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

Philippines FOR I

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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%

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%

財團法人成大研究發展基金會

National Cheng Kung University

>90%

10%-50%

財團法人成大研究發展基金會

National Cheng Kung University

>90%

10%-50%

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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