Environmental Engineering Reference
In-Depth Information
and with calibration by samples collected manually
with reliable bed-load samplers, to infer mass trans-
port. As with the active-acoustic technology, empiri-
cal site-specifi c relations between acoustic signal
strength (or other acoustic parameters) and bed-load
sampler data must be developed and used with the
continuous acoustic signal to compute continuous
bed-load transport. The minimum cost of a passive-
acoustic instrument is about US$5000.
Five types of passive-acoustic system have been
tested: hydrophones, microphones, plate-mounted
accelerometers or velocity transducers, pressure
plates, and velocity transducers as seismic arrays.
Hydrophones, submerged in a relatively quiescent
location, integrate the acoustic energy over a large
area of the streambed, in effect inferring an average
bed-load transport rate. Only single-instrument
systems have been tested, and they may respond dif-
ferentially to changes in the spatial distribution of
bed-load transport. The technology is only appropri-
ate for applications where bed-load particle sizes
range from medium gravel to large boulders. Fine
gravel and sand produce high-frequency noise, which
is computationally diffi cult to separate from ambient
noise. When deployed in slack water areas adjacent
to the main fl ow, the system is relatively robust.
Microphones, which measure acoustic pressure
fl uctuations in air, isolate the instrument's electron-
ics from the water resulting in improved long-term
reliability and maintainability. These systems are
considered robust for monitoring fi ne gravel to small
boulder transport, but their performance is inferior
to other passive acoustic systems at extremely low or
extremely high bed-load discharges.
Plate-mounted accelerometers or velocity transduc-
ers have proven, over a one- to two-decade opera-
tional history, to operate unattended for long intervals
with minimal maintenance. The technology can dif-
ferentiate among grain sizes given suffi ciently high-
frequency data acquisition and advanced processing
techniques. Flume calibration may be suffi cient.
Instrument placement is strongly infl uenced by river
geometry, as some sites may be susceptible to deposi-
tion that could cover the instrument. It is one of the
more expensive of the passive-acoustic technologies
because installation may require excavation.
Velocity transducers as seismic arrays integrate
bed-load transport on the reach-to-basin scale.
Sensors are deployed outside the river channel, with
sensors installed as much as 2 km from the river
channel showing sensitivity to river hydraulics. Two-
dimensional array deployment may allow watershed-
scale transport analysis of regions of high bed-load
transport using seismic tomography techniques. The
system can be expensive to purchase and deploy, and
the effectiveness of its scaled-down performance is
unknown. Only qualitative information is available,
and the minimum particle size to which the system
is sensitive has not been determined.
Pressure plates can be used as either an installed
system or as a manually deployed wading-stick
mounted portable device. System calibration has
been shown to be somewhat stable (within a range
of
20%) for two fl oods on the same stream. It is
effective for grain sizes as small as 4-mm diameter
but the upper size limit is unknown.
A priori
knowl-
edge of size distribution in transport is required. The
instrument projects into fl ow, which changes the
local hydraulics, and subsequently the local bed-load
transport rate, potentially leading to local scour or
deposition.
±
2.4 Prospects for operational
surrogate monitoring of bed-load
transport in rivers
This chapter has described an active hydroacoustic
and several passive hydroacoustic technologies for
monitoring characteristics important to understand-
ing properties of bed-load transport in rivers. Some
characteristics common to these technologies include
the following:
•
All address measurement of bed-load characteris-
tics that are diffi cult, expensive, and (or) dangerous
to directly measure with suffi cient frequency to ade-
quately defi ne their spatial and temporal variability.
•
At least some are relatively affordable, costing
between US$5000 and US$20,000. Some, such as
cross-channel impact-plates installations, may cost
substantially more.
•
Most if not all require site-specifi c calibrations
equating values recorded by the surrogate instrument
to the mean cross-section constituent value.
•
All require additional testing and evaluation
before deployment in operation sediment-transport
programs.
None of the technologies is suitable for monitoring
bed-load transport under all fl ow and sediment-
