Assessing Risks of Breaching of Earth Dams and Natural Landslide …igs/ldh/conf/2010/articles/v010.pdf · · 2017-05-14Assessing Risks of Breaching of Earth Dams and Natural

Assessing Risks of Breaching of Earth Dams and Natural

Landslide Dams

Zhang, L.M. Peng, M. Xu, Y.e-mail: [email protected] e-mail: [email protected] e-mail: [email protected]

Department of Civil and Environmental Engineering, The Hong Kong University of

Science and Technology, Hong Kong

ABSTRACT

Both man-made earth dams and natural landslide dams pose enormous risks to the public because of the potentially

catastrophic floods generated by breaching of such dams. In order to manage the dam breaching risks, it is

necessary to evaluate the structural safety of dams, possible breaching modes and breach size, outflow hydrograph,

flood routing in the downstream river, and potential losses of lives and properties due to dam breach floods. It is

also necessary to prepare emergency plans and evaluate actions to betaken to mitigate the risks. In this paper,

results of recent research on three key issues in dam breaching risk assessment (i.e., failure modes and processes,

breaching parameters, and flood risk assessment) are presented, and a case study on Tangjishan Landslide Dam is

presented to illustrate the risk assessment procedure.

Indian Geotechnical Conference – 2010, GEOtrendz

December 16–18, 2010

IGS Mumbai Chapter & IIT Bombay

1. INTRODUCTION

The world data as of 2000 indicate that there are about

50,000 large man-made dams in operation (ICOLD 2010).

A large dam has a height of not less than 15 m or stores

more than 3 million m3 of water in the reservoir.

Approximately 80% of the world dams are earth or rockfill

dams. In China, the total number of existing dams is as

large as 85,000, of which about 37,000 are distressed dams

(Chen 2008). The potential risks posed by failure of

distressed dams could be enormous. In August 1975, an

extreme storm caused the breach of two large embankment

dams (the Banqiao and Shimantan dams) and 60 smaller

dams in Henan Province, China. Eleven million people

were affected and the death count reached 26,000 (Ru and

Niu 2001; Xu et al. 2009). As social and economical

development goes on, dam safety has drawn increasing

attention from the public in recent years.

Another type of dams is natural landslide dams.

Landslide dams form where narrow steep valleys are bordered

by high rugged mountains (Costa and Schuster, 1988). The

formation of most landslide dams (82%) was trigged by

rainfall, snowmelt, and earthquakes. Over 257 landslide

dams formed during the 2008 Wenchuan earthquake (Cui et

al. 2009). Landslide dams may also pose great risks to the

public. For instance, the formation of Tangjiashan Landslide

Dam in 2008 caused the evacuation of 250 thousands people

living downstream the dam.

When a man-made dam deteriorates seriously or when

a landslide dam just forms, the engineer has many questions

to answer:

(1) Would the dam collapse?

(2) If yes then when would the dam break or under

what conditions would the dam break?

(3) What would be the flood from breaching of the

dam?

(4) How long would the flood last?

(5) How effective would any rescue work be to prevent

or delay the failure of the dam?

(6) What are the consequences of the dam failure?

How many people downstream the dam would be

affected?

(7) How many people should be evacuated and when

should they be evacuated?

(8) What is the accuracy of the above estimates?

In this paper, results of recent research on three key

issues in dam breaching risk assessment (i.e., failure modes

and processes, breaching parameters, and flood risk

assessment) are presented. A case study on Tangjishan

Landslide Dam is presented to illustrate the risk assessment

procedure. The focus of the paper is not the probabilistic

methodologies involved in risk analysis, but the physical

processes involved in assessing the risks of breaching of

earth dams and natural landslide dams.

82 L.M. Zhang, M. Peng and Y. Xu

2. DAM BREACHING DATABASES

Database of Earth and Rockfill Dam Failure Cases

In the past few years, HKUST collected more than 1609

dam failure cases from the literature (Xu and Zhang 2009;

Zhang et al. 2009) and compiled these failure cases into a

database. Among these cases, 66% are earth dams. Details

of the characteristics of the dams (geographic information,

dam type, dam height and volume, construction time etc.),

the reservoir (surface area, volume, and geometry etc.) and

the failure information are collected. The failure cases are

from over fifty countries, including the US, India, and the

UK, but excluding China. The dam failures in China are

compiled in a separate database by China Institute of Water

Resources and Hydropower Research (IWHR). The

information from the database forms the basis for studying

the characteristics of failed dams, dam breaching

parameters, and dam breaching processes.

Database of Landslide Dams

HKUST compiled another database of 1239 landslide dams,

including 257 cases formed during the 12 May 2008

Wenchuan earthquake. Out of all the 1239 landslide dam

cases in the database, 53% (659 cases) occurred in China,

14% (175 cases) in Japan, 8% (95 cases) in USA, 6% (72

cases) in Italy, 2% (28 cases) in New Zealand and 17%

(210 cases) in other places.

3. FAULIRE PROCESSES AND FAILURE MODES

Failure Processes

The breaching of an earth embankment is an erosion

process of the embankment materials by flow of water either

over or through the dam. The former leads to overtopping

of the dam and subsequent external erosion while the latter

gives rise to seepage erosion or piping. Wahl (1998) divided

the whole breaching process into two phases: the breach

initiation phase and the breach development phase. In the

breach initiation phase, the outflow from the dam is small,

consisting of a slight overtopping or a small flow through

a developing pipe or seepage channel. In the breach

development phase, the outflow and erosion develop rapidly.

In the following, breaching characteristics of embankment

dams by overtopping and seepage erosion/piping are

discussed separately.

Overtopping

Overtopping occurs as the result of insufficient spillway

capacity or extreme flood exceeding design criteria. The

water flow over the embankment from overtopping

introduces tractive shear stress on the downstream surface.

The erosion process begins at a weak point where the

tractive shear stress exceeds a critical resistance that keeps

the material in place. Then, it proceeds under the action of

flowing waters with the eroded material being transported

downstream. The extent of breaching depends on the

duration of overtopping and the properties and structure of

the embankment (Chang and Zhang 2010). The erosion

characteristics are different for granular and cohesive

embankments. For granular embankments, surface slips

take place quickly due to the flow on the downstream slope;

and hence granular materials are removed rapidly layer by

layer. Towards the end of the erosion process, the breach

tends to flatten, depending upon the materials and the

general slope of the foundation surface. Fig. 1 shows the

breaching process of a granular embankment by

overtopping. According to the definitions by Wahl (1998),

the breach initiation phase involves stages 1-2 while the

breach development phase involves stages 3-7 in Fig. 1.

1 1 - - 2 2

3 3 - - 7 7

(b) (b)

7 7 6 6

5 5 4 4

3 3 2 2 1 1

间间

(a) (a)

Separating Point

Fig. 1: Breaching Process of a Granular Embankment by

Overtopping: (a) Side View, (b) Downstream View (Breach

Initiation Phase: Stages 1-2 & Breach Development Phase:

Stages 3-7)

For cohesive embankments, erosion often begins at the

embankment toe and advances upstream, undercutting the

slope and causing the removal of large chunks of materials

due to tensile or shear failure of the soils on the steepened

slope. In some cases a series of stairstep headcuts forms on

the downstream face of the dam. Fig. 2 shows the breaching

process of a cohesive embankment by overtopping, in which

(b) (b)

4 4 6 6 7 7 3 3 2 2 1 1

5 5

间间

(a) (a)

1 1 - - 4 4

5 5 - - 7 7

Separating Point

Fig. 2: Breaching Process of a Cohesive Embankment by

Overtopping: (a) Side View, (b) Downstream View (Breach

Initiation Phase: Stages 1-4 & Breach Development Phase:

Stages 5-7)

Assessing Risks of Breaching of Earth Dams and Natural Landslide Dams 83


breach development phase involves stages 5-7. If a granular

embankment contains a vertical cohesive core, the core will

erode in a manner similar to that for a cohesive

embankment. If the core is sloped with structural support

from the downstream shell, the core will fail structurally

after the downstream shell is eroded away.

Seepage Erosion/Piping

Another common cause of failure of embankment dams is

seepage erosion/piping. McCook (2004) emphasized the

distinction between seepage erosion and piping to help

differentiate between the phenomena of flow through cracks

or structural contacts versus flow through granular media.

Piping is defined as the progressive removal of soil particles

from a soil mass by percolating water, leading to the

development of channels. Piping often occurs in granular

embankments. Cohesive embankments offer a greater

erosion resistance than granular fills do, and hence are less

likely to suffer from piping. Seepage erosion occurs when the

water flowing through a crack or defect erodes the soil from

the walls of the crack or defect. Similar to overtopping, the

(b1) (b1)

1 1 2 2

1 1 - - 2 2

(a1)(a1)

(b2)(b2)

33

33--44

(a2)(a2)

44

坍塌坍塌Collapse

(b3)(b3)

55

55--66

(a3)(a3)

66

Fig. 3: Breaching Process of a Granular Embankment bySeepage Erosion/Piping: (a) Side View, (b) Downstream

Materials above the Pipe Becomes Unstable and Collapses,View (Breach Initiation Phase: Stages 1-4 & Breach

Development Phase: Stages 5-6)

breaching characteristics by seepage erosion/piping are also

different for granular and cohesive embankments. At the

original stage of seepage erosion/piping, soil particles are

often removed slowly by seeping water. Then, a pipe is

initially formed in the downstream slope after a significant

amount of material has been carried away. Once the pipe is

formed, the erosion process becomes rapid and the pipe

advances upstream. For granular embankments, the

materials above the pipe collapse when the pipe is sufficiently

large, which is followed by a similar process as in the case of

overtopping. Fig. 3 shows the breaching process of a granular

embankment by seepage erosion/piping, in which the breach

initiation phase involves stages 1-4 while the breach

development phase involves stages 5-6.

For cohesive embankments, the portion of the materials

above the pipe may keep stable, and hence the initial pipe in

the downstream slope continues to advance till the upstream

slope face. A penetrated pipe is finally formed through the

embankment. Fig. 4 shows the whole breaching process of a

cohesive embankment by seepage erosion/piping, in which


breach development phase involves stages 4-5.

(b1)(b1)

11

3322

11--33

(a1)(a1)

(b2)(b2)

55

44--55

(a2)(a2)

44

Fig. 4: Breaching Process of a Cohesive Embankment by

Seepage Erosion/piping: (a) Side View, (b) Downstream View

(Breach Initiation Phase: Stages 1-3 & Breach Development

Phase: Stages 4-5)

Failure Modes

Without channelized spillways or other protected outlets,

landslide dams commonly fail by overtopping, while earth

and rockfill dams fail by both overtopping and piping.

Fig. 5 presents and compares the failure modes of landslide

dams and man-made earth and rockfill dams. Out of the

144 landslide dam failure cases with known failure modes,

91% (131 cases) failed by overtopping, 8% (12 cases) by

piping, and 1% (1 case) by slope failure.


The distribution of the failure modes of landslide dams

is different from that of man-made dams. The corresponding

percentages for 176 man-made earth and rockfill dam

failure cases (Xu and Zhang, 2009) are 58% by overtopping,

37% by piping and 5% by slope failure, respectively.

1%8%

91%

5%

37%

58%

0

30

60

90

120

150

Overtopping Piping Slope failure

Failure mode

Nu

mb

er

of ca

se

s

Landslide dams

Man-made earth and rockfill dams

Fig. 5: Failure Modes of Landslide Dams (144 cases) and

Man-made Earth and Rockfill Dams (176 cases)

Longevity of Dams

Table 1 shows the ages of 1065 man-made earth dams at

the time of failure. A dam is most likely to fail within its

first five-year service, especially during the first year after

construction.

Landslide dams last from several minutes to several

thousand years, depending on factors such as volume, size,

geometry, and sorting of blockage materials; rates of

seepage through the blockage; and rates of sediments and

water that flow into the newly formed lake (Costa and

Schuster, 1988). The longevity of 204 landslide dam cases

in the HKUST database covers a large range from 6

minutes to 20000 years. Fig. 6 shows the longevity

distribution for the historical landslide dam cases. 87%

of the cases lasted less than 1 year; 83% less than 6

months; 71% less than 1 month; 51% less than 1 week

and 34% less than 1 day.

Table 1: Ages of 1065 Earth Dams at Failure

Age Range (Year) Number of Cases Percentage (%)

Under Construction 32 3.0 0-1 89 8.4

1-5 80 7.5 5-10 38 3.6

10-15 38 3.6 15-20 26 2.4

20-30 36 3.4 30-40 21 2.0 40-60 36 3.4 60-80 22 2.1

80-100 10 0.9 100-150 10 0.9

>150 4 0.4 Unknown 623 58.5

Sum 1065 100.0

0

20

40

60

80

100

0 50 100 150 200 250 300 350 400

Age of landslide dams at time of failure (day)

Pe

rce

nta

ge

of la

nd

slid

e d

am

s th

at fa

ile

d

be

low

in

dic

ate

d a

ge

(%

)

87% lasted ≤ 1 year

83% lasted ≤ 6 months

79% lasted ≤ 3 months

71% lasted ≤ 1 month

51% lasted ≤ 1 week

34% lasted ≤ 1 day

8% lasted ≤ 1 hour

Fig. 6: Age of Landslide Dams at Time of Failure (204 Cases)

Differences Between Earth Dams and Landslide Dams

Earth dams and landslide dams differ significantly in their

formation processes and should be treated differently in

assessing their risks. The main differences in the two types

of dams are:

(1) A landslide dam is part of a natural landslide

deposit, while an earth or rockfill dam is

usually well designed and constructed.

(2) Landslide dams consist of heterogeneous rock

and soil materials, so that their mechanical

parameters such as erodibility and

permeability vary significantly across the dams

and are rather uncertain. The geometry of

landslide dams is also irregular.

(3) Soils in landslide dams are not compacted

sufficiently; thus, their erodibility and

permeability are generally higher than those

of man-made earth and rockfill dams.

(4) Given the same reservoir capacity, landslide

dams are usually more massive. When

breaching occurs, more soil materials need to

be eroded, which may increase the breaching

duration and lower the peak discharge.

(5) Landslide dams are usually wider and gentler,

which reduce the chance of piping failure and

slope failure.

(6) Landslide dams often form in remote

mountainous areas, which are difficult to

monitor; so fewer performance data are

available.

(7) Limited flood control measures (e.g. spillways)

and seepage control measures (e.g. corewalls

and filters) are available in landslide dams;

therefore, most of them failed in a short period

after formation.

4. BREACHING PARAMETERS

Typically, dam breaching parameters can be divided into


two groups: geometric and hydrographic parameters. A dam

breach often has a trapezoidal shape, with the geometric

parameters of breach depth Hb, breach top width W

t, average

breach width Wave

, and breach bottom width Wb as shown

in Fig. 7. Any combination of three of these geometric

parameters determines the breach shape and size.

Hydrographic parameters include peak outflow rate Qp

and failure time Tf. After the onset of breaching, the outflow

through the breach increases till it reaches a peak and then

decreases till there is no longer any water in the reservoir

or the breaching process ceases to develop. Failure time Tf

is defined as the period from the inception to the completion

of the breaching process (Singh and Snorrason 1984).

According to Fell et al. (2003), in most cases it has not

been possible to identify the time of initiation of erosion,

and the first signs of erosion tend to be in the progression

phase. Therefore, from a practical standpoint, failure time

is often recorded at the start of the breach development

phase. Failure time Tf in this study is also regarded as the

breach development time. The time for initiation has been

recorded where it is possible, e.g., from increased seepage

flows.

Wt

Wb

Hb

Hd

Hd = dam height

Hb = breach depth

Wt = breach top width

Wb = breach bottom width

Fig. 7: Geometric Parameters of a Dam Breach

Breaching Parameters of Earth and Rockfill Dams

Two sorts of prediction models have been developed:

empirical formulas based on case study data and physically-

based prediction models. Xu and Zhang (2009) summarized

the development of these two sorts of models. Based on the

information from a dataset of 182 earth and rockfill dam

failure cases, Xu and Zhang (2009) proposed a

multiplicative regression model to develop empirical

relationships between five breaching parameters (breach

depth, breach top width, average breach width, peak outflow

rate, and failure time) and five selected dam and reservoir

control variables (dam height, reservoir shape coefficient,

dam type, failure mode, and dam erodibility). These

empirical relations are summarized in Tables 2(a) and 2(b).

The relative importance of each control variable to the dam

breaching parameters is also compared. The dam erodibility

is found to be the most important factor, influencing all

the five breaching parameters. The reservoir shape

coefficient and the failure mode also play important roles

in the prediction models.

Table 2: Summary of Empirical Relations for Estimating the

Breaching Parameters of Earth Dams

(a) Full-variable equations

Parameter Equation and Coefficients1

1.2740.1991/3

5/30.175

p DT FM ERd w

r ww

Q H Ve

H HgV

−

+ +

=

Dam type DT Failure mode

FM Erodibility ER

CW2 –0.503 Ove.

3 –0.705 High

4 –0.007

CF2 –0.591 Pip.

3 –1.039 Med.

4 –0.375

Peak

outflow rate

Qp

HF2

–0.649 – – Low4 –1.362

0.453 0.025b d

d r

H HDT FM ER

H H

= − + + +


FM Erodibility ER

CW2 0.145 Ove.

3 0.218 High

4 0.254

CF2 0.176 Pip.

3 –0.239 Med.

4 0.168

Breach

depth Hb

HF2

0.132 – – Low4 0.031

0.5080.0921/3

1.062 DT FM ERt d w

b r w

W H Ve

H H H

+ +

=


FM Erodibility ER

CW2 0.061 Ove.

3 0.299 High

4 0.411

CF2 0.088 Pip.

3 –0.239 Med.

4 –0.062

Breach top

width Wt

HF2

–0.089 – – Low4 –0.289

0.6520.1331/3

0.787 DT FM ERave d w

b r w

W H Ve

H H H

+ +

=


FM Erodibility ER

CW2 –0.041 Ove.

3 0.149 High

4 0.291

CF2 0.026 Pip.

3 –0.389 Med.

4 –0.140

Average

breach

width Wave

HF2

–0.226 – – Low4 –0.391

1.2280.7071/3

0.304 DT FM ERb d w

r r w

T H Ve

T H H

+ +

=


FM Erodibility ER

CW2 –0.327 Ove.

3 –0.579 High

4 –1.205

CF2 –0.674 Pip.

3 –0.611 Med.

4 –0.564

Breaching

time Tb

HF2

–0.189 – – Low4 0.579

Note:

1 Hd = dam height, H

r = 15 m, V

w = released water volume, H

w =

drop of water level, Tr = 1 hour, g = 9.8 m/s2.

2 CW = dam with corewalls, CF = concrete faced dam,

HF = homogeneous/zoned-fill dam.3 Ove. = overtopping, Pip. = piping.4 High, Med., and Low = high, medium, and low erodibility,

respectively.


(b) Simplified equations

Parameter Equation and Coefficients

1.2761/3

5 / 30.133

p FM ERw

ww

Q Ve

HgV

−

+

=

Failure mode FM Erodibility ER

Overtopping –0.788 High –0.089

Piping –1.232 Medium –0.498

Peak outflow

rate Qp

– – Low –1.433

0.025b d

d r

H HER

H H

= −


Overtopping – High 1.072

Piping – Medium 0.986

Breach depth

Hb

– – Low 0.858

0.5581/ 3

0.996 FM ERt w

b w

W Ve

H H

+

=


Overtopping 0.258 High 0.377


Breach top

width Wt

– – Low –0.288 0.739

1/3

5.543 FM ERave w

b w

W Ve

H H

+

=


Overtopping –1.207 High –0.613


Average

breach width

Wave

– – Low –1.268

1.2460.6541/3

b d w

r r w

T H VER

T H H

=


Overtopping – High 0.038

Piping – Medium 0.066

Breaching

time Tb

– – Low 0.205

Breaching Parameters of Landslide Dams

Breaching of landslide dams is sometimes studied by using

empirical equations for man-made earthen dams. This is

not reasonable because of the aforementioned distinct

differences between these two types of dams. Based on

records of 52 landslide dam cases with detailed breaching

information in the HKUST landslide dam database,

empirical models for estimating the breaching parameters

of landslide dams are developed following the similar

procedure in Xu and Zhang (2009). The empirical relations

are summarized in Table 3.

Comparison of Breaching Parameters of Earth Dams

and Landslide Dams

Figs. 8 and 9 compare the values of peak flow rate and

breach duration for man-made earth and rockfill dams and

landslide dams, respectively. The peak outflow rate, Qp,

increases with dam height or lake volume outflow rate for

both types of dams as shown in Fig. 8. Qp from landslide

dams is less sensitive to dam height, since a larger volume

the models for landslide dams (Table 3) are applied to

Table 3: Summary of Empirical Relations for Estimating the

Breaching Parameters of Landslide Dams

(a) Full-variable equations

Para-

meter Equation

1

Erodibility

Coefficient, α

High 1.276

Med. –0.336 Qp

1/3-1.417 -0.265 -0.471

1/2 5/ 2

1/31.569

( ) ( ) ( )

( )

p d d d

d c d d

al

d

Q H H V

g H H W H

Ve

H

=

Low –1.532

High –0.316

Med. –0.520 Hb

1/30.882 -0.041 -0.099

1/ 30.139

( ) ( ) ( )

( )

b d d d

c c d d

al

d

H H H V

H H W H

Ve

H

=

Low -2

High 1.683

Med. 1.201 Wt

1/ 30.752 0.315 -0.243

1/30.682

( ) ( ) ( )

( )

t d d d

c c d d

al

d

W H H V

H H W H

Ve

H

=

Low -2

High 0.775

Med. 0.532 Wb

1/ 3

1/ 3

0.004( ) 0.050( ) 0.044 ( )

0.088( )

b d d d

d c d d

l

d

W H H V

H H W H

Va

H

= + −

+ +

Low -2

High –0.635

Med. –0.518 Tb

1/30.262 -0.024 -0.103

1/30.705

( ) ( ) ( )

( )

b d d d

r c d d

al

d

T H H V

T H W H

Ve

H

=

Low -2

Note:

1 Hd = dam height, W

d = dam width, H

c = 1 m, V

d = dam volume,

Tr = 1 hour, g = 9.8 m/s2.

2 No records are available for the low erodibility coefficient

cases.

(b) Simplified equations

Parameter Equation Erodibility

Coefficient, α

High 1.236

Med. –

0.380 Qp

1/3

-1.371 1.536

1/ 2 5/ 2( ) ( )

p ad l

d c d

Q H Ve

g H H H=

Low –

1.615 High –

0.500 Med.

–

0.673 Hb

1/3

0.923 0.118( ) ( ) ab d l

c c d

H H Ve

H H H=

Low –

High 0.588

Med. 0.148 Wt

1/30.911 0.271

( ) ( )at d l

c c d

W H Ve

H H H=

Low –

High 0.624

Med. 0.344 Wb 1/ 3

0.003( ) 0.070( )b d l

d c d

W H Va

H H H= + +

Low –

High –

0.805

Med. –

0.674 Tb

1/ 30.293 0.723( ) ( ) ab d l

r c d

T H Ve

T H H=

Low –


predict the peak outflow rate from landslide dams, the mean

bias factors (i.e. the mean of the ratios of measured flow

rate to predicted flow rate) of the models are about 1.01,

which is close to unity. However, if the models for earth

and rockfill dams (Table 2) are used to predict the peak

outflow rate from landslide dams, the mean bias factors

will be 0.34, which means that the equations for earth and

rockfill dams may, on average, overestimate the peak flow

rate from landslide dams by nearly 200%.

10

100

1000

10000

100000

1000000

0 50 100 150 200 250 300

Dam height (m)

Pe

ak

ou

tflo

w r

ate

(m

3/s

)


Landslide dams

Landslide dams induced by the Wenchuan earthquake


Landslide dams

(a)

10

100

1000

10000

100000

1000000

0.1 1 10 100 1000 10000

Lake volume (million m3)

Pe

ak

dis

ch

arg

e (

m3/s

)


Landslide damsLandslide dams induced by the Wenchuan earthquake

Man-made earth and

rockfill dams

Landslide dams

(b)Fig. 8: Comparison of Peak Outflow Rates from Failure of

Landslide Dams and Earth and Rockfill Dams: (a) Influence of

Dam Height; (b) Influence of Lake Volume

The breaching duration of a landslide dam is generally

longer than that of a man-made dam with the same dam

height or lake volume, as shown in Fig. 9. The main reason

is that the volume and width of the landslide dam are

typically larger and the downstream slope is typically

gentler, which reduces the erosion velocity during

breaching. The breaching duration is statistically only

weakly related to the dam height as shown in Fig. 9(a). A

higher dam dose not definitely have a longer breaching

process, because the breach often does not develop to the

bottom of the dam. On average, the actual breaching

duration of landslide dams is 2.76 times of that predicted

using the equations for earth and rockfill dams.

In summary, the direct application of the prediction

models for man-made dams to landslide dams would lead

to overestimation of the peak outflow rate (by about 200%

using Xu and Zhang’s models) and underestimation of the

breaching duration (by 64% using Xu and Zhang’s models).

Separate models should be used.

0

4

8

12

16

20

0 40 80 120 160 200 240

Dam height (m)

Bre

achin

g d

ura

tion (

hour)

Man-made arth and rockfill dams

Landslide dams


Man-made earth and rockfill dams Landslide dams

(a)

0

4

8

12

16

20

0.1 1 10 100 1000 10000

Lake volume (million m3)

Bre

ach

ing

du

ratio

n (

ho

ur)


Landslide dams



Landslide dams

(b)

Fig. 9: Comparison of Breaching Durations of Landslide Dams

and Earth and Rockfill Dams: (a) Influence of Dam Height; (b)

Influence of Lake Volume

5. RISK ASSESMENT AND MANAGEMENT

The procedure of general risk assessment has been described

by many researchers. In the case of dam failure risk

assessment, the following steps may be followed.

Information Collection and Observation

When a dam is reported to be unsafe or a landslide dam

has just formed, it is necessary to set up an observation

plan to closely monitor the safety of the dam, including

but not limited to reservoir water level, inflow into the

reservoir, leakage from the dam, movements of the dam,

and the weather conditions.

At the same time, all necessary information required

for evaluating the structural safety of the dam and possible

consequences of failure should be collected. Such

information includes geologic profiles and the material

zoning within the dam, geotechnical properties of the dam

materials, the river network downstream the dam,

population distribution and statistics, land use, business

operations etc. If such information is not available then

necessary exploration or surveys are required.

Evaluation of Structural Safety of the Dam and

Possible Failure Modes

The structural safety of the dam giving expected loading

conditions should now be assessed. Reliability theory may

be a valuable tool in this step considering the high level of

uncertainties in the dam materials and loadings. For many


landslide dams, it is certain that these dams will be

overtopped at a later time. The questions are when will

overtopping occur and whether the dam will fail by piping

or not before the dam is overtopped. Risk mitigation

measures, such as the construction of division channels,

are usually very effective at an early stage.

Determination of Dam Breaching Parameters

In this stage, the possible breach size and the corresponding

dam breaching hydrograph should be estimated. It is

understandable that any estimates are associated with a high

level of uncertainty.

Flood Routing and Inundation Analysis

How the dam breaching flood travels along the down-stream

river should now be studied so that inundation zones can

be identified and the population to be affected by the flood

can be warned ahead of the arrival of the flood. Analysis

tools such as BREACH (Fread, 1988) and HEC-RAS

(Institute for Water Resources, 2008) are useful in

preliminary flood routing analysis.

Estimation of Risks

The consequences of dam breaching floods can be divided

into four categories (Bonnard, 2009):

(1) Social Impact: Deaths and injuries in both body

and mind.

(2) Physical Impact: Direct impact as property loss

(e.g. buildings, lifelines, and vehicles).

(3) Economic Impact (Indirect impacts): Primary

sector (e.g. agricultural and mining); secondary

sector (e.g. industrial and craft production),

tertiary sector (e.g. service activities and trade).

(4) Environmental Impact: Environmental loss like

forests, air, water, earth.

Accordingly, risks can be divided into four categories.

In each category, the total risk is the sum of the risks caused

by all possible events, and the risk in one event is the

product of the occurrence probability of the event and the

resulting consequence when this even occurs. The

assessment of the four categories of risks is rather difficult.

Particularly, one event may evolve over time and becomes

a disaster chain (Zhang 2009). An example has been

presented by Peng et al. (2010) to illustrate how the loss of

human life can be estimated.

Risk Mitigation Decision

Risks can be reduced by decreasing the probability of

failure of the dam (e.g. decreasing the loadings, i.e., use

of a division channel to decrease the reservoir volume

and in turns the dam breaching flood, or increasing the

resistance, i.e., strengthening the dam), or reducing the

elements at r isk (e.g. evacuating the population

downstream the dam).

All these measures are costly. Optimal decisions on

risk mitigation measures can be made through a cost-benefit

analysis.

6. EXAMPLE: BREACHIING OF TANGJIASHAN

LANDSLIDE DAM, SICHUAN, CHINA

Tangjiashan Landslide Dam, with a dam height of 82 -

124 m, a dam width of 612 m, a dam volume of 20.4 million

m3 and a reservoir capacity of 316 million m3, is the largest

landslide dam induced by the Ms 8.0 Wenchuan earthquake

in May 2008. The dam mainly consists of three layers of

soil and rock materials: gravelly soil 5 - 15 m in thickness,

strongly weathered cataclasite 10 - 15 m in thickness, and

weakly weathered cataclasite as shown in Fig. 10. The

gravelly soil and strongly weathered cataclasite are of

medium erodibility.

Gravelly soil

Strongly weathered cataclasite

Weakly weathered cataclasite

Bed rock

82m

803m

Spillway

Breach

200 400 600 800 1000 1200 14000600

700

650

750

800

740.0mGravelly soil

Strongly weathered cataclasite

Weakly weathered cataclasite

Bedrock

752.2 m

Ele

vat

ion (

m)

Distance (m)

Fig. 10: Cross section of Tangjiashan Landslide Dam: (a)

Across the River; (b) Along the River

The locations of seven towns downstream the dam are

shown in Fig. 11. The landslide dam is located at 3.5 km

upstream of Beichuan Town and about 85 km upstream of

Mianyang City. Mianyang City has a population of 1.2

millions. There was a public panic when such a large

landslide dam was found. Would the landslide dam break?

When would the dam break? How large would the breaching

flood be? How many people would be at risk? When should

the population at risk be warned? How did warning at

different times compare?

The Chinese Hydro-Police Corps excavated a division

channel, with a length of 475 m, a width of 25 m and a

depth of 12 m. The channel lowered the crest elevation

from 752 m to 740 m, and reduced the lake capacity from

316 million m3 to 247 million m3. The water level rose 740

m at 7:08 am on 7 June 2008 with an overtopping flow

rate of less than 1 m3/s. The dam finally breached on 10

June 2008. The outflow rate increased rapidly and the dam

breached in the early morning of 10 June: 497 m3/s at 7:42


am, 2190 m3/s at 10:00 am, and 6500 m3/s (the peak outflow

rate) at 11:30 am. The water level decreased to 714.1 m at

14:00 pm on 11 June from the maximum level of 743.1 m

at 1:30 am on 10 June. The corresponding lake volume

decreased from 247 million m3 to 86 million m3. The final

breach has a depth of 42 m, a top width of 145- 235 m and

a bottom width of 80-100 m.

Fig. 11: Flood Routing in the River Downstream Tangjisshan

Landslide Dam

HEC-RAS 4.0 is used to simulate the dam breaching

progression and flood routing in the downstream river. Dam

breaks and dike breaks can be simulated using this program

given such breaching parameters as breach size, breaching

duration, and breaching progression.

To simulate breaching progression, three cases are

studied as shown in Table 4. In the real case, the dam

Table 4: Three Cases of Breaching Analyses

Item Measured

Values

Full–variable

Equations for

Landslide Dams

Full–variable

Equations for

Earth Dams

Weir elevation

(m) 740 752 752

Wt (m) 145-235 165.1 263.4 Wb (m) 80-100 59.2 177.5 Hb (m) 30 37.1 68.4

Qp (m3/s) 6500 12316 36574

Tb (h) 14 8.1 5.9

breached 47 hours (from 7:08 of June to around 6:00 am of

9 June) after the water level reached elevation 740 m (the

weir elevation of the discharge channel). While in the

second and third cases, no discharge channel is considered,

and it is assumed that the dam breaches 47 hours after the

water level reaches elevation 752 m, which is the lowest

part of the original crest of the natural dam. In the second

case, predictions of the breaching parameters are made

using the models for landslide dams shown in Table 3. In

the third case, predictions are made using the equations

for earth and rockfill dams in Table 2. The mean values of

the breaching parameters for the three cases are presented

in Table 4. As expected, the equations for earth dams grossly

overestimate the breach size and peak flow rate.

For simulating flood routing, 5303 cross sections are

used to represent the river downstream the dam, 26 of which

having detailed elevation and distance data (Liu 2008).

Manning’s roughness factor values for both the river

channel and the floodplains are taken from Chow (1959).

Figs. 12 and 13 show the flow processes at three locations

in the actual case and the medium edodibility case (the

second case). The peak flow rate in the second case is much

larger than that in the actual case and the breaching time

is slightly shorter than that in the actual case. One of the

reasons is that the lake elevation in the second case is

752 m, while it is 740 m in the actual case.

The flood travel time becomes shorter as the peak flow

rate increases. Thus, earlier warning is needed for larger

peak flow rate cases. The flood in Mianyang City takes

longer to fade due to the lower flow velocity there.

0 1000 2000 3000 4000 5000 6000 7000

6/10 5:00

6/10 7:00

6/10 9:00

6/10 11:00

6/10 13:00

6/10 15:00

6/10 17:00

6/10 19:00

6/10 21:00

6/10 23:00

6/11 1:00

6/11 3:00

6/11 5:00

Time

Flow rate (m

3 /s)

Dam Site Tongkou Town Mianyang City

Fig. 12: Flow Rates in the Real Case

0

2000

4000

6000

8000

10000

12000

14000

6/16

23:00

6/17

1:00

6/17

3:00

6/17

5:00

6/17

7:00

6/17

9:00

6/17

11:00

6/17

13:00

6/17

15:00

6/17

17:00

6/17

19:00

6/17

21:00

6/17

23:00

Time

Flo

w r

ate

(m

3/s

)

Dam site

Tongkou Town

Mianyang City

Fig. 13: Flow Rates in the Medium Erodibility Case

Dekay and McClelland (1993) suggested a model to

calculate the loss of life caused by flooding based on a

regression analysis of casualty data in 29 flooding cases.

Three parameters, namely population at risk (PAR),


warning time (WT) and flood force, are used in the model:

0.440/{1 13.277( ) [0.759( )

3.790( ) 2.223( )( )]}

LOL PAR PAR EXP WT

Force WT Force

= + −

+(1)

where “Force” = the flood severity. Equation (1) may

be simplified by substituting zero or 1.0 for Force. If the

PAR is located in a canyon, where the floodwater is likely

to be very deep and swift, “Force” equals 1.0. On the other

hand, if the PAR is located on a plain, where the floodwater

is likely to be shallow and slow, Force equals zero.

The value of PAR is not equal to the total population.

Only those who are exposed to a certain level of flood are

considered as population at risk. In order to obtain PAR, a

physically-based human-water interaction model (Lind et

al. 2004) for human beings to stay stable in a flow is applied.

In the model human beings may loss stability when the

product of water depth, h, and velocity, v, reaches a certain

limit. Based on experimental data, Lind et al. (2004)

obtained the critical conditions of hv to maintain human

stability in water flow: (hv)cr

ranges from 0.65 to

2.13 m2/s with a sample mean of 1.22 m2/s and a COV of

0.27. In the present study, the critical condition of human

stability is selected as 1.22 m2/s. Based on the results of

the inundation analysis, the PAR values in the real case

and the two assumed erodibility cases (see Table 4) are

shown in Tables 5 and 6.

Table 5: Loss of Lives in the Real Case with Different

Warning Times

Warning at Start

of Breach

Warning When Peak

Flow Occurs at Dam

No

Warning Location

Population

(Person)

PAR

(Person) WT

(h)

LOL

(person)

WT

(h)

LOL

(person)

WT

(h)

LOL

(person)

Beichuan 30000 14926 7.83 0 0.17 428 0 691

Tongkou 7300 0 8.67 0 1.00 0 0 0

Hanzeng 10000 0 8.83 0 1.17 0 0 0

Qianlian 16300 903 9.33 0 1.67 1 0 129 Longfeng 15000 1484 9.83 0 2.17 0 0 175

Shima 20500 0 10.33 0 2.67 0 0 0

Mianyang 1127000 0 10.67 0 3.00 0 0 0

Total 1226100 17312 0 430 996

Table 6: Loss of Lives in the Medium Erodibility Case with

Different Warning Times

Warning at Start

of Breach

Warning When Peak

Flow Occurs at Dam No Warning

Location Population

(Person)

PAR

(Person) WT

(h)

LOL

(Person)

WT

(h)

LOL

(Person)

WT

(h)

LOL

(Person)

Beichuan 30000 28619 4.83 0 0.08 792 0 1007 Tongkou 7300 1267 5.50 0 0.67 24 0 159

Hanzeng 10000 2294 5.83 0 1.00 13 0 229

Qianlian 16300 4498 6.33 0 1.50 4 0 342

Longfeng 15000 4991 6.67 0 2.00 2 0 364 Shima 20500 3140 7.17 0 2.33 0 0 276

Mianyang 1127000 0 7.50 0 2.50 0 0 0

Total 1226100 44809 0 835 2377

In the real case, no fatality was caused in the dam-

breaching event because of early warning and mass

evacuation, although the water level increased more than

7 m at Beichuan Town. The flood arrived at Mianyang, the

second largest city in Sichuan Province, with a peak flow

of 7110 m3/s at 17:18 of 10 June. There was not much

damage to property in Mianyang City either, as the design

flood standards are 10800 m3/s for 20-year returned floods,

13000 m3/s for 50-year returned floods, and 14600 m3/s

for 100-year returned floods (Liu 2008).

One may ask what would have happened if no

engineering measures had been taken to lower the failure

risk (e.g. excavation of the division channel) and the

population at risk had not been evacuated. To answer this

question, the losses of life in three cases of assumed warning

times are evaluated: warning at the start of breaching,

warning when the peak flow occurs at the dam site, and no

warning at all. The results of loss of life in the three cases

are shown in Tables 5 and 6. From the results in the two

tables, the following observations can be made:

(1) The PAR is very sensitive to the peak flow rate. It

changes from 17312 people in the real case to

44809 people in the second case, and 232607

people in the third case, with the peak flow rates

of 6526 m3/s, 12316 m3/s, and 36547 m3/s,

respectively.

(2) The LOL is very sensitive to the warning time. If

the warning time is sufficient, very limited LOL

might occur, whereas the LOL becomes large if

no warning time is available. In the real case, the

Beichuan Town was severely flooded, but no

casualty was caused because the PAR was

evacuated ahead of time.

(3) If there is no warning time, the fatality rate (LOL/

PAR) could be over 5.3% even though a large

number of people are located far away from the

dam site and in low force (plain) areas.

7. CONCLUSIONS

Both man-made earth dams and natural landslide dams

pose enormous risks to the public because of the potentially

catastrophic floods generated by dam failures. This paper

looks into three key issues in dam breaching risk

assessment, namely failure modes and processes, breaching

parameters, and flood risk assessment. The following

conclusions can be drawn:

(1) Man-made earth and rockfill dams most

commonly failed by overtopping and piping,

whereas over 90% landslide dams failed by

overtopping. Landslide dams seldom failed by

piping (8% only).

(2) Breaching parameters are key inputs for breaching

flood risk assessment. Separate equations have


been developed for estimating the breaching

parameters of earth dam and landslide dams. Soil

erodibility is an important parameter that governs

the breaching process. The two types of dams differ

significantly. If the equations for man-made dams

are used for landslide dams, the peak flow rate

could be overestimated on average by nearly 200%

while the breaching duration could be

underestimated by about 60%.

(3) The population at risk and the flood travel time

are very sensitive to the peak flow rate. Early

warning is shown to be an effective way of risk

mitigation. If the warning time is sufficient, very

few casualties might be resulted, whereas the loss

of life may become large if no warning time is

available.

(4) Each step of dam risk assessment is highly

uncertain. The reliability of any estimate should

be aware of in making risk mitigation decisions.

ACKNOWLEDGMENTS

The research reported in this paper was supported by the

Research Grants Council of the Hong Kong SAR (622207),

the National Science Foundation of China (50828901) and

the Ministry of Science and Technology (2009BAK56B05).

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