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Nat. Hazards Earth Syst. Sci., 11, 1447–1462, 2011
www.nat-hazards-earth-syst-sci.net/11/1447/2011/
doi:10.5194/nhess-11-1447-2011
© Author(s) 2011. CC Attribution 3.0 License.
Natural Hazards
and Earth
System Sciences
Regional-scale analysis of lake outburst hazards in the southwestern
Pamir, Tajikistan, based on remote sensing and GIS
M. Mergili and J. F. Schneider
Institute of Applied Geology, BOKU University of Natural Resources and Life Sciences Vienna, Peter-Jordan-Straße 70,
1190 Vienna, Austria
Received: 19 November 2010 – Revised: 29 March 2011 – Accepted: 5 April 2011 – Published: 18 May 2011
Abstract. This paper presents an analysis of the hazards em-
anating from the sudden drainage of alpine lakes in South-
Western Tajik Pamir. In the last 40yr, several new lakes
have formed in the front of retreating glacier tongues, and
existing lakes have grown. Other lakes are dammed by land-
slide deposits or older moraines. In 2002, sudden drainage
of a glacial lake in the area triggered a catastrophic de-
bris flow. Building on existing approaches, a rating scheme
was devised allowing quick, regional-scale identification
of potentially hazardous lakes and possible impact areas.
This approach relies on GIS, remote sensing and empirical
modelling, largely based on medium-resolution international
datasets. Out of the 428 lakes mapped in the area, 6 were
rated very hazardous and 34 hazardous. This classification
was used for the selection of lakes requiring in-depth inves-
tigation. Selected cases are presented and discussed in order
to understand the potentials and limitations of the approach
used. Such an understanding is essential for the appropriate
application of the methodology for risk mitigation purposes.
1 Introduction
Natural dams of different size and origin exist in mountain
areas all over the world (Costa and Schuster, 1988). They of-
ten retain lakes which, in the case of a dam failure, may drain
as powerful floods. If the failed dam is a glacier or a fea-
ture of a glacially shaped landscape, such events are called
Glacial Lake Outburst Floods (GLOFs). Sudden drainage
of glacial lakes has been reported from the Himalayas, the
mountains of Central Asia, the North American mountains,
the South American Andes, New Zealand and the Alps.
Correspondence to: M. Mergili
(martin.mergili@boku.ac.at)
(e.g. Clarke, 1982; Hewitt, 1982; Haeberli, 1983; Watan-
abe and Rothacher, 1996; Richardson and Reynolds, 2000;
Vil
´
ımek et al., 2005; Narama et al., 2010). Climate change,
with its impact on the glacial extent, the hydrological cycle
and the condition of ice-bearing dams, may condition the oc-
currence of GLOFs in manifold ways and on different time
scales (Evans and Clague, 1994; IPCC, 2007; Dussaillant et
al., 2009; Haeberli et al., 2010a).
But also other types of lakes – particularly such dammed
by landslide deposits – may be subjected to sudden drainage
(Costa and Schuster, 1991). Lake outburst floods (referred
to as LOFs in the present paper, including both glacial and
non-glacial lakes) often have a highly destructive potential.
A large amount of water is released within a short time, with
a high capacity to erode loose debris, potentially leading to
a powerful flow with a long travel distance. Peak discharges
are often some magnitudes higher than in the case of “nor-
mal” floods (Cenderella and Wohl, 2001).
LOFs can evolve in different ways, possible causes are:
1. landslides, rock/ice avalanches or calving glaciers that
produce flood waves ina pro- or supraglacial lakewhich
may overtop and possibly breach glacial, morainic or
landslide dams (Tinti et al., 1999);
2. rising lake levels, leading to progressive incision and
destabilization of a dam;
3. hydrostatic failure of ice dams or enhanced ground wa-
ter flow (piping), which can cause sudden outflow of
accumulated water (Iturrizaga, 2005a, b);
4. degradation of glacier dams or ice cores in morainic
dams leading to loss of stability and to subsidence re-
sulting in internal failure or progressive erosion if a cer-
tain threshold is reached.
Published by Copernicus Publications on behalf of the European Geosciences Union.
1448 M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir
Richardson and Reynolds (2000) and Haeberli et al.
(2010a) provide overviews of the conditioning and trigger-
ing factors of lake outburst floods, dam failure mechanisms
and process interactions, as well as case studies.
Even though lakes can be identified relatively easily with
remote sensing tools and field work (e.g. Huggel et al.,
2004b; K
¨
a
¨
ab et al., 2005; Quincey et al., 2007), the selec-
tion of potentially hazardous lakes, the prediction of out-
burst floods and modelling of the motion and reach of spe-
cific LOFs still remain a challenge (Kargel et al., 2010;
Mergili et al., 2011). Changes in flow behaviour (e.g. from
clear water flow to hyperconcentrated flow and debris flow or
reverse) imply some difficulties when using computer mod-
els to predict LOF velocities and travel distances.
This paper presents a lake outburst susceptibility and haz-
ard analysis for a selected area of the southwestern Pamir,
focusing on the regional scale: GIS and Remote Sensing ap-
proaches are applied, allowing a quick identification of po-
tentially hazardous lakes and possible impact areas over a
large area. The results of such a study facilitate the selec-
tion of sites requiring more detailed studies and serve as a
baseline for risk mitigation strategies in the region. It is at-
tempted to push the methodology for this type of analysis for-
ward, building on existing approaches (e.g. Reynolds, 2003;
Huggel et al., 2004a).
2 Study area
The study presented covers the Gunt and Shakhdara valleys
in southwestern Tajik Pamir (Fig. 1). It covers a total surface
area of 8430km
2
. The Gunt River originates in the highlands
of the eastern Pamir and joins the Panj River near the town of
Khorog (2075ma.s.l.). The Shakhdara Valley is a southern
tributaryof the Gunt Valley with the confluence just upstream
from Khorog. From north to south, the valleys are sepa-
rated by the Rushan Range (culminating at 6068 m a.s.l.),
the Shugnan Range (5708 m) and the Shakhdara Range
(6723m), all stretching in an east-west direction. The north-
south stretching Ishkashim Range (6095m) follows directly
northwest of the Shakhdara Range. In terms of geology,
gneisses dominate the area, with remnants of a former sedi-
mentary cover particularly in the northern portion. The area
is seismically active with frequent earthquakes (e.g. Babaev
and Mirzoev, 1976).
The area covers the districts of Shugnan (Gunt Valley) and
Roshtkala (Shakhdara Valley) under the Gorno-Badakhshan
Autonomous Oblast of the Republic of Tajikistan with the
regional centre of Khorog. Except for Khorog, the popula-
tion largely depends on agriculture and pastoralism. Indus-
try, service, and tourism are poorly developed. The perma-
nent settlements are concentrated close to the valley bottom
and depend on irrigation with melt water. Transhumance
to distant summer pastures in the upper valleys is common
(Kassam, 2009).
Fig. 1. The study area.
The climate is semi-arid to arid, with an annual mean
precipitation of 288mm. Most of it occurs during winter
and spring. The average annual air temperature is 9.2
◦
C
(Khorog; 1970–2008). High-altitude meteorological data is
missing. The major mountain ranges are glacierized, but
most of the glaciers are in a stage of retreat. Many glacier
tongues are covered by debris, so that it is hard to delineate
their extent fromsatellite imageryor superficial field surveys.
Whilst many glaciers in the northern Pamir have shown a
variable behaviour during the previous decades or surge at
certain intervals (e.g. Kotlyakov et al., 2008), the glaciers
in the southwestern Pamir have been shrinking for several
decades. Numerous lakes have formed in the front or on the
top of retreating glacier tongues, and several more are re-
tained by holocene landslide or moraine dams and in depres-
sions within the glacially shaped terrain.
Altogether, 428 lakes were identified in the study area
(including only those with a surface area ≥2500 m
2
). The
majority of the lakes is found in the direct forefield of the
recent glaciers and on the undulating highlands shaped by
Pleistocenic glaciers. 90 per cent of all lakes are located be-
tween 4200–4900ma.s.l. Some of the larger lakes, however,
dammed by holocene landslides and/or terminal moraines
from the Pleistocene, are located at lower elevation (e.g. Ri-
vakkul,Durumkul, Zardivkul). No lakesat all were identified
below 3341ma.s.l.
Nat. Hazards Earth Syst. Sci., 11, 1447–1462, 2011 www.nat-hazards-earth-syst-sci.net/11/1447/2011/
M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir 1449
3 The Dasht 2002 event
On 7 August 2002, the village of Dasht (Shakhdara Val-
ley) was hit by a GLOF. 10.5km upstream in the headwa-
ters of the valley, a lake with a surface of 37000m
2
, lo-
cated on the decaying tongue of a debris-covered glacier,
had suddenly released an estimated volume of 320000m
3
of
water. Multitemporal analysis of Landsat imagery showed
that the lake had existed for less than two years prior to
drainage. The outburst occurred beneath the glacier surface.
The most likely interpretation of the observed features is that
a drainage channel within the glacier was suddenly blocked
at the end of 2000 or in the beginning of 2001, allowing the
development of the lake. With increasing lake size and there-
fore pressure, the blockage failed and the lake then drained
suddenly.
The volume of debris deposited on the cone was es-
timated at 1.0–1.5millionm
3
, meaning that the ratio be-
tween entrained debris and water would be 3–5. This is a
very high value compared to the ratio of 2–3 suggested by
Huggel et al. (2004b). However, an even higher ratio than
observed for Dasht was reported by Breien et al. (2008) for a
GLOF in Norway. Possibly, subglacial water reservoirs con-
nected to the superficial lake were involved in both events.
These figures show that the characteristics of the Dasht
event underwent pronounced changes during the flow,
converting from a clear water runoff to a hyperconcentrated
flow and finally to a granular debris flow. The event de-
stroyed a large portion of the village of Dasht, killed dozens
of people and dammed the Shakhdara river. It was reported
that the flood wave arrived at Dasht in three stages, a phe-
nomenon that can be explained by temporary backwater in
the canyon of the lower transitional zone due to blockage
of large boulders transported by the GLOF or by lateral
slope failures followed by vigorous breakthroughs (Schnei-
der et al., 2004). According to local people from the village
of Baroj on the opposite side of the valley, the travel time
from the onset of the process to Dasht was at least 45min.
The Dasht event was the only major GLOF reported from
the Pamir in the past decades. However, several events are
known from the Tien Shan, most recently the outbursts of the
Archa-Bashy glacier lake in 1998 and of the western Zyndan
glacier lake in 2008 (Narama et al., 2010).
4 Lake inventory preparation
This study relies primarily on remotely sensed data. In order
to obtain a multi-temporal coverage of the entire study area,
the following sets of satellite images were collected:
1. declassified Corona images (1968; pixel size <5m)
2. Landsat ETM+ images (2000–2003; pixel size 15m
pan-sharpened)
3. ASTER images (2000–2009; pixel size: 15m)
In addition, the SRTM-4 digital elevation model
(Jarvis et al., 2008; approx. 90m pixel size) was used. De-
spite its coarser resolution, it appeared to better represent
the real terrain than ASTER DEMs, a finding in line with
K
¨
a
¨
ab et al. (2005).
All lakes in the area were mapped for three time win-
dows: 1968 (Corona imagery), 2001/2002 (ASTER and pan-
sharpened LandsatETM+ imagery), and 2007/2008 (ASTER
imagery). Except for 1968, it was not possible to cover the
entire area with cloud- and snow-free scenes from one sin-
gle year, let alone day. The verification and collection of
additional information for each lake was supported by high-
resolution Google Earth scenes.
Dam material, type of drainage and the presence of
glaciers calving into the lake were recorded for each lake.
Seepage through the dam was assumed for all lakes without
clearly recognizable surface drainage. Even though combi-
nations and temporal changes of both types of lake drainage
are common, a detailed analysis of this phenomenon is
hardly applicable at the regional scale. Lake area A (m
2
)
was derived and the freeboard F (m) was computed as the
difference between the DEM with filled sinks and the ele-
vation of the lake (value of original DEM for the lake cen-
troid). Table 1 summarizes the key parameters assigned to
each mapped lake. All lakes with A < 2500m
2
in 2007/2008
were excluded from further analysis.
Regression functions relating lake volume to lake area
were suggested e.g. by O’Connor et al. (2001) and
Huggel et al. (2002), and used by Allen et al. (2009). How-
ever, computing lake volume directly from lake area is prob-
lematic. The area is immanent to the volume and autocorre-
lation effects reduce the large scatter in the relation of mea-
sured area and depth and lead to an overestimation of the in-
formation quality (Huggel et al., 2002). Therefore, average
lake depth was first derived from lake area using an empirical
relationship developed by Huggel et al. (2002):
D = 1.04×10
−1
A
0.42
(1)
In the next step, lake volume V (m
3
) was derived from lake
area and average lake depth:
V = D × A = 1.04× 10
−1
A
1.42
(2)
Lake area development was expressed as the lake area in
1968 resp. 2001/2002, related to the area in 2007/2008:
r
A1
=
A
1968
A
2007/2008
(3)
r
A2
=
A
2001/2002
A
2007/2008
(4)
Values below 1 indicate growth, values above 1 shrinkage
of the lake. Values of 0 mean that the lake did not yet ex-
ist in 1968 resp. 2001/2002. Lakes existing in 1968 and/or
2001/2002, but not in 2007/2008, were disregarded in the
analysis.
www.nat-hazards-earth-syst-sci.net/11/1447/2011/ Nat. Hazards Earth Syst. Sci., 11, 1447–1462, 2011
1450 M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir
Table 1. Parameters used as input for the regional-scale lake outburst hazard analysis (see Fig. 2).
Parameter Shortcut Unit Source
Lake drainage boolean Qualitative interpretation of satellite imagery
Dam type nominal (ASTER, WorldView, GoogleEarth)
Possible calving into lake boolean
Elevation a.s.l. z m Computed from lake centroid and DEM
Freeboard F m
Lake area A m
2
Derived from mapped lakes
Average lake depth D m Empirical relationship with lake area (Eq. 1)
Lake volume V m
3
Derived from lake area and average lake depth (Eq. 2)
Lake area development r
A1
ratio Comparison of lake areas derived by multitemporal
1968–2007/2008 analysis of satellite imagery
Lake area development r
A2
2001/2002–2007/2008
Maximum Peak Ground PGA
max
ms
−2
Map of active faults and published relationships
Acceleration (Babaev et al., 1984; Abdrakhmatov et al., 2003)
As lake outburst floods are often related to major earth-
quakes, a surrogate for the seismic hazard was intro-
duced: the maximum possible Peak Ground Acceleration
PGA
max
(cms
−2
) was considered useful for this purpose. It
was derived from a published map of active faults of Tajik-
istan (Babaev et al., 1984) and empirical relationships relat-
ing PGA
max
to the maximum earthquake magnitude assigned
to the fault and the distance to the fault axis (Abdrakhma-
tov et al., 2003).
5 Hazard analysis
5.1 Work flow
The hazard analysis procedure applied to the regional scale
is illustrated in Fig. 2. It aims at the identification of poten-
tially hazardous lakes and possible impact areas of LOFs as a
baseline for in-detail studies and risk mitigation procedures.
The concept includes the hazard
1. of each lake to produce an outburst flood (lake outburst
hazard H );
2. of each pixel to be affected by an outburst flood from a
certain lake (impact hazard HI).
The general difficulty of establishing frequencies for rare
or singular events like lake outburst floods in combination
with sparse historical data in the study area make a strictly
quantitative approach inapplicable. The concept of suscep-
tibility – understood as the tendency of a lake to produce
an outburst flood resp. the tendency of an outburst flood to
reach a certain area or pixel – is used, instead. It is com-
bined with the maximum possible magnitude in order to de-
rive a measure for the hazard. This developed and employed
Fig. 2. Work flow of the regional-scale hazard analysis.
system of scoring and rating partly refers to work done by
Reynolds (2003) and Huggel et al. (2004a) and is explained
in the following sections.
The entire work flow is realized as a system of a shell and
a C script making use of GRASS GIS, which is an Open
Source Geographic Information Software, for the spatial
Nat. Hazards Earth Syst. Sci., 11, 1447–1462, 2011 www.nat-hazards-earth-syst-sci.net/11/1447/2011/
M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir 1451
Table 2. Rating scheme for susceptibility to outburst triggered by external forces S
ext
. Initial values of S
ext
are determined from the
topographic susceptibility index. These values are then increased or decreased according to the possibility of calving, seismic hazard and
freeboard.
Criterion Class Definition S
ext
Topographic susceptibility index
1 Low TSI<10 0
2 Medium TSI ≥10–<40 1
3 High TSI≥40 2
Calving into lake
1 No calving possible mapped ±0
2 Calving possible mapped +1
Seismic hazard
1 Low PGA
max
<500cms
−2
±0
2 High PGA
max
≥500cms
−2
+1
Freeboard
1 High F >25 m −1
2 Low F ≤25 m ±0
analysis components. The major spatial modelling tasks in-
clude the topographic Susceptibility Index TSI (see next sec-
tion), the dam geometry and the area of impact. The DEM,
a raster map with a unique ID for each lake, and a text file
with the parameters listed in Table 1 for each lake are used
as input. The output consists of a table with the lake out-
burst susceptibility and hazard ratings for each lake, as well
as raster maps with lake-specific impact susceptibility and
hazard ratings for each pixel.
The procedure is applied with a pixel size of 60 m. The re-
sults are verified by testing the output of the entire procedure
against the information available for the Dasht 2002 event
and by in-depth investigations of selected cases. For this pur-
pose, several helicopter and field missions were carried out
in 2003 and 2009 in order to ensure a close and up-to-date
survey of the lakes and valleys of interest.
5.2 Lake outburst susceptibility and hazard
5.2.1 Susceptibility to lake outburst triggered by
external forces
The susceptibility to each lake to produce an outburst flood
triggered by any kind of mass movement interfering with
the lake or by an earthquake was investigated. The event
at Laguna 513 in the Cordillera Blanca (Per
´
u; Haeberli et al.,
2010b), where an ice avalanche fromfar upslopecaused a de-
structive flood wave on 11 April 2010, has shown the need to
include the entire catchment in such an analysis, and not only
the portion directly adjacent to the lake. The topographic
susceptibility index TSI is introduced in order to account for
this need: each pixel within the catchment of the lake is as-
signed to one of 25 predefined classes, based on the local
slope and the average slope of the steepest path to the lake.
Depending on the class, a topographic susceptibility rating
TSR (with possible values 0–10) is assigned to the pixel, de-
scribing the tendency of the lake to be affected by a mass
movement originating from the respective pixel. The rating
builds on a combination of classes of local slope for the onset
and of the average slope of the potential flow path between
onset area and lake for the motion of mass movements, fol-
lowing thresholds given e.g. by Corominas et al. (2003). In
order to derive TSI for the entire catchment, the sum of the
TSRs for each class is weighted by the surface area assigned
to the respective class (possible weights: 1–4). The weighted
indices for each class are then summed up. The resulting
maximum possible TSI is 212. This concept does not ac-
count for local geological conditions, but provides a valuable
estimate of where impacts are basically possible and where
they are not.
TSI is the base for rating the susceptibility to lake outburst
triggered by external forces S
ext
(Table 2). The possibility of
ice calving into the lake is accounted for by increasing S
ext
by 1 for lakes with directly adjacent glaciers.
Regarding the probability of triggering events, too little
high-altitude meteo data are available to be included in the
rating. The Maximum Peak Ground Acceleration PGA
max
is used to account for the seismic hazard. PGA thresholds
for the triggering of mass movements in different areas show
a large scatter. The threshold of 0.7ms
−2
found out by
Wang et al. (2010) for the Wenchuan 2008 earthquake relates
to the extremely landslide-prone slopes of that area and is not
applicable to the southwestern Pamir. Murphy et al. (2002)
suggested thresholds between 4.5 and 20.4m s
−2
for the
rocky slopes ofthe Tachia Valley in Taiwan. Holding to these
findings, S
ext
is increased by 1 for PGA
max
≥5ms
−2
.
The freeboard F (m) iscomputed fromthe DEM.Forlakes
with F ≥25m, the rating is decreased by 1 in order to derive
the final rating (see Table 2). Therefore, S
ext
could take val-
ues between 0 and 4 (negative values are considered as 0).
www.nat-hazards-earth-syst-sci.net/11/1447/2011/ Nat. Hazards Earth Syst. Sci., 11, 1447–1462, 2011
1452 M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir
Table 3. Rating scheme for Susceptibility to lake outburst triggered by internal forces S
int
. Initial values of S
int
are determined depending
on the dam material. These values are then increased or decreased according to lake drainage, lake area development and downstream slope
of the dam.
Criterion Class Definition S
int
Dam material
1 Embedded lake mapped 0
2 Block dam mapped 0
3 Debris dam mapped 1
4 Rocky swell dam mapped 0
5 Glacier or fresh moraine dam mapped 2
Lake drainage
1 Clearly recognizable surface drainage mapped ±0
2 No clearly recognizable surface drainage mapped +1
Lake area development
1 Stable or shrinking r
A1
and r
A2
>0.8 ±0
2 Growing r
A1
or r
A2
≤0.8 +1
Downstream slope of dam
1 Gentle tan β<0.02 −1
2 Steep tan β≥0.02 ±0
5.2.2 Susceptibility to lake outburst triggered by
internal forces
As directly measured quantitative data is not applicable at
the regional scale, a qualitative rating for the susceptibility to
lake outburst by internal forces (dam failure) S
int
has to be in-
troduced, building on the following key parameters: (1) dam
material; (2) lake drainage, (3) lake area development, and
(4) dam geometry. Table 3 shows the applied rating scheme.
Dams with seepage are rated more susceptible to failure
than dams with surface runoff only, and growing lakes are
rated more susceptible than stable or shrinking ones. All
lakes with a size of ≤80 per cent of the 2007/2008 surface
area in either 1968 or 2001/2002 are classified as growing.
Dam geometry is expressed as an idealized average down-
stream slope of the dam: the dam width W is defined as the
Euclidean distance between the lake outlet and the closest
pixel along the downstream flow path with a lower elevation
than theaverage lake bottom, using the averagelake depthD.
The tangent of the average slope of the dam in outflow direc-
tion, tan β, is then derived as D/W . For very small down-
stream average slopes (tan β ≤ 0.02), the rating is decreased
by 1 (see Table 3).
Also S
int
can take values between 0 and 4, negative values
are considered as 0.
5.2.3 Derivation of lake outburst susceptibility and
hazard
The ratings for S
int
and S
ext
are combined using the rating
scheme shown in Table 4. In order to derive a measure for
the hazard, theresulting lake outburst susceptibilityS is com-
bined with a measure for the potential event magnitude M.
Lake volume V or the expected peak discharge of an out-
burst flood Q
P
would be most suitable, but they are derived
by empirical equations only and are highly uncertain. The
Table 4. Lake outburst susceptibility rating S: combination of S
ext
and S
int
.
S
int
↓ S
ext
→
0 1 2 3 4
0 0 1 2 3 4
1 1 2 3 3 4
2 2 3 3 4 5
3 3 3 4 4 5
4 4 4 5 5 6
lake volume is directly related to the lake area A (see Eqs. 1
and 2). Therefore, the well-known lake area appears to be the
best surrogate for M. The rating for the lake outburst hazard
H is derived by combining the outburst susceptibility S of
each lake with its area class. The applied scheme is shown in
Fig. 3.
5.3 Impact susceptibility and hazard
5.3.1 Impact susceptibility
Impact susceptibility I is understood as the tendency of an
outburst flood from a defined lake to affect a certain area or
pixel. It disregards the lake outburst hazard H which is in-
cluded in the next step (impact hazard HI).
On the regional scale, empirical-statistical relationships
are suitable for relating the travel distance L or the aver-
age slope of reach ω of a flow to the involved volume V or
the peak discharge Q
p
, or just defining a global ω. Table 5
shows some of these relationships developed for debris flows
in general and lake outburst floods in particular.
Nat. Hazards Earth Syst. Sci., 11, 1447–1462, 2011 www.nat-hazards-earth-syst-sci.net/11/1447/2011/
M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir 1453
Fig. 3. Lake outburst hazard H : combination of lake area as sur-
rogate for the possible lake outburst magnitude M and suscepti-
bility S. Ne =negligible, Lo=low, Mo= moderate, Me=medium,
Hi=high, Vh =very high, Eh=extremely high.
Corominas et al. (2003) found an average angle of reach
of ω = 21
◦
for debris flows on unobstructed flow paths. Rick-
enmann (1999) related the horizontal travel distance L (m) to
the bulk volume of the flow V (m
3
) and the vertical distance
Z (m) (see Table 5). The results obtained by these rules show
a large scatter among themselves and generally underesti-
mate thetravel distance of LOFs(Mergili et al., 2011; Fig.4).
Haeberli (1983) suggested ω =11
◦
specifically for GLOFs,
which was applied by Huggel et al. (2003) in combination
with the Modified Single Flow direction model (MSF).
Several authors have proposed empirical relationships for
deriving the peak discharge Q
p
(m
3
s
−1
) – required as input
for the relationship T3 in Table 5 – based on outburst vol-
ume V (m
3
) and dam height D (m) as predictors, see e.g.
Crosta et al. (2006) for an overview. In the present study, the
outburst volume is set as equal to the lake volume (worst-
case assumption). Table 6 summarizes various approaches,
mainly regression functions. Walder and Costa (1996) and
Huggel et al. (2004a) emphasized the importance to distin-
guish between different drainage modes and different types
of dams, respectively. t in the relationship Q8 stands for the
duration of the outburst, Huggel et al. (2004a) suggest to set
t = 1000 s as a first approximation.
After the deposition of the debris or mud carried by the
LOF, or if not much sediment is entrained at all, the flood
may propagate much farther: Haeberli (1983) suggested an
average angle of reach of 2–3
◦
, but also travel distances ex-
ceeding 200km were reported (e.g. Hewitt, 1982).
In order to achieve a robust estimate of the travel distance
of potential LOFs, the approaches T1–T4 shown in Table 5
are combined. The approach of Corominas et al. (2003) is
disregarded as it is apparently not at all suitable for LOFs.
The susceptibility of each pixel to be affected by an outburst
flood from the respective lake is computed as follows, mak-
ing use of GRASS GIS:
Fig. 4. Matrix for the impact hazard HI rating based on the ratings
for lake outburst hazard rating H and impact susceptibility I . The
abbreviations are explained in the legend of Fig. 3.
1. The lake outburst flood is considered as one single mass
point, starting from the outlet of the respective lake. It is
routed downstream following a random walk approach
weighted for the local slope angle. In order to cover all
possible pathways, 800 random walks are performed for
each lake.
2. At the start of each random walk, the empirical rela-
tionship defining the travel distance is determined ran-
domly, choosing among T1–T4 (see Table 5). This
means that ±200 random walks are performed accord-
ing to each of the relationships. For T1, the volume V
is randomly varied between one time and four times the
lake volume in order to account for bulking with sedi-
ment. Regarding T3, Q
p
is chosen randomly from Ta-
ble 6 for each random walk. Q1–Q9 are used for glacial
lakes (Type 5 in Table 3). For all other lakes, only the
relationships Q1–Q7 are used. This leads to the repre-
sentation of all the relationships in the results (±25–30
random walks per relationship).
3. An impact susceptibility rating I (0 negligible–6 ex-
tremely high) is then assigned to each pixel, based on
the number n of relationships T1–T4 predicting an im-
pact for the corresponding pixel, and on the average
slope ω (Table 7). With n = 1, the corresponding rela-
tionship would always be T4, yielding the longest travel
distance and the largest impact area. For these pixels,
the possible type of impact is considered as a flood (no
debris flow), and the impact susceptibility rating I is
1 low–3 medium, depending on ω. If at least two of the
relationships predict an impact on the pixel, also debris
flow is considered as possible impact type, and the im-
pact susceptibility is defined 4 high–6 extremely high,
depending on n.
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1454 M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir
Table 5. Empirical relationships potentially suitable for estimating the travel distance of lake outburst floods.
Relationship References Remarks
T1 L = 1.9V
0.16
Z
0.83
Rickenmann (1999) for debris flows in general
T2 ω =11
◦
Haeberli (1983),
Huggel et al. (2003),
Huggel et al. (2004a)
for debris flows from GLOFs, applied
with ω =8.5
◦
in the present paper
T3 ω = 18Q
−0.07
p
Huggel (2004) worst case for debris flows from GLOFs
T4 ω≥2
◦
Haeberli (1983),
Huggel et al. (2004a)
for floods from GLOFs
Table 6. Empirical equations relating peak discharge Q
p
to outburst volume V and lake depth resp. dam height D. ρ
w
=density of water
(kgm
−2
), g =gravity (m s
−2
). The examples refer to the computed peak discharges of the Dasht event 2002 (lake area: 37000m
2
) and a
hypothetic complete drainage of Rivakkul (1.2km
2
).
Equation for Q
p
(m
3
s
−1
) Reference Example Q
p
Example Q
p
Dasht 2002 Rivakkul
Q1 672
10
−6
V
0.56
Costa (1985) 354m
3
s
−1
5562m
3
s
−1
Q2 6.3D
1.59
194m
3
s
−1
1955m
3
s
−1
Q3 181
10
−6
V · D
0.43
280m
3
s
−1
4329m
3
s
−1
Q4 1.58· 10
−2
(
ρ
W
· g · V · D
)
0.41
Costa and Schuster (1988) 299m
3
s
−1
4072m
3
s
−1
Q5 1.6V
0.46
Walder and O’Connor (1997) 544m
3
s
−1
5225m
3
s
−1
Q6 6.7D
1.73
278m
3
s
−1
3446m
3
s
−1
Q7 9.9· 10
−1
(
V · D
)
0.40
373m
3
s
−1
4766m
3
s
−1
Q8 2V
t for moraine dams Huggel et al. (2004a) 638m
3
s
−1
Q9 46
10
−6
V
−0.07
for ice dams 50m
3
s
−1
5.3.2 Impact hazard
The ratings for lake outburst hazard H and impact suscep-
tibility I are combined to a rating of the impact hazard HI
according to the matrix shown in Fig. 4. A first step towards
risk analysis is taken by an overlay of the impact hazard map
with a map of settlements and cultivated areas. A further dif-
ferentiation of this map regarding exposure and vulnerability
would be required for a full regional-scale risk analysis.
5.4 Evaluation with the Dasht 2002 event
The hazard analysis procedure is evaluated using the Dasht
2002 event. In its surface appearance, the lake did not differ
substantially from many other glacial lakes in the study area.
The lake outburst susceptibility is rated very high, the hazard
– due to the limited size of the lake – only as medium.
All empirical models shown in Table 5 underestimate
the travel distance of the debris flow of Dasht, which is
characterized by ω =9.3
◦
. Whilst the resulting debris flow
had reached the village of Dasht 10.5km downstream from
the lake, the empirical relationships predict the debris flow to
stop already in the upper portion of the catchment (Fig. 5).
The reasons for the long travel distance might be a mobilized
subglacial water reservoir involved in the flow, a particular
flow rheology, or backwater effects: according to field ob-
servations and interviews with the local population, a block-
age of a narrow channel section occurred at least twice, fol-
lowedby vigorousreleases of water, debris and mud (Schnei-
der et al., 2004).
In order to get more conservative values – as desired in this
type of analysis – a value of ω = 8.5
◦
is used when applying
relationship T2 to the southwestern Pamir (see Table 5).
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M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir 1455
Table 7. Impact susceptibility rating I . n stands for the number of
relationships (see Table 5) predicting an impact on the considered
pixel, ω stands for the average slope angle from the lake to the con-
sidered pixel. Since the relationship designed for floods predicts the
longest travel distance and not allLOFs evolve into debris flows, the
impact type for pixels with n ≥2 can be flood or debris flow.
I n ω (degree) type of impact
0 Negligible 0
1 Low 1 < 4 flood
2 Moderate 4–<6
3 Medium ≥ 6
4 High 2 not applicable debris flow or flood
5 Very high 3
6 Extremely high 4
Fig. 5. Longitudinal profile of the flow path of the Dasht 2002 event
with the upper limits of the flow paths according to different empir-
ical relationships.
6 Results
6.1 Distribution and characteristics of lakes
For the period 2007/2008, 428 lakes with A ≥ 2500m
2
have
been identified in the study area. 20 of them are just widened
portions of riversor shallow swampylakes(recognized byto-
pographic situation, colour and the existence of gravel bars)
and are excluded from further analysis. Among the 408 re-
maining lakes, 187 are embedded in undulating landscapes
most likely formed during the Pleistocene, at some distance
from recent glaciers but still above 4000 ma.s.l (Type 1 in Ta-
ble 3). 20 lakes have dams dominated by coarse blocks, rep-
resenting Pleistocenic terminal moraines, landslide deposits
or a combination of both (Type 2). 13 lakes are dammed by
talus or debris cones (Type 3) and 16 by pronounced rocky
swells (Type 4). The remaining 172 lakes represent glacial
lakes in the strict sense: they are either directly embedded
in the exposed ice, or dammed by debris-covered glacier
tongues, rock glaciers or fresh moraines (Type 5). The tran-
Fig. 6. Distribution of lake types and lake evolution plotted against
elevation. The distribution of the total surface of the study area is
shown as reference.
sition between the latter three types is rather gradual than
sharp and the identification requires geophysical methods not
applicable at the regional scale. No distinction is therefore
made between pro- and supraglacial lakes. Table 8 summa-
rizes the numbers and some geometric characteristics of the
different lake types.
Figure 6 plots the altitudinal distribution of the lakes, or-
ganized by dam types. Less than 10 per cent of all lakes
are found below 4200ma.s.l., but two of them are larger
than 1km
2
(Durumkul and Rivakkul; Type 2). At approx.
4200ma.s.l., the steep valley flanks give way to the un-
dulating plains formed by Pleistocenic glaciers. The latter
landscape hosts numerous lakes of Type 1. Turumtaikul
and Zaroshkul are the largest representatives of this type.
Above 4500ma.s.l., in the zone of recent glaciers and fresh
moraines, lakes of Type 5 become more abundant and dom-
inate the zone from 4700 m upwards. The highest identified
glacial lakes are located at 5060m a.s.l. The highest den-
sity of lakes is found at 4500–4550ma.s.l. Reasons are the
large share of the land area in this class on the one hand (see
Fig. 6) and the presence of favourable conditions for the de-
velopment of lakes of Type 1 and 5 on the other hand.
Figure 6 also shows the trends in lake development. Most
glacial lakes (Type 5), located in a changing environment
with active morphodynamics, are growing, as are many lakes
of Type 1.
6.2 Lake outburst susceptibility
There is no significant differentiation of lake types regarding
the susceptibility to lake outburst triggered by external forces
S
ext
, which rather depends on the topography and state of
the adjacent slopes than on the dams themselves (Fig. 7a).
Glacial lakes are most susceptible to outbursts triggered by
internal forces, as prescribed by the rating scheme (Fig. 7b).
In sum, glacial lakes are by far the most susceptible, with 66
cases of very high susceptibility (Class 5) and 85 cases of
high susceptibility (Class 4). There are also 15 non-glacial
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1456 M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir
Table 8. Lake statistics, organized by lake type. n
l
= number of lakes, p
g
= percentage of growing lakes, A
avg
and A
max
= average resp.
maximum lake area, z
min
, z
avg
and z
max
=minimum, average and maximum elevation a.s.l.
Lake type n
l
p
g
A
avg
(km
2
) A
max
(km
2
) z
min
(m) z
avg
(m) z
max
(m)
1 Embedded lake 187 45 0.143 8.99 3473 4460 5029
2 Block dam 20 55 0.205 1.68 3341 4076 4718
3 Debris dam 13 46 0.085 0.62 3590 4223 4686
4 Rocky swell dam 16 25 0.107 0.48 4418 4692 5021
5 Glacier or fresh moraine dam 172 83 0.018 0.41 3926 4608 5063
lakes with high susceptibility. All other lakes are assigned to
Class 3 (medium susceptibility) or lower (Fig. 7c).
6.3 Lake outburst hazard and Impact hazard
Among the 408 analyzed lakes, the lake outburst hazard H
for 122 is classified as negligible (Class 0) for most of them,
due to their limited size of A < 5000m
2
(Fig. 7d). 35 lakes
are assigned to Class 1 (low hazard), 124 to Class 2 (mod-
erate hazard), 87 to Class 3 (medium hazard), 34 to Class 4
(high hazard) and 6 to Class 5 (very high hazard). No lakes
are assigned to Class 6 (extremely high hazard). Glacial
lakes are more prominent regarding susceptibility than haz-
ard, as they are on average smaller than lakes of the other
classes (see Table 8). In contrast, the large Durumkul with a
surface area of 1.7km
2
and Rivakkul with 1.2 km
2
, dammed
by depositsof large blocks (landslide deposits or Pleistocenic
moraines), are both rated hazardous despite moderate and
medium lake outburst susceptibility, respectively.
Figure 8 shows the hazard indication map for the entire
study area. The lake outburst hazard for each lake and the
impact hazard for each pixel are shown. The maximum
travel distances of potential debris flows and floods emanat-
ing from the lakes are plotted in Fig. 9. Only for 97 out of the
408 lakes, debris flows exceeding a travel distance of 2 km
are predicted in case of an outburst. The maximum value
(15.5km) is computed for Nimatskul (Lake N1 in Fig. 8; see
next section). Flooding could proceed for more than 30km
in the case of 298 lakes, with a maximum of 75.5km. How-
ever, it has to be kept in mind that the only criterion used for
the travel distance of floods is the average slope of the flow
path. Many lakes in the study area are too small to produce
floods reaching that far.
6.4 Case studies
The Dasht event is the only documented LOF in the study
area. There is evidence for earlier outburst floods e.g. of Ri-
vakkul (Schneider et al., 2004), but the observations are too
vague to be used as a reference.
The lakes identified as hazardous or very hazardous can be
organized into two categories:
1. growing glacial lakes (Type 5), mostly with seepage
through the dam;
2. large lakes of various types (Type 1, 2 and 4 according
to Table 3).
Whilst it would be out of scope of the present paper to dis-
cuss all of thoselakes in detail, one example of each type will
be shown in order to illustrate the specific potentials and lim-
itations of the regional-scale analysis procedure when zoom-
ing into a more detailed level.
A growing glacial lake (Lake V1) is located in the up-
per reaches of Varshedzdara at 4513ma.s.l. (Fig. 10a and
b; see Fig. 8), 11 km upstream of the village of Varshedz.
In summer 2007, the lake had a surface area of 155000m
2
.
Lake outburst susceptibility and hazard are rated very high.
Lake V1 serves as an example for many similar glacial lakes
in the study area, most of which are, however, smaller. A
second, larger lake (360000m
2
) of stable size (Lake V2) is
located 2.5 km upstream from Lake V1, at 4795ma.s.l. It is
dammed by a rocky swell partly covered by morainic mate-
rial and bordered by steep, partly glacierized slopes. It has
no permanent surface drainage. The lake outburst suscepti-
bility was rated medium, the lake outburst hazard high. An
ice avalanche into the lake would be the most likely scenario
to cause a flood wave. However, only a fraction of the lake
would drain in such a case.
A LOF from Lake V2would possibly hit Lake V1, which –
depending on the specific characteristics of the flow – could
either level out or amplify the flood. Such effects are not
accounted for by the empirical approaches used for impact
susceptibility at the regional scale. Instead, travel distance,
impact susceptibility and hazard are computed separately for
each of the lakes. The empirical relationships suggest that a
debris flow resulting from an outburst of Lake V1 or Lake V2
would not reach the village of Varshedz. A debris flow from
Lake V2 would proceed farther (11.0 km) than one from
Lake V1 (3.5 km) due to the larger maximum outburst vol-
ume and the steep initial slope. However, substantial flood-
ing would have to be expected in the village of Varshedz
(Fig. 10c). 43.9 km (Lake V1) resp. 50.1 km (Lake V2) are
suggested as maximum travel distances of floods resulting
from lake outbursts. When summing up the impact hazard
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M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir 1457
a)
c)
b)
d)
Fig. 7. (a) Susceptibility to lake outburst triggered by external forces S
ext
. (b) Susceptibility to lake outburst triggered by internal forces
S
int
. (c) Combined lake outburst susceptibility S. (d) Lake outburst hazard H .
HI for all pixels defined as settlement or agricultural area,
Lake V1 and Lake V2 show much higher values than all the
other lakes in the study area, mainly due to the location of
the village of Varshedz directly on the debris cone where the
flow would be expected to spread.
Among all the lakes in the study area, Nimatskul
(Lake N1; Fig. 11a) is closest to the villages in the main val-
ley. Located at 4418 m a.s.l., the horizontal distance from
the lake down to the Gunt Valley is only 5.9 km, with a
vertical difference of 1600 m. The steep average slope of the
flow path (ω =15.2
◦
) and the availability of erodible material
suggest that a possible outburst flood would most probably
severely affect the villages near the outlet of the Nimatsdara
(Fig. 11b).
The lake is dammed by a rocky swell and has a constant
surface area of 475 000 m
2
. Lake outburst susceptibility and
hazard were rated medium and high, respectively. Sudden
drainage of the lake may be caused by a powerful earthquake
weakening the dam or leading to landslides into the lake.
However, a closer on-site inspection of the slopes leading
directly into the lake gave no evidence of large-scale instabil-
ities, so that in reality, S
ext
and H would be lower than sug-
gested in the regional-scale analysis. Even though the slopes
are obviously under permafrost conditions (see Fig. 11a),
mass movements capable of displacing a substantial portion
of the lake would require a very powerful trigger. With some
limitations, similar conclusions can be drawn for Durumkul
and Rivakkul.
7 Discussion
Even though the population of the Pamir directly depends on
the natural environment and a lot of traditional knowledge
exists (Kassam, 2009), lake outburst hazards are often ne-
glected: the source area is usually far away from the area of
impact and events occur at very long time intervals or as sin-
gularities. The Dasht 2002 GLOF hit the village completely
unexpected – there was no awareness of the hazard and no
preparedness for the event (Schneider et al., 2004). Also in
other mountain areas, deficiencies in risk communication are
often responsible responsible for the evolutionof natural pro-
cesses into disasters (Carey, 2005).
A regional-scale hazard analysis for the southwestern
Pamir has been presented in order to highlight potentially
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1458 M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir
Fig. 8. Hazard indication map for the Gunt and Shakhdara valleys.
Fig. 9. Computed maximum travel distances of debris flows and
floods from lake outbursts.
hazardous lakes and possible impact areas, building largely
on medium-resolution satellite imagery and elevation data.
The predictive capacity of such an analysis is governed
primarily by the applicability of various types of input
data: satellite imagery and digital elevation models only pro-
vide information on surface features and patterns, not allow-
ing for insight beneath. Two aspects have turned out to be the
major limiting factors of regional-scale lake outburst hazard
analyses:
1. geological information. Rock types and major faultscan
be obtained from geological maps, but slope stability is
often governed by small-scale dip directions and fault
systems. Such features can be considered for single
slopes or small catchments, but not at the regional scale.
Fig. 10. (a) Aerial view of Lake V1 (August 2009). (b) Devel-
opment of Lake V1 1968–2007. (c) Impact susceptibilities I for
potential outbursts of
Lake V1 and Lake V2.
Therefore, no geological input was used for computing
TSI;
2. information on seepage through the dam and on its
internal structure, particularly sediment consolidation,
porosity (cavities) and subsurfaceice content. Geophys-
ical surveys (e.g. geoelectrics) are not feasible at a level
broad enough to cover all relevant lakes.
Furthermore, the SRTM-4 DEM and the pixel size used
(60 m) are insufficient to represent all relevant features: par-
ticularly for small lakes, the freeboard or the downstream
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M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir 1459
Fig. 11. (a) Aerial view of Lake N1 (Nimatskul) in August 2009,
the Gunt Valley in the background. The viscous creep into the lake
indicates that also the above rock wall is influenced by permafrost.
(b) Modelled impact susceptibility I of a potential outburst of Ni-
matskul.
slope of dams may be blurred. High-resolution DEMs are
hardly applicable to regional-scale studies of this type, but
a more thorough evaluation of the entire analysis procedure
for a small test area with a high-resolution DEM and directly
measured data (e.g. lake depth) would be an important next
step.
The rating schemes used for the hazard analysis were
tuned in a way to provide worst-case estimates, with the pur-
pose of building a reproducible baselineregarding the site se-
lection for in-depth investigations. In the case of Nimatskul
(Lake N1), such an investigation led to a down-rating of the
hazard. In contrast, the rating for Lake V2, with exactly the
same values as for Lake N1, was confirmed in the field.
On the other hand, too conservative ratings had to be
avoided. Due to its limited size, the lake in Dashtdara pro-
ducing the 2002 disaster was rated very susceptible, but only
as medium hazardous. Tuning the rating scheme in a way
to yield a high or very high hazard for that lake would have
led to such a classification of virtually all glacial lakes in the
area, obstructing the purpose of the method for site selection.
It is likely that superficially invisible factors (structure of the
dam, englacial water reservoir) led to the high magnitude of
the Dasht 2002 event. One can learn some major lessons by
comparing the characteristics of that event to the analysis re-
sults:
1. the Dashtlake did notappear morehazardous thanmany
other glacial lakes in the area. This leads to the apparent
conclusion that the location of specific outburst events
is hard – or even impossible – to predict. The proce-
dure shown in the present paper gives way to a possible
strategy of dealing with this problem: identifying poten-
tially dangerous lakes and fostering a broad awareness-
raising and preparedness-building connected with fea-
sible technical measures (e.g. simple early warning sys-
tems) in possibly affected communities;
2. hazardous glacial lakes may evolve within less than one
year. This phenomenon was illustrated not only by the
Dasht event, butalso by theoutburst of the western Zyn-
dan glacier lake in the Tien-Shan (Kyrgyzstan) in 2008
(Narama et al., 2010). This means that careful moni-
toring of the glacial environment is required in order to
keep updated on developing hazards. On the one hand,
such monitoring has to be performed by employing re-
mote sensing techniques. On the other hand, the local
communities have to be trained and encouraged to keep
an eye on relevant environmental changes and develop-
ing lakes.
Empirical relationships were used for estimating the travel
distances and impact areas of lake outburst floods. This is
appropriate at the regional scale as physically-based models
would require multiple input parameters not available over
such a broad area. Mergili et al. (2011) discuss the challenges
and problems connected to physically-based modelling of the
motion of lake outburst floods.
However, it should be emphasized that empirical relation-
ships only provide a first estimate of the impact susceptibility
which can serve as a baseline for in-depth studies. Spe-
cific process chains and interactions regarding the motion
of the GLOF are not explicitly considered in the empirical
relationships. These include entrainment or deposition of
sediment, change between different flow types (flood, hyper-
concentrated flow, debris flow), the amplification or attenua-
tion of flood waves by lakes on the flow path (e.g. Lake V1
in Fig. 10c) or backwater effects. In the case of Dasht, such
interactions are thought to be responsible for the flow not fit-
ting to the empirical rules derived from sets of other GLOFs
(Haeberli, 1983; Huggel, 2004; Huggel et al., 2004a), but in-
stead continuing to the confluence of the Dashtdara with the
Shakhdara Valley where lateral spreading was possible.
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1460 M. Mergili and J. F. Schneider: Regional-scale analysis of lake outburst hazards in southwestern Pamir
The global assumption that an entire lake would drain is
realistic when considering small glacial lakes (in Dasht, it
was the case), but not for larger lakes with rather stable dams
where mass movements may cause an overflow. This prob-
lem leads to an overestimation of the lake outburst hazard of
large lakes (e.g. Durumkul). Huggel et al. (2004a, b) have
related expected outburst volumes to the impact volume – an
adoption of this approach would require in-detail investiga-
tions for each lake.
An important aspect not explicitly accounted for in the
hazard analysis procedure is permafrost, which may signifi-
cantly influence lake outburst susceptibility and hazard (Hae-
berli et al., 2010a): in the southwestern Pamir, almost all
lakes are located in permafrost areas and a detailed analysis
of the condition of the permafrost was out of scope. How-
ever, when applying the methodology presented to study re-
gions with lakes both in permafrost and in permafrost-free
areas, this aspect has to be included.
8 Conclusions
The regional-scale hazard analysis of lake outburst floods,
as shown in the present paper for the southwestern Pamir,
Tajikistan, has proven to be a valuable tool for a rapid and
reproducible identificationof potentially hazardous lakes and
possible impact areas.
The analysis does not require specialized input data, it
largely relies on internationally available medium-resolution
satellite imagery and DEMs. This allows an application in
remote or poorly developed areas with limited availability of
local information. The neglect of such data, however, also
limits the scope of the method: it is clearly confined to the
identification of areas requiring more detailed investigations,
e.g. field studies. The regional-scale analysis has to presume
the unknown parameters as utmost unfavourable in order to
come up with worst-case assumptions. This avoids missing
potentially hazardous lakes and situations during site selec-
tion for detailed studies.
The Dasht 2002 event, the most destructive GLOF in the
documented history of the southwestern Pamir, originated
from a lake rated as very susceptible, but only medium haz-
ardous in the regional-scale analysis. A closer on-site inspec-
tion did not reveal a substantial difference to other glacial
lakes with such a rating, except for the short lifetime of the
lake. This finding underlines the need to detect glacial lakes
immediately after their emergence and to take adequate mea-
sures.
Also, modelling the travel distance of the Dasht 2002
GLOF showed the difficulties of putting lakes and events
into prescribed schemes: it was impossible to reconstruct
the reach of the flow by empirical rules based on previous
events. This, in conjunction with the thoughts presented
above, shows the importance of broad-scale risk mitigation
strategies in potentially affected areas, including awareness-
raising and preparedness-building within the local popula-
tion, in combination with regular monitoring of the glacial
and periglacial environment.
Acknowledgements. The work presented in this paper was part of
the project TajHaz (Remote Geohazards Assessment in Tajikistan)
supported by FOCUS Humanitarian Assistance (an affiliate of
the Aga Khan Development Network), the Swiss Agency for
Development and Cooperation (SDC) and the UK Department for
International Development (DFID). Special thanks go to Demian
Schneider (University of Zurich) for valuable discussions and
contributions and to Anatoli Ischuk (Tajik Institute of Earthquake
Engineering and Seismology) for contributions to the calculation
of PGA
max
.
Edited by: J. M. Vilaplana
Reviewed by: W. Haeberli and another anonymous referee
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