Environmental Engineering Reference
In-Depth Information
2.2.2 Passive-transducer Hydroacoustics
Jonathan S. Barton & Smokey A. Pittman
collected in the cross section or in fl ume studies (e.g.
Barton et al., in press, and Møen et al., in press).
The method of using acoustic energy to derive
bed-load transport rates is predicated on theories of
impact based on that of Hertz (Goldsmith 2001).
Depending on the specifi c application, the appropri-
ate theory may involve: the collision of two irregular
solids (hydrophone, velocity transducer as seismic
array); the collision of an irregular solid with a cyl-
inder (microphone); or the collision of an irregular
solid with a plate (accelerometer, plate-mounted
velocity transducer, pressure plate). In all cases,
empirical calibration is necessary to convert to an
estimate of bed-load transport rate; in most cases,
this calibration must be done
in situ
, though the
accelerometer has been calibrated in a fl ume.
Acoustic measurement of bed-load transport is not
a new idea. The earliest measurements were made by
Mühlhofer (1933), on Austria's Inn River using a
watertight steel box containing a microphone. Bed-
load collisions with the box were counted manually
through the use of headphones. The Grenoble
Laboratory (Labaye 1948) placed a triangular steel
plate on the streambed, with a microphone in a steel
box above it, and the noise of sediment striking the
plate was transmitted to the microphone through a
steel bar connecting the plate to the microphone
membrane (no results were reported). This system
was modifi ed by Braudeau (1951), who used a brass
plate and deployed the microphone in direct contact
with the plate. The resulting sound was amplifi ed
and transmitted to headphones. Braudeau (1951)
was able to determine the critical discharge for incip-
ient motion to within 1 m
3
/s, but did not attempt to
quantify the transport rate. Bedeus & Ivicsics (1964)
used a directional microphone in a boat-mounted
steel housing to remotely record sediment-generated
noise on the Danube River, Hungary. They com-
pared estimates of lateral variability in transport,
and results were compared with sampler data from
the same cross sections. Johnson & Muir (1969)
reported on fl ume experiments with a piezoelectric
microphone, in which they calibrated an empirical
relation between bed-load transport and microphone
output based on the Meyer-Peter & Müller (1948)
gravel-transport relation, the Hertz law of contact,
and a saltation-length formula from Einstein (1950),
which they also showed to improve insignifi cantly on
a power-law fi t to the data.
2.2.2.1 Background and theory
Investigations into the quantifi cation of bed-load
transport using acoustic signals have steadily
increased in number and in complexity as researchers
seek a tractable surrogate for measuring and predict-
ing bed-load discharge. Use of passive hydroacoustic
signals is attractive compared with many traditional
sampling methods because of:
•
relative ease of deployment;
•
lower data-collection cost;
•
lower hydraulic impact, and perhaps most
importantly;
•
continuous measurement capability, a characteris-
tic that enables quantifi cation of the considerable
variability inherent in the bed-load transport process.
Some technologies also offer the potential for
characterizing the bed-load particle size distribution.
Passive hydroacoustic technologies can be grouped
by the transducer type used in the measurement
device. Five acoustic transducer deployments are in
current use for the study of bed-load transport:
hydrophones (measuring acoustic pressure fl uctua-
tions in water), microphones (measuring acoustic
pressure fl uctuations in air), accelerometers (measur-
ing acceleration of a mass), velocity transducers
(measuring velocity of a mass), and pressure plates
(measuring impact pressure). The hydrophone is
usually deployed in a protective enclosure in quiet
water away from the main fl ow. Microphones are
generally deployed within pipes installed on or in the
streambed. Accelerometers are usually deployed on
the underside of metal plates installed on the bed of
the stream. Velocity transducers can be deployed in
one of two ways: In the same fashion as accelerom-
eters, or in geophone arrays, as in seismic surveys,
along the edge of a river. Pressure plates are typically
deployed perpendicular to the streambed (angled to
the fl ow vector), as either an installed system or as a
portable device.
Minimum costs associated with passive surrogate
technologies for monitoring bed load are about
US$5000. These technologies are relatively robust
and, in theory, installations will require minimal fi eld
maintenance. The performance of the instruments
have been calibrated to bed-load samples manually
