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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
695
DOI: 10.4408/IJEGE.2011-03.B-076
THE CASE STUDY OF DEBRIS FLOW HAZARD CAUSED BY TYPHOON
MORAKOT IN TAIWAN, 2009
m
eei
-l
inG
lin
(*)
, k
uo
-l
unG
wanG
(*)
, t
ien
-C
Hien
CHen
(**)
& s
Hen
-C
Hi
lin
(***)
(*)
Department of Civil Engineering, National Taiwan University, Taipei, Taiwan
(**)
Department of Soil and Water, National Pingtung Science and Technology, Pingtung, Taiwan
(***)
Department of Civil Engineering, National Taiwan University, Taipei, Taiwan
wan on August 8, with the monsoon system drawn in
by the Typhoon, continuous high-intensity rainfall re-
sulted in accumulated rainfall as high as 2860 mm in
three days. High accumulated rainfall was measured at
Yushan and Alishan with the recorded rainfall of Al-
ishan station as high as 3059.5mm from 0AM, 5, Au-
gust to 11PM, 10, August, 2009. Typically the highest
rainfall intensity appeared from 8 August to 9 August
and resulted in severe hazards, especially the landslide
and debris flow hazards in mountainous area. Most
landslide hazards occurred during the period from the
afternoon of 8 August to early morning of 9 August as
the rainfall intensity rose. Soil and Water Conservation
Bureau documented 1349 landslide hazards including
46 debris flow cases. The landslide area identified using
FORMOSAT II satellite images by Central Geological
Survey is more than 50000 hectares. Debris flow usu-
ally induces severe hazard due to direct impact to local
communities, thus severe debris flow cases were select-
ed for case study. Field investigation, rainfall records,
damage records, hydrology data collection and analysis
were conducted. Accordingly, the thematic map was
generated and causal factors were discussed. Selected
severe debris flow cases in this research include: Nan-
shalu, Maya, and Dakanuw villages of Namasha Town-
ship, Shinkai and Shinfa area of Liudui Township,
Kaohsiung County, and Chianghuangkern of Nanhua
Township, Tainan County. Results of this study can
provide information for mitigation of secondary hazard
and drafting of mitigation strategy.
ABSTRACT
Typhoon Morakot struck southern Taiwan on Au-
gust, 8, 2009 with high rainfall intensity and accumu-
lated rainfall as high as 2860 mm for 72 hours. Severe
landslides and debris flow hazards were induced. The
debris flow cases resulted in severe impacts to local
communities were selected for case study, for which
field investigation and analysis were conducted. Most
severe landslide and debris flow cases originated from
the rainfall center, which is Alishan mountain, and
distributed with decreasing elevation. The analysis of
debris cases were selected by category of watersheds
including: Nanshalu, Maya, Dakanuwa villages of Na-
masha Township, Shinkai and Shinfa areas of Liugui
Township, Kaohsiung County, and Chianghuangkern of
Nanhua Township, Tainan County, Taiwan. Data analy-
sis of the rainfall records suggested that a linear thresh-
old for triggering of the debris flow could be defined,
and the debris flow associated with low accumulated
rainfall usually occurred at the high rainfall intensity,
but involving smaller amount of debris transportation.
While debris flow occurred at high rainfall accumula-
tion with low intensity usually involved large amount
of debris, which cause sever hazard.
K
ey
words
: Typhoon Morakot; debris flow; case study;
threshold rainfall
INTRODUCTION
Typhoon Morakot struck central and southern Tai-
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M.-L. LIN, k.-L. wANG, T.-C. CHEN & S.-C. LIN
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for the basins originated from these mountains. The
affected areas include Chenyulan river watershed of
Juoshuei river watershed, Bazhang river watershed,
Jenwen river watershed, Chishan river watershed,
Laolun river watershed, Ailiao river watershed, and
Taimali river watershed. Severe landslide, debris flow
and flooding hazards occurred in each affected water-
shed; the distribution of landslide hazards and affected
watersheds are as shown in Figure 2. Figure 2 illus-
trated the distribution of identified severe landslide
cases with accumulated rainfall. Observing Figure
2, it was found that the locations of severe landslide
and debris flow hazards are consistent with high accu-
mulated rainfall area. Several sites located in Taitung
county in the east part of Taiwan as shown in Figure
2 which appeared to be different from the trend of
rainfall distribution.
Careful examinations revealed that those cases
were debris flows originated from the landslides oc-
curred upstream within the area of heavy rainfall. The
results suggest that the major factor causing landslide
hazards induced by typhoon Morakot is high accumu-
lated rainfall. The distribution of high rainfall area and
landslides were plotted along with topographic map
as shown in Figure 3. In Figure 3, it appeared that
the elevation increased rapidly in the high mountain
areas, and such rugged terrain induced topographical
effects which caused high rainfall on high elevation
CHARACTERISTICS OF DEBRIS FLOW
HAZARD
Due to the rainfall carried by Typhoon Morakot,
roads and bridges in the mountain area were severely
damaged. According to the investigation of Directo-
rate General Highway (DGH), damages were caused
mainly by debris flow and landslides as shown in Fig-
ure 1. The rainfall concentrated at Yushan Mountain to
Alishan Mountain and the effects radiated downstream
Fig. 1 - Locations of different types of hazards (from
DGH, 2009)
Fig. 2 - The distribution of landslide hazards and watersheds
with rainfall from August 5 to 10
Fig. 3 - The distribution of landslide hazards with rainfall
from August 5 to 10, 2009 overlaid with topography
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THE CASE STUDY OF DEBRIS FLOW HAZARD CAUSED BY TYPHOON MORAKOT IN TAIWAN, 2009
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
697
and buildings and resulted in 16 people dead, 25 peo-
ple missing, and 80 buildings were damaged. Debris
flow occurred in Maya village at 4 pm, 8, August and
attacked buildings and Sanming junior high school.
About 500 m of the road was covered with debris and
15-20 buildings were damaged. Another debris flow
near Aboriginal Museum destroyed 3 buildings and
about 400 m of road was covered with debris. Debris
flow also occurred in Dakanuwa village and destroyed
the access road to the village. Damages caused by
debris flow were typically located at the lower river
terrace. Communities situated at higher river terraces
were not hit as severely. However, nine bridges in
total along the Highway No. 21 were destroyed by
debris flows, which blocked the access roads to the
three villages and caused isolation and difficulties for
emergency responses and mitigation efforts.
The aerial photo of the Nanshalu village after ty-
phoon Morakot is as shown in Figure 5. Two debris
flows hit the village. The buildings in the village were
buried by debris and boulders as shown in Figure 6.
The roads and riverbed are difficult to identify after
being struck by debris flow especially for the debris
and steep slope. Significant number of severe de-
bris flow hazards located at the west part of Central
Mountain Range, and were mainly in Sanshia group
formation, which included sandstone, shale, and inter-
layered sandstone and shale. The shale is poorly ce-
mented with weak strength and easily crashed when
weathered. Debris was often found filled with clayey
materials during field investigation. These fractured
materials with low shear strength could easily induce
landslide hazards. Severe debris hazards located at
the south and east parts of Central Mountain Range
in Pingtung County and Taitung County are mainly
in Lushan formation. Lushan formation is mainly
slate with highly developed cleavages, which have
significant effects on the strength of the material. The
highly fractured geological condition is one of the
major factors to result in severe landslide hazards
DEBRIS FLOW IN NAMASHA TOWN-
SHIP, KAOHSIUNG COUNTY
Namasha township locates at the north of Ka-
oshiung county with Alishan township in north. Nan-
jishian river flows through the township connecting
Kaoping river in the downstream. Three villages, i.e.
Nanshalu, Maya, and Dakanuwa villages of Namasha
township suffered severe debris flow hazard, and the
distribution of villages in Namasha township is as
shown in Figure 4.
Debris Flow Hazards
The heavy rainfall carried by typhoon Mora-
kot resulted in debris flow at Nanshalu, Maya, and
Dakanuwa villages. Several debris flows occurred
in Nanshalu village since afternoon of August 8,
according to SWCB (2009; b), which destroyed roads
Fig. 4 - The location of the three villages in Namasha
township
Fig. 5 - Aerial photo of Nanshalu village after typhoon
Morakot (provided by National Science and Tech-
nology Center for Disaster Reduction, NCDR)
Fig. 6 - Debris covered entire Nanshalu village (photo:
Meei-Ling Lin)
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Dakanuwa village is consisted with two main resident
areas as shown in Figure 10. The resident areas were
not damaged by debris flow but the access roads were
completely destroyed. The villagers were evacuated
for more than a month due to the roads blockage.
HAZARDOUS FACTORS ANALYSIS OF DEBRIS
FLOw
The accumulated rainfall record of Minshen sta-
tion in Namasha Township was 1778 mm and the
maximum rainfall intensity is 96 mm/hr during ty-
phoon Morakot as shown in Figure 11. The accumu-
lated rainfall and intensity were high which induced
landslide and followed by debris flow. The eleva-
tion varied as much as 1700 meter in the Namasha
Township. Main geological formation of these villag-
es is Sanshia group and formation of Nanjizian river
is mainly Ruifang group. Sanshia group is mainly
consisted with sandstone, shale, and inter-layered
sandstone and shale. Ruifang formation in this area
is formed by Nangang formation, which is consisted
of dark grey shale. The upper part of Nangang forma-
tion is mainly structured by thin layered shale with
thin sandstone. The geological condition in this area is
easy to fracture which results in landslide after heavy
rainfall. Most resident areas of these villages locate
at lower river terrace nearby Nanjishian river except
flow torrent to the south of the village as observed in
Figure 5. The significant change in terrain was caused
by the landslides upstream of the river forming a
dammed-up lake, and with the bursting of the lake at
5pm of August 9, huge amount of debris was flushed
out and carried by the debris flow.
The aerial photo of the Maya village after typhoon
Morakot is as shown in Figure 7. Two debris flows at-
tacked the village from the north and south, and debris
deposited in the Sanming junior high school and main
resident area of this village. The Mingquan elemen-
tary school was filled with debris as shown in Figure
8. Moreover, the highway No. 21 was covered with
debris about 3 meters thick.
The aerial photo of Dakanuwa village is as shown
in Figure 9. Debris flows occurred at the north and
south part of this village. The Markalung area at the
right hand side of the Nanjishian River opposing the
Dakanuwa village was also struck by severe debris
flow. The roads and bridges connecting the Marka-
lung area and the Dakanuwa village were destroyed
by the debris flow and flood of the Nanjishian River.
Fig. 7 - Aerial photo of Maya village after typhoon Mora-
kot
Fig. 8 - Debris flow occupied Minquan elementary school
(photo: Meei-Ling Lin)
Fig. 9 - Aerial photo of Dakanuwa village after typhoon
Morakot (provided by NCDR)
Fig. 10 - The location of Dakanuwa village and surround-
ing debris flow (photo: kuo-Lung wang)
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THE CASE STUDY OF DEBRIS FLOW HAZARD CAUSED BY TYPHOON MORAKOT IN TAIWAN, 2009
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The Tsaihun Temple in the Upper-Sinkai was
struck by debris at about 8:50 pm, August 8. The
in-situ debris flow condition is as shown in Figure
14. The height of landslide is about 250m and trav-
elling distance is about 350 m. The debris flow was
blocked by a big Budda statute and then redirected
into the local community. The length of debris depo-
sition is about 150m covering an area of about 0.5
hectare. Local residents were evacuated before de-
bris flow occurred.
Lower-Sinkai was struck by debris flow at about
8:30pm, August 8. Four people were killed and 24
people were missing during this event. The travelling
velocity of debris flow in this event is too fast to react.
The resident area of Lower-Sinkai is an old deposition
fan and the elevation is 20 m lower than Upper-Sinkai.
The height and width of landslide are 450m and 1200m,
respectively. The length of debris deposition is about
300 m and covering an area of about 20 hectares. The
width of the stream increased to 200 m and the width of
deposition fan is about 600 m. More than 10 buildings
were buried by the debris deposition as shown in Fig-
ure 15. The deposited material flowed into Launon river
and pushed the river channel, which resulted in erosion
of embankment across the river.
Dakanuwa village. Thus the communities were se-
verely hit by debris flow.
DEBRIS FLOW IN LIUGUI TOWNSHIP, KAOHSI-
UNG COUNTY
The Shinkai and Shinfa areas are located in Shinfa
village, Liugui township, Kaohsiung county. The main
road across the area is Highway No. 27 (Shinfa road),
which connects to Highway No. 20 and northbound
toward South Trans-island Highway and southbound
toward Pingtung county. The topographical map of
Shinkai and Shinfa areas is as shown in Figure 12.
The elevation in the area varies from 300m to 1350m.
There is no severe landslide hazard in recent decades
except some soil erosion and flooding conditions dur-
ing typhoon Kalmaegi in 2008.
Debris Flow Hazards
Slope-type debris flow occurred at Upper-
Shinkai, Lower-Shinkai, and Meilunshan stream,
while stream-type debris flow occurred at Bashi-
lan river and Bulau river during typhoon Morakot.
Distribution of the debris flows and landslides of
this area is as shown in Figure 13. The Sinfa com-
munity was struck by debris flow at 8:30 pm, Au-
gust 8, and the Tsaihun temple was struck by debris
flow at 8:50pm at the same date. The outlet of val-
ley, deposition fan and buildings nearby river bank
were damaged by debris flow. Four people were dead
and 24 people were missing at Lower-Sinkai debris
flow site. More than 10 buildings were buried by de-
bris. Bridges connecting the areas including Sinbao
bridge and Sinkai bridge were destroyed by debris
flow. Moreover, Sinfa bridge was damaged by debris
flowing from Bashilan river to Launon river. Sinfa
village was separated into several smaller isolated
areas and lost connections to these areas.
Fig. 11 - Rainfall chart of Minshen station during typhoon
Morakot
Fig. 12 - Location and topography of Shinkai and Shinfa
Fig. 13 - Landslide and debris flow conditions of Sinkai
area (photo: Tien-Chien Chen)
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HAZARDOUS FACTORS ANALYSIS OF DEBRIS
FLOw
The total rainfall in this area is 2418 mm dur-
ing typhoon Morakot. The highest rainfall intensity
occurred at 1pm, August 8, is 103 mm/hr. The rainfall
record in this area is as show in Figure 16. The rain-
fall intensity higher than 50 mm/hr has lasted up to
10 hours.
The Tulunwan fault passed through the Sinfa vil-
lage with lightly metamorphic rock in the east side
and sedimentary rock in the west side of the fault.
The lightly metamorphic rock belongs to lower part
of Changshan formation which consists of slate and
thin sandstone. The fissures and joints are well devel-
oped due to the fault and the material strength is weak.
The main dip of the stratum is 30~50 degrees north
to west with the dip direction along the slope sliding
direction. The valley is eroded and the topography is
mostly steep slope. The river terrace is deposited right
next to the steep slate slope and the deposition materi-
als are transported from upstream areas. Deposition
fans are very common at the outlet of each stream.
The original resident area located at higher river
terrace. Lower river terrace area was developed af-
ter hot spring was found. The new residences and
hotels were located on the deposition fan of stream
including Lower-Sinkai and Chinshan hot spring ho-
tel. The location is originally hazardous area of debris
flow deposition fan. The flooding of Launon river and
debris flow destroyed connecting bridges including
Sinbao, Sinkai and Sinfa bridges. The isolated con-
dition of each resident areas resulted in more severe
conditions.
DEBRIS FLOW IN NANHUA TOWNSHIP,
TAINAN COUNTY
Nanhua township locates at the east part of Tainan
county with Kaohsiung county to the east and south.
Chianghuankern is located in Yushan village of Nan-
hua township. The elevation in the watershed varies
from 210m to 668m. The major road connecting the
site is Highway No. 20. The location and topography
of this area is as shown in Figure 17
.
DEBRIS FLOw HAZARDS
Landslide area in this event covers an area of
about 1.9 hectares according to SWCB (2009a; b).
The stream bed was eroded and the deposition started
at the middle part of stream. The debris flow destroyed
several buildings in the evening of August 8. Local
residents were evacuated before debris flow struck. 15
buildings were damaged and 5 buildings were severe-
ly destroyed. The length of road covered with debris
was about 170m. The width and length of deposition
area were 110m and 150m, respectively. Figure 18 and
Fig. 14 - Debris behind Tsaihun temple (photo: Tien-Chien
Chen)
Fig 15 - Deposition condition of lower Sinkai debris flow
(photo: Tien-Chien Chen)
Fig. 16 - Rainfall chart of Shinfa station during typhoon
Morakot
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THE CASE STUDY OF DEBRIS FLOW HAZARD CAUSED BY TYPHOON MORAKOT IN TAIWAN, 2009
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
701
earthquake. The total affected area is more than 50000
hectares, which is more than the 30000 hectares of
affected area induced directly by the Chi-Chi earth-
quake. The regions struck severely by Typhoon Morak
mainly located in southern Taiwan, which were not
significantly affected by the Chi-Chi Earthquake in
1999. Some hazardous area of Chi-Chi Earthquake
such as the Chenyulan River watershed and upstream
of Chinsuei River watershed were severely struck
comparing with other hazardous areas of Chi-Chi
earthquake. It was observed that some severe hazard-
ous areas in Typhoon Morakot event were recurring
after Typhoons Haitang in 2005, Kalmaegi, and Sin-
laku in 2008. Still most of the hazards caused by ty-
phoon Morakot did not have previous hazard history
in the past decades.
The rainfall records of the debris flow cases in
this study were obtained as discussed previously.
Based on the reported time of debris flow occurrence,
the accumulated rainfall and rainfall intensity at the
time of occurrence was determined for each case and
plotted in Figure 21. A good linear correlation of data
distribution was observed and a linear regression
line was plotted as shown in Figure 21. Such linear
relationship could serve as the triggering threshold
Figure 19 illustrate the overflow position of debris
flow and the buildings destroyed. The original stream
channel of the debris flow path is as shown in Figure
18. However, after the channel was filled with debris,
the debris overflowed in the trajectory direction and
flowed downward in a straight-line direction toward
the lower elevation area as illustrated in Figure 19.
HAZARDOUS FACTORS ANALYSIS OF DEBRIS
FLOw
The total rainfall during Typhoon Morakot is
1326 mm and the highest rainfall intensity is 93 mm/
hr as recorded by the nearby Beiliao station shown in
Figure 20. The elevation of the area decreases from
south-east to north-west. The average slope angle in
this watershed is about 22.5 degrees, which implies
steep slope in the up-stream area. The geological for-
mations in this watershed are mainly consisted with
Tangen sandstone and Beiliao shale. Pingshi fault
passes through the watershed, resulting in fractured
materials and weak strength of rocks. According to
the field investigation, there are some large rocks and
thick debris deposit located within the resident area
before the debris flow occurrence, which suggests that
the resident area has been on an old debris deposition
area of debris flow.
DISCUSSIONS
Due to the high precipitation of Typhoon
Morakot, the accumulated rainfall as high as 2000
mm was observed in many areas, which corresponded
to a return period of more than 300 years according
to the available weather records. The magnitude of
hazards caused by such huge rainfall was larger than
many other events in the past 10 years after Chi-Chi
Fig. 17 - Location and topography of Chianghuankern,
Yushan Village, NanhuaTtownship
Fig. 18 - Building was damaged by debris flow (photo:
Meei-Ling Lin)
Fig. 19 - Downstream of overflow position (photo: Meei-
Ling Lin)
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M.-L. LIN, k.-L. wANG, T.-C. CHEN & S.-C. LIN
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
Shinkai and Shinfa areas of Liugui Township, Ka-
ohsiung County, and Chianghuangkern of Nanhua
Township, Tainan County. Data analysis of the rainfall
records suggested that a linear threshold for triggering
of the debris flow could be defined, and the debris
flow associated with low accumulated rainfall usually
occurred at the high rainfall intensity, but involving
smaller amount of debris transportation. While debris
flow occurred at high rainfall accumulation with low
intensity usually involved large amount of debris,
which cause severe hazard.
The communities struck by debris flows were
mostly located at low river terrace near the debris
deposition area. Moreover, the roads and bridges
connecting the residential areas also passed through
the low river terrace. Both factors caused the risks
and damages to increase during this event. For
the comprehensive mitigation strategy, detailed
investigation of the potential hazardous areas is
required for relocation of communities and connecting
roads and bridges.
ACKNOWLEDGEMENT
The investigation was sponsored by National Sci-
ence and Technology Center for Disaster Reduction
(NCDR), and Soil and Water Conservation Bureau
(SWCB). The data and helps provide by the Direc-
torate General Highways, Township and County gov-
ernment are acknowledged.
rainfall for the debris flows in the study area (H
onG
et alii, 2005, b
Rand
, et alii. 1984, w
ieCzoRek
, 1987).
Observing the data, it was found that the debris
flows occurred with large rainfall intensity usually
occurred at the earlier stage of rainfall record, and
with smaller amount of debris transportation. While
the debris flows occurred at the later stage of the
rainfall record, large amount of rainfall accumulated
with not so large rainfall intensity. In this stage, the
total rainfall accumulation in the watershed is high
and often induced large scale landslides, which lead
to large amount of debris deposition in the upstream
area of the basin. In such cases, the debris flow ended
up involving large amount of debris, which caused
a very severe hazard. The data point of Nanshalu-4
corresponds to the large scale debris flow triggered
by the upstream landslides which dammed-up the
river, and the dammed-up lake burst subsequently and
caused the large-scale debris flow as discussed previ-
ously. The debris flowed downstream and overflowed
to the community located near the outlet of stream.
Some landslide debris still deposit in the upstream
area, and which could be triggered to cause the sec-
ondary hazard.
CONCLUDING REMARKS
Typhoon Morakot struck southern Taiwan and
caused severe landslides and debris flow hazards in
2009. Case study of debris flow resulting into severe
impacts to local communities was conducted. Most
severe landslide and debris flow cases originated from
regions close to the center of high rainfall area, i.e. the
Alishan Mountain, and distributed at the front face with
decreasing elevation. The cases selected for analysis
were by category of watersheds including: Nanshalu,
Maya, Dakanuwa villages of Namasha Township,
Fig. 20 - Rainfall record of Beiliao station during Typhoon
Morakot
Fig 21 - Accumulated rainfall and rainfall intensity
triggering debris flows
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703
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