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| Titel |
Interdecadal trends of sediment discharge in mountain watersheds in Japan |
| VerfasserIn |
Takashi Koi, Norifumi Hotta, Masakazu Suzuki |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250037018
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| Zusammenfassung |
| After sediment production, such as by a landslide, some sediment immediately
discharges from the basin outlet. However, part of the sediment remains in the basin, to
be removed and re-deposited by subsequent rainfall and flooding before finally
being discharged. Such sediment dynamics occur at various timescales, depending
on the frequency and magnitude of sediment production and the various agents
of sediment transport. Thus, interdecadal trends of sediment discharge differ by
watershed. Understanding sediment dynamics over longer timescales is important
for studies of geomorphological processes and for basin management. This study
examined factors associated with interdecadal sediment discharge by comparing two
mountain watersheds in Japan. The study sites were the Nakagawagawa watershed
(39 km2) in the Kanto district, where widespread landslides were induced by a
catastrophic earthquake (M 7.9) in 1923, and the Dogawa watershed (81 km2) in the
Kyushu district, where heavy rainfall events (continuous rainfall of over 1000 mm)
have repeatedly triggered landslides. Long-term reservoir sedimentation data were
available for both watersheds, covering 25 years in the Nakagawagawa watershed and
53 years in the Dogawa watershed. Although both watersheds had high sediment
yield (103 m3km-2year-1 order) for their decadal averages, the interdecadal time
series showed different waveform trends. The waveform of the Nakagawagawa
watershed was nearly constant over 25 years. The waveform of the Dogawa watershed
showed interdecadal fluctuation over 50 years. These waveforms were distinguished
according to the difference between maximum and minimum sediment yields of the
N-year moving average. Annual sediment discharges roughly corresponded with
the rainfall magnitude, yet rainfall was not a main factor controlling interdecadal
variability of sediment discharge. These sediment dynamics can be discussed from the
perspective of temporarily stored sediment. In the Nakagawagawa watershed, the
abundant temporarily stored sediment produced by earthquake-triggered landslides was
discharged with rainfall and, consequently, high sediment discharges continued for a long
time. On the other hand, in the Dogawa watershed, high sediment discharges were
concentrated only after landslide occurrences; in other periods sediment discharge
was moderate, regardless of rainfall magnitude. When temporarily stored sediment
abounded in the watershed, high sediment yields corresponded with rainfall events. In
contrast, when temporarily stored sediment was not abundant, sediment yield was
moderate, regardless of rainfall magnitude. Hence, the interdecadal variability of
sediment discharge was dictated not only by rainfall but also by the amount of
temporarily stored sediment. This study shows that temporarily stored sediment was
closely associated with sediment dynamics on the interdecadal timescale and that
waveforms of sediment discharge reflect fluctuation of the temporarily stored sediment. |
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