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| Titel |
Calibration of a pipe hydrophone through bedload traps in a glacierized mountain basin (Saldur river, Eastern Italian Alps) |
| VerfasserIn |
Andrea Dell’Agnese, Carolina Huincache, Luca Mao, Francesco Comiti |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250075220
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| Zusammenfassung |
| The quantification of sediment transport in small mountain basins is of great relevance to
assess the morphological and ecological dynamics of the entire channel network and as well
as to predict flood hazards. Bedload transport in small mountain basins is highly variable in
space and time due to the complexity of flow resistance sources, to the marked
non-uniformity of bed sediments, to the relevance of bed armouring, as well as to varying
activity and connectivity of the different sediment sources at the basin scale. In
high-elevation, glaciarized basins, seasonal variability in sediment transport is known to be
dramatic, but despite the relevance of such basins in many regions worldwide, very few
investigations have tried to quantify it. Unfortunately, the measurement of bedload transport
via direct methods is time-consuming and practically challenging at high flows. Therefore,
indirect surrogate methods for bedload transport allowing its continuous measurements over
time are highly desirable. Nonetheless these have to be calibrated to provide reliable
estimations.
This study focuses on the calibration of a pipe hydrophone in the recently established
(spring 2011) monitoring station in the Saldur basin, a high-elevation glacierized watershed
in the Eastern Italian Alps (18.6 km2 drainage area, about 3 km2 covered by the glacier). The
hydrophone is a 0.5 m-long steel pipe closed at both ends, air-filled, with one microphone at
one end, and it was developed and built in Japan. Sediment particles hitting the pipe induce
an acoustic wave which is registered by the microphone. The wave is then amplified and
transmitted through 6 channels, each having a different sensitivity, so that small
impacts due to little particles and bigger impacts generated by coarser material are
both recorded. The hydrophone was installed at about 2100 m a.s.l. within a wood
log spanning the 2.9 m wide channel, where a rounded slot had been previously
carved to allow half of the pipe diameter protruding in the flow. The log was then
stabilized by large rocks forming a sort of ramp. The number of impacts detected by
each channel is registered at 1 min intervals in a datalogger powered by a solar
panel. Flow stage is measured continuously by a pressure transducer at the same
section.
The hydrophone signal was calibrated against 53 bedload measurements taken 12.4 m
upstream using "Bunte" bedload traps, featuring a 4mm mesh. Bedload samples were
measured from June to August 2011 during daily discharge fluctuations due to snow- and
glaciermelt flows. Samples were taken at a wide range of discharges (from 1.44 to 3.95 m3
s-1, i.e up to almost bankfull flows and bedload rates (0.01 to 7350 g s-1 m-1). The total
number of impacts registered by the hydrophone for the duration of each bedload trap
sampling was calculated, and subsequently bedload rates and number of impacts were
expressed at 1 min intervals. As expected, the signals of the two most sensitive channels
become dampened at lower discharges, and thus cannot be reliably used for bedload
assessment. Instead, starting from channel 3 upward, power laws relating the number of
impacts per minute to the unit bedload rate were obtained, with correlation coefficients R2
ranging from 0.75 to 0.83, with higher correlations associated to the less sensitive
channels.
Keywords: bedload monitoring, glacierized basins, indirect methods, calibration |
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