Topographical factors of landslides in LEF

Typhoon morakot produced copious amount of rainfall and
triggered enormous landslides in southern Taiwan during it hit
Taiwan from August 8 to 9, 2009. The Liukuei Experimental Forest
(LEF) is located in the peak rainfall region and was inescapable
incurred severe landslides and road destruction. This paper
presents a ststistical approach to study the spatial
relationship between landslides and their geographical factors
in the LEF after the Typhoon Morakot. The Formosat-2 satellite
images, 10*10DEM data and the related coverage of LEF were
examined using the WinGrid computer software for topographical
analysis. The results indicated that the extreme heavy rainfall
is the main factor that triggered landslides huring the typhoon.
There were 204 locations occurred identifiacable landslides and
their total area was about 804.5 ha. The altitude and type of
vegetation covers were not factors related to the failure of
slopes. Stepness, aspect, and distances to roads and streams
were factors that contribute the instability of slopes. About
70.2% of the total area of landslides occurred on slopes with
steepness in the range of 60-80% and this explained that steep
slopes are prone to collapse. landslides having the distance of
lower edge to stream less than 10 m are 49 locations and their
areas account for 47.2% of the total landslide areas. In
addition, there are 34 landslides (24.5%) within 50 m from
forest roads and the average area of those landslides is 5.79
ha. The deep concentrated surface flows resulted from extreme
rainfall in the streams or on road surfaces strike against the
foot of hillslope copes with saturated soil water were the main
factors that caused slides in large area. This also indicated
that landslides in LEF are close related to stream system and
forest road construction. The aspect of slope is not the main
factor that related to landslides for most cases. However,
slopes having southern and southeast aspects were directly faced
to the strong wind and heavy rainfall brought by typhoon Morakot
and were occurred more landslides than slopes with other aspects.