| Summary: | 碩士 === 國立臺灣大學 === 地理學系研究所 === 85 === Typhoon Herb brought about disastrous floods and landslides in Nantou Hsi
en at the end of July and beginning of August of 1996. Landslide disasters
were especially serious along Chen-Yu-Lan-Chi, where debris flows also
occurred several times immediately after the typoon passed, at the month of
one of itstributary, Chu-Shuei-Chi, which have caused special interest among
the scholarsin this field. It is because the TV documented debris flows
occurred at a time when there is less amount of precipitation than that
generally supposed to be. The present study is aimed at the interpretation
of this "unusual" phenomena through literature review and detailed field
study.
Derbis flow is a sort of landslide. It is a mixture of soil, rock and
water which moves as a mass under the gravity force. The appearance and
behavior of debris flow is like wet cencrete. So, it can be formed by adding
more and more solid materials to a fluid until it comes to a state like a
viscous slurry or something like that; it also can be formed by adding more
and more water to solid material to such a degree that it starts to flow as a
viscous mass. Thereis also a transitional phase which is in property between a
debris flow and a high density torrential current, which had causes discusion
about the definitionof "Derbis Flow".
Chu-Shuei-Chi has a wide and more than 15 meters thick regolith at its
downstream riverbed. The present study reveals the follows: During the Herb
typhoon , the channel was surely buried up by sediments coming from its up-
stream, but subsequently reopenned by erosion to form a gorge-like channel
with a depth of more than 15 meters by stream water, which is supplied in
large amount by ground water being originally storedin the thick regolith,
at the time of raining. The collapse of its high andsteep unstable banks had
provided the stream with a sufficient amount of debris that, after completely
mixed with stream water, eventually lead to the formation of the very debris
flows concerned.
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