VARIABLE UNCERTAINTIES

ERDC TN-DOER-I5

August 2000

VARIABLE UNCERTAINTIES: Each variable in the production calculation contains some

measurement uncertainty. The following provides a brief explanation of the potential uncertainty

associated with these variables.

Water Density. The water found within the dredging environment can vary in density due to

dissolved and suspended solids content and temperature changes. The density of the water can

generally vary within the range of 0.98 to 1.030 g/cm3 because of the previously described

conditions. The maximum error introduced into the production calculations from changes in water

density, without compensation, is about 3 percent (Rokosch 1989).

Sediment Mineral Density. The types of sediment minerals found at dredging sites will vary

according to the physical environment. Generally, coarse grained sediments such as sands and

gravel will exist in riverine or coastal environments, while the finer grained materials such as silts

and clays will be found in areas such as ports and bays, which have a more suitable environment

for the settling of finer grained sediments. The density values for sands and gravel will generally

vary within 2.65 to 2.67 g/cm3. Cohesive soils such as silts and clays can vary in density between

2.68 to about 2.75 g/cm3. Assuming a mineral density of 2.65 g/cm3 in production calculations can

generally result in an error range of 0-4 percent.

In Situ Sediment Density. Accurate measurement of the in situ sediment density is essential

for the accurate calculation of volumetric production. The density of saturated sediments is

dependent on the mineral density and the pore volume that the water occupies. A wide variety of

in situ conditions exist which can have a significant influence on the density of the sediments.

Uniform sands existing in a loose or dense state can have densities within the range of 1.89 to

2.09 g/cm3 (Peck and Hanson 1967). Mixed sands (fine, medium, and coarse) in a loose or dense

state can have densities within the range of 1.99 to 2.16 g/cm3. For finer sediments such as soft

silts and clays with organic content, the density can range from 1.4 to 1.58 g/cm3. Fluid mud layers

can be established at densities as low as 1.2 g/cm3, while fine, consolidated sediments such as stiff

clays can have a density as high as 2.07 g/cm3. Dredging in mixed sediments with layers of fine

grained sediments and coarse sediments can produce significant error if in situ density measurements

are not taken and incorporated into the production calculations.

Flow Meter Velocity Measurements. The velocity of the sediment slurry flowing in the dredge

pipe is generally measured by either a Doppler or magnetic flow meter. The Doppler flow meter

is generally considered the least precise of the two. It is a nonevasive ultrasonic flow meter which

attaches to the outside of the pipe. The accuracy of the Doppler meter as claimed by the

manufacturer is 2 percent of full scale. The magnetic flow meter attaches to the dredge pipe, with

an electrode penetrating the pipe lining. The manufacturer of these meters claims an accuracy of

0.25 percent of full-scale.

Nuclear Density Measurements. Nuclear density gauges are devices which measure radiation

particle attenuation through a material. For dredging applications, the gauge attaches to the

discharge pipe. A radioactive source emits particles through the pipe, and a receiver on the other

side of the pipe counts the particles that pass through. The density of the slurry is measured by the

attenuation of the particles as they pass through the slurry in the pipe. These are very accurate devices

for measuring density of a homogeneous fluid, with a calibrated accuracy of 0.001 g/cm3. Fine