10th Asian Regional Conference of IAEG (2015)

Gravitational slope deformation due to river rejuvenation in the Laonung River
catchment, Taiwan
Ching-Ying TSOU(1), Masahiro CHIGIRA(1), Tai-Chieh HO(2)(1) Disaster Prevention Research Institute, Kyoto University, Japan
E-mail: tsou.chingying@slope.dpri.kyoto-u.ac.jp(2) Department of Earth Science, National Cheng Kung University, Taiwan
Abstract
River rejuvenation forms convex, projecting slopes that lead to the development ofdeep-seated gravitational slope deformations (DGSDs) and in some cases transformed
themselves into catastrophic failures. This study analyses these processes in the catchmentof the Laonung River, southern Taiwan, where was severely damaged by 2009 Typhoon
Morakot. At the study area, the landscape comprises paleosurface remnants with slopes
mostly gentler than 30° at higher elevations and separated by convex slope breaks at theirperipheries and incised valleys, in which showing a series of terraces. The slope breaks up
to about 400 m above the current river bed are inferred to have been formed associated withrejuvenation of river. The lower parts of the DGSDs have many slope failures that are along
the rims of the paleosurface and within the incised valleys, showing the toes of the DGSDs
are destabilized by slope failures. Many areas of DGSDs tend to occur along the slope
breaks, suggesting a common mechanism of slope instability, probably owing to riverundercutting.
Keywords: gravitational slope deformation, catastrophic landslide, convex slope break1.

reference to their geomorphic and geological features.
Chigira (2009) and Chigira et al. (2013a) studied
catastrophic landslides induced by rainfalls ingravitationally deformed slopes and suggested thatthe small scarps seen on the deformed slopes become
the crowns of subsequent catastrophic landslides, andso could be used as markers to locate potential sitesof future catastrophic landslides. Such precursory
features are interpreted as exterior expressions related
to internal deformational structures (Chigira et al.,2013a; Hou et al., 2014), and however vary in forms
depending on subsurface condition in different kindsof geologic and topographic settings.
In order to progress in understanding these issues
for the hazard assessment by large, catastrophic
landslides, we studied DGSDs and topographic
features that might imply the regional slope
development in a mountainous catchment of theLaonong River catchment, southern Taiwan (Fig. 1).
Geological structures of the DGSDs are notconsidered here and will be an important part of ourfuture study. A geological map of the study area wascompiled from a 1:50,000 geological map (Sung et al.,2000) published by the Central Geological Survey(Ministry of Economic Affairs, Taiwan), a geological
map published by the (Central Geological Survey.,

Introduction

Deep-seated gravitational slope deformations(DGSDs) in tectonically active mountain areas
typically occur by long-term creep and may failcatastrophically as deep-seated landslides. Suchcatastrophic
landslides
usuallyaccompany
voluminous rock or debris avalanches, travel rapidly
for long runouts, and abruptly bring catastrophe towide areal extent (Crosta et al., 2006; Tsou et al.,2011; Chigira et al., 2013a). These unstable slopes
prior to collapse were inherently controlled bydiscontinuities such as bedding planes, foliation, orfaults and fractures (Chigira, 1992; Nichol et al.,2002). Among these, the number of fractures and/orfaults may develop and propagate that play asignificant role in formation of the gravitational
failure associated with long-term river incision(Chigira et al., 2013b; Hou et al., 2014). Therefore,
DGSD may indicate precursory creeping before their
final failure and must be interpreted from the view
point of geology as well as regional slope
development, because which create a topographic
setting of these landslides over wide area.
Studies have shown that predicting potential sitesof the catastrophic landslides is possible with110th Asian Regional Conference of IAEG (2015)

Fig. 1 Map of the study area within the Laonung River catchment. (a) Map showing an elevation image
superimposed on a slope inclination map derived from a 20-m mesh DEM. The DEM was created byresampling 5-m mesh DEMs obtained from the Ministry of Interior, Taiwan. (b) Map showing thedistribution of landslide scars and geological map superimposed on a slope inclination map.
only 3% of the total number, but for 30% of the totalarea.

2010), and additional data obtained from fieldsurveys. The bedrock in the study area is Miocene inage and mainly comprises argillite, slate, and slatewith thin-bedded sandstone. These beds are faulted
along NNE trending axes. The beds of argillite andslate have well developed cleavage, respectively, thatgenerally trend N31°W and dip SW at 38°. The
cleavage plane forming a discontinuity has been
known to be susceptible to gravitational slope
deformation. Similar geological features could be
widely recognized in the Taiwan slate belt (Fig. 1).
2.

Landslides in the study area

This area was severally devastated by Typhoon
Morakot in 2009, when it caused the worst
rain-induced landslides hazards in 50 years (Lin et al.,2011). Four landslide dams were formed when large
landslides blocked the rivers. Total casualties by thisevent in this catchment were 60 people dead andmissing (National Disasters Prevention andProtection Commission, 2009).
Figure 2 shows the cumulative number-area
distributions for 1257 landslide scars (Fig. 1b), thatare identifiable on Google Earth imagery as bareareas, including source and depositional areas. The
landslides were thus delineated using Google Earthimagery taken after the Typhoon Morakot in 2009.
The landslides indentified here are thus the landslides
occurred before and after the Typhoon Morakot.
These landslides with areas span three orders ofmagnitude from 1.9 × 10–4 to 5.0 × 10–1 km2. Largelandslides (i.e. areas larger than 0.1 km2) account for

Fig. 2 Analysis of the cumulative-area distribution forthe landslide scars.
3.

Topographic features and slope movement

3.1 Paleosurface remnants and terraces andslope breaks at their peripheries
Field surveys showed that slopes mostly gentler
than 30° are widely distributed at higher elevations
and dip into the river valleys, surrounded by steep
slopes with convex slope break in many locations(Fig. 3a). These morphological features suggest thatthe gentler slopes at higher elevations are remnants ofa paleosurface which are preserved within themountain ranges bounded by the slope breaks that up
to 400 m above the present river bed. The2

10th Asian Regional Conference of IAEG (2015)

Fig. 3 Examples of distributions of slope breaks and slope movements along the major trunk Laonung River.
(a) Superimposed contours and a classified slope gradient image. The image shows an example of delineationof slope breaks based on a slope gradient map with a geomorphic break value of 30° and field checks.
Numbers indicate the elevation in meters. An arrow indicates the flow direction of the Laonung River. (b)Distribution of the slope breaks, DGSD, and landslide scars.
paleosurface, the age of which is unknown, has been
dissected by the rejuvenation of the trunk river and its
tributaries in response to base-level lowering
probably associated with tectonic uplift of the area.
This relative elevation of about 400 m might act as aproxy for fluvial dissection in the incised valleys.
Hillslopes adjacent to the higher slope break arecharacterized by incised valleys. The valleys contain
subsidiary slope breaks that aligned the treads of aseries of terraces. The terraces show flat morphology(with slope < 15°) of various sizes and heights,
consisting of fluvial terrace and fan terrace developed
where the tributaries join the main river. The
contemporary terrace tread slopes are commonly
steep showing that they have been enhanced byerosion from shallow soil slips.
The distinct features have been attracted muchattention by previous researchers (Tomita, 1972;
Hsieh et al., 2013), however, have received
insufficient attention in terms of slope movements, inparticular the DGSD. We noted that the higher slope
breaks are commonly blurry by the DGSDs anddisconnected by landslides (Fig. 3b), which might
imply that the river rejuvenation controlled the DGSD
distribution. To find such a causal relationship, we

focused on the spatial relationship between the slope
breaks at the rims of the paleosurface and DGSDs(See Section 3.2).

3.2 Distribution of DGSDs
DGSDs, which are characterized by topographic
features such as scarps, hummocky surfaces, andlinear depressions (Chigira, 1992; Dramis andSorrisovalvo, 1994; Chigira et al., 2013a), weremapped by visual interpretation of Google Earthimagery and stereo-pair ALOS/PRISM images (SceneIDs: ALPSMW090153135 and ALPSMB090153190)
with a resolution of 2.5 m.
The DGSDs were mapped along the major trunk
Laonung River posing great size extending from thecrests of gentle ridges to the valley floor. The DGSDsare concentrated along the rims of the paleosurface,which is attributable to the undercutting of slopes byriver incision. The lower parts of the DGSDs have
many slope failures that are along or below the rimsof the paleosurface, suggesting the toes of the DGSDs
are currently being destabilized by them (Fig. 3b).
This relationship might can be observed further
upstream since the topography suggests that river
incision is propagated upstream along trunk and310th Asian Regional Conference of IAEG (2015)

tributary river. Such relationship is consistent withthat seen in other tectonically active mountain basins(Hiraishi and Chigira., 2011; Tsou et al., 2014).
4.

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Conclusions

We investigated a mountainous catchment of theLaonung River to understand the DGSD process
regarding to rive rejuvenation and has reached thefollowing preliminary conclusions. Hillslope
morphology shows that there are palrosurface
remnants widely distributed at higher elevations andincised valleys below then in which contain a seriesof terraces. Slope breaks are present at the boundary
slopes in contact with the slopes between thepaleosurface remnants and the incised valleys andalong the terrace treads. The higher slope breaks are
up to 400 m above the present river bed, acting as aproxy for fluvial dissection in the incised valleys.
The analysis of the spatial relationship between theDGSDs and the higher slope breaks shows that
DGSDs tend to occur along the slope break,suggesting a common mechanism of slope instability,
probably owing to undercutting by river rejuvenation.
Acknowledgements
We are indebted to Prof. Ching-Weei Lin of
National Cheng Kung University and Prof. Su-Chin
Chen and Mr. Yi-Liang Chien and Kui-Yuan Peng of
National Chung Hsing University for useful
discussion and assistance in the field. This work wasfinancially supported by the Sasakawa Scientific
Research Grant from The Japan Science Society.
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