under Attenuation study later in this report.
settlement of the spoils occurs prior to decanta-
Unfortunately, time and funding considerations
tion, total suspended solids of the supernatant
prohibited conducting moisture and density tests
are comparable to those found during flooding
of the basin liner. This information would have
tides at the Flats (200 to 1600 mg/L vs. 1000 to
been very useful in confirming the cause of the
3000 mg/L, four samples). Particle size is also
sharp increase in hydraulic conductivity of the
very small: Less than 1% is retained by a no. 200
liner.
sieve (0.075-mm particle diameter).
Sample sites were surveyed and sediment
thicknesses were measured within the basin be-
Settling times
fore the onset of dredging in July. Post-dredging
The low salinities in the area to be dredged did
sediment profiles in the basin were surveyed in
in fact cause problems with decanting the super-
early October (see Attenuation study). Although
natant from the retention basin. The supernatant
results of the survey indicate that the basin capac-
remained quite cloudy for days after dredging
ity should be sufficient for at least two more sea-
was temporarily halted. Attempts to utilize the
sons at its current rate of deposition, problems
geotextile fabric to screen out the suspended sol-
once again occurred during dredging due to long
ids resulted in minimal decantation due to fabric
settling times caused by the lack of particulate
clogging. This also occurred in the 1995 season.
flocculation due to the low salinity of the super-
The problem was exacerbated that year as the
natant.
contractor concentrated dredging efforts during
Table 8. Salinity measurements in dredge
area (1996).
Salinity
Temperature
(C)
Location
(ppt NaCl)
Clunie Pad Ramp
3.1
19.8
Clunie Point
2.9
19.1
Canoe Point (N)
4.0
20.1
Off Canoe Point boardwalk
3.4
20.0
Off EOD (EOD pond)
7.0
21.2
Table 9. Settling times for spoils in fresh water.
Silt
WP
Silt
WP
Retention Particle
settling
settling
Pond
settling
settling
Dredge
pond size
size
velocity
velocity
depth
time
time
cycle
(ha)
(cm)
(cm/s)
(cm/s)
(cm)
(hr)
(hr)
(days)*
0.75
0.01
7.0E-1
3.6E-1
40.53
0.02
0.03
0.3
0.75
0.001
7.0E-3
3.6E-3
40.53
1.61
3.14
0.5
0.0003†
0.75
6.3E-4
3.2E-4
40.53
17.89
34.88
1.8
* 8-hr dredging plus retention time. No decanting time included.
† Median particle size (Lawson and Brockett 1993).
Salinity measurements
periods of high tides, with their associated influx
Salinity measurements were taken on 7 June
of low-salinity water.
from several locations around the area to be
Using the model for sediment settling times
dredged (Table 8). This was four days after a 31.8-ft
developed by Walsh and Chamberlain (M.R.
flooding tide, a minor flooding event and the first
Walsh et al. 1995), extended settling times are nec-
flooding tide since March 21. Although Praudic
essary just for the median-sized particles. This
(1970) states that flocculation increases with
model is based on Stoke's Law. The Reynolds
salinity levels from 2 through 6 parts per thou-
number is used to determine if settling is laminar
sand (ppt), our experience has shown that for the
or turbulent. Table 9 shows the results for settling
extremely fine particles dredged from the Flats,
of particles up to the median particle size. Note
the higher salinity levels are necessary to ensure
that the white phosphorus particles take longer to
workable settlement times of less than 12 hours.
settle than the mineral sediments due to the dif-
Work done in 1995 showed that when incomplete
ference in density.
11
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