DEBRIS FLOW

Taiwan-Thailand 

Work Plan for Landslide and Surface 

Erosion Prevention Cooperation 

Potential Debris Flow Torrent 

Identification and Zoning in Taiwan 

Ting-Chi Tsao 

Disaster Prevention Technology Research Center 

Sinotech Engineering Consultants, INC. 

Taipei, Taiwan 

dptrc.sinotech.org.tw 

1

Outline 

I. Debris flows in Taiwan 

II. Legal regulations 

III. Identification of debris flow torrents 

IV. Investigation, zoning and assessment 

V. Public awareness and online information 


2

I. Debris flows in Taiwan 


3

� DEBRIS FLOW 

Water and mixture of sand, gravel, 

cobbles and boulders, driven by gravity 

Look just like readymixed 

concrete 

Debris flow≠Debris flow hazard 

�Debris flow: natural phenomenon 

�The phenomenon could not be 

eliminated 

�Only when hazard likely to occurred 

that mitigation should apply 

�Still long way for accurate forecast 

Hazard 

Risk 

Vulnerability 

Exposure 

�Safety: acceptable risk 


4

Debris flow history in Taiwan 

Typhoon Herb, 1996 

Typhoon Mindule, 2004 

Typhoon Toraji, 2001 

Typhoon Morakot, 2009 


5

Debris flow history in Taiwan 

2004 

2008 

2009 


6

Conditions of debris flow triggering 

Rain 

Debris 

Slope 

Signs of debris flow 

�Strange humming in the 

mountains (Listen) 

�Extremely muddy (Visual) 

�Water level sudden 

decrease (Visual) 

�Stinky smells (Smell) 

�Ground vibration (Feel) 

Debris Flow 

Water and mixture 

of sand, gravel, 

cobbles and 

boulders, driven 

by gravity 

Deposition 

Danger zones 

Landslide area 

Torrent banks 

Alluvial fan 

Initiation 

Transit 

To high 

grounds 


7

Characteristic of debris flow 

�High velocity (2~20m/s), high impact force. 

�Head with boulder front, followed by hyperconcentrated 

flow. 

�Larger size at top, smaller size at bottom 

�Initiation area mostly 

at slope above 

15~30°, deposition 

area 3~6 °. 

�Alluvial fan formed 

at torrent exit, gentle 

slope area. 


8

Types of debris flow 

�Mud flow: 2~20m/sec (72km/hr) 

�Granular flow: 3~10m/sec (36km/hr) 

Mud flow Granular flow 


9

Development of Debris Flow System 

�Initiation zone: Steep slope failure in the headwall or 

side slope of a gully or stream channel. Mostly forms 

a V-shape; the vegetation around often appear 

sparse, for a large amount of landslide fragments 

accumulate here. 

�Transit zone: U-shape, usually 

located in the valley or the 

middle and lower reaches of the 

river, and there are debris that 

collapsed from both side of the 

valley on the riverbank. 

�Deposition zone: Once the flow 

reaches a flatter or less confined 

area, it will spread out, lose 

speed and deposit. 

通道(流動段) 

溢流及堆積區(扇狀地) 

源頭(發生段) 

(redraw from Weber, 1964) 


10

Development of Debris Flow System 

往興隆村 

Alluvial Fan Runout 

distance 

150m 

Runout 

distance 

350m 

Kaohsiung 

DF051 

Waterfall 

2009/9/11 


11

Potential debris 

flow torrent 

� Total 1,660 torrents 

distributed in 17 

counties, 159 

townships, 680 villages 

� Residents 43,782 

Deposition 

Initiation 

Transit 


12

Number of torrents 

�Distribution of debris flow torrents 

250 

200 

150 

100 

50 

0 

139 

138 

135 

124 

40 

40 

宜蘭 

34 

34 

34 

34 

2 

2 

基隆 

219 

219 

218 

215 

50 

51 

50 

50 

49 

16 

16 

42 

42 

51 

51 

42 

18 

18 

台北 

新北 

桃園 

71 

71 

71 

64 

13 

13 

新竹 

76 

76 

76 

70 

61 

32 

苗栗 

101 

101 

95 

85 

74 

15 

台中 

229 

218 

208 

199 

181 

64 

南投 

7 10 

77 

9 

6 7 

899 

3 4 

彰化 

雲林 

62 48 

58 48 

46 47 

43 

40 39 

15 

嘉義 

2 

2010 (1578) 

2009 (1552) 

2008 (1503) 

2001 (1420) 

1999 (722) 

1996 (485) 

台南 

89 

65 

82 

64 

64 

64 

57 63 

4 

4 

高雄 

28 

28 

屏東 

163 164 

163 163 

163 162 

160 160 

110 

77 

110 77 

台東 

花蓮 


13

II. Legal regulations 


14

Procedure of identify potential debris 

flow torrents (1) 

Article 22 of Disaster Prevention and Protection Act: 

To reduce the occurrence or prevent further 

expansion of disaster, various tiers of governments 

depending on its functional authorities shall 

depending on its functional authorities implement 

following mitigation affairs: 

� 7. Investigation of disaster potentials, hazardous degree, 

situation simulation and risk analysis, and publication of 

results as applicable 

According to Debris Flow Hazard Information Public 

Act, Soil and Water Conservation Bureau is 

responsible for identifying, zoning of debris flow and 

organize the certified committee. 


15

Procedure of identify potential debris 

flow torrents (2) 

Debris flow investigation and analysis 

procedure: 

� New ones: Not yet public torrents but with debris 

flow hazards. 

� Update: For torrents already public, request for 

adjustment of potential information. 

Investigation request: 

� Central government: Bureaus or agencies could 

provide the debris flow hazard locations. 

� Local government: Sign up at SWCB website for 

providing the location of debris flow hazards. 


16

Procedure of identify potential debris flow torrents 

Start 

Phase I: 

Request for field 

investigation 

Phase II: 

SWCB conduct 

field investigation 

Phase III: 

Committee to 

review and certify 

Debris flow hazard location 

Township (village heads) report the location 

Local government preliminary investigate 

Accompany by local government or other agencies 

SWCB field investigation 

SWCB conducted field investigation with 

experts, local government, village heads 

and branch office personnel 

Committee for result certificate 

SWCB held committee which consisted 

experts and local government 

personnel to decide if the torrent 

should be enlisted 

Approved 

Information public and local governments were requested to 

establish residential list 

NONE-DF NONE-DF NONE-DF 

To SWCB branch office 

(Engineering method) 

To other agencies 

To other agencies 


17

III. Identification of debris flow torrents 


18

Thresholds of potential debris flow torrents (1/3) 

Location 

Environment 

Hazard 

(at least one) 

1. Central government 

2. Local government 

Exposures 

Torrent terrain 

Historical 

hazard 

Geological 

hazard potential 

Fulfill the conditions and 

confirm if the exposures were 

threaten. Remove if exposures 

were not within possible 

inundation area 

Include the resident housings, 

public facilities that might be 

threaten by debris flow 

Effective watershed area 

greater than 3 Ha (torrent 

slope greater than 10 。 ) 

Remote sensing image interpretation 

or historical hazard reports had 

identified that sediment hazards 

within the watershed 

Information from Central 

Geological Survey suggest 

certain geology hazard 

potential within the watershed 

Phase I 


19


Include the residential housings, public 

facilities that might be threaten by debris flow 

Buildings consider as resident house 

� With address plate 

� With utilities and residents 

(from the fact of residential) 

Buildings NOT include 

� Local temples (no residential area) 

� Warehouse or temporarily rest location 

Phase I 


20


Phase I 

Historical hazard Geological hazard potential 


21

IV. Investigation, zoning and assessment 


22

Phase II 

Field investigation 

SOP 

� Procedures 

established by 

SWCB and refined 

regularly. 

� Accompany by 

experts, local 

government and 

SWCB personnel, 

conduct investigation, 

measurements and 

fill the form. 

d 

c 

e 

ro 

P 

n 

tio 

a 

I s 

tig 

s 

ev 

e 

aIn 

h 

Pld 

ie 

F 

B 

C 

(S 

W 

) 

Preparation 

Equipment Preparation 

‧Camera ‧GPS ‧Notebook 

‧Laser range finder 

‧A4 Printer 

Contact SWCB and others 

1.Bureau and branch offices 

2.Local government officials and 

village head 

3.Project leader 

4.Experts (Professional 

Engineering or Professor) 

Traffic information 

Field investigation 

Fill-up form sheets 

Confirm historical hazard area 

Suggest warning sign location 

START 

(after complete of Phase I) 

Data Preparation 

1.Up-to-date debris flow torrents GIS 

2.Terrain map (1/25,000) 

3.Administrative division map (1/50,000) 

4.Geological map (1/50,000) 

5.Aerial photo 

6.Satellite images 

7.River basin map and catchment layer 

8.Debris flow historical hazard report 

9.Data base of debris flow torrents 

10.Form sheets 

11.Report sheet of local governments 

for debris flow hazard 

12.Up-to-date landslide GIS layer 

Watershed area 

greater than 3 Ha 

YES 

Debris flow hazard 

YES 

To engineering 

method agencies 

Overflow point and affected area zoning 

Consequence factors information 

gathering 

Hazard potential factors information 

gathering 

Summary and draft of debris flow torrent information dptrc.sinotech.org.tw 

NO 

NO 

File and monitoring 

23

Form sheets of potential debris flow torrents basic 

information 

Basic information 

Torrent current condition 

Affected area 

Exposures and 

engineering works 


24

Basic information 

� Hazard information 

sheet from local 

government 

� Location: Bridges, 

overflow location or 

other significant 

landmark. (Ex: Nantou 

M100-51 located at Yin- 

Liao Bridge) 

� Debris flow hazard 

history: Interview with 

village heads or local 

residents 

� After earthquake in 1999, 

debris flow occurred during 

Typhoon Toraji of 2001, 

debris filled the channel and 

affected 1 house 

一、基本資料 

填表人:楊永祺填表人:楊永祺職稱:工程員填表日期:2011/3/7 

行政區域行政區域 

有無溪流 

(野溪) 

溪流編號 

災害歷史(致災原因與時間) 

災害歷史(致災原因與時間) 

鄰近土石堆積(沖刷)可能受 

鄰近土石堆積(沖刷)可能受 

影響範圍內有無保全對象 

災害點GPS座標 

災害點GPS座標 

現地防治工程 

狀況 

照片 

縣市政府回報表 

南投縣草屯鎮雙冬里 

有有 

投縣M100-51 

1.有無歷史災害發生 

2.發生原因 

3.發生時間(幾年幾月) 

4.災害情形大致描述 

無無 

(228161,2655237) 

(228161,2655237) 

溪流名稱 

食水坑 

無無 

現地有無 

土石堆積 

無無 

地調所地質敏感圖資判釋為疑 

似土石流災害潛勢地區 

填表人: 簡榮興填表日期: 100.3.24 天氣: 晴 

行政區域南投縣縣(市) 草屯鎮鄉(鎮) 雙冬里村(里) 

溪流編號投縣 M100-51 溪流名稱銀寮坑溪 

溪流定位 

土石流災害歷史 

(致災原因與時間) 

保全對象所在 

行政區域說明 

坐標系統:□67 ■97 

X:228990 

Y:2655031 

原評定處理順序等級 □高 □中 □低 

1.有無歷史災害發生: ■有 □無 

2.發生原因 ■颱風 □豪雨 □其它【請加描述】 

3.發生時間 90 年 7 月 30 日時事件名稱:桃芝颱風 

4.災害敘述 

颱風豪雨造成土石下移,以致下游橋孔遭堵塞造成溪水溢出,影響左岸雙冬里 

24 鄰石灼巷 10-15 號住戶。 

同行政區域:■是 □否 

Use 

GPS to 

verify 

location 

Investigation with experts and village heads 

25 

dptrc.sinotech.org.tw

Torrent current 

condition 

� Nantou M100-51 (landslide 

ratio 28.02%) 

� Large scale landslide, 9 

landslides totally 14.29 Ha 

Watershed condition 

Bank protection 

works 

Watershed 

area 51 ha 

Boulders intercepted 

by check dam 

26 


Damage type 

� Residential house close to 

torrent, road within 

possible inundation area 

� Damage type: Bury, direct 

impact, re-route 

Garden at the right bank 

Yin-Liao Bridge 

Yin-Liao 1st 

Bridge 

Residential house 

Freeway No.6 


27

Overflow point 

� Location: If no 

exposures 

around, then 

no data were 

recorded 

Overflow point at 

begin of opening 

Yin-Liao bridge 

三、影響範圍修正 

鄰近溢流點之保全對象:□無,「影響範圍修正」與「保全對象及防治設施」部份可免填 

■有,請填寫「影響範圍修正」與「保全對象及防治設施」部份資料 

溢流點位 

■1 點 

□多於 1 點*1 

含保全對象之 

共 1 處 

溢流點總數 

溢流點位置 □上游 ■中游 □下游溢流點編號 BD01 

溢流點定位 

X:229478 

Y:2654768 

溢流點之照片編號投縣 M100-51_MDF10DSC00485 

溢流點之地形位置 □坡度陡變處 ■地形開闊處起點 □谷口 ■障礙物處 

(可複選) □河道轉彎處 □其他位置【請加描述】 

■無 

鄰近保全住戶之 

□見附圖(以具坐標之彩色航照圖或黑白相片基本圖為底圖之附圖) 

修正影響範圍簡圖 

註 1:每處溢流點位填寫一份,需重覆填寫影響範圍修正(第三項)以及保全對象及防治設施(第四項)。 

Residential 

house 

Yin-Liao 1st bridge 

28 


�Common overflow point 

Opening 

Obstacle 

Bending point 

Valley exit 

Slope 坡度陡變處 changing 

Velocity reduced when traveling into opening 

terrain, debris overflew to both banks 

Debris was blocked and overflew at 

bridge or other obstacle location 

Debris flow went straight and overflew at 

river bending point 

Exit of narrow valley become wider and 

shallower 

Velocity reduced when frontal area entered gentle 

slope area, debris were blocked and overflew 


29

Opening 

Velocity reduced when traveling into opening 

terrain, debris overflew to both banks 

Elementary 

school 

Kaohsiung DF069 


30

Obstacle 

Debris was blocked and overflew at bridge or other 

obstacle location, especially in narrow sectors such 

as bridges or box culvert 

Chayi DF054 

Box culvert blocked by tree trunks, 

debris overflew from both side 


31

Bending point 

Debris flow went straight and overflew at river 

bending point 


Debris direction 


32

Valley exit 


Exit of narrow valley become wider and shallower 


33

Slope changing 


Velocity reduced when frontal area entered gentle 

slope area, debris were blocked and overflew 


34

Affected area zoning 

� The apex of the affected area was selected at valley 

exit or overflew point with 105 degree fan area, the 

runout-distance (radius of the fan) was calculated by 

Takahashi equation. 

� Eliminate the un-passable areas, for example if banks 

were 10~12m higher than channel. 

Overflow 

Torrent 

Equation 

Modify 

Takahashi equation 

Log(L) = 0.42× Log(V × tanθ d ) + 0.935 

V=70,992 A 0.61 

(Hsieh, 2000) 

L=run out distance (m) 

A=watershed area (km 2 ) 

Θ d=Slope of torrent 

V=estimated debris volume (m 3 ) 


35

Exposures and 

engineering 

� 1 residential house and 

road, bridge 

� Conditions of check 

dam, bank protection, 

groundsill work were 

fine 

� Suggest the location of 

debris flow warning 

sign 

Check dam 

Groundsill works 

四、保全對象及防治設施 

1.建物 

保全對象說明 

1.住戶 □無 ■1~4 戶 □5 戶以上 

2.與防災相關之公共建築 ■無 □學校 □醫院 □活動中心 

2.交通設施 □無 ■橋梁【_銀寮橋_】 ■道路(含鐵路)【農投草 16 號農路】 

保全對象照片 □無 

編號投縣 M100-51_DDW10DSC00474 

保全對象住戶 □無 

地址草屯鎮雙冬里 24 鄰石灼巷 10-15 號,共 1 戶。 

□上游 □中游 ■下 GPS 定位 X:229642 工程設施 □無 

鄰近保全對象游 

□無 ■有 Y:2654719 照片編號投縣 M100-51_DEF10DSC00469 

之工程設施 □無整治設施 □防砂壩 □潛壩 □固床工 □護坡工程 □擋土牆 □沉砂池 

■整流工 □其他(箱涵) 

■上游 □中游 □下 GPS 定位 X:229182 工程設施 □無 

其他位置之游 

□無 ■有 Y:2655016 照片編號投縣 M100-47_ UEB10DSC00490 

工程設施 □無整治設施 ■防砂壩 □潛壩 □固床工 □護坡工程 □擋土牆 □沉砂池 

□整流工 □其他(_______________) 

現地整治成效 

□暫不需整治 □良好 ■尚可 □待改進 

評估 

警告標誌設置 □合適 □不合適 □暫不需設置 

位置評估 □編號錯誤、牌面毀損【待改進情形說明: Bank protections 

】 

■無 

■無 

■無 

警告標誌設置 

設置照片編 

□有,( ) X: 

位置簡述 

號 

Y: 

危險聚落評估 ■無 

(住戶≧5 戶) □有 

■無 

危險聚落照片 

編號 

建議處置 

設施(可 □無 □遷住 □加強治理 □臨時避難 □暫不需處理 

複選) 

銀寮橋 

農投草16線 

銀寮一石灼巷 

36 


號橋 10-15號

� Risk matrix of SWCB 

assessment method 

1.Consequence (2009) 

2.Hazard (2008) 

3.Treatment priority 

assessment 

Treatment priority assessment risk matrix (2009) 

Priority 

Consequence 

Potential analysis of debris flow 

torrents 

Hazard 

Low Medium High 

Low Low Low Medium 

Medi 

um 


High Medium High High 

e 

ite 

m 

m 

l 

c 

o 

v 

a 

ro 

p 

a 

I d 

n 

s 

ea 

a 

h 

te 

a 

P 

rtific 

c 

e 

ta 

(d 

a 

) 

rity 

rio 

p 

t 

t n 

n e 

e 

m tm 

s a 

9 s 

e re 

0 s T 

(2 

0n 

s 

io a 

rs 

e 

v 

START 

(after completion of Phase II) 

Draft report of potential debris flow torrent 

Zoning of 

affected area 

Residential house 

Consequence scoring 

Location map of potential 

debris flow torrent 

Committee approval 

YES 

Hazard potential scoring and 

treatment priority 

Consequence factor 

scoring and classification 

) Hazard potential scoring 

rts 

t 

e e 

n 

p 

e 

x 

e ite 

m d m 

rn 

te m 

e o 

v is 

s c 

o 

g n 

o ld 

l c e 

a h 

c h 

lo ic B 

l h C 

e d 

n n w W 

n a S 

o 

rs 

e 

p ts 

to n 

e 

d m 

t te 

s rn 

e 

lis e v 

l u 

q o 

g 

tia 

re l 

n a 

e h re c 

id lis e o 

s b w L 

re 

ta 

s 

e 

NO 

Completion of data certificate and approval 

Official numbering of torrents 

Information preserved in data base 

Data base update 

END 

(public by Debris Flow Hazard Information Public Act) 

Data base of 

potential debris 

flow torrents 

(draft) 

Building 

Transportation 

Engineering effectivenes 

Landslide scale 

Slope 

Boulder size 

Geology 

Vegetation 

Frame, legend, compass, 

coordinate system 

Debris flow torrent layer 

Debris flow inundation area 

Color aerial photo 

Delete from debris flow torrent 


37

� Hazard potential analysis 

1. Landslide scale 

2. Slope 

3. Boulder size 

4. Geology 

5. Vegetation 

� The summation of 5 

factors become hazard 

potential score 

2008 

version 


torrents 

Hazard 

potential 

Score 

High ≧62 

Medium 46< X 30cm 20 

Between 8cm~30cm 13 

Smaller than 8cm 2 

Non-significant 2 

Type I (A, D, F Geology zones) 15 

Type II (C, E Geology zones) 15 

Type III (B, G Geology zones) 5 

Bared with rock falls 15 

植被稀疏 15 

Medium vegetation 6 

Summation of hazard potential factors 

植被密集 3 


38

� Consequence factor scoring 

for torrents with exposures 

1. Building 

2. Transportation 

� Engineering effectiveness as 

weighting 

� Summation of all factors for 

consequence factors scoring 

2009 

version 


torrents 

Consequence Score 

High ≧60 

Medium 40< X 5 60 

(Residential house) 1~4 30 

None 0 

Bridge 35 

Road 20 

None 0 

Maximum scores 100 

Weighting 

None or improvement required 1 

Good 0.8 

Fine or no improvement required 0.6 

Result of consequence factors scoring 

(Building+Transportation) x Weighting 


39

Scoring Result 


torrents 

Hazard potential factors and scores Consequence factors scores 

Torrent type Creek Building 

Geology D (Sedimentary rock) 15 Transportation 

Slope of source 

area 

Landslide ratio None-significant 

(0.97%) 

60%~120% 15 Engineering 

effectiveness 

Boulder size Average size 8~30cm 13 

Vegetation Scattered 6 

Resident house:1~4 (1) 

Public building: 0 

Bridge: Yes 

Road: Yes 

Engineering treatment: Yes 

Effectiveness: Good 

Summation 54 Summation 52 

Classification Medium Classification Medium 

5 

Treatment priority (from Risk Matrix) 

Treatment Priority Medium 

Treatment priority assessment risk matrix 

Priority 

Consequence 

30 

35 

0.8 

Hazard 


Low Low Low Medium 

Med 

ium 


High Medium High High 


40

V. Public awareness and online information 


41

Debris flow disaster prevention information 

�System information 

� Torrent location 

� Typhoon condition 

� Real-time rainfall 

� Debris flow alerts 

� Evacuation route map 

� Evacuation threshold 

� Historical hazards 

� Historical drawings 

http://246.swcb.gov.tw 


42

Where to find debris flow torrents information? 


43

Where to find debris flow torrents information? 

� Mesh-up 

� High 

resolution of 

Google map 

images 

� Overlaying 

other torrent 

information or 

photos 

� Synchronize 

worldwide on 

internet 

� User-friendly 

uploading 

hazard area 

photos 

Google Map API 


44

Thank You 


45

DEBRIS FLOW

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