Environmental Engineering Reference
In-Depth Information
(a)
(b)
1,000
10
5
1,000
10
5
Silt and clay concentration
Sand concentration
Water discharge
10
4
10
4
500
500
10
3
10
3
0
0
10
2
10
2
3-Freq. acoustic
Pump
LISST-100
LISST-25X
10
1
10
1
3-Freq. acoustic
Pump
LISST-100
LISST-25X
Water discharge
10
0
10
0
1-25-20051-26-20051-27-20051-28-20051-29-20051-30-2005
1-25-2005 1-26-2005 1-27-2005 1-28-2005 1-29-2005 1-30-2005
(a)
Date
(b)
Date
Fig. 1.21
Comparisons of SSCs from three-frequency acoustic backscatter, calibrated pump, and LISST measurements (a)
suspended-silt and -clay concentration and (b) suspended-sand concentration.
From Topping
et al.
(2007).
technique, considerable time and effort for a user to
compute a time series of SSC from ABS may be
required. The cost of a single-frequency
in situ
instrument is about double that for a fully equipped
turbidimeter, but the fi eld maintenance cost is
expected to be less than that for a turbidimeter.
shaped beams (acoustic technology) in streamfl ow.
The capability for providing computed time series of
SSC is a major advantage over the relatively sparse
data produced by traditional methods for collecting
and computing records by conventional methods
described by Porterfi eld (1972), Edwards & Glysson
(1999), and Nolan
et al.
(2005). The routine need to
estimate SSC values for periods lacking sample data
and to interpolate between known or estimated SSC
values interjects an unquantifi able degree of uncer-
tainty in traditionally derived sediment-discharge
values. The reduction in uncertainty associated with
the availability of continuous surrogate data likely
will result in a more accurate computation of
sediment discharges even considering uncertainties
associated with instrument-measurement realm or
cross-section calibration of surrogate measurements.
Spatial correlations between any surrogate meas-
urement and its respective mean value in the cross
section are still required. However, because of the
relatively large ensonifi ed volume associated with
acoustic surrogate techniques, correlations associ-
ated with the acoustic-backscatter technology are at
least theoretically less variable than those for the
single-vertical pressure-difference technology, which
in turn are theoretically less variable than those for
the at-a-point measurements obtained by bulk, laser,
or digital-optics technologies.
The most common surrogate technology is turbid-
ity (bulk-optics). Turbidity has been shown to
provide suffi ciently reliable data for computing SSC
1.3 Summary and conclusions
Five surrogate technologies for monitoring sus-
pended-sediment-transport characteristics have been
or are being tested and evaluated by the USGS
toward deployment in operational sediment-trans-
port monitoring programs. The fi ve technologies are
bulk optics (turbidity), laser optics, digital optics,
pressure difference, and acoustic backscatter. None
of the
in situ
technologies measures the surrogate
constituent of interest over the entire cross section.
Hence, most if not all of the technologies require
cross-section calibration. Although most of the
in situ
instruments are routinely calibrated, this step
is sometimes bypassed in favor of cross-section
calibration.
Table 1.2 summarizes selected attributes of the fi ve
suspended-sediment-surrogate technologies pre-
sented herein. All of the technologies, with suitable
calibration, provide time series of computed SSC at
sub-daily sampling frequencies at-a-point (three
optical technologies), in a single vertical (pressure-
difference technology), or along one more cone-
