An attempt was made to utilize a geotextile
central mast with a total height of 3 m.
filter fabric as a secondary containment and
The most pertinent parameter available from
decontamination structure in the basin drop inlet
the basin datalogger stations for the retention
structure. The fine particles quickly clogged the
basin performance is supernatant levels. As men-
fabric, and repeated scraping of the fabric surface
tioned above, the long settling times of the ex-
increased flow through the fabric only marginally.
tremely fine suspended particles in the super-
The fabric did work well as a secondary backup,
natant, combined with the slow decantation rates
holding more than a meter of supernatant back
due to clogging of the geofilter fabric, resulted in
before we slowly lowered the top edge for decan-
long retention times in the basin. This also resulted
tation. However, our attempts to use it to filter out
in high heads. These factors, plus the reduction in
residual particles failed because of the extremely
the performance of the basin liner, led to increased
slow settling times due to low salinity. A better use
percolation of supernatant through the liner into
of the filtering fabric may have been in screening
the EOD pad below. The drop in surface level of
out particles in excess of 0.5 mm, half the size of
the supernatant, seen in Figure 11, demonstrates
the lethal particles that are sieved by ducks.
this. The 500-mm drop, due to infiltration as well
as passage through the geotextile fabric, occurred
Basin monitoring instrumentation
over the course of 81 hours. This translates to a per-
colation rate of about 1.6 104 cm/s. This is a "fall-
During the initial trip to the Flats in 1996, data-
logger instrumentation stations were reinstalled in
ing head" rate, and assumes all loss is through the
the retention basin. These stations autonomously
basin liner. Rates near the start and end of this peri-
od are 2.1 104 and 8 105 cm/s, respectively,
monitor sediment moisture and temperature, air
temperature, and water levels in the basin. These
averaged over a 3-hour period. At the end of the
parameters can then be used in the natural attenu-
dredging season, the falling head percolation rate
with over 1 m of spoils in the basin was 8.8 105
ation studies to determine the basin performance
cm/s, with start and stop 3-hour rates of 1.6 104
and to monitor dredging activities. The datalog-
cm/s and 5.5 105 cm/s, respectively. Unfortu-
gers used for the four sites are the Model CR10
datalogger system manufactured by Campbell
nately, we were unable to monitor the effects on the
Scientific, Inc. (CSI) of Logan, Utah. This system
water table below the EOD pad surface because
consists of the CR10 Measurement and Control
the EOD pad hydrology monitoring well project
was terminated in 1996.
Module, the CR10 Wiring Panel, the PS12 12-volt
Power Supply and Charging Regulator, and the
Liner organic content
SM716 Storage Module. All of the components are
One final test was performed to determine the
housed in a weather-resistant fiberglass-
basin liner characteristics: an organic carbon con-
reinforced polyester enclosure that in turn is
tent analysis of the peaty-silt liner. Organic content
attached to the central mast of a galvanized steel
helps determine the ability of the liner to densify
tripod that consists of three adjustable legs and a
1200
End of Dredging
1000
Start of Final Decanting
800
600
400
Start of Dredging
200
0
Accumulated Spoils, Station No. 1
200
May 21
Jun 18
Jul 16
Aug 13
Sep 10
Oct 8
Day
Figure 11. Basin supernatant levels (1996).
12
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