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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
675
DOI: 10.4408/IJEGE.2011-03.B-074
DEBRIS FLOWS AND LANDSLIDES CAUSED BY TYPHOON
MORAKOT IN TAIWAN
C
Hyan
-d
enG
JAN
(*)
, y
u
-C
Hao
HSU
(**)
, J
i
-s
HanG
WANG
(***)
& w
en
-s
Hun
HUANG
(*)
(*) Dept. of Hydraulic and Ocean Engineering, National Cheng Kung Univ., Tainan 70101, Taiwan, R.O.C.
fall duration but also on total area covered with high
rainfall. For example, the maximum 1-hour, 12-hour,
24-hour and 48-hour rainfalls at Alishan rainfall sta-
tion were 123 mm, 934 mm, 1623 mm and 2,361 mm,
respectively. This paper will report the rainfall condi-
tions when the debris flows and landslides occurred
and the characteristics of debris flows in Gaoping riv-
er watershed basing on the data of field investigations
and satellite image analyses.
K
ey
worDS
: Typhoon Morakot, Debris Flow, Landslide, Gao-
ping river watershed, Taiwan
INTRODUCTION
Taiwan is an island of 36,000 km
2
in size, oval in
shape with a length of 394 km and a maximum width
of 144 km. The island was formed by the collision of an
island arc with the Asian continental margin. Orogene-
sis resulted in two-thirds of the island being covered by
rugged mountains and hills, and about 31% of the total
island area has an elevation exceeding 1,000 m (G
io
,
2003). Most mountains are quite steep with slope gradi-
ents exceeding 25° (as shown in Figure 1). The ongoing
orogenesis also causes frequent and strong earthquakes.
In addition, significant denudation results in landslides,
debris flows, and soil erosion in such mountainous
terrain. Landslides generate a large amount of loose,
weathered materials, which are delivered to creeks,
thus forming the dominant sediment to supply mecha-
nism for sediment-related hazards (H
uanG
, 2002).
ABSTRACT
Typhoon Morakot was "born" on August 4, 2009
at approximately 22.4° N and 133.8° E in the North
Pacific Ocean, about 1,000 km far from northeastern
Philippines, moving west at a speed of 10-30 km/hr
towards Taiwan, landing on Hualien, eastern Taiwan
on August 7, and then moving across over northwest-
ern Taiwan on August 8 with a wind speed up to 40
m/s. Unexpectedly, Typhoon Morakot brought severe
rainfalls and caused catastrophic disasters, such as
a large number of debris flows, shallow landslides,
deep-seated landslides, debris dams and inundations
in the mountainous areas of central and southern Tai-
wan. The catastrophic disasters killed more than 700
people. The rainfall brought by Typhoon Morakot was
record-breaking, not only on rainfall amount and rain-
Fig.1 - Geographical location of Taiwan Island in
SPOT image
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CHYAN-DENG JAN , YU-CHAO HSU , JI-SHANG WANG & WEN-SHUN HUANG
676
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
tershed. Over 60% debris flow events and 50% land-
slide areas in Taiwan occurred in that watershed. The
present paper focuses on and studies this area. The
rainfall characteristics, debris flow events, landslide
areas, and the relationship between the rainfall char-
acteristics and debris flows as well as landslide areas
will be discussed by means of rainfall data, field in-
vestigations and satellite image analyses.
STUDY AREA
Gaoping River is the largest river in southern Tai-
wan with a 3257 km2 drainage area and a 171 km long
main stream in southwest Taiwan (as shown in Figure
2). The river originates in the south of the Jade Moun-
tain with high altitude of 3952 m and has four main
tributary drainages, Cishan river watershed, Laonong
river watershed, Ailiao river watershed and Gaoping
downriver watershed, whose drainage areas are 833
km2, 1375 km2, 623 km2, and 414 km2, respectively
Gaoping river watershed receives an annual rain-
fall of 3,000 mm, about 90% of annual rain falls from
May to October, especially during typhoons. Gaoping
river possess a high average slope gradient of 1/150
and high rainfall which result in the highest erosion
rates in the world (10,934 ton/ km2 year) (Chung et
al., 2009). Weak geological conditions, steep slopes,
high potential rainfall and frequent earthquakes make
severe debris-flow hazards and landslides disasters
occur frequently in the watershed with years (see Tab.
1). The sediment-related hazards, debris flows, shal-
low landslides, deep-seated landslides caused about
572 deaths, 29 injured, and about 650 houses damaged
in Gaoping river watershed during Typhoon Morakot.
In this paper, severe debris flow events and landslides
disasters brought by Typhoon Morakot within Gaop-
ing river watershed were analyzed by the examination
of Formosa Satellite II Image and field investigations.
Furthermore, Taiwan possesses plentiful rain-
fall. It receives an annual rainfall of 2,500 mm, and
the maxima rainfall exceeds 5,000 mm in some high
mountainous regions. About 80% of the annual rain
falls from May to October, especially during typhoons.
As the result of heavy rain brought by typhoons, steep
topography, young and weak geological formations,
strong earthquakes, loose soils, and land develop-
ment in mountainous terrain, many areas in Taiwan
are susceptible to landslides and debris flows (TGRU,
2001). Among these contributors, typhoons are espe-
cially the major agent of landslides, debris flows and
sediment-related hazards in Taiwan. On average, three
typhoons hit Taiwan annually. Rainfall intensity dur-
ing some destructive typhoons may exceed 100 mm/
hr and 1,000 mm/24 hr.
On August 8, 2009, Typhoon Morakot landed on
Taiwan, It covered almost the entire southern region
of Taiwan and parts of eastern and central Taiwan and
brought heavy rainfall. A large number of debris flows,
shallow landslides, deep-seated landslides, debris dams
and inundations in the mountainous areas are caused by
its record-breaking rainfall. According to the estimation
of CEOC (Central Emergency Operation Center), Ty-
phoon Morakot resulted in 665 people deaths, 34 people
missing, and about NT$141 billion economic losses.
Typhoon Morakot brought catastrophic damage
in southern Taiwan, especially in Gaoping river wa-
Fig. 2 - Location map of the Gaoping river watershed,
locations of landslides and 20 debris flow events
caused by Typhoon Morakot.
Tab. 1 - Number of severe debris flow events and landslide
areas from 2004 to 2009 in the Gaoping river wa-
tershe
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DEBRIS FLOWS AND LANDSLIDES CAUSED BY TYPHOON MORAKOT IN TAIWAN
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
677
est typhoon that had stricken Taiwan in the past four
decades, Therefore, Typhoon Morakot had not been
considered a serious threat before it struck Taiwan.
However, in contrary to the prediction, after landing
on Taiwan, it caused much more damage than any
other typhoons because of record-breaking severe
rainfall conditions, especially in the mountainous ar-
eas of central and southern Taiwan. Figure 5 shows
the spatial distribution of rainfall depth from August
6 to August 11, 2009. The duration of the rainfall is
about 108 hours in Taiwan.
Among all rainfall stations in Taiwan, the maxi-
mum 1-hour, 6- hour, 12-hour, 24-hour, 48-hour and
72-hour rainfalls were recorded at Alishan Station, lo-
cated above Gaoping river watershed, the rainfall depth
RAINFALL BROUGHT BY TYPHOON
MORAKOT
Typhoons and heavy rainfalls are major agents of
sedimentrelated hazards in Taiwan. The rainfall data
of typhoon events was collected and analyzed from
2004 to 2009, as shown in Figure 3. The results show
that the rainfall characteristics, rainfall depth and rain-
fall intensity have an increasing trend in recent years.
The rainfall depth of Typhoon Morakot reached 2884
mm, which is the largest rainfall depth within a single
typhoon event from 1960 to 2009. According to the
estimation from SWCB (Soil and Water Conservation
Bureau), the number of severe debris flow events and
landslide disasters in Taiwan has increased, as shown
in Table 2.
Typhoon Morakot was born on August 4, 2009
in the North Pacific Ocean, about 1,000 km far from
north-eastern Philippines, moving west at a speed of
10-30 km/hr towards Taiwan, landing on Hualien,
Eastern Taiwan on August 7, and then moving across
over north-western Taiwan on August 8 with a wind
speed up to 40 m/s. The moving path is shown in
Figure 4. Considering wind speed and effect radius
of a typhoon, Typhoon Morakot was not the strong-
Fig. 3 - Rainfall depth and maximum rainfall intensity
brought by Typhoons in Taiwan from 2004 to
2009
Tab. 2 - Number of severe debris flow hazards in Taiwan
from 2004 to 2009
Fig. 4 - Path of Typhoon Morakot ( from August 6 to Au-
gust 10, 2009)
Fig. 5 - Isohyetal map of the 108-hour rainfall during Ty-
phoon Morakot from
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678
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
over a period of 108 hours during Typhoon Morakot.
This total amount equals about 70% of the regional
annual rainfall at Alishan Station. The hourly rainfall
exceeding 78 mm persisted for 12 hours, from 16: 00
on August 8 to 03:00 on August 9, 2009, at Alishan
Station (see Fig. 8). The maximum hourly rainfall ex-
ceeded 120 mm at Alishan and Weiliaoshan rainfall
stations.
LANDSLIDES CAUSED BY TYPHOON
MORAKOT
The strong rainfall during Typhoon Morakot
caused 370 km
2
landslide areas in central and southern
Taiwan. Over 50% landslide areas occurred in Gaop-
ing river watershed, for example, Xiaolin, Xinfa, and
Nanshalu Villages having a lot of severe catastrophic
disasters are also involved in this watershed. The po-
sitions and distributions of landslide areas after Ty-
phoon Morakot were showed in Figure 2.
The landslide areas were confirmed and docu-
mented after each strong typhoon event from 2004 to
2009 by Formosa Satellite II Image analyses and field
investigations. The results showed that the landslide
areas in Gaoping river watershed were at an average
value below 5000 ha/year before 2009. The areas im-
mediately increase up to 18314 ha from 4190 ha, New
landslide areas about 14124 ha were made after Ty-
phoon Morakot, see Table 2. According to the estima-
tion from SWCB (Soil and Water Conservation Bureau,
2009), landslide disasters in Gaoping river watershed
were collected and documented, as shown in Table 3.
The locations of landslide disasters, rainfall records and
damage,adopted from rainfall stations are described.
In Figure 9, the Isohyetal map was plotted in dif-
were 123 mm, 548.5 mm, 934.5 mm, 1,623.5 mm,
2,361 mm and 2,748 mm, respectively (see Figure 6).
The previous rainfall records in Taiwan, 48-hour rain-
fall (1,986 mm) and 72-hour rainfall (2,243 mm) were
broken. The 24-hour and 48-hour recorded rainfall ap-
proach the world records (2,467 mm and 1,825 mm,
respectively). For comparison, Sinan, Welliaoshan,
and Yuyoushan rainfall stations within Gaoping river
watershed were plotted in Figure 6 as well.
Figure 7 shows the rainfall intensity-duration
graph for the estimated return periods for 1-hour to
72-hour rainfall at Alishan Station well exceeded 200
years. In total, 2,884 mm rain fell at Alishan Station
Fig. 6 - Magnitude-duration graph of rainfall at Alishan,
Sinan, welliaoshan and Yuyoushan rainfall sta-
tions during Typhoon Morakot
Fig. 7 - Intensity-duration graph of rainfall at Alishan Si-
nan, welliaoshan and Yuyoushan rainfall stations
during Typhoon Morakot
Fig. 8 - Hourly rainfall measured at Alishan Sinan,
welliaoshan, and Yuyoushan rainfall stations dur-
ing Typhoon Morakot
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DEBRIS FLOWS AND LANDSLIDES CAUSED BY TYPHOON MORAKOT IN TAIWAN
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
679
ferent gradation of blue colors to show the rainfall
depth from 27 rainfall stations records in Gaoping riv-
er watershed during Typhoon Morakot from August 6
to August 11, 2009. According to the estimation from
CWB (C
entRal
w
eatHeR
b
uReau
, 2009), there are 8
rainfall stations with rainfall depth of over 2,000 mm.
The maximum rainfall depth reached to 2,701 mm
at Weiliaoshan Station. Its return periods for 1-hour
to 72-hour rainfall exceeded 200 year. Considerable
rainfall depth, high intensity and long duration caused
a lot of landslides and disasters in Gaoping river wa-
tershed. The most miserable hazard of all landslides
is in Xiaolin Village in Cishan river watershed see
Figure 10 and 11. It caused the deep-seated landslides
with 190 ha landslides and 160 ha sediment deposited
which destroyed 200 houses, 3 bridges, 3 km road,
490 deaths, and about 3.5 km
2
inundation areas. Ty-
phoon Morakot also brought significant landslide haz-
ards in other villages as well, see Tab.3.
The relationship between the rainfall depth and
landslide areas is analyzed as well as topographic con-
ditions to understand the impact of Typhoon Morakot.
By mans of the Isohyetal map (in Fig. 9), the repre-
sentative rainfall depth can be calculated from iso-
hyetal map and landslides map in overlap program
with GIS (Geographic Information System) system.
The rainfall depth and landslide areas in the
Gaoping river watershed have been analyzed in Ta-
ble 4. The results show the landslide areas have an
Fig. 9 - Isohyetal map of the 108-hour rainfall during Ty-
phoon Morakot from August 6 to August 11, 2009
Tab.3 - Landslde disasters bought by Typhoon Morakot
in Gaoping river watershed
Fig. 10 - The catastrophic landslides and debris flow haz-
ards in Xiaolin Village and Xinfa Village in Gaop-
ing river watershed
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CHYAN-DENG JAN , YU-CHAO HSU , JI-SHANG WANG & WEN-SHUN HUANG
680
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
increasing trend with rainfall depth. In Figure 12, the
maximum landslide areas is 72.8 km
2
with rainfall
depth of 1900~2200mm. The maximum landslide rate
is 11.6% with rainfall depth over 2500 mm. The high
landslide rate indicates that landslide areas within
high Isohyetal areas are more than low Isohyetal.
The slope and landslide areas are selected to
study as well. From Figure 13, the results show that
the landslide areas increase as the slope rises. While
the slope is between 24.8° and 45.0°, both landslide
area and landslide rate are at maximum values, which
are 109 km
2
and 8.83%, respectively. The relationship
between elevation and landslide areas was shown in
Figure 14, in which the major landslides took place
at the elevation from 400 m to 2000 m, and the maxi-
mum landslide rate is within 800~1000 m, 12%.
DEBRIS FLOWS CAUSED BY TYPHOON
MORAKOT
Typhoon Morakot brought heavy rainfall which
triggered 45 debris flows in central and southern
Taiwan. About 45% debris flows took place in Gaop-
ing river watershed. These severe debris flow events
caused serious disasters to some villages in this wa-
Tab. 4 - The relationship between Isohyet areas and land-
slides areas
Fig. 12 - The relationship between accumulated rainfall
and landslides in Gaoping river watershed
Fig. 11 - The catastrophic deep-seated landslides in Xiaolin
Village in Gaoping river watershed after Typhoon
Morakot
Tab. 5 - Consequences of Typhoon Morakot in kaoping
river watershed
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DEBRIS FLOWS AND LANDSLIDES CAUSED BY TYPHOON MORAKOT IN TAIWAN
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
681
river watershed), where 20 debris flow disasters oc-
curred. The most wretched hazard of all debris flows
is in Xinfa Village in the Laonong river watershed (see
Figg. 10 and 15). The debris flows with 20 ha land-
slides and 1,000,000 sediments deposited which dam-
aged 80 houses, destroyed 1 km roadway, 41deaths.
Furthermore, Typhoon Morakot also brought signifi-
cant debris flow hazards in other villages, see Tab.5.
The running path and widespread sediment deposition
volume of debris flows play an important role to en-
danger the habitants who live close to the debris flow
potential streams, high probability of debris flow oc-
currence streams. In order to further understand the
debris flow deposited volume, the catchment areas
and the debris flow deposited volume are selected to
analyze and estimate the relationship of catchment ar-
eas and deposited volume (f
Ranzi
& b
ianCo
, 2001).
The debris flow deposited volume is roughly pro-
portional to the catchment in Chenyoulan river wa-
tershed, and get the empirical functions at low limit
tershed, such as, Nanshalu, Xinfa, and Maya Villages.
The positions and distributions of debris flow events
after Typhoon Morakot were showed in Figure 2.
The debris flows were confirmed and documented
after each single strong typhoon event hit Taiwan from
2004 to 2009 by Formosa Satellite II Image analyses
and field surveys. The results showed that the number
of severe debris flows in Gaoping river watershed was
below 5 before 2009. However, the number increases
up to 20 from 6 after Typhoon Morakot, see Table 1.
According to the outcomes of SWCB, debris flow
disasters in Gaoping river watershed were estimated
and documented after Typhoon Morakot, as shown in
Table 5. The locations, rainfall records, and damages
of debris flow disasters adopted from rainfall stations
have been described and calculated.
From the rainfall data brought by Typhoon Mora-
kot in Fig. 9 or Tab.5, the record-breaking rainstorm
mainly concentrated on the upper region of Gaoping
river watershed (Laonong river watershed and Cishan
Fig. 13 - The relationship between slop and landslides
in Gaoping river watershed
Fig. 14 - The relationship between elevation and land-
slides in Gaoping riverwatershed
Fig. 15 - The debris flow hazard in Xinfa village in the
Laonong river watershed in Gaoping water-
shed after Typhoon Morakot
Fig. 16 - The relationship between watershed area and
debris flows deposited volume in Chenyoulan
and Gaoping river watershed
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CHYAN-DENG JAN , YU-CHAO HSU , JI-SHANG WANG & WEN-SHUN HUANG
682
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
the Kaoping river watershed. The most catastrophic
landslide and debris flow hazard could be observed in
Xinfa Village (Xinkai Community) and Xiaolin Vil-
lage (Kaohsiung County).
Extreme rainfall events and recurrent debris
flows have had significant socioeconomic conse-
quences whose impacts on the wealth of Taiwan
may eventually exhaust the available resources to
cope with the aftermath of such disasters. Although
the central government has been making efforts to
develop debris flow and landslide countermeasures
since Typhoon Herb, many debris flow and land-
slide mitigation measures and warning systems are
still inadequate. For evacuation in the case of emer-
gency, it will be necessary to enhance public aware-
ness of many debris flow and landslide hazards and
educate people on how to react to debris flow and
landslide hazards.
ACKNOWLEDGEMENTS
The writers appreciate the supports from the Na-
tional Science Council in Taiwan (NSC 99-2625-M-
006-002), and from the Soil and Water Conservation
Bureau Council of Agriculture, Executive Yuan in Tai-
wan. The writers are also grateful to the reviewers for
their constructive comments and suggestions.
V= 50A and up limit V= 2,000A, as showed in Figure
16 (J
an
& C
Hen
, 2005). To comprehend the impact of
Typhoon Morakot on debris flow deposited volume in
Gaoping river watershed, the deposited volume events
during Typhoon Morakot were plotted in Figure 13
.The results show that the low and up limits in Figure
13 have an increasing trend, which are V= 500A and
V= 7,500A, respectively.
CONCLUSIONS
The sediment-related hazards have been more
severe due to extreme rainfall events. On August 8,
2009, Typhoon Morakot stroke Taiwan and brought
huge amounts of rainfall. The rainfall of Typhoon
Morakot is found to be high-intensity and long-
duration. All Taiwan was covered under the rainfall.
Severe disasters, including landslides, debris flows,
landslides dams, and floods, occurred in the Central
and Southern Taiwan. Large amount of sediments and
driftwoods have severe impacts on downstream rivers
and reservoirs The heavy rains during Typhoon Mora-
kot triggered 27 debris flows and 370 km
2
landslide
areas in the central and southern Taiwan. The 60%
debris flows and 50% landslide areas occurred in the
Kaoping river watershed. Typhoon Morakot triggered
31 large cases of debris-flow and landslide hazards in
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