Environmental Engineering Reference
In-Depth Information
produce high frequency noise, which may be prob-
lematic to separate from fl ow noise.
Microphone:
• Technique has limited applicability for extremely
low or extremely high sediment discharges. Long-
term averaging at low discharges can improve signal-
to-noise ratio.
• High-fl ow performance depends upon half-bury-
ing the pipe in the bed.
Plate-mounted accelerometer or velocity transducer:
• Selection of placement site is strongly infl uenced
by river geometry, as some sites may be susceptible
to deposition at certain fl ows, which could cover the
instrument.
• Installation may be expensive, and possibly require
excavation.
Velocity transducer as seismic array:
• An array such as that used by Burtin
et al
. (2008)
is expensive to purchase and deploy. Effectiveness of
the technology is uncertain if scaled down.
• Studies thus far have focused only on qualitative
evaluation of transport. No quantitative information
is available yet.
• The minimum particle size to which the system is
sensitive has not been determined.
Pressure plate:
• Instrument projects into fl ow, which changes the
local hydraulics, and subsequently the local bed-load
transport, leading to scour or deposition.
•
Technique requires
a priori
knowledge of size dis-
tribution in transport.
the
in situ
technologies do not directly measure the
constituent of interest over the entire cross section.
Hence, the technologies require cross-section calibra-
tion with reliable bed-load samplers.
The technique of monitoring bed load using active
acoustics has been tested in sand- and gravel-bed
systems. Like the passive acoustic technology, site-
specifi c, empirically derived relations using data
from an ADCP and a bed-load sampler are required.
For active acoustics, the calibration is a function of
the sediment size and the operating parameters of the
ADCP.
Stationary measurements of apparent bed velocity
utilizing manually deployed ADCPs have been cor-
related with concurrent measurements of bed-load
transport and bed shear stress in sand and gravel
reaches, and to dune tracking in sand-bed rivers.
Distributions of apparent bed velocity measured by
ADCP from a moving boat have been correlated to
concurrent distributions of near-bed water velocity,
depth-averaged water velocity, shear velocity, and
channel depth. Instrument measurement variance
constitutes the majority of error in the technique.
The variance of the bottom track velocity for a
mobile bed is the same order of magnitude as that
for water velocity.
Apparent bed-velocity measurements made by
using active acoustics is a fast and non-intrusive tech-
nique for computing bed-load transport. One advan-
tage of using an ADCP to characterize bed-load
transport is the ability to measure the spatial distri-
bution of apparent bed velocity. The method also
benefi ts from substantial averaging of measurements.
However, lack of spatial homogeneity of apparent
bed velocity in the region sampled by the acoustic
beams may cause increased variance in bed-load
computations. The cost of the technology (ADCP)
is about US$20,000, in addition to the costs of a
GPS, boat, and other equipment necessary for
deployment.
Passive acoustic techniques are limited to applica-
tions in gravel-bed systems where bed-load particles
are suffi ciently large for the acoustic energy emitted
by contacts to be measured. In all cases, this particle
size is not clearly defi ned; in many cases, size thresh-
olds may depend on the specifi cs
2.3 Summary and conclusions
One active (ADCP) and several passive (hydrophone
or geophone) acoustic surrogate technologies for
monitoring bed-load transport that have been
described in this chapter are being tested and evalu-
ated for use in large-scale operational sediment-
transport monitoring programs. Active and passive
hydroacoustics are but two of more than a dozen
bed-load surrogate technologies described in the lit-
erature. However, hydroacoustics technologies are
considered by the editors to be among the most
promising of the bed-load surrogate technologies
with which they are familiar.
With the potential exception of some passive bed-
load hydroacoustic technologies in selected streams,
of
the
installation.
Many of these techniques, designed to function
remotely, can be used to infer incipient bed motion,
