These two sites would be replenished from SO-9. The silt/clay content from this borrow site ranged from
2 to 11 percent with an average of 8 percent. Native sediments in the Oceanside Cell average 3 percent
Page 4.3-2
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