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| Titel |
Bedload transport in steep glacier-fed streams: from incipient motion to floods |
| VerfasserIn |
Francesco Comiti, Andrea Dell'Agnese, Ana Lucía, Gianluca Vignoli, Silvia Simoni, Walter Bertoldi, Luca Mao, Pierpaolo Macconi, Bruno Mazzorana, Roberto Dinale |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250107595
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| Publikation (Nr.) |
 EGU/EGU2015-7301.pdf |
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| Zusammenfassung |
| The current understanding of bedload dynamics in mountain channels is rather scarce, and
the capability to predict it over a range of discharges and under different morphological
conditions is still very poor despite the headways made during the last decade.
Indeed, there has been an increased recognition of the highly stochastic nature of
bedload transport in steep streams, especially at low to medium flows (i.e.Âup to
ordinary events). On the other hand, considerable efforts have been made to model the
effective energy available for bedload in steep channels, in order to reduce the large
overestimation in bedload rates produced by transport capacity equations. Nonetheless,
because high-gradient channels are notoriously sediment supply-limited, largely
varying bedload rates can be observed at the same stream cross-section under nearly
identical morphological and hydraulic conditions, as a consequence of different
sediment supply regimes/events. Therefore, the use of a single bedload transport
equation even for the same stream is becoming strongly questioned by researchers,
whereas most river agencies and consultants – and numerical models – still rely
on "classical" transport capacity equations. Remarkably, glacial streams offer the
possibility to investigate how seasonal changes in sediment supply at the basin scale –
deriving from the periglacial and glacial areas – affects bedload transport rates in the
main channel. However, little quantitative bedload data from these systems are
available.
This contribution intends to share the recent results obtained in two glaciarized basins in the
Eastern Italian Alps, which range from about 10 km2 (upper Saldur river basin) to 130 km2
(Sulden river basin) in drainage area. Different monitoring methodologies encompassing
PIT-tagged clasts tracking (by both portable and stationary antennas), geophone plates,
acoustic pipe sensor and direct sampling by portable traps have been deployed in these two
mountain streams. Our results indicate that numerous and well-conducted direct bedload
samples are key to assess bedload discharges, and rates up to 0.2 and 0.5 kg m-1 s-1
were directly measured in the Saldur and Sulden rivers, respectively. Based on the
acoustic pipe/geophone plate calibration curves (based on the number of impulses
exceeding a given voltage threshold), max unit bedload rates up to about 5 and
15 kg m-1 -1 were estimated in the two rivers during the highest flood events
occurred so far, featuring recurrence interval of 5-10 yr. Despite these unit bedload
discharges rates are seemingly high, they are at least one order of magnitude lower than
those predicted by several bedload equations, also accounting for reduced energy
slopes. Indirect monitoring devices proved also extremely useful to investigate
incipient motion conditions and the variation of transport intensities over the years.
Indeed, the upper Saldur River (monitored at nearly proglacial conditions) exhibited a
marked seasonal trend, with higher bedload rates (at identical water discharges),
lower motion thresholds and counter-clockwise hysteresis loops (between water
and bedload) during glacier melt flows. Preliminary data indicate for the Sulden
River a more complex bedload temporal dynamics, likely in response to its larger
basin featuring several active sediment sources beside the glacier and the proglacial
areas. |
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