4.4 Biological Resources
within open coastal areas and the potential for material trapping would be low. The impact would be less
than significant.
Indirect Impacts of Dredging
As discussed in Section 4.3, dredging of the borrow sites would cause localized turbidity plumes. Indirect
impacts may occur if turbidity plumes and siltation affect sensitive resources beyond the boundaries of the
dredge areas. Turbidity results from suspended particles in the water column that can reduce ambient light
levels and lead to siltation. Turbidity can have a number of adverse effects on marine biota. Turbidity can
reduce ambient light levels in the water, which can impact primary production of plankton and inhibit kelp
and algae recruitment and growth. Turbidity can also affect the feeding efficiency of filter feeding organisms,
including zooplankton.
The location and footprint of the dredge area for each borrow site has been designed to minimize both
direct and indirect impacts from the dredging operations. A minimum 500 foot buffer has been provided
between the dredge area and kelp or reef, except at SO-7 where there is one artificial reef approximately
350 feet distant. Table 4.3-1 provides estimates of the average turbidity plumes from each borrow site,
assuming the hopper dredge which is the worst-case condition for generating turbidity. As shown, they
would range from an estimated 72 to 544 feet and average about 260 feet. The borrow sites with higher
sand content (SO-7, MB-1 and SS-1) would generally have the smaller plumes. The plume estimates and
the nearby known resources of reefs and kelp (Table 3.4-6) were used to evaluate the potential indirect
impacts for each of the borrow sites. A detailed analysis by borrow site is provided in Appendix D;
however, since the impact conclusions would be identical for all sites, only a summary is provided in this
document.
In general, turbidity plumes from the borrow site are predicted to be small for this project as the sands to
be dredged are sandy sediments with low percentages of fines. All reefs and kelp would be beyond the
estimated length of average turbidity plumes. Site SO-6 would be the closest to any resources with
nearshore reefs approximately 500 feet distant and historicalkelp shown as 500 feet distant. Kelp mapping
conducted in 1999 shows the nearest kelp was 3,000 feet away. While the probability for some turbidity
to occasionally reach reefs and/or kelp cannot be entirely eliminated, turbidity is a natural occurrence at
these sites. The artificial reefs at all borrow sites naturally experience increased turbidity from storms, rip
currents, and plumes from river and harbor discharges. Therefore, even though there is some potential for
turbidity plumes to reach reefs, the duration would be limited, and the actually amount of turbidity reaching
Regional Beach Sand Project EIR/EA
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