Environmental Engineering Reference
In-Depth Information
a signifi cant contribution to the seismic signal from
sources other than streamfl ow.
Downing & Ryan (2001), Downing
et al
. (2003)
and Downing (in press) describe a manually deployed
pressure-plate device that, when impacted by a
moving sediment grain, produces a charge that is
proportional to the force applied, which through
integration yields the momentum fl ux. They derived
a pulse-count record of the bed-load interaction with
the plate above a minimum threshold impact value.
Application of the device requires
a priori
knowledge
of the size distribution in motion. Unlike the other
devices discussed here, this device interacts with the
fl ow, and so a calibration involving the hydraulic
effi ciency is required. Downing (in press) showed, for
two fl oods on the same river, that assuming a con-
stant calibration coeffi cient would result in an error
in the calculated transport rate of only
acoustic power ranging from 0.01 to 14.8 kHz over
1-minute intervals was calculated from the data.
Sample data collected using a Toutle River-2 (TR-2)
bed-load sampler, a modifi ed version of the BL-84-
type bed-load sampler capable of collecting medium-
size gravel (Childers 1999; Pittman 2005) (Fig. 2.4)
deployed from a tethered raft system, were compared
with the temporal average of acoustic data collected
during a sampling interval (Fig. 2.8). The resulting
regression was applied to the 1-minute data
(Fig. 2.9). Barton
et al.
(in press) indicate that the
range of the acoustic data is consistent with the range
of most Toutle River-2 bed-load sampler data.
Spectral analysis of the 1-minute data shows discrete
frequency peaks, the lowest of which falls within the
frequency range reported for bed-load sheet
movement.
±
20%.
2.2.2.3 Summary: passive hydroacoustics as
bed-load surrogate technology
2.2.2.2 Example fi eld application
A single hydrophone (Geospace Technologies MP-
18) system was installed 250 m downstream from the
USGS streamgage on the Trinity River at Douglas
City, California, USA (Barton
et al
. in press). Acoustic
data were collected from May 6 to May 19 2005; total
This technology is applicable for continuous bed-load
monitoring in gravel-bed systems where the acoustic
energy emitted by contacts of bed-load particles larger
than a minimum grain-size threshold can be meas-
ured. In all cases, this minimum size is not clearly
15
300
Acoustic prediction
TR-2 samples used in regression
Other Helley-Smith and TR-2 samples
Water discharge
250
12
200
9
150
6
100
3
50
0
0
6789 0 1 2 3 4 5 6 7 8 9 0 1 2
Date (May 2005)
Fig. 2.8
Predictions of coarse (
8 mm) bed-load transport rates from one-minute-averaged acoustic power (small dots) over the
study interval plotted with the water discharge (solid line), Trinity River at Douglas City, California, USA, and bed-load transport
rates from data collected by Helley-Smith and TR-2 bed-load samplers (solid and hollow stars). The solid stars represent data used
in the least-squares regression shown in Fig. 2.9.
From Barton
et al.
(in press).
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