image, only landslides greater than about 625 m2 (2×2 pixels) can be
mapped accurately.
The steep topography of the study area makes for thin layers of soil
and colluvium with thicknesses of b2 m. The field investigation of
landslides shows that the depth of failure in most of the landslide
areas exceeds 3 m. A prevailing landslide type in this area is a complex
type composed of soil and rock failures.
Besides, in order to understand the change of landslides in
different events, we calculate the landslide ratio (total landslide
area to the whole catchment area), newly-formed ratio (newly formed
landslide area to the total landslide area after a given event),
and reactivated ratio (reactivated landslide area to the landslide area
that existed before a given event) by comparing the landslide map
with the maps of earlier landslide events (Lin et al., 2008a; Chuang
et al., 2009). Newly-formed landslides and reactivated landslides can
be surveyed by contrasting the post-event satellite images with the
pre-event images. In this study, the time spans between pre-event
and post-event satellite images are less than two months. A newly formed
landslide will not have appeared in the pre-event satellite
image, which means that the area was not affected by landsliding in
the two months before the typhoon events. In contrast, reactivated
landslide refers to an area that had been affected by a prior typhoon
event, which was again disturbed in the subsequent typhoon event
(Lin et al., 2008a).
image, only landslides greater than about 625 m2 (2×2 pixels) can bemapped accurately.The steep topography of the study area makes for thin layers of soiland colluvium with thicknesses of b2 m. The field investigation oflandslides shows that the depth of failure in most of the landslideareas exceeds 3 m. A prevailing landslide type in this area is a complextype composed of soil and rock failures.Besides, in order to understand the change of landslides indifferent events, we calculate the landslide ratio (total landslidearea to the whole catchment area), newly-formed ratio (newly formedlandslide area to the total landslide area after a given event),and reactivated ratio (reactivated landslide area to the landslide areathat existed before a given event) by comparing the landslide mapwith the maps of earlier landslide events (Lin et al., 2008a; Chuanget al., 2009). Newly-formed landslides and reactivated landslides canbe surveyed by contrasting the post-event satellite images with thepre-event images. In this study, the time spans between pre-eventand post-event satellite images are less than two months. A newly formedlandslide will not have appeared in the pre-event satelliteimage, which means that the area was not affected by landsliding inthe two months before the typhoon events. In contrast, reactivatedlandslide refers to an area that had been affected by a prior typhoonevent, which was again disturbed in the subsequent typhoon event(Lin et al., 2008a).
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