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The 1985 Catastrophic Drainage of a Moraine-Dammed Lake, Khumbu Himal, Nepal: Cause and
Consequences
Author(s): Daniel Vuichard and Markus Zimmermann
Source:
Mountain Research and Development,
Vol. 7, No. 2 (May, 1987), pp. 91-110
Published by: International Mountain Society
Stable URL: http://www.jstor.org/stable/3673305
Accessed: 04-11-2015 13:51 UTC
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source: https://doi.org/10.7892/boris.72507 | downloaded: 22.12.2016
Mountain
Research and
Development,
Vol. 7, No. 2, 1987, pp. 91-110
THE 1985 CATASTROPHIC DRAINAGE
OF A MORAINE-DAMMED
LAKE,
KHUMBU
HIMAL,
NEPAL:
CAUSE AND CONSEQUENCES
DANIEL
VUICHARD1
AND MARKUS ZIMMERMANN2
ABSTRACT On 4 August 1985 Dig Tsho, a moraine-dammed
glacial lake in
the Khumbu area of eastern
Nepal, burst
above Thame.
For the region close to the origin
of the outbreak the
consequences were catastrophic.
The destruction
of a newly
built
hydroelectric power plant, 14 bridges, about 30 houses, and many hectares of
valuable arable
land, as well as a heavily
damaged trail
network,
resulted
from 5 million
m3
of
water
plummetting
down the Bhote Kosi and Dudh
Kosi valleys. The breaching of the moraine was triggered by wave action following
an ice avalanche of 150,000 m3
into the lake.
The surge
had a peak discharge
of
1,600 m3/sec;
3 million
m3
of debris were moved within a distance of less than
40 km. However,
only 10-15 percent
of the material left
the region as suspended load.
The potential
hazard of
glacial lakes persists
and increases. A hazard assessment
including
an identification
of source areas and
subsequent monitoring
of
glacial
lakes
is
proposed. It should be incorporated
into
any development
concept
for the
Himalayan region.
RESUME Cause
et
consiquences
du
drainage
catastrophique
d'un lac
morainique
au Khumbu
Himal,
Nipal, en
1985. Le 4 aofit
1985, le lac Dig
Tsho, un lac morainique situi au-dessus
de Thame dans le Khumbu au Nepal oriental,
s'est deverse subitement
par suite de la rupture
du barrage naturel. Les consequences ont et6 catastrophiques pour la region
avoisinante.
Un volume d'eau d'environ
5 millions de mitres
cubes s'est devers6
rapidement
dans les vallees du Bhote Kosi et du Dudh Kosi,
detruisant
sur
son
passage une nouvelle centrale
hydrodlectrique,
14
ponts,
environ
30 maisons,
une grande quantite
de terres
arables,
et endommageant serieusement le reseau de sentiers battus. La rupture
de la moraine a ete causee par une lame de fond
die a un
'boulement de glace de 150.000 metres cubes dans le lac. Le d6versement maximum a &t de 1.600 mitres cubes par seconde et
3 millions
de metres cubes de debris ont
ete emportis sur une distance de moins
de 40 kilometres.
Neanmoins, au plus 10 a 15 pour
cent des materiaux sont sortis de la region sous forme de charge en suspension.
Les risques
associes aux lacs glaciaires persistent
et
augmentent.
Une 6tude est
propos6e pour
evaluer ces risques; elle
comprendra
l'identification
des zones vulnerables et la surveillance ult6rieure des lacs. Cette etude devra etre incorporee
dans tout projet
de
developpement de la region himalayenne.
ZUSAMMENFASSUNG
Zerstiirerischer
Gletschersee-Ausbruch 1985
des
moranengediimmten
Dig
Sees
im
Khumbu
Himal,
Nepal.
Ursachen
und
Konsequenzen.
Am 4. August
1985 entleerte sich
Dig Tsho,
ein
moriinengedimmter
See oberhalb
Thame, in
Ost-Nepal. Fiir
die
Region in
unmittelbarer
Nihe der Ausbruchsstelle
waren die Konsequenzen katastrophal.
Die rasante sozio-6konomische
Entwicklung
des Sherpa-Landes
erlitt einen empfindlichen
Einbruch.
Ein neu erstelltes
Wasserkraftwerk,
14
Briicken,
etwa
30 Hduser und viele
Hektaren
kostbares Kulturland
gingen
verloren.
Zusitzlich
blieb ein stark
in Mitleidenschaft
gezogenes Wegnetz
iibrig.
5 Millionen m3
Wasser
wilzten sich innerhalb
weniger
Stunden das Bhote Kosi und Dudh Kosi Tal hinunter. Der Moranenbruch
war durch eine Schwallwelle, welche von einer Eis-Lawine (150,000 m3) in den See herrfihrte,
ausgel6st worden. Die Wasserfront
erreichte einen
Spitzenabfluss
von 1,600
m3/sec.
3 Millionen
m3
Geschiebe
wurden innerhalb einer Distanz von
weniger
als 40 Kilometer
umgelagert. Jedoch nur 10 bis 15% davon verliessen
das Gebiet als Schwebfraktion.
Die potentielle
Gefahr
von Gletschersee-Ausbriichen
bleibt
allgegenwiirtig
und ist im Zunehmen
begriffen.
Eine Gefahren-Erhebung,
die genaue Festlegung
der Ursprungs-Gebiete, sowie eine Ueberwachung der Gletscherseen
wird hier
vorgeschlagen. Ein solches
Vorgehen wird als integraler
Bestandteil
jeglicher Entwicklungskonzepte
in himalayischen
Gebieten betrachtet.
INTRODUCTION
The present report
is a follow-up
paper
to Vuichard
and
Zimmermann
(1986) which
gave
a preliminary
account
of
the 1985 glacial-lake
outburst
in Khumbu Himal, Nepal.
The purpose is to discuss the
possible mechanisms
of the
catastrophic
flood,
including
the
triggering
event
that
led
to the
bursting
of the moraine dam, the lake and breach
characteristics,
the form
of
the
hydrograph
downstream,
and the
runoff
characteristics. In addition,
a mass balance
calculation is presented.
These investigations
lead to a proposal for
the assess-
ment of the
potential
hazard of other
glacier-dammed
water
masses in the Himalaya.
Within
the last several
decades world
population
has in-
creased enormously. Concurrently
a rapid technological
'Mineralogisch-petrographisches
Institut,
Universitdit
Bern,
Baltzerstrasse
1,
3012
Berne,
Switzerland.
2Geographisches Institut,
Universitiit
Bern, Hallerstrasse 12, 3012 Berne, Switzerland.
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92 / MOUNTAIN RESEARCH AND DEVELOPMENT
evolution
has
led to
more
intense use of
natural
resources,
even in
remote
mountainous
regions
such as the
Himalaya.
Today the
Himalayan region,
sensu
lato,
supports
more
than 300 million
people in seven different nations
and is
facing
severe
pressures
from continued
population growth
which is destabilizing the natural environment.
This is
exacerbated
by
rapid,
and sometimes
misguided, develop-
ment,
often
supported by foreign
aid programmes.
One
consequence of this
rapid development
is the
large
invest-
ment in infrastructure
(for
example, hydroelectric power
plants
and roads), which not
only
serve the local popula-
tion but, as in Nepal, a growing
tourist
industry.
Very
often, however,
people and agencies are unaware of
the
natural
dynamic processes
that
can pose serious
threats to
modern technological
intervention.
Natural dam failures
are known
throughout
the
world;
the
great potential
hazards
of man-made
dams
are also
gen-
erally
recognized
(Hagen, 1982). Thus, the sudden cata-
strophic discharge of large volumes of water in moun-
tainous and glacierized regions, such as the Himalaya,
must
be anticipated.
In many
inhabited mountain
regions
accounts
of
glacial-lake
outbursts have led to considerable
awareness and abundant literature.
This includes events
in Iceland (Thorarinsson, 1940, 1954, 1957; Nye, 1976),
the
European Alps
(R6thlisberger,
1981; Haeberli, 1983),
Norway (Liestol, 1956; Theakstone, 1978; Spring and
Hutter,
1981), Alaska (Post and Mayo, 1971), the Cana-
dian Arctic
(Maag, 1969; Blachhut and Balantyne,
1976),
Yukon Territory
(Young, 1977; Clarke, 1982), British
Columbia (Marcus, 1960; Mathews, 1965; Clague and
Mathews, 1973; Clague et
al., 1985; Blown and Church,
1985), the
Pacific Northwest
(Richardson, 1968; Gallino
and Pearson, 1985), and the
Andes (Lliboutry
et
al., 1977;
Patzelt, 1983). These studies should be of
great
value for
similar
studies in the Himalaya.
Throughout
the
Nepalese Himalaya and neighbouring
mountain ranges glacial-lake
outbursts have occurred
in
geological and historical
times
(Visser, 1932; Lombard,
1954; Hewitt, 1964, 1965; Gansser, 1966; Krenke and
Kotlyakov,
1982). Background
information resulted
mainly
because of scientific interest that was attracted
to the
spec-
tacular landforms created
by
such events
(Fort
and Freytet,
1982; Yamanaka, 1982; Buchroithner, 1985;
Fushimi et
al.,
1985). In recent
years the many dammed water
masses
have become a major threat to regional development
projects
(Hewitt, 1982; Gansser, 1983; Xu, 1985; Galay,
1985a, 1985b; Ives, 1986).
Not only
must the immediate
physical impact
of
a lake
outburst be taken into
account, but also the
downstream
effects,
which may trigger
the acceleration of morpho-
dynamic processes
for
many years.
Because of the
possible
effects
on the lowlands
it is
most
important
to understand
the
natural conditions of
the
High Himalaya. In effect,
the
possibilities
for
glacial-lake
outbursts
throughout
the
entire
Himalaya-Gangetic system
warrant careful assessment.
Study
of the
highland-lowland
interrelationships along
a north-south transect from the High Himalaya to the
Gangetic Plain formed the central tenet of the United
Nations University
(UNU)/Nepal MAB Mountain Haz-
ards Mapping Project (Ives and Messerli, 1981). The
Khumbu region
was selected as one of the test areas for
this project. A Hazards Index Map (scale 1:50,000,
Zimmermann et
al., 1986) provided an overview of the
degree
of hazard of
the
local area. It was concluded that
the
greatest
single
threat
was the
possible
outburst of ice-
dammed and moraine-dammed lakes. Several
such events
have occurred
within
living memory,
with extensive
geo-
morphological activity
and damage to cultivated
land,
property,
and vegetation,
as
well
as
loss
of
life. The knowl-
edge acquired through
the Mountain Hazards Mapping
study
prior
to the
glacial-lake
outburst
in 1985 has pro-
vided
a unique opportunity
for
the evaluation of its
effects.
THE 1985 GLACIAL-LAKE OUTBURST (CHHU-GYUMHA)
In the
summer of 1985 a catastrophic
flood
(Sherpa: chhu-
gyiimha;
Icelandic:
j6kulhlaup)
hit the
Khumbu region
when
a glacial
lake (Dig Tshol) in
the
Langmoche valley
above
Thame broke through
its moraine dam (Figure 1). The
area was in
full bloom and the
potatoes
were
ready
for har-
vesting.
A colourful
Sherpa festival
was in
progress
when
enormous amounts of
water
plummetted
down the Bhote
Kosi and Dudh Kosi valleys
(Figure 2), destroying
houses,
bridges,
trails,
and much of
the scarce agricultural
land.
Many families
lost their entire
personal property.
The
region
was cut
off
from the rest of
Nepal for several
days.
The nearly
completed hydroelectric power plant (worth
US$3 Million) located
opposite
Thame on the Bhote
Kosi
was virtually destroyed.
Loss of
human lives was remark-
ably
low; in some places livestock
were killed. The social
and economic
impact,
as well as the
effects on the natural
environment,
were severe.
The natural
processes
characteristic of
this
spectacular
mountain environment
must
be understood before a full
assessment of the
impacts
of
the
event can be attempted.
GLACIATION AND MORPHOLOGY
The natural
environment
of
Khumbu Himal is
marked
by
its
deep
pre-glacial valley system,
consisting
of four main
valleys
which
are framed
by
enormous mountains. Cover-
ing an area of
about 1,200 km2
it lies in the uppermost
parts
of
the
watershed of
the Dudh Kosi (4,100 km2),
one
of
the
headstreams of
the Sun Kosi in eastern
Nepal. The
physiographic
setting
has been influenced
extensively by
the tectonic and lithologic disposition
resulting
from
the
Tertiary Himalayan orogeny and its on-going activity
(Vuichard, 1986). The broad
morphological
characteristics
have been moulded by the high-relief
energy
(Zimmer-
mann et
al., 1986) and fluvio-glacial
activity
since late-
Cenozoic time.
As a first
approximation, two contrasting
altitudinal
zones can be distinguished
in the
upper
Dudh Kosi water-
1
Tsho:
lake.
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D. VUICHARD AND M. ZIMMERMANN / 93
FIGURE 1.
Dig
Tsho
(4,400
m
a.s.1.),
the
pear-shaped glacial
lake lies embedded
in the
uppermost Langmoche valley,
one of the
tributary valleys
of the Bhote
Kosi valley,
above
Thame, Khumbu
Himal. Note the dead-ice tongue.
Photograph by
Tj. Peters,
14
October
1982.
FIGURE
2. The flash-flood at the site
of
the
Namche Small
Hydel
Plant. Five
million m3
water were drained
out of
Dig
Tsho within 4-5
hours
and
caused
a flash-flood down the Bhote Kosi and
Dudh Kosi
valleys
on 4 August
1985.
Photograph by
G. B. Shrestha,
4
Au-
gust
1985.
shed
(Haffner,
1979). The lower zone
(below
3,400
m) con-
tains
narrow,
deep-cut valleys
of
fluvial
origin exhibiting
typical
V shapes. Level land on terraces close to the
main
rivers derives
from a combination of
ancient-to-sub-recent
depositional processes,
very
difficult
to assign to specific
chronological stages
(Vuichard,
1986:49). Fluvial
processes
during
the
progressive
uplift
of
the
region
have severely
eroded these terraces
which
are
composed
of
fanglomerates.
In the
second,
higher
zone
(above 3,400
m)
a characteris-
tic
glacial morphology
has
evolved,
with
U-shaped troughs,
cirques,
and well-developed
moraine and terrace
systems.
These features
may
be correlated to
Pleistocene and Holo-
cene glacial advances (Heuberger, 1956; Iwata, 1976a,
1976b; Fushimi, 1977, 1978).
GLACIAL STAGES AND FLUCTUATIONS
The formation of
glacial
lakes
depends
largely
on
gla-
cial
activity.
Glacier fluctuations are
induced
by
changing
climatic conditions and
controlling
factors include altitude
and
aspect, precipitation, temperature,
and
radiation,
as
well as the extent of debris cover.
Khumbu
Himal
is one
of the areas in the
Himalaya
which
has been
comparatively
well
investigated
in
terms of its
glacier inventory
(Mfiller,
1970;
Higuchi
et
al., 1980).
The lack
of detailed historical
records, however,
hinders the
development
of
a
chronology
of
glacial
fluctuations on a regional Himalayan
scale.
Nevertheless,
several authors have
attempted
to correlate
glacial stages
in the
Himalaya,
especially
for the
Little
Ice
Age,
with
Europe,
Antarctica,
South and North America
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94 / MOUNTAIN RESEARCH AND DEVELOPMENT
(R6thlisberger
and Geyh, 1985a, 1985b; Ono, 1985; Heu-
berger
and Weingartner,
1985; Kuhle, 1986).
Three major stages
in the
glaciation,
with
several
sub-
stages, have been identified
in the Khumbu area (Heu-
berger,
1956; Iwata, 1976a; Fushimi, 1978; Mfiller,
1980;
Heuberger, pers. comm., 1986).
After the
Little Ice Age maximum advance, a general
glacial retreat,
interrupted by local re-advance phases,
seems
to
have occurred
throughout
the
Himalaya (Mayew-
ski and
Jeschke,
1979; Ono, 1985). Today glacial fluctua-
tions
may
result
in
the
formation of new
moraines in front
of
the termini
of small glaciers
within
one year, destroy-
ing
older moraines
(Fushimi and Ohata, 1980). Most gla-
ciers of
the debris-free
type
(Moribayashi and Higuchi,
1977:
C-type
glaciers)
in
the
upper
part
of Khumbu Himal
were
in retreat
during
the
period
of 1960-1975 (Higuchi
et
al., 1980).
THE LANGMOCHE
GLACIER
The Langmoche glacier belongs to the
category
of
the
C-type glaciers. In the glacier inventory
of the Mount
Everest
region
(MUiller,
1970) it is referenced
as number
13. A uniform medium-to-small-sized
glacier
(> 200 m),
it is located in a sub-basin of the
Nangpo-Tsangpo area
(Bhote Kosi valley). It has its
origin
at 5,400 m a.s.l. at
the
foot of the
1,500-m-high
northeast
face
of
Tangi Ragi
Tau (6,940 m), and is
extensively
nourished
by
avalanches
falling
off
this
rock face
(Figure 3). The glacier
lies in an
east-west-trending valley.
Conditioned
by
the surface
mor-
phology
of the
granitic
and gneissic
rocks,
the
glacier
flows
over a steep rock step after
its origin, falling
over a
30-degree
ramp toward
the Langmoche valley, where it
levels off within
a large arc of moraines
at 4,400 m (Fig-
ure 4).
Although
the
region
is controlled
by
a typical
monsoonal
climate,
the
Langmoche valley
receives
very
little
precipi-
tation
because of the
rain-shadow effect
of the
high
moun-
FIGURE 3. The receding Langmoche glacier
at the foot of the
1,500-m-high,
steep
northeast
face of
Tangi Ragi
Tau
(6,940
m).
Note
the
active ice fall
on the
rock
step
in the middle of the
photograph. Photograph
by
M. Zimmermann,
21
October
1985.
FIGURE 4. The arcuate moraine dam
and
lake before
the outbreak.
Note the
2
scree cones
from outlets
overtopping
the moraine: (1) recent,
(2) old. The
fresh scree
at the foot
of the moraine
(3)
points
to
an intramorainal
drainage
sys-
tem.
Photograph
by
M. Zimmermann,
3 October 1982.
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D. VUICHARD AND M. ZIMMERMANN / 95
tain topography
(Zimmermann et
al., 1986). The glacier
tongue
is
exposed
to
heavy incoming
solar
radiation.
These
factors have contributed
to
the
rapid
retreat and thinning
of
the Langmoche glacier
tongue in recent
decades.
The glacier
must have
changed drastically
since the time
of the
glacier
inventory.
On all former
maps it is shown
as a single
intact
tongue
of
glacial ice with
minor debris
cover on its terminal
part. Today, however, the glacier
tongue barely
reaches
to
the rock
step.
The ramp
in
front
of it serves as the cone for ice material to avalanche down
into
the lower basin surrounded
by the
end moraines. A
fissured dead-ice mass was lying
at the foot of the
ramp
in 1985, in contrast to its almost intact condition of
1982
(Figure 5).
DIG TSHO, THE MORAINE-DAMMED LAKE
Before
its
outbreak this
pear-shaped
lake had been im-
pounded
between
the
well-developed
end moraines and the
receding
terminus of the
Langmoche glacier. Space Shuttle
imagery
(STS 9/ESA Metric Camera Experiment:
28
Nov-
8 Dec 1983) shows
a maximum extent of
the
lake surface
of about 50
ha, one-third of which was covered
by
the dead-
ice body mentioned
above. This dead-ice mass (which
is
also visible on photographs
taken in 1982: Figure 1) was
about 300 m wide and 500 m long.
The famous Schneider map Khumbu
Himal (1968/78)
does not show a lake within the moraine boundaries. A
stereoscopic pair
of black-and-white
photographs,
taken
by
E. Schneider
(1961), shows a very
large
body
of
dead-ice
slightly
covered with debris.
To assume that
a small
pond
between the
end moraine
and the
glacier
terminus
already
existed at that
time is
purely hypothetical,
as such cannot
be recognized
on the stereo
pair. Nevertheless,
a lake is
depicted
on the
Survey
of
India map
(721/9,
scale 1:63,360)
surveyed
in 1963 and printed
in 1974. At least it
can be
assumed that
there were two
ponds
as two outlets
over the
end moraine can be recognized. The right-hand
outlet
showed more channel
activity
in 1962. The Dig Lake may
have been formed
within
the
last 25 years.
Large differences
in
water
supply
between
the wet
sum-
mer seasons and the
dry,
cold winters must
have resulted
in a marked fluctuation
in
water level.
An air
photograph
taken
by
a
Japanese research
party
in
April
1978
(Higuchi
et
al., 1976) shows a largely empty
lake.
On 4 August 1985 particularly
fine weather
prevailed
and radiation was intense.
The supply
of melt-water
must
have been very
high.
Before the outbreak
the
water reser-
voir was full to its
rim
(Figure 5). The maximum depth
of the lake was 18 m and the
maximum volume of
stored
water behind the moraine was estimated
as approximately
Sketch Map:
lpte.ral moraine
0 *. moraine
Dig
Tsho 0
+l.
..avalanche
+cone.2'. . . _ . .. o
I o
0 "
o0' 0
.. * s..
fan
deposit
o * . ....
after
outburst'
+ +'Is
Langmoche " lateral moraine
0 200 400 m
+ dead ice lake level former
outlet
bed rock - -
. + outl
et? O
Cross Section:
FIGURE 5. Diagram showing
cross-section
of
the
lake.
Length
of the lake:
1,500
m;
average
width: 300
m;
maximum
depth:
18
m;
lake surface: 50
ha; dammed
water masses:
without dead-ice
mass,
6.75 x 106
m3,
with dead-ice
mass,
5.1 x 106
m3.
Calculated
wave
height
5 m.
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96 / MOUNTAIN RESEARCH AND DEVELOPMENT
FIGURE
6. Icebergs from
the broken,
once
floating,
ice sheet near the
outlet
after
the
outbreak.
Compare
the thick-
ness of the ice
with the
person
in
the
foreground.
Photograph by D. Vui-
chard,
21 October 1985.
FIGURE 7. The
V-shaped
trench in
the
Langmoche
terminal moraine
(height:
60 m; width:
200
m). Photograph by
D. Vuichard,
21 October 1985.
6.75 x 106
m3
without the
dead-ice mass and 5 x 106 m3
with the dead-ice mass.
THE MORAINE DAM
The arcuate end moraine is assumed to have been
formed
during
the
Little Ice Age maximum
advance (thus
roughly
corresponding
to the
1850 moraines in
the Euro-
pean Alps). It was 60 m in height
at its lowest
point
and
its outer
slope
was partly
covered
by grass
and sods
(grass
and shrub
vegetation
reached
only
the
foot of
the
moraine),
with a gradient of 25-30 degrees. Lichen growth
and
weathering (frost-shattering)
were observed on surface
material. The inner side was bare and unstable, with
a
sharp, very steep
edge. The morainic
material
consisted
of
very large,
sometimes
angular,
boulders
(2-5 m in
diam-
eter)
and gravel
within
a non-plastic mica-sandy
matrix.
During the summer monsoon the
lake overtopped
the
lowest
point
of the moraine
and drained
via a steep
channel
down
the outer
slope. A debris
cone about 30 m
wide
was
observed at the foot of the moraine
prior
to the
outburst;
it showed
very
few
signs
of
high
water flow
(Figure 4). A
second
cone on
the
right
of the former outlet indicated
that
another
overflow
had been active at the
beginning
of
the
1960s. A seep
was located
at the
foot
of
the
moraine,
indi-
cating
the occurrence of
significant
subterranean
drainage.
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D. VUICHARD AND M. ZIMMERMANN / 97
DAM FAILURE
The event
which
triggered
the
catastrophic
drainage of
Dig Tsho was an ice avalanche from
Langmoche glacier,
inducing
an impulse
wave on the lake. A Sherpa farmer
from
Thame came down from
Langmoche valley a few
weeks
before the
event. He thought
that
drainage of
the
lake
was imminent
as waves
were
seen
to
overtop
the
lower
moraine rim
every
time
ice avalanched off the
mountain
behind.
Continuation of ice avalanching can be demonstrated
even
after
the
event:
a photograph
from
the
glacier
tongue
taken
in
January 1986 shows
that there
had been activity
since October 1985. Rock avalanches
from
high
on the
face
of
Tangi Ragi Tau can be excluded as a possible trigger-
ing
mechanism. The incorrectly
assumed source
area for
a larger
rockfall
(bare rock
cliff)
below
the summit
(Galay,
1985a; Ives, 1986) can be detected
on Schneider's
older
photographs
(1961).
An ice
mass of
between
100,000
m3
and 200,000 m3
was
detached
from
the
glacier
tongue
at a height
of
4,600-4,800
m a.s.l. (site
of
ice fall
over
rock
step; Figure
3), and slid
over
the
large
ice-and-snow
cone with
a gradient
of 30 de-
grees.
It overran
the
floating
dead-ice mass at the
bottom
of
the ramp and plunged into
the lake.
From this
point
two
different
triggering
mechanisms
can
be envisaged
that would
produce
increased
runoff
over
the
moraine dam:
1. Most of
the
material
of the
ice avalanche was deposited
on the
floating
and fissured
dead-ice body. This mate-
rial
resulted
in a water
displacement
of
135,000 m3
to
produce
a static
lifting
of the
lake
level of about 40 cm,
accompanied by a superposed wave of
unknown
size.
2. The dead-ice body
was already
broken
into
single
ice-
bergs (many ice blocks, 5-10 m in diameter, were
located
near
the
outlet
after
the
outburst;
Figure
6). The
ice
avalanche
produced
an impulse
wave
which travelled
across the
lake surface.
The second triggering
mechanism
is believed to
be the
most
likely
cause of
the
outburst.
Thus impulse
wave cal-
culations
were made after Huber (1980, 1984) using the
formula:
Hi/h = aMb
(where Hi = wave height; a = 0.13; b = 0.81). This
would yield
a wave height
of 4.4 m. Parameters
used for
the
calculation:
M: (displacement number): M = V/bh2
Vo: 150,000 m3 (volume of ice-avalanche 100,000-
200,000 ms) roughly
estimated
V: 135,000 m3
(water
displacement
volume
of
plunging
ice)
b: 200 m (width
of
avalanche) estimated
h: 18 m (lake depth) measured
x: 1,500 m (length
of
lake) measured
Hi: (wave height
at the
distance x/h = i)
Taking into
account the
narrowing
of the
lake (from
300
to 100 m
in
width)
and the continuous
rise of
the lake
floor
toward the
outlet,
the
assumed wave height
could be in-
creased up to 9 m. However, assuming that
the
floating
ice absorbed a certain
amount
of
energy,
the
wave height
could
be reduced
to
a maximum
of 4-6 m
(average of
5 m
used here). The presence of
several debris slides on the
inner bend of the
moraine
(north
side) suggests
that
they
were a subsequent response
to the
impulse
wave and the
drainage rather than
caused by
the
triggering
mechanism
itself.
The 5-m
wave
which
overtopped
the
moraine dam
would
have had an enormous capacity to erode it. The initial
down-cutting
of the moraine
would cause an increased dis-
charge
from
the
lake. A critical
parameter
is the
peak dis-
charge,
Qp. This depends upon the
volume
of
water,
the
erodibility
of
the
dam material,
the
geometry
of
the
breach,
and the
initiating
event
(Blown and Church, 1985:555).
Various authors
(Hagen, 1982; MacDonald and Lang-
ridge-Monopolis, 1984; Blown and Church, 1985) elabo-
rated
mostly
empirical
relations
between dam and breach
parameters
and peak
discharge
(breach
formation
factors).
At
Dig Tsho the
calculations,
using
the breach
formation
factors,
would
yield
a peak
discharge
of
6,000-8,000 m3/s.
However, analysis
of
the
runoff
characteristics
a few
kilo-
metres downstream
(see below) shows a maximum dis-
charge of not more than 2,000 m3/s.
On one
hand, it
is
supposed
that the
proportion
of
large
boulders
within
the
easily
erodible matrix
was
high
enough
to prevent
a total
collapse down to the base of the dam.
On the other
hand, the
number of
large
blocks
was insuf-
ficient
to
armour the
channel after
the
impulse
wave had
run
over the
dam. Only temporary
armouring
is
envisaged.
Thus, it
is
assumed that the
process
of
lowering
of the dam
was intermittent:
collapses
of
parts
of the dam with
periodic
sudden increases of outflow,
alternated
with
periods of
slowly decreasing
outflow
during
which
a fairly
uniform
discharge
was maintained.
The amount
of eroded material
from the moraine was approximately
900,000 m3.
All the
material
of the breach was deposited
within the first
two
kilometres
(see also section
on Mass Balance below). Very
coarse
material
(2-5 m in
diameter)
was deposited
imme-
diately
below the
V-shaped trench
(Figure 7). Fine mate-
rial
(fine
gravel
and sand) was deposited
at
the termini
of
a large cone (Figures 8 and 9).
THE FLASH FLOOD
During the afternoon
of
the
event
and the
days
preced-
ing
the
weather
was clear
and the levels of
the
main
rivers
in Khumbu Himal were low. The sudden extreme
dis-
charge,
accompanied by
an enormous
noise, shocked
the
many Sherpas living
close
to
the
river.
Although,
for
them,
natural events
are
the
work of
the
gods
and beyond
human
control
(Bjenness, 1986; Zimmermann et
al., 1986:34),
statements
from
the local Sherpa people were
very helpful
for
understanding
the main characteristics
of the
ensuing
flood.
Interviews,
together
with the
interpretation
of
geo-
morphological features and changes along the river,
enabled the reconstruction
of the
timing
and duration
of
the
flood,
its
peak
discharge
and hydrograph,
and changes
of
the
river
bed during
the
event.
The river
gauge station
at
Rabuwa Bazar (460 m
a.s.l., 90 km
below the
lake) had
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98 / MOUNTAIN RESEARCH AND DEVELOPMENT
FIGURE 8. Dig (Langmoche
valley),
3
October
1982, 4,200 m a.s.l., km
2.
The summer
settlement
Dig
and its
sur-
rounding pastures
are located in the
immediate
vicinity
of the
lake down
valley.
Note the intact
end moraine
with the
lake
outlet
draining
over it.
Photograph by
M. Zimmermann.
FIGURE 9. Dig (Langmoche
valley),
21
October 1985.
The end
moraine of
the
former Langmoche glacier was
trenched
by the
impulse
wave. The
whole
material of the
breach
(880,000
m3)
was
immediately
deposited
within
a river
section
of
2 km
length.
Photo-
graph
by
D. Vuichard.
been out of
order
for
several months
prior
to the
event.
Therefore,
it was not
possible
to
obtain an official
hydro-
graph
chart.
Nevertheless,
with all
the
background
infor-
mation,
a
hydrograph
could be
reconstructed
(Figure
10).
In
the
early
afternoon
of
4
August
1985
drainage
of
Dig
Tsho was
initiated and
within
4-6 hours
the
lake
emptied
into the
valleys
below.
The surge
reached the
Namche
Small
Hydel
Plant
(12
km
below)
at about
2
p.m.,
the
area
of
Ghat
(25 km
below)
between
2:30 and 3 p.m., and
Jubing
(40
km
downstream)
at about
3:30
p.m.
This
yields
a mean
velocity
for the
surge
front of 4-5
m/sec.
Witnesses
described
several
successive
surges
with a progressive
in-
crease in
discharge.
In some
places
(for
example,
Jorsale)
people
were
still
able to
cross
the river
over
suspension
bridges
while
the
water masses
rushed below.
The
bridges
at
Jorsale, Phakding,
and
Jubing
were
not
destroyed
until
30-90 minutes
after
the
passage
of the
initial
surge.
At these
sites
heavy
accumulations of
debris
were
ob-
served,
so that the increases in discharge
were only
apparent
and due to
raising
of
the river
bed. However,
the witnessed
surges may
have
been
caused
by
variations
in outflow
from
the lake
and/or from
temporary
damming
of the river
channel
through
slumps
and
slides from
under-
cut river
terraces.
Near
Langmoche
village
a distinct
cross-section of
the
river
channel
could be
used to
provide
an
estimate of
the
peak
discharge.
The river
was about
21 m in width
and
9-10
m
in
depth
with a gradient
of 9
percent.
The
Mannig-
Strickler
formula:
v, = k
R2/3
J1/2
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D. VUICHARD AND M. ZIMMERMANN / 99
m3/s
1500
1200
900 -
600
300
1 2 3 4 5 hours
FIGURE 10. Constructed
hydrograph
with
peak
discharge
(Qp)
of
1,600
m3/sec,
duration of
5
hours and an
average discharge
of
300 m3/sec.
Note the
steep
increase
phase
in
the first
30
minutes,
followed
by
a short
period
of maximum runoff and the subse-
quent decreasing phase.
with
a k-value of 6-10 (very rough
bed with
large
unmoved
boulders) gives a maximum flow
of
about 1,400 m3/sec
three
kilometres
below the moraine dam. An application
of
the formula
by Smart and Jaiggi
(1983) gives
a maxi-
mum flow of
1,500-1,700 m3/sec,
which seems to be quite
realistic.
Figure 10 shows the reconstructed
hydrograph
based on a total amount of
water
of
5 x 106
m3, a peak
discharge
of 1,600 m3/sec
(averaged from the Smart and
J
iiggi
formula),
a duration of
5
hours,
and ste'pped
increase
and decrease phases with an average discharge of 300
m3/sec.
This hydrograph
is closely comparable with that
of
the
1977 event
(Zimmermann
et
al., 1986:36). There is
a steep
increase phase within 30 minutes,
a short
period
of
maximum runoff,
and a long intermittent
decreasing
phase. In 1977 the peak discharge dropped from
1,200
m3/sec
below the moraine dam to
800 m3/sec
at the
gauge
station 90 km
downstream
(Rabuwa Bazar). Now, in
1985,
the
peak
discharge
below the breach
was
calculated
at 1,600
m3/sec.
Thus it
follows
that
the
peak discharge
all
the
way
to
the
gauge station at Rabuwa Bazar should also exceed
that
of 1977. Precipitation
over the whole
area during
the
days preceding
the 1985 event was slight.
The base runoff,
despite
the monsoon season, therefore,
was low and com-
parable to that of
the
1977 event. This is
why
in
the lower
sections
of the
Dudh Kosi the
channel capacity
was suffi-
cient to absorb the water masses without
major
disruption.
All the local informants told
of an "earthy"
smell asso-
ciated with the
discharge
of "black
water"
which was full
of
debris; large boulders appeared to bounce around.
FIGURE
11.
Thamo
(Bhote
Kosi
valley),
4 April
1985, 3,300
m
a.s.l.,
km
12.
At the site of the Namche Small
Hydel Project
(on
a sub-recent debris
fan)
at
Thamo,
the
widening valley
bottom
is
U-shaped.
Settlements are located on
fluvio-glacial
terraces.
Photograph
by
D. Vuichard.
FIGURE 12.
Thamo,
19
October
1985. The
nearly complete
Small
Hydel
was washed
away
and 6-10
m of
debris
in
depth
was ac-
cumulated
in
the
river bed over a distance of
1
km
(volume
of
accumulated
material
660,000
m3).
Photograph
by
D. Vuichard.
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100 / MOUNTAIN RESEARCH AND DEVELOPMENT
FIGURE 13.
Thamo
Teng
(Bhote
Kosi
valley),
3
October
1982, 3,800
m
a.s.l., km 7. The main
access route
to Tibet
follows
the
right
bank. The
villages
are located on
fluvio-glacial
terraces on both sides.
Photograph
by
M. Zimmermann.
FIGURE 14. Thamo
Teng,
19
October
1985. In the
vicinity
of Thamo
Teng
the river
bed widens
and flattens
out. This
led
to
vast
accumulations
(6-8 m in
depth,
with a total volume of
638,000
m3).
Due to the
irregular
flow
pattern
the water
changed
its course from
one
bank
to
the
other,
causing
lateral
erosion.
Photograph
by
D. Vuichard.
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D. VUICHARD AND M. ZIMMERMANN / 101
FIGURE 15. South of
the confluence of Bhote Kosi and Dudh Kosi
the river eroded the
previously
accumulated debris
(8 m
depth)
step by
step.
Three erosional
stages
from
the
receding surge
can
be
depicted.
Note
the stratification of the terrace
deposits,
with
debris coarseness
increasing
in
upward cycles. Photograph by
D.
Vuichard,
24 October 1985.
The surge
mobilized large amounts
of
debris from the
river
terraces,
debris
cones,
and the river
bed
itself
although
this material
was redeposited
within
a short distance. The
movement
of
bed load was strongly
pulsating
due to sudden
changes in transportation
capacity. In steep channels
(gradient > 10 percent)
erosion of
the river bed and the
banks predominated (for example, below Mingbo and
below
Thame). Slide scars
at
heights
of
20-50 m above the
river
bed and terrace failures with
100,000 m3
of
moved
material were common. In widening
channel
sections,
with
a gradient
of 5-7 percent
(for example, Thamo Teng,
Thamo, and
Jorsale),
aggradation prevailed
often to
depths
of more than 10 m (Figures 11 and 12). Large boulders
(1-1.5 m
diameter)
were
embedded
in
a fine
sandy
matrix.
In some places the sediments were distinctly
stratified.
Within such
broad accumulation sections the river
had an
inconsistent flow
pattern,
mainly
depending
on river bed
changes
through
debris
deposition.
Since the river
changed
its course from one side of its flood
plain to the
other,
it
retained the capacity to undercut and erode adjacent
terraces
(for
example,
at Thamo Teng: Figures
13
and 14).
Accumulations from earlier
phases
of the
event
were
sub-
sequently
eroded step
by
step
during
the
subsiding
phase
(Figure 15).
MASS BALANCE
The catastrophic glacial-lake
outburst
produced severe
morpho-dynamic
changes
in
the Khumbu region.
Because
of the authors' documentation material collected both
before and after the
event,
changes
in
and around the Dudh
Kosi river could be assessed. In the mass balance calcula-
tion the volume of eroded material
(slides,
river
banks,
ter-
races,
and river
bed) is
compared
with
accumulation
forms
along the river
section from its origin down to Jubing
(42 km). The volume of abraded material
was included
(Table 1).
From the data presented
in Table 1 it
is apparent that
enormous
amounts of material
were
eroded
and deposited.
The frequency
of
negative
balances suggests
that estimates
of the
eroded
material were too
low,
or of the accumulated
material
were
too
high. Since the
assessments of material
volumes involved in the slides and bank
failures
were
espe-
cially
difficult
due to their
very
irregular
geometry,
it is
assumed that estimates
of the eroded material must
have
been too low.
As the
mass
balance calculation
always
reaches the value
of
zero (with
a negative total
in brackets,
Table 1), it is
concluded
that the material was transported only
over short
distances
(3-5
km
at the
most).
In
the Khumbu
area most
of
the
eroded material was
redeposited
within
a
river
sec-
tion
within
the first
25 km
(Figure 16). A helicopter
sur-
vey
farther
downstream
(Galay,
1985a,
1985b)
has demon-
strated that alternation of erosional and accumulation
activity
continued
below 42 km,
the end of the terrestrial
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102 / MOUNTAIN RESEARCH AND DEVELOPMENT
TABLE 1
Langmoche glacier
lake
outburst
flood:
mass balance
(in
1,
000
m3)
Location km Erosion Total Abrasion Total Accumulation Total Balance
Dig
moraine 0 880 880
rbl 0 - 0.5 50 930
rbr 0 - 1.0 40 970
b 0 - 1.0 805 165
b 1.0- 2.0 178 0 (-13)
970 983
Langmoche
b/rb 2.0- 2.6 81 6 75
b/rb 3.1- 3.5 32 107
b 3.3- 3.5 16 91
1,083 6 999
Mingbo
b/rb 3.5- 4.8 240 7 324
rbl/r 4.8- 5.5 140 4 460
b 4.8- 5.5 28 488
1,491 17 999
Thamo
Teng
sl 5.8 8 496
b 5.5- 7.0 638 0 (-142)
sl 7.2 200 200
sl 7.5 38 238
rbr 6.0- 8.1 126 364
b 7.0- 8.1 422 0 (-58)
1,863 17 2,059
Thame
sr 8.1- 8.3 11 1 10
sl 8.1- 8.3 58 5 63
sr 8.6 11 1 73
sr 8.3- 8.5 30 2 101
b 8.3- 9.0 1 102
sl 9.5-11.0 74 2 174
sr 9.8-10.5 108 4 278
b 9.5-11.0 71 3 346
2,227 35 2,059
Thamo
s 11.8-12.0 6 352
b 11.0-12.0 660 0 (-308)
s/b 12.0-16.5 150 17 133
b 16.5-16.8 18 115
2,383 52 2,737
Jorsale
b 16.8-18.1 59 56
b 19.1-20.0 111 0 (-55)
2,383 52 2,907
Bemkar
sl 20.2 4 4
b 20.0-22.0 90 2 92
2,477 54 2,907
Phakding
sr 22.0 10 102
sl 22.4 4 106
b 22.0-22.4 153 0 (-47)
sr 22.5-23.0 11 11
b 22.4-23.0 21 32
2,523 54 3,060
Ghat
sl 24.2-24.5 43 75
b 23.0-24.6 213 0 (-138)
sr 24.8 4 4
sl 25.0 40 44
sr 25.2 36 80
b 24.6-25.2 41 39
16 23
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D. VUICHARD AND M. ZIMMERMANN
/
103
TABLE 1 (Continued)
Location km Erosion Total Abrasion Total Accumulation Total Balance
sl 25.4 10 4 29
sl 26.9 13 5 37
b/rb 25.2-27.4 165 61 141
2,834 140 3,314
Lukla
sl 28.0 12 4 149
b 27.4-28.2 40 14 175
sl 28.9 200 68 307
sr/1 29.8 35 11 331
sr 30.0 34 11 354
b 30.1-30.4 22 7 369
sr 30.4 5 1 373
3,182 256 3,314
Jubing
b/rb 37.0-39.0 20 4 389
sl 39.2-39.9 11 400
sl 41.5 10 2 408
sr 42.0 40 6 442
sl 42.3 10 1 451
3,273 269 3,314
b:
bed;
rb: river
bank;
s:
slide;
1:
left;
r:
right.
The abrasion was calculated
using Sternberg's
formula:
v = vo x ec-x c: coefficient
of abrasion
(0.02) x: distance
of
transport
(km)
Abrasion
was not
considered where the material was
transported
less than
1
km. Volumes of
less
than
1,000
m3
were not taken into
account.
900
Sz
Soo :
Eroded
Mass
80 0
-
C :Accumulated
Material
Soo
: Abrasion
0
0
O
"-4
fLCL
300
200
100 0
0 5 1 0 1 5 20 25 30 35 40 -45
RIVER KILOMETRES
FROM
THE ORIGIN
FIGURE 16.
Correlation
of
accumulation,
erosion,
and abrasion
over a surveyed length
of
42
km
from
the
flood
outbreak
point
to
Jubing.
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104 / MOUNTAIN
RESEARCH
AND
DEVELOPMENT
survey.
Even though
the
transported
material of the first
40 km (3.3 x 106
m3) almost corresponds
to the entire
water volume discharged (5.1 x 106 m3), only 10-15
percent
of the sediment
load, most of it resulting
from
abrasion, will have left the region
as suspended load. It
is unlikely that the huge deposits of coarser material
remaining
in the river bed will be moved out of
the area
within the next several
decades. The average transportation
capacity
of monsoonal rains is not
sufficient
in
the Khumbu
area to achieve this.
Because of the extensive lateral
erosion,
river
banks,
ter-
races, and slopes remain in a very
unstable condition so
that
secondary activity
can be expected
to
continue for
an
indefinite
period.
SOCIAL AND ECONOMIC EFFECTS
For the Khumbu region,
which is
undergoing
extensive
development,
the result of the flood was a socio-economic
disaster.
In addition to a great
psychological
impact,
the
economic losses
were
the most
important consequences of
the
event. The Sherpa community
has been affected eco-
nomically
on two levels: (1) Individual families,
directly
hit
by
the
surge,
lost their entire
property
(cattle
and 30
houses; only
in a few
cases was it
possible
to
salvage
house-
hold effects).
One of the severest
consequences was that
whole
villages
were
deprived
of a large part
of their sub-
sistence base through
the loss and destruction
of
valuable
cultivable land and forest. The reclamation of debris-
FIGURE 17. Mingbo (Langmoche val-
ley),
3 October
1982, 4,100
m
a.s.l.,
km 4. The
summer
settlement
Mingbo
is located on a glacio-fluvial
terrace.
Photograph
by
M. Zimmermann.
FIGURE
18. Mingbo, 20 October 1985.
On a river section of 2 km a total
amount of
400,000
m3
were eroded and
30 percent
(1 ha) of the arable
land,
together
with 13 houses, were de-
stroyed.
Photograph
by
D. Vuichard.
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D. VUICHARD AND M. ZIMMERMANN / 105
covered land
will be impossible
in
most cases
(Table 2; Fig-
ures 17-20). Many of the
newly
built tea
shops
and lodges
along the main trail were cut off and are now some dis-
tance from the
new trail;
the individual owners
will
lose
income from tourists.
(2) The entire
Sherpa community
is
dependent
on an efficient trail
network
especially
for
sup-
plies
of
staple
goods
and tourist
activity. Disruption
of trails
and destruction
of
bridges
result
in
severe economic diffi-
culties
(Table 3; Figure
21). The problematic supply
situa-
tion,
due to the difficult access to the area in the first weeks
after the
event,
caused a drastic
price
rise
up to 50 percent
(for
rice,
sugar,
oil,
etc.). Rebuilding
of
trails and bridges
will
cost enormous amounts
of
money
for
the
Sherpas,
even
though
there
might
be assistance from the government,
from
trekking
agencies,
and from
foreign
aid programmes.
As a result of the destruction of the
hydroelectric project
the
promised
electrical
power,
to be a partial
substitute for
fuelwood,
will remain uncertain for
many years.
FIGURE 19.
Ghat
(Dudh Kosi valley),
1
June
1966, 2,500
m a.s.l., km
25.
Most of the level land of Ghat is located
near the river where
potatoes,
buck-
wheat,
maize,
millet and
vegetables
are
grown. Photograph
by
H. Heuberger.
FIGURE
20.
Ghat,
26
October
1985.
In
the
permanent
settlement
of
Ghat 11
houses
and 10
percent
of the cultivable
land
(2 ha) were
destroyed by
lateral
erosion
(undercutting
of river
banks,
progressive
sliding
of slopes). The
Chorten
(arrow) fell
apart through
vibrations of the
ground
caused
by
the
surge. Photograph
by
D. Vuichard.
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106 / MOUNTAIN RESEARCH AND DEVELOPMENT
TABLE 2
Damage
to
property,
cultivable
land,
and
forest
Dig (km
2) Most
of
the flat
grazing
land
(5-10 ha) and several
hay
fields
are covered with
a thick
layer
of
debris. Recultivation
and reuse
will not be possible
for the next
generation.
Langmoche
(km 3) Damage
to
two summer
houses. One-third
of
the
fields for
potatoes
and
hay
are
covered
with
stones
and sand.
Recultivation
and reuse
is possible
within the
next
2 or 3 years.
Mingbo (km 4) Six summer
houses on the
left and at least
6 on the
right
side
are lost. 30
percent
of the
arable
land
(3.2 ha) used for
potatoes
and buckwheat
is lost. No possibility
to find
new
land.
Thamo
Teng/Yulajung
(km
7) At
least
2 houses
(permanent
settlement)
lost.
2-4 ha of
fertile land
lost.
1-2
ha of flat
grazing
land covered with sand
(reuse
possible
within
2 or 3 years).
Jorsale (km
18) One house
lost.
Chumo
(km
19) One tea
shop
lost.
Bemkar
(km 20) One house
(permanent
settlement
and
hotel) completely destroyed.
Forest
lost.
Phakding
(km 22) Damage to
a hotel
(permanent
settlement),
reconstruction took
place.
Tsermadingma
(km
23) Arable
land
and forest lost.
Ghat
(km 25) 2 houses
(permanent
settlement)
completely destroyed,
1
house
with
hotel
and
mountaineering
equipment
store,
6
houses on the
right
side,
1
or
2 shacks lost.
1-2
ha fields
(millet,
maize,
potatoes)
and forest lost.
Nakjung/Surke (km 30) Several terraces
with
flat,
arable land
were eroded on both sides of the
river.
TABLE 3
Damage
to
infrastructure facilities
Mingbo
(km
4) The
bridge
which
connected the
village
on
the
right
side with
the
part
on
the left side was
washed
out.
Below the
village,
washed
out trails
on both sides
for
Y2
km.
Confluence
(km 5.5) Washed
away
bridge
of
main trail to
Tibet,
eroded
trails,
dangerous
detours.
Thamo
Teng
(km 6-8.5) Washed
away
bridge connecting
Thamo
Teng
with
Yulajung.
Destroyed
trails
(2 km)
on
right
side
(main
route
to
Tibet).
Thame
(km 9-10.5) Bridge
to Thame
was
washed
away.
Main trail
slid into
the river
bed
(1.5 km).
Thamo
(km 10-12) Thamo
Hydel
completely
destroyed.
Trails
on
right
side
to
summer
village
interrupted,
cumber-
some detours.
Bridge
washed
away.
Confluence
(km
16.5) Hillary
Bridge
to
Namche
washed
away.
Trails on short
stretches
interrupted,
very
long,
diffi-
cult and
dangerous
detours.
Jorsale
(km
18) Suspension bridge destroyed,
dangerous
detour.
The
village
ofJorsale
on
right
side
cut off
(empty
lodges).
Bemkar
(km
19.5-21) Carried
away
bridge.
Destroyed
main
trail on 2 km
dangerous
detours.
A new
lodge
isolated
because of
interrupted
trail.
Phakdingma
(km 22.5) Suspension
bridge
carried
away.
Eroded
main
trails
(1 km).
Cumbersome
detours.
Gyuphede
(km 23) Suspension bridge
destroyed.
Ghat
(km
25) Bridge
destroyed.
Main trails on
both
sides
eroded
and
interrupted
(3 km).
Nakjung/Surke (km
30) Two
bridges
washed
away.
Jubing
(km
39.5) Main access
bridge
destroyed.
Trail
stretches
endangered.
CONCLUSIONS
Sudden and destructive outbreaks of
glacial
lakes occur
throughout
the
Himalaya and future occurrences
must be
anticipated.
Moreover,
glacial
lakes
are
increasing
in
num-
ber and volume because of the continued thinning
and
recession
of the
glaciers
(Figure 22). Thus it
is
likely
that
frequency
of
glacial-lake
outbursts
will
increase.
Dig Tsho
in
the
Langmoche valley
was probably
formed
within the
last 25 years
(volume of
water: 5 million
m3). As many
of
the glaciers
occur in deeply eroded basins at the
foot
of
high
mountain
cliffs,
they
and their
lakes are
within
the
reach of
avalanching material from
the slopes above.
Avalanching or surging
of glaciers generate impulse
waves
which
can trigger
the
rupture
of
natural dams. The
avalanching ice mass which
triggered
the
Dig Tsho out-
burst in 1985 was estimated
to
be 150,000 m3
in
volume.
Other
mechanisms,
such
as piping,
also must
be
taken
into
consideration,
because each glacial lake
has its
own char-
acteristics.
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D. VUICHARD
AND
M. ZIMMERMANN
/
107
FIGURE
21.
Jubing (Dudh
Kosi
valley),
1,563
m
a.s.l., km
39.5. The bridge,
which crossed the
Dudh Kosi with a
free-board of
14
m,
served as the main
access to the Khumbu area. This tem-
porary
bamboo
bridge
has been in
use
for over
a year
since the
surge.
Photo-
graph
by D. Vuichard,
13 October
1985.
For
the areas
in close
proximity
to the
origin
of a lake
outbreak the
consequences
can be
catastrophic. Although
in
the
Khumbu
area human casualties
were
rather
low
(due
to the coincidence
of
benevolent
circumstances:
daytime
occurrence,
few
people
near the river because of
a festival,
timing
in an
off-tourist
season),
a
natural environment with
its
superimposed
cultural and economic structures
was
severely disrupted. Large
amounts of debris were
eroded,
transported,
and
redeposited
within
a short
distance
(2.6
million
m3
within the first
25
km).
The landscape along
the
river was
drastically
changed
and with it the sites
of
the
Sherpas'
subsistence
base and much infrastructure were
lost.
It is
regrettable
that new investment
projects,
such
as the Namche Small
Hydel Project
and the
suspension
bridge
at
Jorsale,
were
destroyed.
Such destruction
might
have been
prevented
or
averted had the
potential
hazard
of
glacial-lake
outbursts
been taken into account
during
the
planning
stages.
The economic
consequences
of this
type
of
inadequate,
if
not
irresponsible, planning
will be
a burden on the
Sherpa community
for
many
years
to
come.
Downstream effects
were
not
as
severe,
because the river
channel
(beyond
50-60
km)
was able to absorb most of the
surge
and the
river level at the moment of
occurrence
was
unusually
low.
Generally
the sediment load
reaching
the
Himalayan foredeep
does not
originate
from the
High
Himalaya
(Zimmermann
et
al., 1986:39);
greater signifi-
cance is
attributed
to the
deeply
weathered strata of the
Nepalese
Middle Mountains
and Siwaliks as a source area
for downstream
sedimentation.
Most
of
the sediment load
from the
drainage
of
Dig
Tsho
remained
in the Khumbu
region. Only 10-15 percent
(270,000
m3)
of the material moved
is
assumed to
have left
the
region
as suspended
load.
In
light
of the
foregoing
discussion it
would seem that
a systematic
and co-ordinated
response
to the
potential
hazard
of
moraine-dammed and ice-dammed lakes is
imperative
(see
also
Ives,
1986).
Thus
hazard assessment of
glacier
lakes
should be incor-
porated
into
any development concept
for
the
Himalayan
region.
In
Khumbu Himal and
adjacent
areas
alone,
many
lakes of
comparable
size to
Dig
Tsho can be identified:
Tshola Tsho
(ice-dammed);
Pareshaya
Tsho on
Imja
Glacier
(moraine-
and
ice-dammed);
Gokyo
Tsho
(ice-dammed);
Trakarding
Tsho,
Rolwaling
(moraine-dammed);
Lumding
Tsho and Dudha Khund,
Numbur
(both
moraine-
dammed);
Sabai Tsho and Dudh
Khund,
Hinku
Valley
(both
moraine-dammed),
and
many
unnamed
ice-
and
moraine-dammed lakes in
the Makalu/ Baruntse
area,
Arun
Valley.
These are
just
a few
(Figure 22) of the
hundreds
of
dammed water masses
in
eastern
Nepal.
Therefore,
a lake
inventory
must be established. It
should include
identification
of
source
areas
(mapping)
and
evaluation of the
potential
hazards
(Ives,
1986).
This
can
be done
by
interpretation
of
terrestrial, air,
and satellite
imagery.
Remote
sensing
is
important,
but
a final
assess-
ment must
include
a
rigorous
field
investigation
to
provide
the
following
information:
- size
and volume of lake
- seasonal
changes
of water level
- changes
in lake size
during
recent
years
- dam characteristics
(moraine,
ice,
assessment
of
stability
against
erosion or
piping)
--
probability
for
avalanching
(causing
impulse
waves)
- quantification
of
potential
sediment
load that
might
be
transported by
an
outburst
(volume,
material charac-
teristics,
transportation
distance)
- evaluation of the
consequences
for the channel
system
below the lake
(capacity
of
the river
bed,
erodible mass
potential,
potentially endangered
stretches
through large
accumulations)
- a
detailed hazard assessment for
specific
sites
(potential
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108 / MOUNTAIN RESEARCH AND DEVELOPMENT
FIGURE 22. There is the
potential
for
catastrophic
discharge
of
large
volumes
of water
throughout
the Himalaya:
above,
Tshola
Tsho
(4,500 m),
one of
the
major
lakes
in
the
Khumbu
area;
below,
Sabai Tsho (4,450 m) in the
Hinku Drangka valley
between the
Dudh and
Arun rivers.
destabilization of river
banks and terraces with subse-
quent
destruction of vital
installations;
where and why?
what countermeasures can be taken?)
Due to the
rapid glacial changes that have been a fea-
ture of recent
decades a monitoring programme
will have
to be developed for
selected localities. Remote sensing
should be
an essential
component,
but this would be
useful
primarily
as the basis for a rapid
overview reconnaissance
and for the selection of sites for detailed
survey
and regular
monitoring;
a final
assessment
will
depend upon careful
field
investigations
as indicated above. Co-ordination be-
tween local and foreign
scientific
and development
agen-
cies, as well as making
accessible the available documen-
tation
material,
would greatly
augment the value of
the
results of such a programme.
Preventative
measures,
such
as artificial
lowering
of lake levels, will
only
be possible
at considerable expense. However, compared to the ex-
tremely
high
cost of
investment
in
infrastructure,
pre-con-
struction
surveys
and monitoring
are inexpensive
and are
rather
easy
to
put
into
practice.
Such an approach
will con-
tribute to the
prevention
or
minimization of disasters exem-
plified by the Langmoche lake outburst of 1985.
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D. VUICHARD AND M. ZIMMERMANN / 109
ACKNOWLEDGEMENTS
The results
presented
here have evolved since our first
report
(Vuichard and Zimmermann, 1986). This is due to
the
many
specialists
from all
over
the
world who have pro-
vided advice and information. We thank Dr. R. Gautschi,
Professors
J. D. Ives and B. Messerli, as well
as our col-
leagues at the
departments
of
Geography
and Mineralogy
of
the
University
of
Berne,
for
providing
financial
support
and for
editing
and general guidance.
We are also indebted
to J. Niederer, P. S. Pradhan, and the members of
S.A.T.A. and ICIMOD, who
provided
us with invaluable
support
in Nepal. We are most
grateful
to the
Sherpas of
Khumbu Himal who
generously responded
to
our
requests
for information
and faithfully supported
our
work,
and also
provided
outstanding hospitality.
We wish to record
special
personal
thanks to
Dr. V. Galay, who
made his
first-hand
information
and material available. Professors H. Heu-
berger,
K. Higuchi, E. Schneider,
Tj. Peters,
and Dr. J.
Meyer
provided
access
to
their
ground
and air
photography.
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