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Titel |
Mountain rivers may need centuries to adjust to earthquake-triggered sediment pulses, Pokhara, Nepal |
VerfasserIn |
Amelie Stolle, Oliver Korup, Wolfgang Schwanghart, Anne Bernhardt, Basanta Raj Adhikari, Christoff Andermann, Hella Wittmann, Silke Merchel |
Konferenz |
EGU General Assembly 2017
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 19 (2017) |
Datensatznummer |
250138149
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Publikation (Nr.) |
EGU/EGU2017-1077.pdf |
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Zusammenfassung |
Mountain rivers respond to strong earthquakes by not only adjusting to changes in
local base level, but also by rapidly aggrading to accommodate excess sediment
delivered by co- and post-seismic landslides. A growing number of detailed sediment
budgets suggests that it takes rivers several years to decades to recover from such
seismic disturbances, depending on how recovery is defined. We test this notion and
study how rivers adjusted to catastrophic sedimentation triggered by at least three
medieval earthquakes in the central Nepal Himalaya. In the vicinity of Pokhara, the
nation’s second largest city, rapid aggradation formed a large fan covering 150 km2 of
mountainous terrain over a length of some 70 km. The fan prograded into several
tributary valleys, rapidly infilling their lower reaches with several tens of meters of
sediment from a major point source tens of kilometers away. A robust radiocarbon
chronology of these valley fills provides an ideal framework for gauging average rates
of fluvial incision and adjustment. We use high-resolution digital elevation data,
geodetic field surveys, aerial photos documenting historic channel changes, and
several re-exhumed tree trunks in growth position to define dated geomorphic marker
surfaces. We compare various methods of computing the volumes lost from these
surfaces to arrive at net sediment yields averaged over decades to centuries. We find
that contemporary rates of river incision into the medieval earthquake debris are
between 160 and 220 mm yr−1, with corresponding sediment yields of 103 to 105
t km−2 yr−1, several hundred years after the last traceable seismic disturbance.
These rates greatly exceed the density-adjusted background rates of catchment-wide
denudation inferred from concentrations of cosmogenic 10Be in river sands sampled in
different tributaries. The lithological composition of active channel-bed load differs
largely from local bedrock and confirms that rivers are still busy with excavating
medieval valley fills. Pronounced knickpoints and epigenetic gorges at tributary
junctions add to the picture of a drawn-out fluvial response, while the re-exhumed tree
trunks indicate that some distal portions of the earthquake-derived sediment wedge
have been incised to near their base. Our results challenge the notion that mountain
rivers recover within years or even decades following earthquake disturbance. We
caution against generalizing the spectrum of fluvial response in this context, as the
valley fills around Pokhara document the possibility of a more protracted fluvial
response that may have been ongoing for as long as 900 years despite the high and
aggressive erosion that characterizes Himalayan rivers. Beyond the scientific community,
our results may motivate some rethinking of post-seismic hazard appraisals and
infrastructural planning during the rehabilitation phase in earthquake-struck regions. |
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