days required for freshwater sediments. More rig-
100
100
2 hrs.
orous laboratory settlement tests following the
80
80
method of Montgomery (1978) confirmed the pre-
liminary tests. These sedimentation tests were con-
60
60
ducted in a 15-cm-diameter polycarbonate tube
50% Settlement
with a length of about 1.8 m. The tube had a po-
40
40
rous stone and drainage port in its base (see Fig.
Dose No. 1
2). Water was prepared at a salinity of 5 ppt and
20
20
mixed with sediment at a ratio of four parts water
0
0
to one part wet sediment by volume (70% water
content by weight). The water content for the sedi-
a. Dose #1.
ment was obtained from an estimate of the water
0
0
content in the sediments that the dredge would
100
100
2 hrs.
pump into the settling pond. The sediment was
80
80
thoroughly mixed to its initial water content in a
laboratory blender (in contrast to hand mixing in
60
60
the preliminary tests) and then mixed in a barrel
with the saline water using a stirring agitator. It
40
40
was then quickly poured into the sediment tube.
The elevation of the interface at the top of the sedi-
20
20
Dose No. 1R
ments was then monitored for at least 24 hours. A
0
0
definite interface formed within a few minutes,
100
100
the water above the interface being relatively trans-
b. Dose #1R.
parent. Table 2 and Figure 3 show the test results.
0
0
In all cases, 50% of the total settlement occurred
100
100
2 hrs.
in about two hours. In the preliminary tests, 50%
of the total settlement occurred in about 15 min-
80
80
utes. The difference between these results is prob-
60
60
ably related to the size of the soil aggregates used
in the two tests and the quality of the mixing. The
40
40
aggregates were much coarser in the hand-mixed
test than the machine-mixed tests, thus settling
20
20
Dose No. 2
more quickly. The two cases probably bracket the
0
range of results that would be achieved in the
0
500
1000
1500
dredging operation.
Elapsed Time (min)
These tests confirmed our preliminary study
c. Dose #2.
and gave us confidence that we could readily con-
duct the dredging, filling and decanting operations
in daily cycles. However, we had to be assured
100
100
that any white phosphorus in suspension would
80
80
drop from suspension in the two-hour window of
time. We conducted a fourth sedimentation test,
60
60
this time with white phosphorus particles in sus-
pension. We spiked the sediment with white phos-
40
40
phorus particles obtained from the Flats by
Marianne Walsh of CRREL. White phosphorus
20
20
2 hrs.
Dose No. 3
particles of the size considered to affect the health
of the waterfowl were used (≈ 0.1 mm [Walsh
0
0
100
200
300
400
500
personal communication*]). Water samples were
Elapsed Time (min)
taken through ports located at 15-cm intervals in
d. Dose #3.
*M.E. Walsh, Applied Research Division, U.S. Army
Figure 3. Sedimentation test results in Montgomery
Cold Regions Research and Engineering Laboratory,
tube.
Hanover, New Hampshire.
7
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