MEASUREMENT IN THE MEANS OF CONVEYANCE

ERDC TN-DOER-T6

September 2004

MEASUREMENT IN THE MEANS OF CONVEYANCE: Basically two parameters are

measured to quantify hopper load contents: volume and weight. Either one of these two

parameters is used as a production evaluation tool, or both are measured simultaneously to

calculate the dredged material's average density or some derivative thereof (i.e., tons dry solids).

Methods and equipment used to measure these parameters range from manually sounding the

hopper volume with a weighted tape, to measuring and calculating hopper density with an

automated measuring/monitoring system i.e., the Silent Inspector.

Volumetric Measurement

Hopper manual sounding methods. Since the

first hopper dredge, the General Moultrie, was

used to dredge the Charleston, South Carolina

Bar in 1857, the volume of material in the

hopper has been measured to calculate

production (the General Moultrie with a 19-in.-

diam drag pipe averaged 328 yd3 per day).

Sounding of the hopper at various locations with

a lead line was the primary method of measuring

the volume of material being transported in the

hopper, and this method is still in use today (see

Figure 2).

This photograph was taken on a hopper dredge

working in coarse-grained material (medium-

sized sand) on the Columbia River Bar, managed

by the Portland District. Soundings, or ullage

measurements, are usually taken at six to eight

different locations around the hopper using

conveniently located reference points (i.e.,

walkways or coaming tops). The soundings are

then averaged together. The resulting value is

applied to an ullage table or equation that Figure 2. Sounding hopper to measure load

equates ullage distance to volume of material in

volume

the hopper. Depending on the number of

soundings taken and care taken by the sounder, this measurement method can be quite accurate

as long as the dredged material provides a surface with sufficient bearing strength to support the

sounding lead (i.e., in a sand load). After loading is completed and the sand settles out quickly

with a layer of water over it, this measurement method compensates for the water layer by the

lead penetrating through the water layer till it rests upon the sand surface. Rullens (1993)

concludes that when sand is dredged, the half-sphere "sounding" method (a sounding lead with

specific mass and dimensions that will be described later) gives an unambiguous indication of

the top of the solids mass and that this method is suitable to measure payable dredged quantities.

Another advantage of this method is that the measuring equipment is inexpensive and simple.

Disadvantages of the half-sphere sounding method include: