Environmental Engineering Reference
In-Depth Information
Gage house
2 MHz EZQ
Pump
shelter
LISSTs
on cable
1 MHz EZQ
600 kHz Aquadopp
Acoustic
instruments
on bracket
(a)
(b)
Fig. 1.20 Photograph of an array of the three acoustic Doppler current profi lers used to estimate SSCs and PSDs in the Colorado
River in Grand Canyon, Arizona, USA.
From: Topping et al. (2007).
three single-frequency (1.0 and 2.0 MHz, and
600 kHz) side-looking ADCPs (Fig. 1.20). A post-
processing technique is applied to analyze (1) acous-
tic attenuation to compute the suspended silt-clay
size fraction, and (2) acoustic backscatter to compute
the suspended-sand fraction in a size range applica-
ble for each frequency. Topping et al . (2007) indicate
that the approach is applicable for monitoring SSC
over the ranges of 0.01-20 g/L (silt-clay) and 0.01-
3 g/L (sand); results are within 5% of those com-
puted by conventional methods. In addition, the
method calculates median grain size within 10% of
that measured by conventional means. Topping et al .
(2007) infer a greater accuracy with this technique
than with a conventional sampling regime largely
due to the substantially greater sample frequency and
volume. Figure 1.21 shows comparisons of SSC from
three-frequency acoustic backscatter, calibrated
pump, and LISST measurements.
surrogate technologies. Unlike point measurements,
profi les of acoustic backscatter measurements from
Doppler velocity instruments can cover a substantial
part of the water depth or river cross section; they
can integrate orders of magnitude more fl ow than
other methods that rely on at-a-point or single-
vertical measurements. Sediment fl uxes in the beam
can be computed and empirically indexed to the
mean cross-sectional SSC value. These data in turn
can be used with continuous water-discharge data
to compute unit- and daily-value sediment fl uxes at
the monitoring site. Unlike optic-based surrogate
instruments, biological fouling is not a problem.
In addition to some major advantages over other
surrogate techniques, the acoustic backscatter
method has some limitations. Similar to optical sur-
rogate techniques, a single-frequency source cannot
differentiate between change in PSD and change in
SSC without calibration and there is an appropriate
frequency for a given particle size and a somewhat
narrow frequency range for which the method is
appropriate for a given size distribution. A series of
calculations are required for the reduction and analy-
sis of the acoustic signals; thus until standard operat-
ing procedures are developed and adopted for this
1.2.5.3 Summary: acoustic backscatter as
suspended sediment surrogate technology
As a surrogate for SSC, acoustic backscatter holds
several advantages over other suspended-sediment-
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