Environmental Engineering Reference
In-Depth Information
in several varied fi eld settings so as to warrant USGS
endorsement for use in operational sediment-moni-
toring programs. However, instrument-sensor satu-
ration can result in failure to record usable data
during periods of high SSCs associated with higher
streamfl ows, which tend to be the most infl uential in
sediment-transport calculations. SSC computed from
at-a-point turbidity data may not be representative
of the mean cross-sectional SSC, particularly when
sand-size material composes an appreciable fraction
of total suspended-sediment transport. The presence
of biofouling can cause bias in signal accuracy or
render the data unusable if the optical surface is not
kept clean manually or by using a mechanical wiper.
Two fully equipped turbidimeters and one optical
backscatterance meter purchased in the summer
2008 each cost about US$5000. This cost can be a
small fraction of the annual cost associated with
monitoring suspended-sediment transport using tra-
ditional techniques. However, the potential for addi-
tional site visits for maintenance, cleaning, or the
collection of calibration samples can result in
increased operating costs.
Similar to bulk-optical sensors, laser-optic instru-
ments also are prone to biofouling and signal satura-
tion at high SSC. However, these instruments have
the major advantage in providing continuous PSDs
from which volumetric SSC can be calculated, as well
as mass SSC if particle density is known or can be
confi dently estimated. The cost of the LISST suite of
instruments (the only commercially available
in situ
instruments using forward (multi-angle) laser light
scattering measurements) ranges from two to six
times that of a fully equipped turbidimeter.
The digital-optic surrogate technique determines
volume SSC by enumerating and summing the volu-
metric characteristics of individual sediment particles
from a digital image of a fi lament of sample in a
fl ow-through cell. Real-time measurements of parti-
cles between 4 and 4000
ment parts is one to two times that for a fully
equipped turbidimeter.
Research on the pressure-difference technology
(Double Bubbler) implies that its use should be
limited to SSCs exceeding at least 10 g/L, which is
generally larger than the suitable SSC range for the
other surrogate techniques examined herein (with
the exception of the LISST-Infi nity laser instrument).
This relatively robust technology, the cost of which
is similar to that of a fully equipped turbidimeter,
measures SSC in a fi xed water column. The theoreti-
cal underpinnings of this technology are straightfor-
ward and its fi eld application is relatively simple.
However, performance of the pressure-difference
technology has been marginal at best in fi eld tests in
Puerto Rico (maximum SSCs approaching 20 g/L)
and Arizona, USA (maximum SSCs 10
2
-10
3
g/L).
Nevertheless, potential remains for use of this tech-
nology because it may provide time series of very
high SSC that cannot be resolved using other sur-
rogate techniques.
The acoustic backscatter technology shows the
most promise for meeting the needs of suspended-
sediment monitoring programs. Mounted
in situ
in
a side-looking (or, less often, upward-looking) ori-
entation, the technology is relatively robust and can
integrate several orders of magnitude more fl ow than
those technologies that make point measurements.
Results using a three-frequency instrument array at
the USGS streamgage on the Colorado River at
Grand Canyon, Arizona, USA, have compared well
with manually collected calibration data for sand-
size material in the range 0.01-3 g/L and for fi ner
material in the range 0.01-20 g/L. At present, the
cost of using a three-frequency Doppler array (three
separate instruments such as used at the USGS
streamgage on the Colorado River at Grand Canyon)
is about sixfold that for a fully equipped turbidim-
eter. Although at least one multi-frequency ABS is
commercially available, it lacks Doppler (velocity)
capability. Research and development efforts toward
production of a reasonably priced multi-frequency
hydroacoustic instrument are underway.
m are possible and the
system requires no routine calibration. The technol-
ogy's performance is currently limited to laboratory
analyses, although it may have applications for
bank-operated pumping systems or for manual
deployment in rivers. Similar to the LISST instru-
ment, results are expressed in volume/volume rela-
tions and not the more common mass/volume units.
Indistinct particle boundaries can reduce measure-
ment accuracy, as can high turbidity from organic or
colloidal material. The cost of off-the-shelf instru-
μ
1.4 Prospects for operational
surrogate monitoring of suspended-
sediment transport in rivers
This chapter has described fi ve surrogate technolo-
gies for monitoring characteristics important to
