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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
31
DOI: 10.4408/IJEGE.2011-03.B-004
RAINFALL CONDITIONS FOR THE INITIATION OF DEBRIS FLOWS
DURING TYPHOON MORAKOT IN THE CHEN-YU-LAN
WATERSHED IN CENTRAL TAIWAN
J
inn
-C
Hyi
CHEN
(*)
, w
en
-s
Hun
HUANG
(**)
, C
Hyan
-d
enG
JAN
(***)
& y
uan
-f
an
TSAI
(****)
(*)
Associate Professor, Department of Environmental and Hazards-Resistant Design, Huafan University, Taipei 22301, Taiwan
(**)
Ph.D. Candidate, Department of Hydraulic and Ocean Engineering, National Cheng Kung University, Tainan 70101, Taiwan
(***)
Professor and director, Department of Hydraulic and Ocean Engineering, National Cheng Kung University, Tainan 70101,
(****)
Professor and director, Department of Social and Regional Development, National Taipei University of Education, Taipei
10671, Taiwan
cal formations, strong earthquakes (25 earthquakes
with magnitude greater than 6 on the Richter scale
in the 20th century), loose soil, and land develop-
ment in mountainous terrain, many areas in Taiwan
are susceptible to landslides and debris flows (TGRU,
2001; J
an
& C
Hen
, 2005; C
Hen
et alii, 2009), espe-
cially in the study area of the Chen-Yu-Lan water-
shed in central Taiwan. Many debris-flow events
in the watershed have been documented (e.g., y
u
& C
Hen
, 1987; y
u
, 1997a,b; DPRC, 2000; DPRC,
2001; Lin & Jeng, 2000; C
HenG
et alii., 2005; J
an
& C
Hen
, 2005). The more severe events were those
caused by Typhoon Herb in 1996 and Typhoon Toraji
in 2001, as indicated in Table 1. On August 7, 2009, a
moderate Typhoon Morakot landed on Taiwan. After
Morakot landed in the midnight of August 8, almost
the entire southern region of Taiwan (Chiayi, Tainan,
Kaohsiung, and Pingtung counties) and parts of east-
ern and central Taiwan (Taitung and Nantou counties)
were flooded by record-breaking heavy rain. Massive
flooding have destroyed bridges and roads, and sub-
merged many homes forcing residents to evacuate
and stay in shelters. Typhoon Morakot brought cata-
strophic damage in Taiwan, leaving 665 people dead
and 34 others missing, and roughly NT$141 billion
(US$4.4 billion) in damages. Like previous typhoons
events, Typhoon Morakot also caused several debris
flows in the the Chen-Yu-Lan watershed.
This study presents the critical rainfall conditions
for the initiation of debris flows in the Chen-Yu-Lan
ABSTRACT
In the August of 2009, Typhoon Morakot brought
heavy rainfall and caused several landslides, floods
and debris flows in central and southern Taiwan. This
work presents the critical rainfall conditions for the
initiation of debris flows in the Chen-Yu-Lan water-
shed of central Taiwan during the action of Typhoon
Morakot, where had many severe debris–flows events
during the actions of Typhoon Herb in 1996 and Ty-
phoon Toraji in 2001. Thirty-seven debris flow events
induced by the heavy rainfalls brought by Typhoon
Morakot in the Chen-Yu-Lan watershed were iden-
tified by the examination of aerial photographs and
field investigations. The rainfall conditions, including
the rainfall intensity, the cumulative rainfall, the rain-
fall intensity-duration curve and the return period of
the rainfall, for debris-flows initiation were analyzed
and compared with the rainfall conditions of histori-
cal debris-flows events in the watershed. The average
rainfall intensity that triggered abundant debris flows
in the watershed were determined and the relation be-
tween the density of debris-flows occurrence and the
rainfall characteristics were also presented.
K
ey
words
: rainfall condition, debris flow, Chen-Yu-Lan wa-
tershed, Typhoon Morakot
INTRODUCTION
As a result of heavy rain accompanying ty-
phoons, steep topography, young and weak geologi-
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J.-C. CHEN , w.-S. HUANG , C.-D. JAN
& Y.-F. TSAI
32
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
occur frequently in the watershed. Over ten rainfall
events caused numerous debris flows in the past fif-
teen years. The more severe debris-flow events oc-
curring in the Chen-Yu-Lan watershed were brought
by Typhoon Herb in 1996, Typhoon Toraji in 2001
and Typhoon Morakot in 2009. In this paper, 37 de-
bris flows brought by Typhoon Morakot within the
Chen-Yu-Lan stream watershed were identified by the
examination of aerial photographs and field investiga-
tions, as shown in Figure 1.
RAINFALL BROUGHT BY TYPHOON
MORAKOT
Typhoon Morakot formed early on August 2,
2009 as a tropical depression over the Pacific Ocean
about 1,000 km east of the Philippines. It then moved
west towards Taiwan, and landed on August 7 near
Hualien city, eastern Taiwan. After Morakot landed
in the midnight of August 8, almost the entire south-
ern region of Taiwan (Chiayi, Tainan, Kaohsiung, and
Pingtung Counties) and parts of Taitung County and
Nantou County were flooded by heavy rain. Hourly
rainfall measured at 4 rain-gauge stations, including
Longshen Bridge, Xinyi, Shenmu Village, and Al-
ishan, in the Chen-Yu-Lan stream watershed during
Typhoon Morakot were collected. Locations of these
rain-gauge stations covering the downstream and up-
stream of the watershed were indicated in Figure 1.
The largest rainfall record appears near headwaters
of the Chen-Yu-Lan watershed at the Alishan station
and the smallest one appears at downstream of the
watershed at the Longshen Bridge station. An hourly
rainfall exceeding 60 mm persisted for 13 hours from
15:00 on August 8 to 4:00 on August 9, 2009 at Alis-
watershed of central Taiwan during the action of Ty-
phoon Morakot. The rainfall conditions, including the
rainfall intensity, the cumulative rainfall, the rainfall
intensity-duration curve and the return period of the
rainfall, for debris-flow initiation were analyzed and
compared with the rainfall conditions of historical
debris-flow events in the watershed.
STUDY AREA
The Chen-Yu-Lan watershed is located in Nan-
tou County in central Taiwan, about 12 km east of
Chi-Chi, the town closest to the epicenter of the 1999
earthquake. The Chen-Yu-Lan stream is 42 km long
with a mean gradient of 4°, and elevations ranging
from 310 to 3952 m. The catchment of the Chen-
Yu-Lan stream area is 449 km
2
with a mean annual
rainfall about 3000 mm. The Chen-Yu-Lan stream fol-
lows a major fault, the Chen-Yu-Lan Fault, which is a
boundary fault dividing two major geological zones of
Taiwan. In addition to the boundary fault separating
geological zones, the watershed of the Chen-Yu-Lan
stream also contains many other faults, accompanied
by fractured zones. Consequently, fractured rock mass
prevails over the study area, accounting for enormous
landslides and providing an abundant source of rock
debris for debris flow (l
in
& J
enG
, 2000). Since the
weak geological condition, heavy rainfall and accom-
panying frequent earthquakes, debris-flow hazards
Fig. 1 - Location map of Chen-Yu-Lan watershed, Nantou,
Taiwan, and locations of debris-flow gullies ana-
lyzed in this paper
Fig.2 - Hourly rainfall measured at 4 rain-gauge stations
in the Chen-Yu-Lan stream watershed during Ty-
phoon Morakot. The maximum hourly rainfall at
Alishan rain-gauge station exceeded 100 mm
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RAINFALL CONDITIONS FOR THE INITIATION OF DEBRIS FLOWS DURING TYPHOON MORAKOT IN THE CHEN-YU-LAN WATER-
SHED IN CENTRAL TAIWAN
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
33
Chen-Yu-Lan stream watershed exceeded the design
standard of 50 years. The extreme amount of rain
triggered enormous debris flows and severe flooding.
The consequence caused several river embankments
damaged, bridges crashed, houses buried, and nu-
merous sections damaged along Highway Route 21.
For example, the flood in the Chen-Yu-Lan stream
caused Soshan Bridge crashed and Highway Route
21 broken, and over 20 houses damaged. The Long-
hwa elementary school built near the riverbank of
the Chen-Yu-Lan stream was almost destroyed by
flood. In Shenmu and Tongfu villages, Xinyi Town-
ship, over 20 houses were buried by debris flows or
washed away by floods.
The rainfall produced by Typhoon Morakot,
peaking at 2,747 mm in three days, surpassing the
previous record of 1,736 mm set by Typhoon Herb
in 1996. However, the casualties caused by Typhoon
Morakot were much lighter than those by Typhoon
Herb in 1996, because many structural and non-
structural countermeasures against debris flows
were made after 1996. There was no death in this
Morakot event but 21 deaths in Typhoon Herb in the
Chen-Yu-Lan watershed. Especially, the non-struc-
tural countermeasures for the planning of evacuation
routes and shelters and education of hazard preven-
tion have been conducted in mountainous villages.
Many mountainous villages have conducted debris-
flow disaster prevention and evacuation practices to
enhance inhabitants’ hazard-mitigation knowledge
and emergency-response swiftness as well as their
familiarity with evacuation routes and shelters.
han station (Fig. 2). The maximum rainfall intensity,
123 mm/hr, appears at 1:00 to 2:00 on August 9.
The maximum 1-hour, 12-hour, 24-hour, 48-hour,
and 72-hour rainfall at Alishan station were 123mm,
935mm, 1,624mm, 2,361mm, and 2,747mm, respec-
tively, as shown in Figure 3. The cumulative rainfall
in three days is up to 2,500 mm, exceeding the average
annual rainfall in Taiwan. The previous 48-hour (2280
mm) records in Taiwan were broken. The new 48-hour
rainfall records in Taiwan approach the world records
(2,490 mm)(TGRU, 2001). Figure 4 shows the rain-
fall intensity-duration graphs for Typhoons Morakot,
Toraji and Herb at Alishan station. For comparison,
the corresponding relations for various return periods
T (WRAT, 2001) are also shown. The estimated re-
turn periods form 10-hour to 40-hour rainfall at Alis-
han station during Typhoon Morakot approached 200
years. This record-breaking rainfall is believed to play
a major role in triggering debris flows.
PROPERTY LOSS CAUSED BY DEBRIS
FLOWS
The design codes of culverts, bridges, and riv-
erbanks in mountainous area consider floods with a
recurrence interval of 50 years. The maximum rain-
fall return period brought by Typhoon Morakot in the
Fig. 3 - Rainfall magnitude-duration graph for Typhoon
Morakot at 4 rain-gauge stations in the Chen-Yu-
Lan stream watershed. For comparison, maximum
rainfall magnitude-durations relations in Taiwan
and the corresponding world are shown
Fig. 4 - Rainfall intensity-duration graphs for Typhoons
Morakot, Toraji, and Herb at Alishan station. For
comparison, the corresponding relations for vari-
ous return periods T are also shown
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J.-C. CHEN , w.-S. HUANG , C.-D. JAN
& Y.-F. TSAI
34
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
INITIATION THRESHOLD OF DEBRIS-FLOwS
Rainfall characteristics that are often considered to
affect the triggering of debris flows are rainfall dura-
tion, rainfall intensity, and cumulative rainfall (C
aine
,
1980; k
eefeR
et alii., 1987; w
ieCzoRek
, 1987). The
characteristics of rainfall intensity and rainfall duration
for debris-flow gullies in the watershed are discussed in
this paper. Because not each gully has a local rain gauge
to record the hourly rainfall or cumulative rainfall, the
rainfall at an ungauged point was computed using the
reciprocal-distance-squared method; that is, the influ-
ence of the rainfall at a gauged point on the computation
of rainfall at an ungauged point is inversely proportion-
al to the distance between the two points (C
How
et alii.,
1988; w
ei
& m
C
G
uinness
, 1973). Hydrometeorologi-
cal thresholds of debris-flow initiation can be defined
by the rainfall intensity versus rainfall duration relation
(C
aine
, 1980; k
eefeR
et alii., 1987; w
ieCzoRek
, 1987;
J
an
& C
Hen
, 2005). In the Chen-Yu-Lan watershed,
the maximum return periods of rainfall brought by Ty-
phoons Herb, Toraji, and Morakot were over 100 years
in the Chen-Yu-Lan watershed. These typhoons caused
numerous debris flows and severe properties damages
in the watershed. A rainfall intensity-duration formula
for abundant debris-flows initiation threshold based on
a lower bound of the rainfall data in the Chen-Yu-Lan
watershed from Typhoons Herb, Toraji, and Morakot, is
plotted in Figure 6 as a Line A and can be expressed as
where I
A
is the average rainfall intensity (mm/hr) and
D is the continuous rainfall duration (hr). Equation (1)
reveals that the average rainfall intensity that triggered
abundant debris flows were at least 14 mm/hr for du-
ration 24 hr, 10 mm/hr for 48 hr and 8 mm/hr for 72
hr. The abundant debris-flows initiation threshold are
also compared with the initiation thresholds based on a
lower bound of the rainfall triggered at least one debris
flow before and after the 1999 earthquake, shown in
Figure 6 as a Line B and a Line C, respectively.
The thresholds of the Lines B and C are notably
less than those identified for abundant debris flows.
The initiation threshold for abundant debris-flows, Eq.
(1), is around one to four times of that for at least one
debris-flow before the 1999 earthquake, Line B. Fig-
ure 6 also finds that the 1999 earthquake significantly
lowered the threshold of the rainfall amount required
to trigger debris flows, especially after the earthquake
RAINFALL CONDITIONS FOR TRIGGE-
RING DEBRIS FLOWS
THE RELATION BETwEEN DEBRIS-FLOwS
OCCURRENCE DENSITY AND RAINFALL CHA-
RACTERISTICS
Most debris flows caused by Typhoon Morakot
generally occurred at the time of the maximum hourly
rainfall intensity Im in the Chen-Yu-Lan watershed.
Similar results also found in the watershed for abun-
dant debris flows caused by Typhoon Herb and Ty-
phoon Toraji. In order to understand the rainfall char-
acteristics for triggering numerous debris flows, the
relationship between the maximum rainfall intensity
I
m
and a parameter named debris-flow occurrence den-
sity NA were analyzed.
The debris-flow occurrence density NA is defined
as the number of debris flows per unit area, i.e. NA=N
/ A
. In the study area, the watershed can be viewed as
consisting of five subwatersheds as shown in Figure 1.
The average rainfall characteristics, such as the hourly
rainfall and the maximum rainfall intensity, in each
subwatershed area A was computed at the centroid
point of A using the reciprocal-distance-squared meth-
od (C
How
et alii., 1988; w
ei
& m
C
G
uinness
, 1973)
from two adjacent rain-gauge stations. The number of
debris-flow gully N in each subwatershed area A was
also determined. The relationship between the occur-
rence density of debris flow NA and maximum hourly
rainfall Im at each subwatershed area in the Chen-
Yu-Lan watershed was shown in Figure 5. The results
showed that the value of NA has an increasing tendency
with increasing in I
m
,, especially when I
m
exceeds 60
mm/hr. High rainfall intensity has a high probability to
trigger high occurrence density of debris flows.
Fig. 5 - Relationship between the occurrence density of
debris flows and maximum rainfall intensity in the
Chen-Yu-Lan watershed
1
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RAINFALL CONDITIONS FOR THE INITIATION OF DEBRIS FLOWS DURING TYPHOON MORAKOT IN THE CHEN-YU-LAN WATER-
SHED IN CENTRAL TAIWAN
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
35
threshold based on rainfall data in the Chen-Yu-Lan
watershed from Typhoons Herb, Toraji, and Morakot
was presented (Eq. (1)). The average rainfall intensity
that triggered abundant debris flows were at least 14
mm/hr for duration 24 hr, 10 mm/hr for 48 hr and 8
mm/hr for 72 hr. The initiation threshold for abundant
debris-flows is around one to four times of that for
at least one debris-flow before the 1999 earthquake.
The study results also showed the non-structural coun-
termeasures for the planning of evacuation routes and
shelters and education of hazard prevention can effi-
ciently reduce the casualties caused by debris flows.
ACKNOWLEDGEMENTS
The writers sincerely appreciate the help of grad-
uate students, J.H. Huang and Y.H. Yang, of Huafan
University in conducting field investigation and draw-
ing the figures. This research was financially support-
ed by the National Science Council of the Republic of
China under research projects (NSC. 99-2218-E-006-
238 and NSC. 99-2625-M-006-002).
five years, 2000-2004. Apparently, the earthquake
caused an abundant supply of loose material to accu-
mulate on hillslopes and in stream channels, and thus,
material moved more easily than previously under
light rains following the earthquake.
CONCLUSIONS
Typhoon Morakot brought heavy rainfall fall with
maximum hourly rainfall of 123mm and maximum
cumulative rainfall of 2361 mm in 48 hours at Alis-
han rainfall station where near the headwater of the
Chen-Yu-Lan watershed. The return periods of maxi-
mum hourly rainfalls from 10-hour to 40-hour rainfall
approached the 200 years and the breaking cumula-
tive rainfall of 2361 mm in 48-hour approached the
world records. This record-breaking rainfall plays a
major role in triggering abundant debris flows in the
Chen-Yu-Lan watershed. Thirty-seven debris flows
triggered by Typhoon Morakot in the watershed were
collected and investigated. Most debris-flow gul-
lies occurred at the time of peaking hourly rainfall.
The occurrence density of debris-flow gully NA de-
pends on the maximum rainfall intensity I
m
, and NA
increases with increasing in Im. A rainfall intensity-
duration formula for abundant debris-flows initiation
Fig. 6 - Empirical relations between average rainfall in-
tensity and duration for initiation thresholds of
debris flows in the entire Chen-Yu-Lan watershed
Tab. 1 - Significant debris flow events occurred in the
Chen-Yu-Lan watershed
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J.-C. CHEN , w.-S. HUANG , C.-D. JAN
& Y.-F. TSAI
36
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
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