Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanINITIATION OF HILLSLOPE DEBRIS FLOW
AFFECTED BY CHI-CHI EARTHQUAKE, TAIWAN
Tien-Chien CHEN1, Hsiao-Yuan YIN2, Zhen-Yu WANG3, Xuan JIANG3, Bo-Long CHEN3,and Zong-Lian LI3
1

Associate professor, Department of Soil and Water Conservation, National Pingtung University ofScience and Technology, Pingtung, Taiwan, tcchen@mail.npust.edu.tw.
2Section chief, Bureau of Soil and Water Conservation, Taiwan, sammya@mail.swcb.gov.tw.
3
Student of graduate school, National Pingtung University of Science and Technology, Taiwan.
ABSTRACT: A reconnaissance was conducted to hillslope debris flows (HDF) which
affected by Chi-Chi earthquake and Typhoon Morakot. Historical PGA and cumulative and intensity rainfalls at debris-flows initiation time were studied to figure out theinitiating characteristic of debris flows. Result shows that the earthquake related HDF not
only initiation in high PGA region but also in moderate PGA region (100 to 250 gal). As
PGA increasing, the triggering cumulative - and intensity rainfalls of HSDF aredecreasing. Shocking by Chi-Chi earthquake, rock mass developed dense cracks
enhancing landslide potential and stored the material for the circumstance of HSDF.
Key Words: hillslope debris flows, Chi Chi earthquake, rainfall

INTRODUCTION
Taiwan is situated at the juncture zone of the Euro-asia Continental Plate and the Philippine Sea Platewith a northwest tectonic movement of the Philippine Sea Plate. As the consequences of the tectonic
actions, the Central Mountain Range as well as the Coastal Mountain Ranges are produced, and theprevailing geological formations and structural patterns align approximately parallel to thelongitudinal axis of the island. The topography and geological conditions are highly related to thetectonic activity, where the mountain area composes about 77% of the whole area with highly
fractured geological conditions. As the results of the tectonic activity, Taiwan is among the most active
seismic districts in the world.
During the 1999 Chi-Chi earthquake, extensive slope failures were triggered by the earthquake incentral Taiwan. Among those, numerous of hillslope debris flows were triggered by Typhoon Torajiand Typhoon Nari. Those hillslope debris flows were identified that affecting by Chi-Chi earthquake.
Been heavy and intense rainfall, Typhoon Morakot attacked Taiwan during August 6 to August 10 in2009, 10 years after the earthquake, which brought the worst record to the history of Meteorology insouth Taiwan. More than 3,000 mm of precipitation was accumulated including 1,500 mm of rainfall
collected in one day and up to 130 mm/hr in intensity in the mountainous area. (Chen et al., 2009)
A reconnaissance was conducted to hillslope debris flows (HDF) which were affected by Chi-Chiearthquake and Typhoon Morakot. PGA and cumulative- and intensity rainfalls on hillslope debris

1800flows were studied to figure out the initiating characteristic of the debris flows.

CHI-CHI EARTHQUAKE AND TYPHOON MORAKOT EVENTS(1) The Chi-Chi earthquake
The Chi-Chi earthquake struck central region of Taiwan on September 21, 1999, with a moment
magnitude of 7.6, had caused severe ground failures and loss of lives and properties. The earthquake
was triggered by the faulting action of the Chelungpu fault, with a fault rupture length of 105 km. The
Chelungpu fault is a shallow thrust east-dipping fault which moved westward. The focal depth of theearthquake was only 8 km, which meant a tremendous amount of energy was released near the groundsurface. The maximum peak horizontal ground acceleration recorded was about 1 g, and the maximum
peak vertical ground acceleration recorded was 0.7 g. The PGA contour of motion caused by the main
shock is shown in Fig. 1.
The Chi-Chi earthquake caused extensive slope failures in central Taiwan from Miao-Li County toJai-Yi County. The Soil and Water Conservation Bureau surveyed the landslides based on aerial photos
and SPOT satellite images taken before and after the earthquake. More than 21,900 items of groundsurface variation involving a total area of more than 8,600 hectares were identified. The distributionsof the documented ground surface variations are shown in Fig. 1.

Fig. 1. Distribution of the PGA and ground surface variations after Chi-Chi earthquake in central
Taiwan.(Lin et al., 2002)

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(2) The Typhoon Morakot
Typhoon Morakot attacked Taiwan during the days of August 6 to August 10 in 2009, which brought
the worst record to the history of Meteorology in south Taiwan. More than 3,000 mm of precipitation
was accumulated including 1,500 mm of rainfall collected in one day and up to 130 mm/hr in intensity
in the mountainous area. The maximum intensity and accumulation rainfall are shown in Fig.2 and Fig.
3.
The high-intensity and –accumulation event caused flooding and triggered ten thousands of soils
slips, debris flows and large, complex landslides. Total area of landslide is more than 60,000 ha andmore than 500 victims were killed. (Chen et al., 2009; Lin et al., 2011)

Fig. 2 Maximum rainfall intensity inTyphoon Morakot (Chen et al., 2009)

Fig. 3 Accumulated rainfall in Typhoon
Morakot (Chen et al., 2009)

Most landslide hazards occurred during the period from the afternoon of 8 August to earlymorning of 9 August as the rainfall intensity rose. Soil and Water Conservation Bureau documented
1349 landslide hazards including more than 100 debris flow cases. The landslide area identified usingFORMOSAT II satellite images by Central Geological Survey is more than 50000 hectares. (Lin et al.,2011)
Kaoping River, the major river in south Taiwan for 171 km in length and 3285 square km in basin,
was severe damaged in Typhoon Morakot. A reconnaissance was conducted shown the sever slope
failure and debris flows were triggering during the typhoon. 60 stream debris flows and 29 hillslopedebris flows were documented. Stream type debris flows were recognized and mitigated by official
authority before typhoon mostly; oppositely few hillslope type debris flows were detected from
authority hence causing serious hazard to community and residents.
Fig. 4 illustrated 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

1802consistent with high accumulated rainfall area (as shown in Fig. 5). (Lin et al., 2011)

km

LegendSWCB
DGH

LegendAcc. Rainfall (mm)Others
Debris Flow
Foundation L oss
Landslide

Fig. 4 Locations of hazards and typesal., 2011)(Lin etFig. 5 The distribution of landslide hazards
with rainfall from August 5 to 10, 2010
overlaid with topography. (Lin et al., 2011)

DISTRIBUTION OF HILLSLOPE DEBRIS FLOWA reconnaissance on hillslope debris flows affected by Chi-Chi earthquake and Typhoon Morakot wasconducted. More than 80 hillslope debris flows were documented in the events. The investigation is shownin following.

(1) Occurrence of a hillslope debris flow
Hillslope debris flow (HDF) originating on steep slopes have the short gully length, smallwatershed and the narrow shape of the watershed of the topographic characteristics and gully
length is related to watershed area. (Lin et al., 2000)
Hillslope debris flows (Brunsden 1979) are one of the most common geomorphic phenomena inmountainous areas. A shallow landslide evolves s into a debris flow (Bathurst et al, 1997). They have been
described as the “rapid mass movement of blocky, mixed debris of rock and soil by flow of wet, lobatemass”, as “the downslope flow of debris mixed with a minor, yet significant, amount of water” (Innes1983). Brunsden (1979) pointed out that they typically occur on slopes with abundant nonconsolidated
sediments, steep gradients, scarce plant cover, and no previous rills or incised channels. Scars develop at
the rupture area, and a tongue develops with lateral levees ending in a frontal lobe with imbricated,nonsorted clasts (Johnson and Rodine 1984). The earthquake and typhoon related HDF are shown as Fig. 6and Fig.7.

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Fig. 6 HDF in Moi-Li after Chi-Chi earthquake.

Fig. 7 Lui-Que HDF in Typhoon Morakot.

Fig. 8 The Distribution of HDF of the two typhoon
events in a year after Chi-Chi earthquake.

Fig. 9 Total 29 cases HDF cases were
documented in Typhoon Morakot and themaximum PGA of south Taiwan in lastdecade.

(2) The hillslope debris flow after Earthquake event
The reconnaissance was conducted in two typhoon events in a year after Chi-Chi earthquake. The

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Distribution of HDF is shown in Fig. 8. The Locations of HDF are around in major earthquake affected
area, region of PGA 150 gal to 800 gal, in central island. About 80% of 50 HDF is located in the region ofPGA large than 200 gal. Half of HDF were distributed in sedimentary rock; the others were in metamorphicrock. Documented data of each HDF included watershed area, stream length, average slope, landslide area,initiation time, surround rainfall gauge records of HDF, and so on. The small and shallow landslides weremostly found in the HDF catchment as referring to Fig. 1.

(3)

The hillslope debris flow of Typhoon event

The reconnaissance was also conducted in Typhoon Morakot event, 10 year after Chi-Chi earthquake.
The Locations of HDF are in Kaoping River, the major river in south Taiwan, where was minoraffected, PGA lower than 50 gal, by Chi-Chi earthquake. 30 HDF were documented in this typhoon
event, the distribution of HDF is shown in Fig. 9. Because the consideration of earthquake effect, wecollect the earthquake PGA record in the region and surround area in last decade. Fig.9 shows thedistribution of PGA, contour is range from 100 gal to 250 gal in Kaoping River. About 20% of HDF islocated in the region of PGA large than 200 gal. 30% of HDF were distributed in sedimentary rock; theothers were in metamorphic rock. Same data was documented for each HDF included initiation time,
rainfall gauge records. The small and shallow landslides were also found in the HDF catchment.
despite its landslide area larger than earthquake event.
 The triggering rainfall of hillslope debris flow
Hours rainfall records of three typhoon events, overall gauges in Taiwan Island, were collected anddeveloped in a database for the HDF triggering rainfall determination. We surveyed the HDF initiation
times by investigation in field, News records, and reports of government agent and considered with thegeomorphic factors and rainfall history. The effective rainfall composed of antecedent and present
rainfall was introduced to character the triggering rainfall of HDF. The calculation of rainfall Intensity
I (mm/hr) and effective accumulation P (mm) at the most possible initiation moment of HDF areillustrated in Fig. 10. There are 74 cases out of the 80 HDF cases (~ 92 %) related to high intensity
rainfall with rainfall intensity exceeding 30 mm/hr. Most of Chi-Chi earthquake relating HDF cases (~48 %) with the effective accumulation >400 mm, oppositely, the effective accumulation are higher
than 400 mm for most of Typhoon Morakot cases.

Fig.10 The schematic of trigging intensity and effective accumulation rainfall

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EARTHQUAKE EFFECT ON HILLSLOPE DEBRIS FLOW

(1) Intensity I – PGA
As shown in figure 11, the HDF location distributes from 100 gal to 800 gal in Chi-Chi earthquake
events, the HDF is rare in the region of PGA lower than 100 gal. There are 14 cases out of the 50 HDFcases (~ 28 %) in which PGA between in 100 to 250 gal (Level 5 earthquake intensity of CWB). Itpoint out the potential of HDF not only initiation in high PGA region but also in moderate PGA
region, the HDF potential area for disaster mitigation seen be considered to spread to the regions ofPGA 100 gal and up. The threshold of HDF triggering intensity decreased, as earthquake’s PGAincreased. The lowest triggering intensity of is about 20 mm/hr in which PGA was 208 gal. HDFs
located in the region lower than 200gal have a tendency towards higher triggering intensity, oppositely
the one in high PGA region tend to lower triggering intensity.
It also showed in the figure, the HDF location distributes from 100 gal to 250 gal in Typhoon
Morakot events, the HDF is also rare in the region of PGA lower than 100 gal. There are total 29 cases
HDF cases in which the maximum PGA lower than 250 gal, included the Chi-Chi earthquake event, inlast decade. There are not clear tendency on PGA and intensity because the HDF are also influenced
by effective accumulation rainfall. Most of HDF triggering intensity was higher than 35 mm/hr despite
3 cases of effective accumulation more than 1000 mm.

Fig. 11 The distribution on the triggering rainfall
intensity and PGA of HDFs

Fig. 12 The distribution on the triggering
effective accumulation and PGA of HDFs

(2) Effective accumulation P - PGA
The figure 12 illustrated the 400 mm of effective accumulation rainfall seen to a bound between
Chi-Chi earthquake and Typhoon Morakot events. Overall HDF related to Chi-Chi earthquake was
low than effective accumulation 400 mm, the lower effective accumulation P is 95 mm and 29 cases

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out of the 50 HDF cases (~ 58 %) in which P lower than 250 mm, in small to moderate rainfall scale.
The HDF has a tendency towards the threshold of HDF earthquake’s PGA increased, the triggering
lower effective accumulation P decreased.
The HDF located lower than 300gal has a tendency towards the triggering P was higher 200 mm,
oppositely the one in high PGA region tend to lower triggering P. In contrast, most of HDF located inhigh triggering accumulation region, there are 23 cases out of the 29 HDF cases (~ 79 %) in which P
was higher than 1000 mm.
Fig. 13 shows the distribution of triggering intensity and effective accumulation of HDF. Twogroups are observed in the figure, the earthquake related group shows a tendency to74% of HDF’s
intensity higher than 50 mm/hr despite there were lower accumulation rainfall; in contrast, only 59%
of Typhoon’s HDF that the intensity was lower than 50 mm/hr but the P was higher than 1000 mm
mostly. It also demonstrates the difference of HDF related to earthquake and typhoon.

landslide area (ha) log10100
Morakot
Chi-Chi

1010.11Fig. 13 The magnitude of HDF of Chi-Chi
earthquake and Typhoon Morakot related

101001000catchment (ha) log10Fig. 14 The catchment and landslide area ontwo types HDFs

(3) Magnitude of hillslope debris flow
The magnitude for HDF was difference influenced by the factor, the earthquake related HDF tends tosmall landslide area, and the typhoon related HDF is tending large landslide magnitude. Fig. 14
showed the catchment and landslide area on two types HDF, there is a tendency towards the landslidearea increasing with the catchment increased. The catchment of earthquake HDF range from 3.4 to 56
ha with the landslide area from 0.14 to 12 ha. The catchments were range from 10 to 165 ha with thelandslide area from 1 to 20 ha. 75% of earthquake HDF case is landslide area small than 2 ha, incontrast, it is only 12 % did for the typhoon case.

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CONCLUSIONS
Hydrological and geomorphic characteristics study focuses on earthquake and Typhoon Mprakot
related types of hillslope debris flow, and cumulative- and peak intensity rainfalls on debris-flows
were studied by spatial analysis to figure out the initiating characteristic of debris flows. We conclude
the results as following:：(1) The earthquake related HDF not only initiation in high PGA region but also in moderate PGA
region (100 to 250 gal, Level 5 earthquake intensity of CWB), the HDF potential area fordisaster mitigation seen be considered to spread to the regions of PGA 100 gal and up. The
threshold of HDF triggering intensity decreased, as earthquake’s PGA increased.
(2) The 400 mm of effective accumulation rainfall seen to a bound between Chi-Chi earthquake andTyphoon Morakot events. There is a tendency towards the threshold of HDF earthquake’s PGA
increased, the triggering lower effective accumulation decreased.
(4) There are not clear tendency on PGA and intensity for HDF in Typhoon Morakot, those HDF
are also influenced by effective accumulation rainfall. Most of HDF triggering intensity was
higher than 35 mm/hr but strongly influence by the higher effective accumulation rainfall.
(5) Two groups are observed in the figure, the earthquake related group shows a tendency to74% of
HDF’s intensity higher than 50 mm/hr despite there were lower accumulation rainfall; it also
demonstrates the difference of HDF related to earthquake and typhoon.
(6) The magnitude for HDF was difference influenced by the factor, the earthquake related HDF
tends to small landslide area, and the typhoon related HDF is tending large landslide magnitude.

ACKNOWLEDGMENTS
This research was supported by the National Science Council, project number: NSC 99-2218-E-002-033. The Bureau of Soil and Water conservation provided part of the data used in this research. Their
helps were gratefully acknowledged.

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