ORI GIN AL PA PER

Causes of catastrophic failure of Tam Pokhari morainedam in the Mt. Everest region

Rabindra Osti • Tara Nidhi Bhattarai • Katsuhito Miyake

Received: 8 August 2010 / Accepted: 11 January 2011 / Published online: 22 January 2011� Springer Science+Business Media B.V. 2011

Abstract The moraine dam of the Tam Pokhari glacial lake breached on 3 September

1998 and caused a catastrophic flood in the downstream areas. To learn from the event, a

field survey was conducted. The survey team found that a landslide, which is considered to

be responsible for the outburst flood, occurred in the northeast-facing slope of the moraine

dam. The dam internal structure played a crucial role in forming a landslide that triggered

the excess overflow and finally the breach of the dam. The internal structure of the dam

was made of alternating layers of finer and coarser sediments inclining at 30� downstream

and layers are truncated in the upslope direction by a huge pile of unconsolidated and

structureless moraine materials. Since the upstream slope angle of the dam i.e., 40� is

larger than the angle of repose i.e. 35� of sediments, the increased pore water pressure in

the dam triggered a landslide. The rainfall and seismological activities of that particular

day, which hit the record high, were crucial in triggering the failure. It is estimated that the

dam’s north and northeast-facing slopes completely slid involving about 30,000 m3 of

sediment mass of unconsolidated moraine materials above the shear plane. A slope stability

analysis was also performed. The calculated safety factor was 0.85, and the calculated slip

circle agreed with the shear plane marked in the dam. About 18 million cubic metres of

water was swiftly released due to the sudden breach of the moraine dam.

Keywords Climatic change � Mountain hazard � Moraine dam formation and failure �Slope stability � Landslides � Himalaya

R. Osti (&) � K. MiyakeInternational Centre for Water Hazard and Risk Management, Public Works Research Institute,Minamihara 1-6, Tsukuba 305-8516, Japane-mail: osti55@pwri.go.jp

T. N. BhattaraiDepartment of Geology, Tribhuvan University, Tri-Chandra Campus, Ghantaghar, Kathmandu, Nepale-mail: tnbhattarai@wlink.com.np

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Nat Hazards (2011) 58:1209–1223DOI 10.1007/s11069-011-9723-x

1 Introduction

The high mountain regions around the globe are geologically weak and known as global

spot of natural hazards. The frequency and magnitude of such mountain hazards have been

significantly influenced by climatic change (IPCC 2007; UNEP 2007). According to the

reports (IPCC 2007; UNEP 2007), glaciers in the region are rapidly melting due to climatic

change at almost double the rate reported in the early 1970s. Melting process including

sub-arial melting, mechanical fracture, preferential exploitation of structures within buried

ice bodies etc. enlarge existing lakes or creates new lakes (Benn et al. 2000; Richardson

and Reynolds 2000; Watanabe et al. 2009). Many glacial lakes are dammed by uncon-

solidated moraine materials. Such moraine dams can breach for various reasons (Post and

Mayo 1971; RGSL 2003; Hambrey and Alean 2004; Clague 2003; Korup and Tweed 2007)

and in various modes (Costa and Schuster 1988; Grabs and Hanisch 1993; Wahl 1998) and

pose immediate threats of flash floods to downstream communities. Different triggering

factors are proposed in the aforementioned publications to explain possible moraine-dam

breaches in different geological settings; however, it is very challenging to verify rec-

ommended criteria for each event based on field observations mainly due to a lack of

relevant data and difficulties in conducting field investigation in a high-altitude environ-

ment. Further, it is still not clear whether mentioned reasons and modes of dam failure are

the all that represent the context. Indeed, there are only a few scientific reports on GLOF

events, and other available discussions are also primarily focused on historical accounts of

past events, satellite observations, and early warning and possible control measures.

Therefore, there is a knowledge gap, despite the fact that GLOF issues are very much

highlighted in recent years. Before coming to any conclusion on prediction and mitigation

tools, many cases need to be analysed particularly to understand the moraine dam failure

mechanism and GLOF development process. In this paper, a case study of Tam Pokhari

Glacial lake outburst flood in Nepal will be discussed.

Khumbu in Solukhumbu District, Nepal, also known as the Everest region, is one of the

highest altitude regions in the Himalayas, which consists of large number of glaciers and

glacial lakes and is considered a highly GLOF prone area. The Tam Pokhari glacial lake,

locally known as Sabai Tsho, is one of many glacial lakes in the region and located at

27�440N, 86�500E at an altitude of 4,400 m (Fig. 1). A small stream, the Inkhu River,

originates from the lake. The moraine dam of Tam Pokhari breached on 3 September 1998

and resulted in a catastrophic glacial lake outburst flood, which is widely known as glacial

lake outburst flood (GLOF). The event killed a few people and damaged million dollars

worth of property (Osti and Egashira 2009). This is one of such events that have not been

well reported despite the fact that it is one of the serious GLOF events in Nepal (WWF

2005).

Geologically, the region belongs to the ‘‘Higher Himalayan Zone’’ extending

throughout the Himalayas. The Main Central Thrust (MCT), i.e., a low angle reverse fault,

and the South Tibetan Detachment System (STDS), i.e., a normal fault system, mark their

southern and northern boundaries, respectively. The zone consists of a 10–12-km-thick

succession of high-grade metamorphic rocks, such as gneisses, migmatites, schiests,

quartzites and marbles (Gansser 1964). These rocks have developed a steep, rugged

topography throughout the Himalayas.

The region is also characterized by humid climate with snowfall in winter (November–

April) and south-west monsoon in summer (June–September), which together dominates

the regional hydrology. The main type of summer monsoon precipitation is drizzle with

sporadic, more intense events. Rainfall usually starts around 16:00 and continues until

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around 6:00 with its peak around mid night (Bollasina et al. 2002; Ueno et al. 2001).

According to Ueno et al. (2001), 79% of the annual rainfall in the region was observed in

the monsoon season during the period from 1995 to 1999 (observation station located at

86�430E, 27�490N, 3,833 m asl). The region’s daily low and high temperatures for the same

period fluctuated between -11 and 11�C, and January and August were the coldest and

warmest months, respectively.

The main objective of this study was to identify triggering factors and explain a

breaching mechanism of the Tam Pokhari moraine dam aiming at improving the under-

standing and techniques of GLOF disaster assessment, prediction and mitigation. This was

possible only through integrated field observation and practical assessment of the event

Fig. 1 Locations of glacial lakes(circled) in the Inkhu River basinand topographical map of thesurrounding area (red circle is aTam Pokhari lake and bluecircles are other glacial lakes)

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based on field evidences. This paper describes observational evidence of the Tam Pokhari

moraine-dam failure and discusses triggering factors and a dam failure mechanism.

2 Methodology

A field survey including geological investigation was conducted during the late summer,

2008. The drawdown depth of the Tam Pokhari glacial lake was measured in reference to

the permanent water mark observed on the vertical slope located in the northwest part of

the lake. The lake width and length were measured by using an inclinometer and mea-

surement tape. The surface inspection of the moraine dam involved on-the-around

observations of grain size and sediment composition, geometry of the breached part, in-

and out-ward slopes of the dam and other geometric features. On-site sediment size

samplings were done by referencing a measurement tape laid on the exposed surfaces of

the dam. The locations and elevations of observation points were identified with the Global

Positioning System (GPS) and GPSmap hand tool kits. Dam cross sections were measured

at several locations to capture representative dam structures. Rainfall data recorded at three

nearby stations were also analysed to understand any possible influence of rainfall in

triggering the dam failure. Since the region is highly prone to earthquakes, relevant his-

torical seismic data of the region was additionally used to check whether seismic activities

could have had any impact on the event. The surface discharge of the lake was measured at

about 500 m downstream from the lake by using the float (distance travelled by the cork

divided by total time required to travel) method. The discharge measured during field

survey in September indicates roughly the outflow condition before the dam breach. It was

difficult to measure discharge at the outlet of the lake due to the existence of boulders and

an excess amount of seepage flow. However, the discharge measured at 500 m is also

indicative only as it might have been influenced by secondary discharges such as incoming

and outgoing seepage flows. Group and individual interviews were conducted with local

people of Thangnag Village, which is located approximately 1 km downstream from the

lake, especially to cross-check the observations.

The two-dimensional numerical programme namely GeoStudio developed by GEO-

SLOPE International Ltd. in Canada was used to roughly assess the stability of moraine

dam in a given geological and hydrological conditions. GeoStudio is useful to solve a

wide variety of practical geotechnical problems and can be used to simulate both sat-

urated and unsaturated ground-water flows under steady-state and transient conditions

(GEO-SLOPE International Ltd. 2010). The SEEP/W module of GeoStudio can be used

to simulate, in particular, the pore-water pressure changes inside the dam body due to the

accumulation of rainfall or the surge from the lake. However, the SLOPE/W module of

GeoStudio was only used to quantify the stability of the slope at a given pore water

pressure condition, conceptually and quantitatively represented by the factor of safety. In

SLOPE/W, a wide variation of trial slip surfaces can be specified with a defined grid of

circle centres and a range of defined radii. In the modelling process, the surface areas can

be specified on the top and on the toe of the dam, where the trial slip surfaces will enter

and exit respectively. The number of entries and exits can also be specified as the

number of increments along these two lines. The inbuilt Morgenstern-Price Method,

which uses two factors of safety equations: one with respect to moment equilibrium and

another with respect to horizontal force equilibrium, was used to access the stability i.e.

the factor of safety of the slope.

1212 Nat Hazards (2011) 58:1209–1223

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3 Site inspection

Tam Pokhari is a kidney-shaped lake, 1 km long at the surface along the centreline (north–

south) and 350 m wide at its widest point (east–west). The lake is formed within an end

moraine composed mainly of loose and unconsolidated boulders, gravels and sand with a

subordinate amount of silt and clay. No ice component was found within the sediment mass

of both outer and inner parts of the dam. Except the northern side, where massive gneiss is

exposed and forms a steep cliff, all the remaining lake sides are made up of loose moraine

materials laid on the top of the rocky slopes. Since the altitude of the region is above

4,500 m, there is no vegetation cover in the dam and surrounding areas. The end moraine

dam at the outlet is about 100 m high and takes a curvilinear shape. Its top width is

estimated to be 50 m with its upstream and downstream faces inclining at 38 and 25� on

average, respectively (Fig. 2). The frontal or lake-facing slope of the dam is steeper (38�)

than the angle of repose of similar loose moraine materials (30�–35�). These are common

characteristics of unstable moraine dams (Costa and Schuster 1988), and it suggests that

moraine dams can easily deteriorate due to excess rainfall or earthquakes without obvious

surface manifestations. The steep upstream slope of the dam was maintained by the

pressure exerted by propagating glaciers.

Figure 3 shows the upstream facing pictures of the existing lake. As shown in Fig. 3a,

the lake receives melt water from the existing glacier (the Sabai Tsho glacier), which snout

is located about 150 m upslope to the north from the lake head. In the space between the

glacier tongue and the lake is a steep and narrow cliff composed of massive gneiss.

The present outlet, the origin of the Inkhu Khola, lies at the end of the lake and exactly at

Fig. 2 Geometrical feature of Tam Pokhari moraine dam

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the dam breached section (Fig. 3b). The outlet discharge is estimated at 1.4 m3/s, i.e., the

discharge measured at 500 m downstream from the outlet. This is not the time for annual

peak discharge due to the snow melt, and this discharge is far less than the total capacity of

the outlet.

The breached portion of the dam has a trapezoidal shaped opening that has an average

width and height of 60 and 50 m, respectively (Figs. 2c, 3b). The existing outlet (trape-

zoidal opening) is marked with two distinct banks cross-sectioned along the dam alignment

(Fig. 4). The dam materials in the west (right) bank, which is the inner part of the bend in

Fig. 3 Lake level down-draw (50 m), damaged moraine dam and debris plunged into the lake (camerafacing upstream (a) and downstream (b))

1214 Nat Hazards (2011) 58:1209–1223

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the kidney-shaped lake alignment, are comparatively finer than those in the east (left) bank

as clearly seen in the photographs (Fig. 4). Gravels and boulders are predominant in the

left bank while such materials are less populated in the internal structure of the right bank.

This difference in sediment composition, especially in the internal structure of the left and

right banks, resulted from variation in deposition by glacier as the dam was formed at the

end of the curvilinear alignment of the glacier path. As a result of such variation in

sediment compositions, the left-side slope of breached section remains steeper (more than

60�) and continuous, resembling a part of the U-shaped valley, whereas the right-side slope

is gentler (less than 40�) and irregular (Fig. 4). In the pocket (lake facing side) of the dam

on either side of the outlet at the present lake level, fine materials mixed with gravels are

seen predominantly deposited nearby while boulders are in the lake body and spread up to

30 m from the foot of the dam (Fig. 3a). The sediment size distribution of the deposited

sediment mass at the mouth of the lake is illustrated in Fig. 5.

As observed on the right slope of the breached portion (Fig. 6), the dam is made up of

alternating layers of finer and coarser sediments. These sediment layers incline at the same

angle (about 30�), parallel to the downstream-facing slope of the moraine dam. The longer

axis of gravels and boulders found in these layers are also parallel to the inclination of the

sediment layers. These sediment layers are truncated in the upslope direction by a huge pile

of unconsolidated moraine materials in which boulders are randomly oriented and a

Fig. 4 Difference in sediment composition in the left and right banks of dam breached section

Fig. 5 Grain size distribution of deposited sediment mass at the lake mouth

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layering structure is also lacking. An inclined thin layer (dip amount 45� due northeast) of

clayey (crushed) materials marks the contact between the edges of underlying inclined

sediment layers and the overlying unconsolidated moraine materials. The clayey materials

consist of a very few gravels with their longer axis aligned parallel to the 45-degree

inclination of the clay layer. It is to note that the layering structure of dam extended up to

the upstream face of the dam in the past, but the structure was damaged along with the

downward movement of the upstream-facing slope (a mass slide without detachment from

the dam), whose stability became critical after glacier melt. These features of the upstream-

facing part of the dam suggest the previous movement of sediment mass within the dam

and further indicate the fact that the contact plane could have been the share plane of the

landslide.

4 Hydro-meteorological conditions

In order to analyse rainfall trends, 28-year rainfall data (1974–2002) observed at the

Chaurikharka Weather Station in Lukla, the nearest available permanent weather station

monitored by the Department of Hydrology and Meteorology of Nepal, were examined.

Another set of data is also available for 5 years (1994–1999) from short-term weather

monitoring conducted by Ueno et al. (2001) at Syangboche (27�490N, 86�430E,), located

closer to Tam Pokhari than Chaurikharka. The trend of rainfall during the peak monsoonal

months (July, August and September) at the Chaurikharka station for the period of

1974–2002 indicates that there was no significant change in the 3-month total rainfall

(Fig. 7). However, August monthly rainfall was noticeably in an increasing trend, while

July and September monthly rainfalls were in a decreasing trend over the same period

(Fig. 7). The data from Ueno and his colleagues clearly shows that the rainfall in the area

was exceptionally large in August 1998. The sharp increase in rainfall in the month of

August was also confirmed based on the data set from the Chaurikharka Station (Fig. 7).

Furthermore, at Chaurikharka and also at another nearest weather station in Chepuwa

(25�900N, 87�250E), the daily rainfall was also found to be the highest during 29 August–

2 September 1998 (Fig. 8). Therefore, the August monthly rainfall at the nearest

Fig. 6 Internal structure of the Tam Pokhari moraine dam

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observatory surpasses the historical high from 1974 to 2002, while the 3 days rainfall from

29 August to 2 September 1998 was the highest in 1998.

5 Seismological condition

It is also important to note that the study area is highly prone to earthquake. The earthquake

history of the region (USGS 2009), particularly the marked epicentres within 300 km

radial distance from the lake, indicates that about twenty high-magnitude earthquake

events (over 4 Richter Scale) were recorded during the last week of August to the first

week of September 1998 (Table 1), which is extremely high in the earthquake history of

the region. The first biggest trigger of magnitude 5.9 was recorded on 25 August, 1998

followed by 11 consequent earthquake events on the same day. The other events of

magnitude over 5 were also recorded on 28 and 30 August followed by two other triggers

of magnitude over 4 on 30 August, 1998. There are eight recorded earthquake events in

total during 1973–2009, which have epicentres within 15 km aerial distance from the lake,

and four of them were recorded on the day of the dam breach. Referring to the trend of

occurrences of earthquake events during that time period, there is also a chance that several

low-magnitude (less than magnitude 4 i.e. besides four major events of the day) earth-

quakes could have hit the area on 3 September 1998 prior to the dam breach. The events

with magnitude less than 4 are not documented in the report (USGS 2009). In fact, a report

by Sponsortrek (2008) quoted the local people as well as the government sources and

claimed that three earth quakes hit this region on September 2nd and 3rd and the one that

took place at 5.50 am (about 10 min prior to the dam breach) on 3 September had a

magnitude of 3.77 on the Richter Scale. Although none of the recorded timings of the

Fig. 7 Observed monthly rainfall at Chaurikharka Hydrological station

Fig. 8 Observed 6 days daily rainfall at Chepuwa and Chaurikharka (after Ueno et al. 2001)

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earthquakes exactly coincides with the timing of Tam Pokhari moraine dam failure i.e.

6.15 am in the morning of 3 September 1998 as quoted by the local people, the record of

that particular period clearly shows the evidence that the earthquake is one of the factors to

destabilize the moraine dam structures prior to the breach.

6 Result and discussion

The observation results suggest that the surface area of the lake reduced by 44% due to the

1998 dam breach event. The total water volume discharged during the event was

approximately 18 9 106 m3, which was calculated based on the observed drawdown depth

Table 1 Earthquake events recorded within 300 km distance from the Tam Pokhari Glacial Lake during 15August to 15 September 1998 (USGS 2009)

No Date Time Location Magnitude

(UTC) Latitude Longitude (Richter Scale)

1 August 24, 1998 7:32:21 30.001 87.919 5.0

2 August 25, 1998 7:41:40 30.079 88.109 5.9

3 August 25, 1998 7:59:00 30.024 88.188 4.6

4 August 25, 1998 8:01:45 30.061 88.172 4.1

5 August 25, 1998 8:13:04 30.172 88.134 3.9

6 August 25, 1998 9:43:06 29.998 88.099 4.1

7 August 25, 1998 9:56:44 29.831 87.910 4.4

8 August 25, 1998 10:25:06 29.983 88.103 4.4

9 August 25, 1998 12:29:45 30.022 88.060 4.6

10 August 25, 1998 12:43:04 29.961 88.091 4.4

11 August 25, 1998 13:39:49 29.926 88.124 3.8

12 August 25, 1998 13:50:06 30.099 88.054 4.1

13 August 25, 1998 15:16:02 29.976 88.105 4.4

14 August 28, 1998 22:01:55 30.191 88.150 5.0

15 August 30, 1998 3:37:48 30.044 88.077 5.0

16 August 30, 1998 3:49:48 29.904 87.989 4.0

17 August 30, 1998 4:11:35 30.034 88.081 4.6

18a September 3, 1998 18:15:56 27.85 86.941 5.6

19 September 3, 1998 18:22:19 27.973 87.040 –

20a September 3, 1998 18:51:38 27.65 86.802 4.4

21a September 3, 1998 21:00:53 27.729 86.880 4.1

22a September 3, 1998 23:02:28 27.6 86.733 4.6

23 September 4, 1998 0:36:17 27.455 86.464 4.1

24a September 4, 1998 1:10:18 27.777 86.793 4.2

25 September 4, 1998 21:16:13 27.651 86.720

26a September 6, 1998 21:35:49 27.859 86.895 4.6

27 September 10, 1998 22:57:16 27.199 88.341 4.7

28a September 12, 1998 5:08:52 27.852 86.900 4.2

a Epicentre located within 15 km radius from the Tam Pokhari Glacial Lake

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(50 m) and the difference in surface area. According to local people, the overflow from the

lake continued for several hours, while the maximum flow lasted for about half an hour.

Snow avalanches are often considered to have caused many historical GLOF events in

the region. Some researchers (e.g. Vuichard and Zimmermann 1987) have also pointed out

a fact that the glacial lake had breached in the past due to snow or ice avalanches in the

region. In the case of Tam Pokhari Lake, a snow avalanche was considered as the sole

cause of dam failure (Dwivedi et al. 2000). A clear evidence of snow avalanche (not

necessarily on the same day of the dam breach) is also the snout of the existing Sabai Tsho

glacier, located in the northern part of the lake, which has shifted about 150 m upslope than

it was in the past (Figs. 3b, 9). However, if a huge mass of snow or Ice avalanche had

fallen into the lake from a glacier, it would not have been able to cause much impact

because it would have rather formed a pile of snow or ice mass at the glacier foot where the

lake is narrow and shallow. It is also to note that the lake shape does not permit any wave

originating from the lake-head to propagate in a single direction; such a wave usually

reduces its strength due to to-and-fro motion between east and west banks. Therefore, it is

possible to say that a snow or ice avalanche plunged into the lake, displaced the lake water

and generated surges that propagated up to the dam and raised the water level for a while,

but this cannot be the direct cause of the dam breach. Lacking any evidence of impact on

the eastern and western sides of the lake also does not permit to justify the occurrence of

huge surge induced by a snow or ice avalanche. Additionally, the left and right sides of the

lake are steep (more than 60�) and stable mountains (Fig. 3), where a huge snow mass may

not be able to retain for a long period; therefore, chances of snow avalanche generation

from either side slope is also minimum. Similarly, there were also no signs of the

occurrence of rock avalanches from the side slopes around the lake, which indicates that

the dam breach was not caused by a rock avalanche, either.

The moraine dam location, lake shape, surrounding topography and outlet condition

indicate that the lake shape and volume have little changed in recent decades even if there

is an increment in glacier melting rate. The stability of lake size and shape highlights the

fact that the dam did not breach as a consequence of the increase in water volume in the

lake.

Fig. 9 Sabai Tsho Glacier before glacial lake outburst (left—Sponsortrek 2008; right—Photo by MichaelDurham dated October 2005 with permission)

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It is important to note that the continuous rainfall during 29 August to 2 September 1998

had contributed to a significant amount of infiltration into the dam. Since the dam sedi-

ments are heterogeneous, the infiltration may have caused a substantial increase in pore

water presser inside the dam, especially where finer materials are predominant. It is also to

note that summer infiltration is much higher than infiltration due to snow melt in late

winter as the ground is frozen, and snow on the ground start melting from top to bottom.

The seismic activities of that particular period had also helped weaken the soil strength

and increased the pore water pressure inside the soil mass within a short period of time.

Since the upstream slope angle of the dam (e.g. 40� on the lake side) is larger than the

angle of repose (35�) of sediments, the increased pore water pressure in the dam triggered a

landslide. Usually, a landslide is also preceded by saturation of the slope. This is com-

monly caused by heavy rainfall events or periods of extended precipitation that saturated

the upper part of the slope and raised groundwater level. Research results presented by

many researchers such as Fleming and Varnes (1991), Reid (1994), USGS (2004), Iverson

(2000) and Sassa et al. (2010) have shown that a rise in pore water pressure level above a

potential sliding surface can increase and sometimes double the potential for a landslide.

When the pore water pressure at the slip surface increases, it reduces the effective normal

stress and thus diminishes the restraining friction along the slip line. This is combined with

increased soil weight due to the added groundwater. Additionally, the seepage water a)

increased the weight of materials on a slope above their point of gravitational equilibrium,

b) increased pore pressures within a zone of weakness in the materials underlying a slope,

c) decreased the coefficient of friction on a potential sliding surface d) caused soil to

hydrate and expand increasing slope instability and e) dissolved minerals that were binding

the particles of the materials that make up the slope. The mentioned factors decreased the

overall strength of the materials making up the slope.

The geological features of the right bank of the outlet, especially the presence of a shear

plane that inclines towards the lake (Fig. 6), can be taken as evidence of the occurrence of

a landslide. The deposition of finer sediment (silty sand) overlain by coarser sediment and

deposited boulders on the lake bed around the outlet observed at the existing lake level are

an indication of a landslide occurrence on the dam. Landslide materials that plunged into

and deposited on the lake after segregation formed an inverted U-shape debris pile with a

vertex height of about 30 m from the foot of the dam (Fig. 3), as described in the previous

section of this paper. It is estimated that the dam’s north- and northeast-facing slopes

completely slid involving about 30,000 m3 of sediment mass of unconsolidated moraine

materials above the shear plane. The landslide may have led to the following

consequences:

i) The landslide reduced the height and width (thickness) of the dam, which in turn

increased the driving/shearing load and decreased the resisting/normal load in the dam. As

a consequence, the remaining portion of the dam became critically unstable and ultimately

collapsed.

ii) The snow avalanche that developed at the glacier snout plunged into the lake,

displaced the lake water first and then created dynamic waves in the lake. The surges or

seiches of any magnitude reached at the dam site and further increased in pore water

pressure, in addition to the one caused by continuous infiltration, in the dam materials. The

sudden increase in pore water pressure reduced the shear strength of the dam materials and,

consequently, triggered a landslide on the north- and northeast- facing slope of the dam

(near the outlet).

iii) The landslide mass also displaced the lake water and created back water waves.

Returning surges or seiches of any magnitude may have also further enhanced the failure of

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the dam whose stability had been already in a critical state due to the increase in pore water

pressure and decrease in height and width. A huge mass of slid moraine materials still

remain deposited on the foot of the moraine dam as described. The sediment mass could

not have been fully flushed except the part at the centreline of the flow.

iv) The increased discharge at the outlet due to the increased outlet capacity further

helped the dam failure process by rapid erosion. As a result, the dam breach occurred

within a short time inducing a huge water flux, which might have lasted for about 30 min

as noticed by local people.

The stability analysis based on the SLOP/W module in the GeoStudio model results in

the landslide with a slip circle radius of 170 m (Fig. 10) and a safety factor of 0.85. This is

quite reasonable for a case of failure. The detail of acting forces (in kN) and momentum

calculated by using the Morgenstern-Price Method for slice number 13 (shaded in Fig. 10)

as an example is illustrated in Fig. 11. It has been found that the slip circle coincides with

pre-existing share plane in the dam. In this result, the change in pore water pressure due to

Fig. 10 Calculated landslide slip circle

Fig. 11 Free body and force polygon for Morgenstern-Price method

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rainfall infiltration and earthquake are not taken into account. The results agree with the

field evidence and help deduce the conclusion that the main cause of the moraine dam

breach in Tam Pokhari Lake on 3 September 1998 was due to the landslide that occurred in

the north-facing slope of the moraine dam.

7 Conclusion

Moraine dam breaches and associated GLOF disasters have been a long scientific debate

without reaching any concrete conclusions. In order to understand the Tam Pokhari

moraine dam breach mechanism, a 2-week-long field survey was conducted in the Tam

Pokhari glacial lake in the Khumbu region, Nepal. The field observation found that the

internal structure of the Tam Pokhari moraine dam was made of alternating layers of finer

and coarser sediments inclining at 30� downstream. The layering structure extended up to

the upstream face of the dam in the past, but afterwards the upstream facing part of dam

structure slid with the downward movement possibly during glacier melting process.

Therefore, the upstream part of the dam lost its internal structure and remained as huge pile

of unconsolidated moraine materials, in which boulders are randomly oriented and the

layering structure is also lacking. An inclined thin layer of clayey (crushed) materials

marks the contact between the edges of underlying inclined sediment layers and the

overlying unconsolidated moraine materials. The contact plane was a share plane of a

landslide. Noticeably, the August monthly rainfall at the nearest observatory surpasses the

historical high from 1974 to 2002, while the 3 days rainfall from 29 August to 2 September

1998 was the highest in 1998. There were over twenty recorded earthquake events in

7 days time and four earthquake events recorded within the same day of dam breach. On

3 September 1998, due to the increase in pore water pressure in the share plane resulting

from continuous and excess rainfall as well as seismic activities and surge generated

through the snow avalanche, sediment pile above the shear plane slid and generated a

landslide that plunged into the lake body. Returning surges or seiches generated during the

landslide caused further damage to the dam, whose stability had already been critical due

to the landslide. The landslide occurrence is confirmed in the field observation and

numerical simulation. As a result, the Tam Pokhari glacial lake partially emptied, gener-

ating a huge water discharge, which became a catastrophic GLOF in the downstream areas.

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