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 ERDC TN-DOER-I3
July 2000
The load and volume method of computing load density is accomplished by measuring the hopper
volume in conjunction with the hopper load. The hopper volume can be measured by hand or
through automated methods such as acoustic sensors. Traditionally, the hopper load is determined
through pressure sensors. Bubbler tubes that vent in the keel of the dredge measure changes in
hydrostatic pressure as the dredge drafts under load. The load is measured by relating the pressure
changes to displacement of the dredge. The average density in the dredge is therefore computed as
the change in volume over the change in displacement:
VOL2 - VOL1
ρh =
(7)
DISP2 - DISP
1
where
VOL1
= volume of material in hopper before filling
VOL2
= volume of material in hopper after filling
DISP1
= displacement of dredge before filling
DISP2
= displacement of dredge after filling
With the hopper dredge production computed by multiplying the percent solids in the hopper by
the hopper volume or:
b
g
ρ - ρw
H pro = h
(8)
Hvol
ρi - ρw
where
Hvol
= the load volume
ρw
= water density
ρi
= in situ sediment density
DISCUSSION: Both of the monitoring methods discussed have been automated by the Engineer
Research and Development Center (ERDC) to provide real-time data and production/efficiency
reports. Sensor data are acquired over a set time interval. Software computes the variables described
by Equations 1-8 and provides graphical and tabular summaries. These data provide a real-time
evaluation of dredge performance, with reports generated on a time-line determined by the user.
During the conduct of a dredging project, dredge performance can be affected by changes in site
conditions such as unforeseen changes in sediment characteristics, foreign bodies in the waterway
that impede progress, and inefficient dredge operation. Sediment characteristics in the project area
may be substantially different than presented in the contract specification, thus affecting dredge
performance. The sediments may be more consolidated or the sediment size may be greater than
anticipated. In some cases, the digging depth or discharge pipe length may be excessive for the
dredge design, thus limiting dredge performance. In all cases, delays in dredging are costly both to
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