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| Titel |
Paleo-basin hydrology and sediment transport in the Pisco Valley, Peru |
| VerfasserIn |
Toufik Bekaddour, Fritz Schlunegger |
| 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 |
250074883
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| Zusammenfassung |
| Quantitative data on sediment transport and climatic changes allow the understanding of
alluvial river processes such as aggradation, degradation, vertical and lateral incision and
valley widening. Here, we focus on the ca. 4.5*10^3 km^2-large Pisco drainage basin on the
western Pacific side of the Andes, central Peru, which is currently a desert with precipitation
rates decreasing form 800 mm/yr on the Altiplano, to 10 mm/yr at the coast. While higher
precipitation rates than at present have been inferred from lake level highstands on the
Altiplano between 48 and 36 Ka PB (Minchin period), no quantitative estimates about water
budgets for that time have been presented so far. Here we use the stratigraphic
archives from the Pisco Valley and explore relationships between: (i) the particle size
of bedload and the shear stress conditions needed for their entrainment, and (ii)
the basin hydrology and channel adjustment for the present time and the Minchin
pluvial period between 48 to 36 ka BP when sediment aggradation resulting in the
build-up of ca. 50 km-thick terrace sequences. We find that the bedload material is
generally more coarse-grained in the terrace deposits than in modern longitudinal bars.
In particular, grain size measurements reveal that the mean D50 and D84 of the
modern deposits are about 100 and 170 mm, respectively. Meanwhile the mean D50
and D84 inferred from measurements of the terrace deposits are ca. 150 and 310
mm. Our results reveal that the current mean annual discharge Q=60 m^3/s of the
Pisco River is just refreshing the flat middle reaches and transporting the coarse
particles in the steep higher reaches at near critical threshold. On the other hand, the
hydraulic analyses based on the D50 of the Pisco River reveal that a critical discharge
of Q=300 m^3/s is required to transport the D84 along the entire channel, and a
discharge of this magnitude could rearrange the entire channel morphology except
for the downstream reaches where the coarse bed particles remain immobile. For
the terrace deposits, however, the critical discharge inferred from grain size data
yield strong prevailing hydraulic conditions during the Minchin and more powerful
formative discharge (Q=900 m^3/s) that exceeded the threshold of motion of the
D84 along the entire Pisco River. Our results first underscore the importance of
the channel geometry in general, and gradients in particular, as one of the most
important controlling parameter for sediment transport (i.e. where erosion, by-pass and
sediment accumulation occurs). Most important, the data presented here suggest
that in the Pisco Valley, the channel forming discharge was at least 3 times higher
between 48-36 Ka PB than at present. For the Minchin period, we anticipate similar
changes for the Altiplano, and potentially also for other regions in South America. |
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