Rainfall-based Debris-Flow Warning Model and Its ... Deng Jan...Jan, C. D., and C. L. Chen (2005),...
Transcript of Rainfall-based Debris-Flow Warning Model and Its ... Deng Jan...Jan, C. D., and C. L. Chen (2005),...
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Prof. Chyan-Deng JanDirector of the Disaster Prevention Research Center National Cheng Kung University
Rainfall-based Debris-Flow Warning Model and Its Application
2007 International Training Program for Typhoon and Flood Disaster Reduction
References:Jan, C. D., and C. L. Chen (2005), “Debris flow caused by Typhoon Herb in Taiwan,”Chapter 21 in the book of Debris-Flow Hazards and Related Phenomena,
edited by Matthias Jakob and Oldrich Hungr, pp. 539-563, Springer.Jan, C. D., et. al. (2004) “A Rainfall-Based Debris-Flow Warning Model and Its application in Taiwan,”Proceedings of the 2004 International Conference on Slopeland Disaster Mitigation, Taipei, Taiwan
References:Jan, C. D., and C. L. Chen (2005), “Debris flow caused by Typhoon Herb in Taiwan,”Chapter 21 in the book of Debris-Flow Hazards and Related Phenomena,
edited by Matthias Jakob and Oldrich Hungr, pp. 539-563, Springer.Jan, C. D., et. al. (2004) “A Rainfall-Based Debris-Flow Warning Model and Its application in Taiwan,”Proceedings of the 2004 International Conference on Slopeland Disaster Mitigation, Taipei, Taiwan
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Location of Taiwan
Taiwan is located at the convergent boundary of the Eurasian Plate and the Philippine Sea Plate.
Taiwan was formed by the collision of an island arc with the Asian continental margin.
Taiwan is separated from the southeast coast of China by the Taiwan Strait.
Taiwan has an oval shape with a length of 394 km and a maximum width of 144 km.
China
Korea
Taiwan
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Basic Conditions for Debris Flow Occurrence
Loose SedimentsSteep SlopeLarge amount of Water
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Characteristics of Taiwan IslandPopulation: 23 millions
Total area: 36,000 km2
Slope lands: 70% (Elevation >100 m)(steep slope) 10% (Elevation >1000 m)
Geology:young (3 million years)and weak geological formations, active earthquakes(abundant loose soils )
About 3 typhoons, annually attacked Average annual rainfall: 2,500 mm(large amount of water)
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Dept. of Hydraulic and Ocean Engineering
Typhoons as agents of debris flows
On average, three typhoonshit Taiwan annually.
Rains brought by typhoons may exceed 100 mm/hr and 1,000 mm/24 hr.
Recorded maximum rainfalls are 300 mm/hr and 1,749 mm/24 hr.
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because Taiwan has the basic conditions for debris flow occurrence: steep topography, abundant loose soil, and large rainfall.
It is natural for Taiwan having debris flows,
By Prof. Lin
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2004.7.25 Typhoon Aere2004.7.02 Typhoon Mindulle 2001.9.17 Typhoon Nari2001.7.29 Typhoon Toraji 1996.7.30 Typhoon Herb1990.6.23 Typhoon Ofelia
2004.7.25 2004.7.25 TyphoonTyphoon AereAere2004.7.02 2004.7.02 TyphoonTyphoon Mindulle Mindulle 2001.9.17 2001.9.17 TyphoonTyphoon Nari2001.7.29 2001.7.29 Typhoon Toraji Typhoon Toraji 1996.7.30 1996.7.30 Typhoon Typhoon HerbHerb1990.6.231990.6.23 Typhoon OfeliaTyphoon Ofelia
Recent Typhoons Causing Significant Debris Flows in Taiwan
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First well documented debris flow in Taiwan
The first well documented debris flow would be the one occurred in Hualian, Eastern Taiwan, during Typhoon Ofelia in 1990, with rainfall intensities exceeding 106 mm/hr.
This debris flow (with estimated volume of 56,000 m3 and average speed of 8.4 m/s), killed 35 people and destroyed 24 houses.
Before 1990, debris flow was just called by people in Taiwan as a “sediment disaster” instead of debris flow.After 1990, some (but not a lot) researchers in Taiwan
started to study debris-flow phenomena, mechanism, and mitigation methods.
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52 Debris Flows Caused by Typhoon Herb in 1996.
Typhoon Herb hit Taiwan in 1996 and triggered 52 debris flows (killed 43 people), in which 47 debris flows occurred in the watershed of the Chenyoulan Stream in central Taiwan.This event made people realize debris-flow hazards, and started extensive debris flow studies.
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Debris Flow Caused by Typhoon Herb in 1996Debris Flow Caused by Typhoon Herb in 1996
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Catastrophic earthquake with magnitude 7.3 Catastrophic earthquake with magnitude 7.3 on the Richter scale on September 21, 1999on the Richter scale on September 21, 1999
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Debris Flow caused by Typhoon Toraji in 2001Debris Flow caused by Typhoon Toraji in 2001
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Houses were destroyed by severe debris flow during Houses were destroyed by severe debris flow during Typhoon Typhoon Mindulle Mindulle & 72 Flood in & 72 Flood in SongheSonghe , , Heping Heping Township, Central TaiwanTownship, Central Taiwan
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Debris flows caused by Typhoon Debris flows caused by Typhoon AereAere in 2004in 2004
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土場土場 VillageVillage
Shangp
ing
Shangp
ingrive
rrive
r
Rd 122Rd 122
CheckpostCheckpost
landslidelandslide
landslidelandslidelandslide
2004.08.262004.08.262004.08.26
Height 343mWidth 201mDepth 30~40mVolume1,010,000m3
Debris flows caused by Typhoon Debris flows caused by Typhoon AereAere atat TaoshanTaoshanvillage,village, HsinchuHsinchu in 2004in 2004
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Number of debris flows per year in TaiwanNumber of debris flows per year in Taiwan
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
Year
0
30
60
90
120
150
180
210
240
Num
ber
of d
ebri
s-fl
ows
1 3 1 1 1 2 1 2 1 57 2 3 1 1
47
7 11 3
100
192
23
0
163
Special Events 1996 - Typhoon Herb1998 - Typhoon Zeb and Babs1999 - 921 Earthquack
2001 - Typhoon Toraji and Nari2004 - Typhoon Mindulle and Aere
2000 - Typhoon Xangsane
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0
20
40
60
80
100
120
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 1 2 3 4 5 6 7
Rainfall Duration (hr)
Rainfall Intensity (m
m/hr)
1996/7/31 8.1
阿里山
A-Li Mt.1,994mm
溪頭Si-Tou
1,090mm
和社610mm
More than 5 hrs.
More than 5 More than 5 hrs.hrs.
More than 13 hrs.
More than 13 More than 13 hrs.hrs.
Why Typhoon Herb could cause so many debris flows?Rainfall Intensity During Typhoon Herb in 1996More then 5 hours continually having rainfall intensity larger than 100 mm/hr.More then 13 hours continually having rainfall intensity larger than 80 mm/hr.
Why Typhoon Herb could cause so many debris flows?Why Typhoon Herb could cause so many debris flows?Rainfall Intensity During Typhoon Herb in 1996More then 5 hours continually having rainfall intensity larger than 100 mm/hr.More then 13 hours continually having rainfall intensity larger than 80 mm/hr.
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Debris-Flow Mitigation Strategiesin Taiwan
Structural debris-flow countermeasuresThe structural countermeasures include the installation of debris barriers, debris breakers, debris basins, slit dams, Sabo dams in debris-flow gullies and alluvial fans.
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Debris-Flow Warning/Monitoring System
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Non-structural debris-flow countermeasures
Zoning of potential debris-flow hazard areas,Identification of high debris-flow-prone streams, Debris-Flow evacuation education,
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Rainfall threshold criterion for debris flow initiation
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Concept
Abundant loose soils, steep slope, and large amount of water are three basic conditions for debris-flow occurrence.
For a specified watershed of a debris-flow gully, the changes of the topographical and geological conditions in a period of time are small as compared with the change of rains.
If one can analyze the rain’s condition and its relation with debris-flow occurrence, one estimates the debris-flow-occurrence potential under a rainfall event.FO
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Rainfall-based debris-flow warning model
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Dept. of Hydraulic and Ocean Engineering
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Time (hours)
0
10
20
30
40
Hou
rly R
ainf
all (
mm
)
A Rainfall EventAntecedent Rainfall
Ending-time:less than 4mm andlasting at least 6 hours
Starting-time:larger than 4mm
Subsequent rainfall
Debris flow occurrence time
Definition of a rainfall event
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Rainfall-based warning models
I-T Modelwithout consideration of antecedent rainfall
I-R Modelwith consideration of antecedent rainfall
Rainfall intensity IRainfall duration TAccumulated rainfall RAntecedent rainfall P
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Dept. of Hydraulic and Ocean Engineering
I-T model
For the same rainfall duration, the rainfall intensity needed to trigger debris flows after the 1999 earthquake is only about a half of that before the earthquake.
Before the 1999 earthquake
I = 1.35 T -0.2
After the 1999 earthquakeI = 6.7 T -0.2
For Chenyuland watershed
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Effective accumulated
rainfall
rainfall intensity
Where a, b are coefficient needed to be determined
baIRt =+
Linear I-R model
Effective accumulated rainfall (mm)
Rainfall intensity(mm/hr)
Rainfall Triggering index (mm2/hr)
RTIRI t =×Hyperbolic I-R model
I-R model
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How to calculate the values of RTIfor historical rainfall events
R(t) is the amount of the accumulated rainfall at time t in the considered rainfall eventRi is the amount of the antecedent i day’s rainfallα is a weighting factor and is set to be 0.8
Debris-flow rainfall events :Hourly rainfall at the debris-flows occurrence time
No debris-flow rainfall events :Peak of hourly rainfall in the rainfall events
RTIRI t =×
∑=
+=7
1
)()(i
ii
t RtRtR α
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How to calculate an instant RTI at time t
( ) ( ) ( )tRtItRTI t×=
12:00
13:00
14:00
15:00
16:00
17:00
18:00
19:00
20:00
21:00
22:00
23:000:0
001
:0002
:0003
:0004
:0005
:0006
:0007
:0008
:0009
:0010
:0011
:00
0
10
20
30
40
50
60
70
50
40
30
20
10
0
Hou
rly ra
infa
ll(m
m)
Time t
10 m
inut
es ra
infa
ll(m
m)
July 20 July 21
R(t)ii
i R∑=
×7
1
8.0
ii
it RtRtR ∑
=
×+=7
1
8.0)()(60 minutes
I(t)
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A lower critical line (RTI10 ) is defined as the lowest RTI-values of rainfall events that had triggered debris flows
An upper critical line (RTI90 ) is defined as that 90% of RTI-values for the historical rainfall events no matter triggering and not triggering debris flows is smaller than it .
Other debris-flow occurrence probability
0 10 20 30 40 50 60 70 80 9010
100
1000
10000
100000
×: No-debris-flow events○: Debris-flow events
Series rainfall events N
RTI
Upper line
Lower line
RTI90
RTI10
)(8.01.0)(1090
10
RTIRTIRTIRTIRTIP−−
+=
Determine the critical RTI-values for debris-flow-occurrence potential
Calculate RTI-values for previous rainfall events no matter having debris flows or not.
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Rainfall Events N
High Potential(>90%)
Medium Potential(10%~90%)
Low Potential(10%
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0 100 200 300 400 500Rt (mm)
0
10
20
30
40
50
I (m
m/h
r) High potential
RTI90Medium potential
Low potentialRTI50RTI10
Debris-flow occurrence potential
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0 10 20 30 40 50 60 70 80 9010
100
1000
10000
100000
Warning line
Rainfall Events N
Rai
nfal
l Tr
igge
ring
Inde
x R
TI
Deb
ris-f
low
Occ
urre
nce
Prob
abili
ty P
RTI90
RTI10
RTI50
Approaching warning line
0 100 200 300 400 500Rt (mm)
0
10
20
30
40
50
I (m
m/h
r)
RTI90
RTI10
RTI50
Warning line
Approaching warning line
Pre-warning time is set at RTI=RTI10Warning time is set at RTI=RTI50
Warning diagrams
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Application
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Many debris flows caused by Typhoon Mindulle and its succeeding rainstorms in 2004
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Data:Central Weather Bureau
Station Accumulated rainfall(mm) City溪南 2093.5 Kaohsing
御油山 1940.5 Kaohsing小關山 1786.5 Kaohsing南天池 1762 Kaohsing阿里山 1733.5 Chiayi新集 1693.5 Kaohsing梅山 1620 Kaohsing雪嶺 1619 Taitung新發 1604 Kaohsing稍來 1584 Taichung阿眉 1574.5 Nantu
神木村 1406 Nantu高中 1340 Kaohsing
尾寮山 1332 Pingtung上谷關 1311 Taitung
93.07.2-07.04
Rainfall distributionFO
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3350
2670
5000
2650
RTI90
1920490Guosing國姓鄉
4
1525380Renai仁愛鄉Nantou
3
37502500Dongshi東勢鎮
2
1535420Heping和平鄉
1Taichung
RTI50RTI10
Rainfall Triggering IndexesTownshipCountyNo
Critical RTI-values for four area
Critical RTI-values
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Dept. of Hydraulic and Ocean Engineering
Temporal Variations of Debris-Flow Warning at Heping, Taichung County
I Rt
90%
50%
10%
Debris-flow occurrence time
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I Rt
90% 50% 10%
Debris-flow occurrence time
Temporal Variations of Debris-Flow Warning at Dongshi, Taichung County
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I Rt
90%
50%
10%
Debris-flow occurrence time
Temporal Variations of Debris-Flow Warning at Ren-ai, Nantou County
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I Rt
90%
50%
10%
Debris-flow occurrence time
Temporal Variations of Debris-Flow Warning at Guosing, Nantou County
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Time of debris-flow warning and time of debris-flow occurrence for four debris-flow-prone mountainous areas
07/02 10:50
07/02 09:50
07/02 16:10
07/0210:40
Time of debris-flow warning , based on the proposed model
28 hrs 20 min38055.507/03 09:10
Ren-ai
21 hrs 10 min419125.007/03 08:00
Guosing
15 hrs 50 min42134.007/03 08:00
Dongshi
26 hrs 50 min47281.507/03 08:30
Heping
Rt(mm)I (mm/hr)
Rainfall intensity and effective accumulated rainfall at the time of debris-flow occurrence
Time of debris flow occurrence
Length of warning time ahead the time of debris-flow occurrence
Location
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Temporal debris-flow warning process for Da-Gin County during a rainstorm in June, 2006
6/8/06 6/9/06 6/10/06 6/11/06 6/12/06Date
0
20
40
60
80
100
Hou
rly R
ainf
all (
mm
)
0
100
200
300
400
500
600
700
800
Accu
mul
ativ
e R
ainf
all (
mm
)
100
1000
10000
100000
RTI
Occurrence of debris flow
Occurrence of debris flow
RTI50RTI90
17.5 hrs
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Rainfall Critical Values for debris-flow warning
at mountainous counties in 2005
9 1 1 9
1 3 0
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
1 4 0
2 0 0 2 5 0 3 0 0 3 5 0Rainfall Critical Values:
200 ~ 350 mm
Rainfall Critical Values:200 ~ 350 mm
Number of CountiesNumber of Counties
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Spatial distribution of rainfall-based debris-flow warning
at Nantou County for rainfall amount of 100 mm
>90%
Debris-flow Occurrence Probability
10%-50%
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財團法人成大研究發展基金會
National Cheng Kung University
>90%
10%-50%
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財團法人成大研究發展基金會
National Cheng Kung University
>90%
10%-50%
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Spatial and Temporal distribution for debris-flow warningbased on critical values and rainfall distribution
200000 220000 240000 260000 280000 3000002580000
2600000
2620000
2640000
2660000
2680000
2700000
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Spatial and Temporal distribution for debris-flow warningbased on critical values and rainfall distribution
7月2日 20:00 7月3日 8:00 7月3日 20:00
200000 220000 240000 260000 280000 3000002580000
2600000
2620000
2640000
2660000
2680000
2700000
200000 220000 240000 260000 280000 3000002580000
2600000
2620000
2640000
2660000
2680000
2700000
200000 220000 240000 260000 280000 3000002580000
2600000
2620000
2640000
2660000
2680000
2700000
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Rainfall critical values for debris-flow warning at mountainous counties in 2005
9 1 1 9
1 3 0
0
2 0
4 0
6 0
8 0
1 0 0
1 2 0
1 4 0
2 0 0 2 5 0 3 0 0 3 5 0Rainfall Critical Values:
200 ~ 350 mm
Rainfall Critical Values:200 ~ 350 mm
Number of CountiesNumber of Counties
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Rainfall critical values for debris-flow warning at mountainous counties in 2006
9 311 6 9
14
130
9
54
73
0
20
40
60
80
100
120
140
200 250 300 350 400 450
調整前 (200mm-350mm)
調整後 (200mm-450mm)
Number of CountiesNumber of Counties
Rainfall Critical Values:200 ~ 450 mm
Rainfall Critical Values:200 ~ 450 mm
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Rainfall critical values for debris-flow warning at mountainous counties in 2007
95
8 83
138
53
5
71
15
44
76
0
10
20
30
40
50
60
70
80
200 250 300 350 400 450 500 550
調整前 (200mm~450mm)
調整後 (250mm~550mm)
Number of CountiesNumber of Counties
Rainfall Critical values:200 ~ 550 mm
Rainfall Critical values:200 ~ 550 mm
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ConclusionA method to define a rainfall event and its antecedent rainfall used to define the rainfall parameters such as the rainfall intensity, the rainfall duration, the accumulated rainfall and the effective accumulated rainfall is proposed for analyzing the risk of debris flows caused by rainstorms. The product of the hourly rainfall intensity and the effective accumulated rainfall is defined as a rainfall triggering index (RTI) that is used to set up a rainfall-based debris-flow warning model. A method has been proposed to determine the lower critical RTI-value and the upper critical RTI-value, basing on the RTI-values of historical rainfall events.
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Conclusion
Once the two critical RTI-values are determined, one can estimate the instant debris-flow occurrence potential, with a diagram having instant RTI-values at time t on the ordinate and the variation of time t on the abscissa in a rainfall event. The rainfall-based debris-flow warning model has been applied to estimate the debris-flow occurrence potential for mountainous areas in Taiwan by the SWCB since TyphoonMindulle and is following rainstorms and typhoons in 2004.FO
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Dept. of Hydraulic and Ocean Engineering
Thanks for your Thanks for your attentionattention
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