Environmental Engineering Reference
In-Depth Information
adequacy of its calibrations. Two general types of
calibration are used: instrument calibrations and
cross-section calibrations. Instrument calibration
refers in a statistical sense to the precision and vari-
ance of data derived from the surrogate measurement
in the sampled region (the instrument-measurement
realm) to an actual value in the corresponding realm
ascertained by independent measurement. Cross-
section calibration refers to correlation of the derived
data to the mean constituent value occurring in the
full stream cross section or stream segment at the
time of the measurement, typically using FISP sam-
plers and sampling techniques. Although the instru-
ment-measurement realm generally corresponds to a
volume, it is referred to herein in practical terms with
respect to the instrument sensor as a point for a local,
minute-volume measurement; a water column; or a
beam (or average of multiple beams).
Derivations of true mean cross-section constituent
values are unlikely from consistently false instru-
ment-measurement-realm values, similar to the axi-
omatic “garbage in, garbage out” concept in
computer science. On the other hand, inferences of
false mean cross-section constituent values from true
instrument-measurement-realm values can and often
do occur. False inferences from true surrogate data
can result from heterogeneity typically associated
with the occurrence and transport of suspended sedi-
ment in the cross section, and is the reason for the
need for cross-section calibrations. Therefore, the
most meaningful measure of a surrogate technology's
reliability is derived from calibrations performed
within the instrument-measurement realm. Hence,
criteria to evaluate sediment-surrogate technologies
should be based solely on instrument calibrations in
the instrument-measurement realm, if possible.
However, the ultimate measure of the effi cacy of a
surrogate technology to monitor suspended sedi-
ments in rivers is its ability to quantify adequately
the sedimentary characteristics of interest over the
entire cross section.
Validation of a suspended-sediment-surrogate
technology requires evaluation criteria and a well-
conceived and -administered testing program (Gray
et al.
2002; Gray & Glysson 2005). The following
are some qualitative criteria for selecting and deploy-
ing a surrogate technology:
•
capital and operating costs should be affordable
with respect to the objectives of the monitoring
program in which the surrogate instrument is
deployed;
•
the technology should be able to measure SSCs,
and in some cases, PSDs, throughout the range of
interest (but not necessarily throughout the entire
potential environmental range);
•
the equipment should be robust and reliable, that
is, prone to neither failure nor signal drift;
•
the method should be suffi ciently simple to deploy
and operate by a fi eld technician with a reasonable
amount of appropriate training;
•
the derived data should be relatively simple and
straightforward to use in subsequent computations
and (or) accompanied by standard analytical proce-
dures as computational routines for processing the
data.
Quantitative criteria for acceptable accuracies of
the derived data are diffi cult to develop for all
potential applications, in part because of substantial
differences in river sedimentary and fl ow regimes.
For example, accuracy criteria for rivers transport-
ing mostly silt and clay should be set more strin-
gently (intolerant of larger-magnitude uncertainties)
than those for rivers that transport comparatively
large fractions of sand. However, there is a clear
need for consistency in PSD and SSC criteria on
the part of instrument developers, marketers, and
users.
To this end, quantitative acceptance criteria devel-
oped for PSD and SSC data produced by a laser-
diffraction instrument (Gray
et al.
2002) have been
generalized for evaluating data from other sus-
pended-sediment surrogate instruments. At least
90% of PSD values between 0.002 and 0.5 mm
median diameter are required to be
25% of true
median diameters. In the absence of a more rigorous
evaluation, this criterion has been applied to all par-
ticle sizes in suspension.
SSC acceptance criteria range from
±
±
50% uncer-
tainty at lowest SSCs to
15% uncertainty for SSC's
exceeding 1 gram per liter (g/L). The criteria pre-
sented in Table 1.1 are adapted from Gray
et al
.
(2002).
These criteria pertain solely to the performance of
a surrogate technology within its physical realm of
measurement. Routine calibrations to correlate
instrument signals to mean cross-sectional SSC
values are required for all of the
in situ
instruments
presented herein.
±
