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Erosion and Sedimentation Caused by Glacial Lake Outburst Floodsin the Nepal and Bhutan Himalayas
Daisuke HIGAKI1* and Go SATO2
1

Faculty of Agriculture and Life Science, Hirosaki University
3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan2
Department of Modern Life, Teikyo Heisei University,
2-51-4, Higashi Ikebukuro, Toshima-ku, Tokyo 170-8445, Japan*e-mail: dhigaki@cc.hirosaki-u.ac.jp

Abstract
Various erosion and sedimentation disasters have been induced by glacial lake outburst floods (GLOFs)
occurring along various segments of rivers in the Nepal and Bhutan Himalayas. This paper discusses suchphenomena from the point of view of GLOF hazard management. In a U-shaped valley formed by past glaciation, a GLOF occurs as a debris flow from breached moraines. GLOFs may cause the collapse of banks along
hills and terraces of glacial and fluvioglacial origin by toe erosion, and the vegetation may not recover fromthe damage caused by the GLOFs for a long time and may act as a sediment source for the downstream river.
The breaching of temporary dams by the sediment-loaded flow of a GLOF or during associated landslides
could aggravate the secondary damage along narrow river courses in V-shaped valleys. In the Lesser andGreater Himalayas, a dense distribution of old deep-seated landslides and rock creep indicates there-activation of landslides on slopes of rivers undercut by GLOFs. Woody debris may be produced fromriparian forests in V-shaped valleys. In the case of gently sloping rivers in the basins of such valleys, bankerosion and the river blockage caused by woody debris from those valleys often pose serious risks, particularly, in terraces along the lower reaches of the rivers. Sand deposition in rivers and irrigation canals may
adversely affect the downstream alluvial plain areas. In the case of topographical units such as river terraces
that are prone to bank erosion, planning should be done for constructing bridges with appropriate dimensions.
Geomorphological classification maps of the regions along rivers and DEM maps of the basins and alluvial
plains are essential for planning for GLOF hazard management.
Key words: erosion, GLOF, Himalayas, landslides, sedimentation, woody debris

1. Introduction
In recent years, retreating glaciers and associatedexpansions of glacial lakes in the Himalayas have increased the risk of catastrophic flooding and are becoming a great threat to mountainous developing countries
with large populations (Bajracharya et al., 2007). In
particular, glacial lake outburst floods (GLOFs) thatfrequently occur owing to the formation and expansion of
glacial lakes in the upper catchments of the Himalayanrivers often cause serious damage downstream along therivers (Yamada, 1998; Mool et al., 2001a, 2001b;
ICIMOD, 2010). However, it is rather difficult to predict
the onset of GLOFs and the resulting damage. Risk
management of GLOFs is an urgent task not only forsecuring communities in the Himalayas but also for
ensuring careful planning and design of various development and watershed management projects. In addition,
post-disaster surveys of GLOFs have indicated extreme
Global Environmental Research
16/2012: 71-76
printed in Japanerosion and sedimentation (Vuichard & Zimmermann,1986). Large-scale landslide morphologies that are concentrated along thrusts such as the Main Boundary Thrust(MBT) and Main Central Thrust (MCT) (Yagi, 1995;
Abe et al., 2001) may become unstable because of toe
erosion caused by a GLOF. Thus, hazard management of
sedimentation and erosion caused by GLOFs requires
detailed knowledge of the relationships among these
phenomena and the different segments of a river.
This paper discusses the erosion and sedimentation
caused by GLOFs in the Nepal and Bhutan Himalayas
along different segments of rivers which are characterized by different geological/geomorphological development such as past glaciation and crustal up-lifting along
the river course.

©2012 AIRIES

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2. Segment-wise Erosion and Sedimentation
along Rivers
2.1 Erosion and sedimentation caused by GLOFs
A GLOF originating from the Dig Tsho glacial lake
(Fig. 1), located at 4,400 m a.s.l., occurred in eastern
Nepal in August 1985. This GLOF washed away a newhydropower station, 14 bridges, and 30 houses (Vuichard& Zimmermann, 1987). It transported 900,000 m3 ofdebris about 2 km away from the breach point anddeposited it in a U-shaped valley (Vuichard &Zimmermann, 1986) (Fig. 2). In October 1994, a GLOF
occurred at the Lugge Tsho glacial lake (Fig. 1) in thecentral part of Bhutan, and the lake discharged about
18 million tons of water that flowed 80 km downstream
along the Pho Chhu River at an average speed of
16.6 km/s (Motegi, 2001). It killed 20 people around
Punakha (Fig. 1) , located at 1,200 m a.s.l., which used tobe the country’s winter capital, and destroyed parts of ahistoric building constructed in 1637 (Motegi, 2001). The

Fig. 1 Map of the locations.
a: Dig Tsho glacial lake, b: Lugge Tsho glacial lake, c: Phunakha,
d: Nare glacial lake, e: Bjizam.

maximum flow rate of this GLOF was 1,600 m3/s at theorigin, (Motegi, 2001). Classification of the terrain units
in the vicinity and flood simulation of GLOFs haveillustrated the primary impacts of GLOFs themselves andtheir secondary impacts such as bank erosion andlandslides (Bajiracharya et al., 2007). Such phenomena
can aggravate various erosion and sedimentation
problems, depending on the topography and geology of
the river segment (Fig. 3).
2.2 U-shaped valleys in upper catchments
Although extreme erosion occurred at the breachpoint of the Dig Tsho GLOF, the debris traveled only ashort distance, possibly because of the gently sloping
riverbeds in the U-shaped, wide valley. The same phenomenon was also observed at the Lugge Tsho GLOF in
Bhutan (Watanabe & Rothachar, 1996).
In 1977, GLOF deposits discharged from the Nareglacial lake (Fig. 1) on a tributary of the Bhote Koshi
River accumulated near the confluence. The deposits
were characterized by randomly distributed gravel of awide range of sizes and a convex shape along the transverse sections, clearly indicating a debris flow.
Areas above the forest line (about 3,900 m a.s.l.) have
not recovered from repeated bank collapses even
30 years after the occurrence of the GLOF from the Nareglacial lake (Fig. 4). These collapses are caused by the
distribution of highly erosion-prone glacial deposits,
fluvio-glacial deposits and thick river-terrace deposits
from past glacial expansion on the U-shaped valley floor.
Deposits from past glaciation due to scouring erosion

Fig. 2 Breached end moraine of Dig Tsho (A) and deposition of debris flow deposits (B) (photos taken by J. Umemura in 2002).

Erosion and Sedimentation by GLOFs in the Nepal and Bhutan Himalayas

73

Fig. 3 Schematic diagram of topographical segments and their images along the main river.

Fig. 4 Bank collapses in glacial deposits originating from the Nare GLOF in 1977.

have been reported to cause landslides along the KaliGandaki River in the Mustang District, Nepal (Higaki,2005). Lateral erosion by a GLOF may cause theselandslides to be reactivated.
2.3 V-shaped valleys
In the downstream segments of the U-shaped valleys,
narrow V-shaped valleys formed by river incision are
dominant. Local witnesses claim that a temporary
dam-up of the Dig Tsho GLOF was formed in the narrowgorge around the confluence of Dudh Koshi and Bhote
Koshi, which was subsequently breached (Fig. 5).
Because the GLOF deposits in the downstream rivercontained boulders with diameters of 1 to 1.5 m amongfine sand with stratification, the sediment transportation

process has been assessed as sediment flow. Bridges overthe downstream river were not destroyed during the
GLOF but reportedly collapsed 30 to 50 minutes later(Bajiracharya et al., 2007). Their collapse may have beencaused by the breaching of the temporary dam. In thissegment, the thick forests along the valley side slopes
tended to produce woody debris (Watanabe & Rothacher,1996). The Dig Tsho GLOF caused large-scale landslides
along the outer bends of the valley (WECS, 1987).
Because many deep-seated landslide configurations
are distributed in the middle reaches of a river (Yagi,
1995; etc.), lateral erosion by a GLOF may induce
instability in these slopes. The possibility of bank erosion
is an issue that needs to be examined on the basis of thespatial relationships among rivers, roads and bridges. It is

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necessary to evaluate the erosion susceptibility of materials that constitute banks such as terraces and talus
slopes. Geomorphological classification maps are necessary for GLOF hazard assessment.
As part of the “Study on GLOFs in the Bhutan
Himalayas” supported by the JST/JICA- SATREPS
project to clarify the characteristics of landslide phenomena, a landslide distribution map was obtained for the
Mangde Chhu basin, Bhutan, by interpreting satellite
images and conducting field surveys. The map shows thatthe mountain slopes are widely occupied by many
deep-seated landslides along the Mangde Chhu basin(Fig. 6) and that many of the landslides have a width ofseveral kilometers. These landslides are topographicallyclassified into two types: slide type and rock-creep type(Fig. 6). It is thought that some of the landslide slopes
located along undercut slopes of the main stream of the
Mangde Chhu may be reactivated by a GLOF and may
exist as unstable slopes.
2.4 Basins
In basins such as Punakha, central Bhutan, densely
populated terrain with intensive agricultural land use may
face various problems owing to erosion or sedimentation
caused by GLOFs (Fig. 7). The Lugge Tsho GLOF in
1994 was studied by viewing video films shot near
Punakha (currently owned by Bhutan Broadcasting
Company) and by interviewing local residents (Higaki& Umemura, 2002), and it was found to have had thefollowing characteristics. The valley floor along the Pho
Chhu River has a large width of about 500 m. The
riverbed slope is about 1/100. The water level rose by
about 2 m and reached 4 m at bends or narrow passes in
the river. The floodwater flowed into the area near
Punakha Dzong (a historic building used both as aprefectural office and monastery) along the old rivercourse. Initially, a large volume of gravel hit the area withthe flood wave. It was followed by a large amount ofwoody debris, mostly from the central part of the river
stream. River blockage by woody debris was reported
near a bridge located downstream (Higaki & Umemura,2002). The flow contained a large amount of sand. Bankerosion occurred on the lowest river terrace surfaces
about 2 m above the river channel consisting of sandy
deposits. However, there were only a few instances of
erosion among the talus slopes and tributary fans because
they consisted of boulders with diameters of 1 m or more.
GLOFs have occurred along the Pho Chhu River in 1960
and 1968 (Motegi, 2001), and a large amount of sand ispresent on the river terraces and along the river path.
Thus, the area is vulnerable to bank erosion.
2.5 Alluvial plains
In alluvial plains such as the Terai Plain, Nepal,
sandy/silty material sedimentation is the dominant characteristic of GLOFs, owing to extremely gentle river
gradients and wide riverbeds. Consequent coursechanges and riverbed aggradations can be expected along
the river.

Fig. 5 Point of temporary damming by the Dig Tsho GLOF.

Fig. 6 Landslide distribution map in and around Bjizam(Fig. 1), Bhutan. 1) Rock-creep type landslides, 2)
Slide type landslides, 3) Scarps induced bylandslides, 4) Terraces.

Fig. 7 Gently-sloping Pho Chhu River and the area of the L2terrace eroded by the Lugge Tsho GLOF in 1994.

Erosion and Sedimentation by GLOFs in the Nepal and Bhutan Himalayas

3. Hazard Management of Erosion and
Sedimentation
Here, we discuss hazard management of erosion andsedimentation phenomena expected along different riversegments in terms of recent land use and infrastructuredevelopment (Fig. 8).
3.1 U-shaped valleys in the Great Himalayas
This segment is located just downstream of glaciers or
glacial lakes, adjacent to them. Huge amounts of sediment are expected to be transported from a moraine
breach by debris flow, which is expected to reach as far as
riverbeds with gradients of 2° or 3° (Ikeya, 1974). Bankerosion and associated slope collapses and landslides
along the river-facing slopes of terraces and river-facing
hills of glacial or fluvio-glacial origin are highly hazardous. Owing to the presence of high-altitude rangeland,
unstable slopes tend to exist and act as a sediment source
for the downstream river for a long time.
3.2 V-shaped valleys in the Great and Lesser
Himalayas
This segment is located in the downstream of
U-shaped valleys. Large-scale bank collapses (slope
failures) are expected at the outer bends. Landslides
widely distributed along the rivers, especially in theLesser Himalayas, and rocks with discontinuity plane in
the Great Himalayas may be reactivated. Terraced
cultivation and houses built on gentle slopes of landslide
origin near the river may also be exposed to the hazardsof a GLOF. Temporary landslides damming narrow
riverbeds and subsequent breaches induce secondary
impacts.
Thus, bridges of sufficient height and length should75

be constructed in consideration of the erosion-prone
topography of this region, such as river terraces (Fig. 9).
Hydropower projects resistant to GLOFs, such as underground power stations, must be designed.
3.3 Gently sloping rivers in basins
In basins surrounded by high mountains, riverbed
gradients are gentle and river terraces are distributed at
different levels. Woody debris blocked by a bridge or
narrow path can cause inundation along the downstream
river. Bank erosion is expected mainly along the lower
terraces where construction of houses and various land
uses are carried out. Sand deposition tends to occur inwide riverbeds.
3.4 Alluvial plains
In large-scale alluvial plains such as the Terai Plain in
Nepal, a GLOF with an extremely large discharge may
cause bridge collapses and siltation of irrigation canals
supplying river water, as well as sedimentation ofagricultural lands. However, GLOF flood simulation ofthis segment is required for risk assessment, because thewater level may not rise considerably due to the long
distance of more than 150 km from the breach point.

4. Conclusion
By studying case histories and by carrying out field
investigations in the Nepal and Bhutan Himalayas, wefound that various erosion and sedimentation disasters
have been induced by GLOFs occurring along the various segments of rivers. The risk of a disaster may be
increased not only by the GLOF itself but also by its
secondary impacts such as bank erosion and consequentlandslides and woody debris from the riparian forests.

Fig. 8 Schematic diagram of hazard types along different river segments.

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Fig. 9 A newly constructed bridge crossing the Dudh Koshi
without the risk of bank erosion of the river terrace.

In U-shaped valleys formed by past glaciation, a
GLOF may take the form of a debris flow from thebreach point. The GLOF may cause bank collapses along
the hills and terraces of glacial and fluvio-glacial origins,
and the vegetation in these regions may not recover for along time, thus acting as a sediment source for thedownstream river.
The breach of temporary damming by the sediment-loaded flow of a GLOF or associated landslides
could aggravate secondary damage along the narrow
river course of V-shaped valleys. Thus, there is highprobability of landslides on the deep-seated landslide
slopes in the Great and Lesser Himalayas.
Along the gently sloping rivers in basins, bank erosion and river blockage by woody debris transported
from the V-shaped valleys often pose serious risks, especially on terraces along the lower reaches of the river.
Sand deposition in the river and irrigation canals may
adversely affect alluvial plain areas.
Geomorphological classification maps, which indicate the locations of landslides, river terraces, anderosion-prone terrains such as moraines, are required forassessing hazards of erosion and sedimentation caused by
GLOFs. Digital elevation model (DEM)-based maps
produced from satellite data (Fujita et al., 2008) can also
be used for detailed assessments of river terraces andalluvial fans and plains.
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Daisuke HIGAKI
Daisuke HIGAKI is a Professor of Erosion Control
Engineering at the Department of Agriculture andLife Science, Hirosaki University, Japan. He
received his Ph.D. in Science from Tohoku
University. His research mainly focuses onlandslide hazard management based on engineering geomorphology. He has been trying to reduce disaster risks induced byerosion and sedimentation in Nepal and Bhutan Himalayas through both
evaluation of geomorphic processes and a community-based approach.

Go SATOGo SATO is an Associate Professor of Geography
at the Department of Modern Life, Teikyo Heisei
University. He received his Ph.D. in Earth
Science from Chiba University. His researchmainly focuses on landslide geomorphology anddisaster prevention studies. Recently, he has beenin charge of landslide distribution mapping to reduce landslide disasters in
Tegucigalpa metro area, Honduras as a JICA expert.

(Received 1 December 2011, Accepted 13 February 2012)