Environmental Engineering Reference
In-Depth Information
Microphone:
• Isolation of electronics from the water leads to
improved reliability and maintainability.
• Sensors can operate unattended for long intervals
with minimal maintenance.
• Method is robust for monitoring fi ne gravel to
small boulder transport.
2.4
2.2
2.0
10
-6
G
b
+ 1.41
10
-5
2.13
×
×
1.8
1.6
1.4
95% CI
Plate-mounted accelerometer or velocity transducer:
• Sensors can operate unattended for long intervals
with minimal maintenance.
• Technique has a 15-year operational history;
• Technique has ability to differentiate grain sizes
with suffi ciently high-frequency data acquisition and
advanced processing techniques.
• Flume calibration may be suffi cient.
1.2
1.0
0.8
0.6
1.0
1.5
2.5 3.0 3.5
Coarse bedload transport rate (kg/s)
2.0
4.0
4.5
5.0
5.5
6.0
Fig. 2.9
Correlation plot between temporally averaged
total acoustic power (totaled over the frequency range of
0.01-14.8 kHz) and bed-load transport rate from the Toutle
River 2 sampler. Error bars show
Velocity transducer as seismic array:
• Sensors are deployed outside the river channel;
Burtin
et al.
(2008) showed that sensors as much as
2 km away from the river channel still showed sig-
nifi cant sensitivity to river hydraulics.
• Integrated bed-load transport measurements are
on the reach-to-basin scale.
• Two-dimensional array deployment may allow
watershed-scale transport analysis of regions of high
transport using seismic tomography techniques.
2 standard errors of the
temporal mean. The Pearson's correlation coeffi cient
R
is
0.758, with a
p
-value of 0.0180. Confi dence interval for
the regression parameters assumes Gaussian error.
From Barton
et al
. (in press).
±
defi ned; in many cases, size thresholds may depend on
the specifi cs of the surrogate technology installation.
The technique relies entirely on calibrations to
cross-section bed-load samples. This technology can
be used to infer the incipient motion, and with
calibration by reliable bed-load samplers, to infer
mass transport. Most parts are available off the shelf
at a cost similar to that for a fully equipped
in situ
turbidimeter. Specifi c advantages and limitations of
each type of sensor follow.
Pressure plate:
• Technique can be used as either permanent
(installed) system or portable (wading-stick mounted)
system.
• Calibration has been shown to be fairly stable
(
20% variation) for two fl oods on the same stream.
• System is effective for grain sizes as small as 4 mm
in diameter (the largest size that will not damage the
instrument has not been reported).
±
2.2.2.3.1 Advantages of passive hydroacoustic
technologies
2.2.2.3.2 Limitations of passive hydroacoustic
technologies.
All passive hydroacoustic technologies
for bed load require site-specifi c calibrations. Other
limitations include the following.
Hydrophone:
• By integrating over a large area of the streambed,
the hydrophone allows estimation of average trans-
port rate, compensating for spatial variability in the
transport rate.
• Taking advantage of the high acoustic conductiv-
ity of water, the hydrophone can be placed in slack
water adjacent to the main fl ow.
• The hydrophone can be installed at minimal cost,
requiring no excavation of the bed and can be
installed during high fl ow.
Hydrophone:
• Only single-instrument systems have been tested,
and evidence suggests that this arrangement may be
sensitive to changes in spatial distribution of bed-
load transport. Array deployment may help to reduce
this sensitivity.
• Technique is only appropriate for medium-gravel
to large-boulder applications. Fine gravel and sand
