4.3 Water Resources
Table 4.3-1
Estimated Average Turbidity Plumes Resulting from Dredging
Estimated Average Downcurrent
Plume Distance (ft)(2) Range
Overall Mean
Grain Size
Mean Settling
According to Depth and Average
Depth (ft)
Diameter
Velocity
Current Speed
(mm)(1)
Borrow Site
MLLW
(ft/sec)
0.2 (0.34)
SO-9
48 to 58
0.16
0.06
272-329
SO-7
50 to 80
0.44
0.18
94-151
SO-6
60 to 80
0.27
0.10
204-272
SO-5
50 to 80
0.15
0.05
340-544
MB-1
55 to 65
0.57
0.26
72-85
SS-1
40 to 53
0.28
0.10
136-180
(1)
Overall median of the mean grain size diameters reported for cores (and/or core layers) characterizing the dredge
area.
(2)
The estimated plume distances were calculated by the following formula: water depth/particle settling velocity x
current speed (ft/sec).
The duration of turbidity plumes would relate to equipment used and the duration of dredging. Generally,
turbidity associated with the cutterhead dredge is estimated to be less than half that of the hopper dredge
and generally restricted to near the bottom. Because of the continuous operation of the cutterhead dredge,
turbidity would occur for the entire period the dredge is operating. In contrast, the hopper dredge involves
cycles of dredging interspersed with transit time to the mono buoy to pump material onto the beach. The
turbidity plume at the dredging location would be maximum at the surface during dredge or hopper
overflow and dissipate once the hopper dredge moved away. Overflow of the hopper dredge while
underway to the mono buoy location would extend the width of the plume shoreward.
Table 2-2 identifies the various time periods of dredging (including mobilization and demobilization) for each
borrow site. The longest duration would be SO-9 because it has the greatest quantity of material to be
dredged. This site also has a shallow overburden of silt. Therefore, this site represents the greatest
potential for turbidity impacts.
Elevations in turbidity would not result in significant impacts to water quality at SO-9. This conclusion is
based on the relatively localize d nature of the expected turbidity plumes for the majority of the dredge
period, and rapid diluting capacity of the open ocean. The larger turbidity plumes associated with use of
a hopper dredge would occur in pulses and not represent a continuous impact at the borrow site, and the
Page 4.3-6
Regional Beach Sand Project EIR/EA
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