Post-event reconstruction of the short-lived Siaolin landslide dam

Advisor： J.J. DongPresenter： Y.S. LiDate： 2010/10/14

莫拉克颱風引致之堰塞湖地形與水文參數分析－以小林村堰塞湖為例

Outline

• Introduction• Methods• Results• The characteristics and failure mechanisms of

the Siaolin landslide dam• Conclusions• Future works

Introduction(1/3)• Formation of landslide

dams ：

• Longevity of landslide dams：

(Casagli and Ermini,2003)

(Costa and Schuster,1986 )

Stability

Introduction(2/3)

• Stability of landslide dams :

(Dong et al.,2009)

(Dong et al.,2009)

DBI :

log b d

d

A HDBI

V

bA

dH

dV

： catchment area

： dam height

： dam volume

Discriminant analysis： 2.94log 4.58( ) 4.17( ) 2.39log( ) 2.52s d d dD P H W L

P

dH

dW

dL

： peak flow

： dam height

： dam width ( along the valley)

： dam length( across the valley)

Ds ＞ 0 ： Stable

Ds ＜ 0 ： Unstable

(Dong et al.,2009)

(Ermini and Casagli,2003)

Introduction(3/3)• Why ? The database of landslide dams is incompleteness.

→ Lacked for systematic data gathering on process, characteristics, control measures, and effects of landslide dams. (Costa and Schuster, 1986)

→ The forming and breaching of rainfall triggered landslide dam is beneficial to understand the impact of extreme weather on surface process relates to landslide dam. (Kroup, 2004)

→ Detailed documentation of processes of dam failure remained scare. (Kroup, 2002)

• So … Morakot event.

→ The Siaolin landslide dam.

→ Reconstruction of the shape of landslide dam.

ASTM→ Grain size distributions of dam materials .

2. Field investigation

MethodsReconstruction of dam geometry

TopographyDebris budget of

landslideInflow rate and

overtopping time

1. Aerial photographs 1. Laboratory works

2. Debris budget

DTM.

Field investigation & Laboratory works.

Kriging.

2. Inflow rate is 2974cms. → overtopping time.

1. Water level. →Water volume. (based on DTM)

, , ,1ls channsl ls i F E ls slopeV V F V V (Hungr and Evans,2004)

Volume of debris entered the channel.

Initial volume of a landslide.

volume expansion due to fragmention.

Volume of debris deposit on the slope.

Volume of the entrained materials.

Results(1/5)• Topographic Analysis

• Laboratory works Sample NO. 1 2 3 4Wet Unit Weight (kN/m3) 20.1 20.6 21.3 21.7 Water Content (%) 3.4 1.7 1.8 1.8Specific Gravity 2.85 2.56 2.80 2.71Atterberg Limits (%) LL=24.1

PI=7.5LL=23.1PI=3.6

LL=25.6PI=7.7

LL=29.5PI=7.8

Fines Content (%) 40 50 39 52Soil Classification SC SM SC SC

t

wW

sG

cF

Fine grain particles

Lower Hydraulic conductivity

Overflow

●

●

EL. 440

EL. 420

Results(2/5)• Volume and debris budget of landslide

, , ,1ls channsl ls i F E ls slopeV V F V V 25.21 million m3 2.23 million m3

13.33% 15.41 million m3

15.31 million m3

Reference value

Results(3/5)• Dam height

●

EL. 410

●EL. 440

●

EL. 420No.11 Bridge

0.0E+000 2.0E+007 4.0E+007 6.0E+007W ater storage (m 3)

370

380

390

400

410

420

430

440

450

460

470

Ele

veat

ion

of

lake

's w

ater

leve

l (m

)

12.4E+06 m 3

W ater storagewhen the elevation of dam crest

= 415 mPredicted overtopping time

0 100 200 300Time (m in)

370

380

390

400

410

420

430

440

450

460

470

Ele

veat

ion

of

lake

's w

ater

leve

l (m

)

P redicted overtopping tim ewhen the elevation of dam crest

= 415 mPredicted overtopping tim e

69 m in

Q=2974 cms

Results(4/5)• Reconstruction of the shape of landslide dam

0 100 200 300 400 500 600 700 800 900D istance (m )

350

400

450

500

550

600

650

Ele

vatio

n (m

)

AD-AD Cross-SectionTopography before the forming of the landslide dam

Topography after the breaching of the landslide dam

D am crest

O vertopp ing po in t (EL.415m )

Results(5/5)The total dam volume is 15.34 million m3 .

15.31 million m3

The maximum thickness of deposition is 60 m.

The breached dam volume is 7.36 million m3.

The maximum thickness of erosion is 41 m.

The characteristics and failure mechanisms of the Siaolin landslide dam

• Characteristics欄位名稱 結果堰塞湖天然壩座標 X=120°38'52" ; Y=23°10'19"

發生年 2009/08/07-2009/08/09

名稱 旗山溪堰塞湖誘因 降雨導致崩塌土石堵塞淇山溪河道集水區面積 354067960.758m2

水系次數 3

平均流量 30.14cms

尖峰流量 2974cms

上游坡降 約 7.6%下游坡降 約 1.0%壩體組成材料與地質 SC 、 SM 、糖恩山砂岩 ( 上新世 )山崩移動土塊量 25210743.35m3

山崩發生源面積 627294m2

山崩水平滑距 2916.69m

比高 875m

壩高 damA=27.36m ( 平均壩高 )damA=54m ( 縱剖面最高點的壩高 )damA=44m (Breaching point 壩高 )damA=60 m( 最大壩高 )

壩長 damA=1553.6m

壩寬 damA=486.68m ( 平均壩寬 )damA=760m( 縱剖面最高點的壩寬 )

湖水深 35m

湖面積 73350 m2

壩體積 damA=13179396.26m3 ; damB=2161888.972m3

湖體積 9876559.36 m3

壽命 約 44 分鐘

• Failure mechanisms DBI=3.2 ； Ds= -0.37 Unstable

Matrix-supported.

Overtopping

FS=3.46

Piping

FS=3.46

SlopeXX

Conclusions

• The total volume of landslide dam is estimated to be 15.34 million m3, the dam height of breaching point is 44m, the maximum dam height is 60m, the dam length(along valley) and dam width(across valley) is 1554m and 760m.

• According to dam materials and estimated geometry and hydrologic parameters：→ The Siaolin landslide dam is classified as unstable and failure

mechanism is dominated by overtopping.

Future works

• 3D landslide dam.

• Collecting the data of landslide dams during Morakot typhoon.

Thanks for your attention ！