Environmental Engineering Reference
In-Depth Information
1.2.3.2 Status of laboratory evaluation
tifi ed and the number of complicating environmental
variables minimized, it may be feasible to achieve
practical quantitative results for measuring SSC and
PSDs in riverine environments.
Research in quantitative digital-optic analysis for
suspended-sediment particles has so far been limited
to laboratory conditions at the USGS Cascades
Volcano Observatory, Vancouver, Washington,
USA. The technology calculates, enumerates, and
sums volumes of individual moving particles photo-
graphed in a fl ow-through cell. There are no routine
requirements for validation of the technology,
although cross-section calibrations will be required
if deployed in the fi eld in the future.
Several challenges remain in rendering this labora-
tory-based technology acceptable for laboratory or
riverine deployment. Partly hidden particles, aggre-
gates, and other anomalies can result in less-accurate
measurements, as can higher turbidity levels. The
multi-port fl ow-through cell design reduces these
problems; however, imaging bias can still occur, such
as at very large SSC of clay-size particles. Analytical
results are expressed in volume/volume units and not
in more commonly used mass/volume units, requir-
ing assumptions on the value of particle density or
collection and analysis of samples for SSC and (or)
particle density. Reliable PSD and SSC estimates can
be diffi cult to obtain when the image becomes
“noisy” because of several factors. Aggregates,
organics, air bubbles, and stagnant material within
the viewing area can cause the image to become cor-
rupted and numerically unstable. Special safeguards
incorporated into the software help overcome these
obstacles.
If the source of the imaging problems is identifi ed,
then there may be geometric and statistical solutions
to the problem. For example, image-to-image com-
parisons can be used to check for stationary particles
that have adhered to the fl ow-through cell windows
viewing area. This particular group of pixels becomes
useless for analytical purposes until the area has
cleared. The software recognizes the recurring blob
and will not use the occupied pixels in sequential
calculations until the area clears or changes. Air
bubbles could be counted as particles, but with their
distinctive geometric attributes the software can
easily identify them as such and remove them from
subsequent SSC calculations.
There are inherent diffi culties for digital-imaging
systems to perform well in real-world environments.
However, if the problems can be identifi ed and quan-
1.2.3.3 Summary: digital optical imaging as a
suspended sediment surrogate technology
Digital-optic imaging technology remains in the
research and development phase and has yet to be
deployed for testing beyond the laboratory. Other
than the fl ow-through cell and lens extensions, the
technology is composed of off-the-shelf parts avail-
able at a cost similar to that of a fully equipped
turbidimeter. Routine instrument calibrations are
unnecessary.
Pending completion of testing and development,
several inferences on limitations based on its
attributes can be made:
•
The technology can be affected by some of the
same drawbacks as those for the bulk-optic and laser
technologies. These drawbacks include issues associ-
ated with samples drawn from a single point, bio-
fouling of the optic lenses, and upper measurement
limits;
•
Assumptions or measurements of mean particle
density are required to convert volume SSC values to
mass SSC values;
•
Because the fl ow-through cell system is designed
to separate aggregated sediments, it is not suitable
for ascertaining SSCs of fl occulents.
1.2.4 Pressure difference
John R. Gray, Nancy J. Hornewer, Matthew C.
Larsen, Gregory G. Fisk, & Jamie P. Macy
1.2.4.1 Background and theory
The pressure-difference technique for monitoring
SSC relies on measurements from two precision pres-
sure-transducer sensors arrayed at different, fi xed
elevations in a water column. The difference in pres-
sure readings is converted to a fl uid-density value,
from which SSC is inferred after correcting for water
temperature (dissolved-solids concentrations in
fresh-water systems are rarely large enough to be of
consequence in the density computation). One of the
fi rst uses of the pressure-difference technique for
measuring fl uid density was applied to crude oil in
