地震造成新山崩地質環境特性之研究─(子計畫三)集集地震所引發山崩地下游河流環境的監測(3/3)

We examined aerial photographs taken
in 1976, 1980, 1985, 1990 and 1997-2001 to
inspect the response of the Gen and
Ten-wei-keng rivers (Ninety-nine mountain
terrain) upon disastrous typhoons and the
1999 Chi-Chi earthquake, which triggered
great landslides and rock falls. A total of 24
transects also was repeatedly surveyed along
the Ten-wei-keng river to monitor its
evolution since the earthquake. We find: (1)
The 2001 Toraji Typhoon resulted in debris
flows, the greatest since 1976, in almost all
low-order rivers in the study area. Since the
typhoon did not bring about particularly
severe rainfall, this debris-flow event
apparently resulted from the accumulation of
debris during the 1999 earthquake, which
increased the slope of the rivers. The great
supply of debris from the tributaries also
caused aggradation in the main valley (ca.
120,000 m 3 debris was deposited in the
lower 2 km-long Ten-wei-keng River and its
river mouth). Without heavy rainfall, the
river became stable within one year after the
typhoon, during which minor deposition was
only locally detected (downstream of
Check-dam VII [1.6 km from the river
mouth] of the Ten-wei-keng River and at its
river mouth). (2) Construction of Check
dams and artificial banks, which stabilizes,
steepens river channels and facilitates
sediment transport, controlled the evolution
of the studied rivers. These constructed river
reaches were less widely alluviated during
the Toraji Typhoon than during the previous
typhoons. The Toraji Typhoon, however,
resulted in greater deposition downstream of
these river reaches: at the place downstream
of Check-dam II (0.9 km from the river
mouth) of the Gen River, where erosion
dominated during the previous typhoons,
and at the mouth of Ten-wei-keng River
(creating a debris fan).