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IJEGE-11_BS-Yune-et-alii

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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
159
DOI: 10.4408/IJEGE.2011-03.B-019
ANALYSIS OF DEBRIS FLOW CHARACTERISTICS THROUGH
DATABASE CONSTRUCTION IN KOREA
C
Han
-y
ounG
YUNE
(*)
, k
yunG
-s
uk
KIM
(**)
, n
a
-J
ae
YOO
(***)
,
H
eunG
-s
eok
SEO
(****)
& k
younG
-J
ea
JUN
(****)
(*)
Assistant Professor, Gangneung-Wonju National University, Gangneung, South Korea
(**)
Researcher, Korea Expressway Corporation, Hwasung, South Korea
(***)
Professor, Kangwon National University, Chuncheon, South Korea
(****)
Associate Professor, Department of Construction Engineering, Gangneung Yeongdong College, Gangneung, South Korea
(*****)
Ph. D Candidate, Gangneung-Wonju National University, Gangneung, South Korea
Recent disaster reports show that there were a total
321 casualties and a 4.3-billion dollar property-loss
damage during Typhoon ‘Rusa’ in 2002 (NIDP,2002)
and 132 casualties with 3.99-billion dollar property-
loss damage during Typhoon ‘Maemi’ (NIDP, 2003).
In 2006, regional intensive rainfall following Typhoon
‘Ewinia’ caused 61 casualties with a 1.8-billion dollar
property-loss damage. These damages are frequent in
Gangwon province where 90% of the land is moun-
tainous (Fig. 1). Considering that Gangwon occupies
only 17% of Korean territory, the area is quite vulner-
able to disasters such as landslides or debris flows.
Korean government agencies and other organiza-
tions have recognized the severity of the slope disas-
ters and have set forth national projects for prevent-
ing and mitigating these disasters. NIDP (National
ABSTRACT
In recent years in Korea, Typhoon Rusa (2002),
Typhoon Maemi (2003), and the localized extreme
rainfall followed by Typhoon Ewiniar in 2006 devas-
tated residential areas, roads, and agricultural lands in
Gangwon province where 90% of the area is in moun-
tainous regions. Most of the economic losses and casu-
alties were concentrated in the area near the mountain
valleys and creeks due to the floods and debris flows.
In this study, DATABASE, which includes a total 153
debris flow events in the Gangwon area, was created
by collecting the hazard records and field investiga-
tions of existing debris flow sites. Based on these data,
we analyzed geomorphic characteristics of the initia-
tion zone, the initiation and transportation mechanism,
and the threshold of rainfall for an early warning sys-
tem. Analysis results showed the potential for further
use of the DATABASE in the development of hazard
mapping to predict debris-flow susceptible areas and
the assessment of hazard severity in those areas.
K
ey
words
: debris flow, DATABASE, field investigation, rain-
fall threshold
INTRODUCTION
In Korea, over 70% of the land is in mountain-
ous regions. There are many casualties and significant
property loss due to slope failures or debris flows
resulting from typhoons or regional intensive rain-
fall during the summer between June to September.
Fig. 1 - Research area. (Gangwon province, korea)
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C.-Y. YUNE, k.-S. kIM, N.-J. YOO, H.-S. SEO & k.-J. JUN
160
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
flow occurred and damaged the roads. For the selected
sites, field investigation mainly focused on where de-
bris flows were initiated and transported. For the ini-
tiation zone, the types of debris flow being initiated
were investigated, and the slope angle and the volume
of this zone were measured. For the transportation
zone, the average gradient at every 20-30 m distance
and the direction of the debris flow were measured.
Using topographic maps, the area, average gradi-
ent, and direction of the drainage basin were obtained.
The effective soil depth and soil type in the drainage
basin were also obtained using geologic maps.
Rainfall data were obtained from the AWS (Au-
tomatic Weather Stations) which are operated by the
KMA (Korea Meteorological Administration). Rain-
fall data used in used in constructing DATABASE
were maximum hourly rainfall, continuous rainfall,
3-day cumulative rainfall, and 28-day cumulative rain-
fall data. Table 2 shows the structure of DATABASE.
CHARACTERISTICS OF DEBRIS FLOW
VOLUME OF INITIATION FAILURE
Slope failure-initiated debris flows occurred at
150 sites whereas debris flow initiated by gully ero-
sion were only at three sites among 153 sites. For the
slope failure type, the volume of the initial failure is
shown in Fig. 2. The minimum, maximum, and aver-
age volume were 16 m
3
, 11,310 m
3
and 969 m
3
, re-
spectively. As shown in Fig. 2, the volume at 117 sites,
78% of 150 sites in total were less than 1,000m
3
.
Institute for Disaster Prevention) and KIGAM (Korea
Institute of Geoscience and Mineral Resources) have
been carrying out researches on slope disaster preven-
tion. NIDP and KEC (Korea Expressway Corporation)
are jointly researching the mitigation of debris flows.
The authors of this paper, members of these national
research institutes, have been involved in research
projects on the debris flow DATABASE construction
and have also participated in the analysis of character-
istics of debris flow by field survey.
The main objective in this study was to create
DATABASE, a widely accessible source of disaster
cases in Gangwon province, where slope failure and
debris flow have occurred most frequently in Korea.
DATABASE’s records include field survey data, rain-
fall data, topographical, and geological data. Using
DATABASE, debris flows in Gangwon province are
analyzed especially in the aspect of initiation, trans-
portation, and triggering rainfall.
DATABASE CONSTRUCTION
Field investigations on initiation and transportation
zones of debris flow including rainfall data at 67 basins,
which contains 153 sites where debris flows have oc-
curred, have been carried out. DATABASE is continu-
ously updated with ongoing investigation results.
Investigations focused on the sites where debris
Tab. 1 - Statistics of Damages by Heavy Rainfall
Tab. 2 - Structure of DATABASE
Fig. 2 - Volume of initiation failure in Gangwon province
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ANALYSIS OF DEBRIS FLOW CHARACTERISTICS THROUGH DATABASE CONSTRUCTION IN KOREA
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
161
sorted by 10 degrees, the most probable frequency (80
sites, 52.3%) of gradient of a transportation zone oc-
curred between 10 to 20 degrees as shown in Fig. 4,
while the most probable frequency (28.5%) of gradient
was between 30 to 40 degrees in the Swiss Alps region
(Fig. 5). We accordingly concluded that debris flows
occurring in Gangwon province are transported at rela-
tively gentler slopes than those of the Swiss Alps.
LENGTH OF TRANSPORTATION
We investigated the length where debris flow initi-
ated and then moved through the transportation zone.
The result suggests that the lengths of transportation
ranged from 41 to 1,451 m and the average transpor-
tation length was 408 m. When it was sorted by the
length of 200 m, the most probable frequency (55 sites,
35.9%) occurred at 200-400 m as shown in Fig. 6. The
frequency on which debris flow moved more than 600
m was very low (19.6%).
The results of previous studies showed that in the
Kamikamihori valley of Mt. Yake in Japan, 57.1%
of total debris flows occurred at 600-800 m while in
the Swiss Alps, most of the debris flows occurred at
ANGLE OF INITIATION ZONE
Slope angles at the initiation zones of 153 sites
were analyzed. The result reveals that the angles of
the initiation zones ranged from 10 to 56 degrees. The
most probable angle was 38 degrees and the average
value was 33.5 degrees. Sorting by every 10 degrees of
angle, the most probable angle ranged from 30 degrees
to 40 degrees (79 sites, 51.6%) as shown in Fig. 3.
Considering the facts from previous studies that
the most probable frequency of debris flow initiation
occurred at 26 to 30 degrees in Korea (k
im
et alii,
2006) and 10 to 20 degrees in Japan (Ikeya, 1989), it
is apparent that debris flow in Gangwon province is
occurring at relatively steep slopes.
AVERAGE GRADIENT OF TRANSPORTATION
ZONE
Gradients of a total of 153 transportation zones
based on the database were analyzed. The result in-
dicates that gradients of transportation zones ranged
from 2 to 55 degrees. The gradient on which debris
flow was frequently transported was 13 degrees and
the average gradient was 16.9 degrees. When it was
Fig. 3 - Angle of initiation zone in Gangwon province
Fig. 4 - Average gradient of transportation zone in Gan-
gwon province
Fig. 5 - Average gradient of transportation zone in Alps
region. (Swiss, 144 sites) (Data from Z
immermANN
et alii, 1997)
Fig. 6 - Length of transportation zone in Gangwon provin-
ce
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C.-Y. YUNE, k.-S. kIM, N.-J. YOO, H.-S. SEO & k.-J. JUN
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
range of 0-0.2 km
2
. The rest of the basin area indicates
a very low frequency without significant tendency.
Compared with the Swiss Alps (Fig. 9), basin areas in
Gangwon province are approximately ten times small-
er than those of the Swiss Alps where the number of
debris flows tend to decrease remarkably in proportion
to the increase of basin area.
AVERAGE BASIN GRADIENT
The average basin gradients at 67 sites where de-
bris flows occurred were obtained by taking the average
gradient of the mesh (10 m x 10 m) from the Digital
Elevation Map data of each site. The gradient of basins
ranged between 5 and 46 degrees as shown in Fig. 10.
The frequency tended to decrease as the gradient of the
basin increased. The highest frequency was at 6 degrees
and the average of all data was 12.8 degrees.
BASIN AREA AND AVERAGE GRADIENT
Fig. 11 shows the relationship between basin area
and average basin gradients with different lithological
200400 m, which is similar to the Korean cases. Based
on the slope angles of the initiation zone, it is judged
that the debris flows in Japan have volcanic charac-
teristics so that they occur at a relatively gentle slope
compared with the steeper slopes in Gangwon prov-
ince which move a longer distance.
BASIN AREA
Figure 8 shows a total of 67 basins sorted into 0.2
km
2
with frequency of debris flow occurrence. The
most probable frequency (76.1%, 51 sites) is in the
Fig. 7a - Japan, Mt. Yake, kamikamihori Valley (14 Debris
flows) (Data from o
KuDA
& S
uwA
, 1984)
Fig. 7b - Length of transportation zone. Swiss, the Alps (12
Debris flows) (Data from VAw, 1992)
Fig. 8 - Basin area of the debris flow in Gangwon province
Fig. 9 - Basin area of the debris flow in Swiss. (Swiss, 144
sites)(Data from Zim
mermANN
et alii, 1997)
Fig. 10 - Average basin gradient in Gangwon province (67
sites)
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ANALYSIS OF DEBRIS FLOW CHARACTERISTICS THROUGH DATABASE CONSTRUCTION IN KOREA
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
163
age basin gradient in the western United States are
greater than those values from Gangwon province.
This might be caused by different characteristics be-
tween runoff-initiated debris flow related to fire and
landslide-initiated debris flows.
RAINFALL TRIGGERING DEBRIS FLOw
Using the available rainfall data at each of the de-
bris flow sites, the relations of the maximum hourly
rainfall intensity and antecedent cumulative rainfall
before occurrence of debris flow are shown in Fig. 13.
In this figure, cumulative rainfall tends to increase with
the maximum hourly rainfall intensity. All of data ex-
conditions in the Western United States where basins
were reported to have experienced fire-related debris
flows (C
annon
& G
aRtneR
, 2005). The heavy line
indicates threshold conditions, above which runoff-
initiated debris flows can be expected. However, it is
known that the threshold for landslide-initiated debris
flow is not delineated because of insufficient data. De-
bris flow producing basins range in area from 0.02 km
2
up to 25 km
2.
Debris flows were not observed at the
outlets of basins larger than about 25 km
2
, indicated by
the dotted vertical boundary. Rock types are not likely
to significantly affect the characteristics of the basin
area and average gradient. Fig. 12 illustrates those data
from debris flow sites in Gangwon province.
Comparing those figures, we see that the distribu-
tions of data and threshold conditions are quite simi-
lar except the scales of basin area and average basin
gradient. The average values of basin area and aver-
Fig. 11 - Relations between basin area and average basin
gradient of basins producing fire-related debris
flows in the western U. S. (c
ANNoN
& G
ArtNer
,
2005)
Fig. 12 - Relations between basin area and average ba-
sin gradient of basins producing debris flows in
Gangwon province
Fig. 13 - Relations of the maximum hourly rainfall inten-
sity and antecedent cumulative rainfall before oc-
currence of debris flow. (Rainfall threshold lines
representing landslide watch and warning respec-
tively proposed by kFS)
Fig. 14- Cumulative rainfall for regions in Gangwon province
Tab. 3 - Landslide forecast by kFS
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C.-Y. YUNE, k.-S. kIM, N.-J. YOO, H.-S. SEO & k.-J. JUN
164
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
cept a single point in Fig. 13 are within the landslide
threshold lines which are proposed by the Korea Forest
Service, as shown in Table 3. From this figure, it can
be noticed that debris flow in Gangwon province usu-
ally occurs during a storm event having greater rainfall
than the threshold triggering landslide.
As shown in Fig. 14, rainfall events with cumu-
lative rainfalls during three days and from four to
twenty-eight days before occurrence of debris flow for
regions in Gangwon province were analyzed to inves-
tigate the effect of antecedent cumulative rainfall on
the occurrence of debris flow. Amounts of cumulative
rainfall during three days are greater than those from
four to twenty-eight days before occurrence of debris
flow; thus, rainfall events during three days appear to
be dominant to initiate debris flows. In addition, the
amounts of rainfall from four to twenty-eight days be-
fore the occurrence of debris flows, ranging from 55
mm to 301 mm, might increase the degree of saturation
in the ground and subsequently result in the occurrence
of debris flows. More work in the future is needed to
verify details of rainfall effects on debris flow.
CONCLUSIONS
Based on data obtained from 153 sites where de-
bris flow occurred recently in Gangwon province in
Korea, geomorphic characteristics of the initiation
zone, initiation and transportation mechanism, and the
thresholds of rainfall for an early warning system were
analyzed and the results are summarized as follows.
1. Volumes of the debris flow initiation zone are in the
range of 16-11,310 m
3
and most of them are less
than 1,000 m
3
.
2. Debris flows are initiated in relatively steep slopes,
and are associated with the geomorphic charac-
teristics of Gangwon province. They travel along
mountain streams having a relatively gentle gradi-
ent of 16.9 degrees on average and relatively short
distances averaging 408 m.
3. Basin areas are less than 1.2 km
2
being quite small
in comparison with the scale in Western United
States and the Swiss Alps. The average basin gradi-
ent is 12.8 degrees.
4. Rainfall events triggering debris flows have greater
rainfall than threshold boundaries, categorized to
induce landslides, proposed by the Korea Forest
Service. Amounts of cumulative rainfall during
three days are so severe that it may be the dominant
factor in inducing debris flow.
ACKNOWLEDGEMENTS
This research was supported by a grant (Code#’08
RTIP B01-01) from The Regional Technology Inno-
vation Program (RTIP) funded by Ministry of Land
Transport and Maritime affairs of Korea government.
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