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The Pokhara Valley: A Product of a Natural Catastrophe

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The Pokhara valley in the central part of Nepal is one of the few Himalayan intramontane valleys that permits one to decipher the way the land-forms of the world’s highest mountain range evolve. The valley is attractive to tourists for the scenic majesty of its glaciated mountains, gorges, caves, and lakes, the formation of which results from a complex yet recent and dramatic evolution of the valley. For a long time, most of the inhabitants believed that the valley originated from the drying up of a huge lake similar to those of the Kathmandu and Kashmir valleys. Careful observations of the sediments filling the basin indicate that the Pokhara valley was affected by a giant, catastrophic debris flow five centuries ago. It is an emblematic site, where the steepness of the still rising front of the very Himalaya (“the abode of snow”) is maintained by sporadic collapses of the mountain walls controlled by a combination of both glacial and seismo-tectonic dynamics.
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265
Abstract The Pokhara valley in the central part of
Nepal is one of the few Himalayan intramontane
valleys that permits one to decipher the way the land-
forms of the world’s highest mountain range evolve.
The valley is attractive to tourists for the scenic maj-
esty of its glaciated mountains, gorges, caves, and
lakes, the formation of which results from a complex
yet recent and dramatic evolution of the valley. For a
long time, most of the inhabitants believed that the
valley originated from the drying up of a huge lake
similar to those of the Kathmandu and Kashmir val-
leys. Careful observations of the sediments filling the
basin indicate that the Pokhara valley was affected by
a giant, catastrophic debris flow five centuries ago.
It is an emblematic site, where the steepness of the
still rising front of the very Himalaya (“the abode
of snow”) is maintained by sporadic collapses of the
mountain walls controlled by a combination of both
glacial and seismo-tectonic dynamics.
Keywords Debris ows • glaciation • Himalaya •
mountain building
27.1 Introduction
The Pokhara valley at the foot of the Annapurna Himal
presents one of the sharpest contrasts in relief in the
world. Located in the central part of Nepal, it stands
out as a distinctive feature of the Himalayan landscape.
It is an abnormally broad plain with an area of ~125
km
2
, and confined by hills ranging from 1,200 to 3,000
m in elevation. Towering above it to the north is the
Annapurna Himal (8,091 m), only 35 km as the crow
flies from Pokhara town (800 m a.s.l.). It is drained by
the Seti Khola (khola meaning riverin Nepali language)
and its tributaries, originating from the glacial cirque
of Sabche, surrounded from west to east by the peaks
of Macchapuchare (6,993 m), Annapurna III (7,555 m),
and Annapurna IV (7,525 m). Well known for the
outstanding scenic majesty of these “snowy mountains”
(“Himalin Nepali), the Pokhara valley is also attractive
to tourists for its gorges, caves, and lakes, the forma-
tion of which results from the complex yet recent and
dramatic evolution of the valley.
In fact, for a long time, the formation of the Pokhara
valley was a mystery. Inhabitants were struck by the
very many large rocks found in the upstream part of
the valley, and had their own explanation of these fea-
tures, referring to legends involving the action of God.
Most of them believed that the Pokhara valley origi-
nated from the drying up of a huge lake similar to those
of the Kathmandu and Kashmir valleys (Hagen 1961).
Careful observations of the sediments filling the valley,
together with a better understanding of the geomor-
phological processes in action, have led to new inter-
pretations involving the very recent evolution of the
glaciated mountain front of the Annapurna Range.
This makes the Pokhara valley one of the few
Himalayan basins that permits one to decipher the way
the landforms of the world’s highest mountain range
evolve and explains why this particular part of the
Himalaya has been selected for this volume.
27.2 Geographical and Geological
Setting
The Pokhara valley is located in the midland region
(or Pahar zone) of the Himalayan Range, a low-lying
belt, which is sandwiched between the Lesser
Himalaya (or Mahabharat Range) in the south and the
Chapter 27
The Pokhara Valley: A Product of a Natural Catastrophe
Monique Fort
P. Migon´ (ed.), Geomorphological Landscapes of the World,
DOI 10.1007/978-90-481-3055-9_27, © Springer Science+Business Media B.V. 2010
266
M. Fort
Great (or Higher) Himalaya to the north. It belongs
to a series of intermontane basins, the formation of
which is closely related to the formation of the
Himalayas as a whole.
The Himalayan Arc results from the collision
between the Asian and Indian plates, which occurred
about 50–45 million years ago. The compressional
motion between the two plates has been, and continues
to be, accommodated by slip on a suite of major thrust
faults, connected at depth along a major detachment
plane. The extreme elevations were acquired by stack-
ing of crust units in a continuing continental subduc-
tion régime. The Himalayan Range is still rising at a
rate estimated to be between several millimeters per
year to more than 1 cm/year. The collision process cre-
ated a series of elongated, more or less parallel and
asymmetrical ridges, developed to the north of the
Indo-Gangetic plains as a fold-and-thrust belt (Lesser
Himalaya), which controls the drainage pattern.
To the north, the steep barrier of the Greater Himalaya
reaches more than 8,000 m above sea level (Fig. 27.1).
It represents a more than 10 km thick thrust unit made of
crystalline, gneissic rocks, bounded at depth by the
Main Central Thrust, and overtopped by sedimentary
rocks, mostly limestones, that now underlie the
Himalayan peaks. It separates two very contrasted ter-
rains: the humid, tropical, monsoon-influenced Indian
subcontinent and the cold, arid, and rugged Tibetan and
Central Asian High Plateaus. Nowhere in the Himalayan
Range is this bioclimatic gradient greater than along the
Annapurna Range, north of Pokhara.
The Pahar zone has developed between the Lesser
and the Higher Himalayas, and may locally widen in the
form of intermontane depressions such as the Pokhara,
Kathmandu, or Kashmir valleys. They correspond to
large confluences of river valleys and/or to fluvio-lacus-
trine basins, developed on the back of thrust units of the
Lesser Himalaya. They were initiated by tectonic dam-
ming as a result of a relatively faster uplift rate in the
Lesser Himalaya than along the Pahar, transitional zone.
In contrast to the flat, perched, lacustrine basins of
the Kashmir and Kathmandu valleys (Burbank and
Raynolds 1984), the Pokhara valley floor is charac-
terized by an extensive, gravel-covered surface, the
formation of which has been influenced by a long
period of fluvial modeling and dissection, and was the
Fig. 27.1 Northern part of the Pokhara valley, developed at the
foot of the High Himalayan Front, dominated by Macchapuchare
(6,993 m), Gangapurna (7,454 m), Annapurna III (7,555 m),
Annapurna IV (7,524 m), Annapurna II (7,937 m), and Lamjung
Himal (6,983 m), from left to right. The Seti Khola, in the fore-
ground, issues from deep gorges cut below the glaciated cirque
of Sabche. The river is responsible for the accumulation of the
indurated Gaunda conglomerates, which underlie the two high-
est and very flat terrace levels, for the catastrophic accumulation
of Pokhara gravels, deposited about 500 years ago, and for their
recent dissection into several (here at least five) strath terraces
(Photo M. Fort)
26727 The Pokhara Valley: A Product of a Natural Catastrophe
locus for catastrophic events and processes developed
in relation to the proximity of the steep Main Central
Thrust Front (Fort 1987).
27.3 Landforms and Landform Diversity
Besides magnificent mountain views, the Pokhara basin
is characterized by several specific landforms. Firstly,
the wide, flat morphology of the Pokhara “plain” is
striking, a view reinforced by the sharp contact between
the plain and the surrounding hills. The plain slopes to
the south; the general longitudinal gradient varies from
32‰ upstream to 9‰ downstream. The plain also slopes
laterally from a central axis, a feature that has caused
diversions of the tributary streams. The surface of the
plain displays a braided-channel morphology (Fig. 27.2).
These characteristics are those of a large alluvial out-
wash fan. In fact, this plain is underlain by the so-called
Pokhara gravels (Gurung 1970), excavated by the pres-
ent Seti Khola River, hence providing numerous sec-
tions that permit one to analyze and interpret the nature
of the gravels and their mode of deposition.
Another distinctive landform of the Pokhara valley is
the dramatic set of terraces shaped by the Seti Khola and
its tributaries by both vertical incision and lateral erosion
(Fig. 27.1). The number of terraces increases down-
stream, whereas their relative height above the Seti
thalweg increases upstream, from 60 to 50 m in the
south to >100 m to the north. Most of the terrace levels
are unpaired, a feature distinctive of meandering streams
and rapid incision, this last statement being reinforced
by the fresh appearance of the topographic surface and
the absence of soil developed on terrace surfaces.
The canyons of the Seti Khola and its tributaries are
among the most intriguing features of the valley
(Fig. 27.3). Interrupting the long sections of terraces,
they occur along limited reaches, a few hundred meters
to 1 km long, and are deeply (up to 50 m) entrenched into
a material made of gravels and boulders, cemented
together into a hard conglomeratic bedrock with a rich
limestone matrix, locally known as “gaunda” (Hormann
1974). These gorges are locally so narrow that only the
sound of water can be heard; in some cases the stream
has even disappeared in underground tunnels. One of
these canyons is crossed by the Mahendra Pul (bridge),
along the main Kathmandu road before entering the old
bazaar of the city. These gorges are often associated with
potholes and caves, such as the Mahendra and Chamere
caves, close to Batulechaur village, the Jogi cave in
Balam Hill, or the Gupteshwor and Davis falls caverns in
Fig. 27.2 Surface, braided morphology of the Pokhara gravels, as highlighted by the rice-fields pattern, east of Pokhara airport
(Photo M. Fort)
268
M. Fort
Chorepatan, south of the Phewa lake: there, the Pardi
River disappears into a tunnel, 200 m long, with a natural
bridge on which the Sidartha highway passes. The devel-
opment of these karst-like features provides additional
evidence for the combined action of water dissolving the
limestone and abrading the conglomerates.
The last feature that makes Pokhara famous is the many
lakes (“tal” in Nepali) that are found along the edges of the
valley (Fig. 27.4). Most of them (Dipang, Maidi, Khalte,
Kamalpokhari, and Gunde lakes) have virtually disap-
peared due to siltation. The lakes Rupa, Begnas, and
Phewa are larger, and their sinuous shorelines clearly sug-
gest that they are drowned valleys (Gurung 1970). Close
to Pokhara city, the largest of them, the Phewa Tal, is an
important tourist attraction, with its waters reflecting the
entire Annapurna Himalayan peaks during clear days. The
occurrence of these lakes in juxtaposition with the grav-
elly plain of Pokhara reflects their origin, as a response to
the deposition of the Pokhara gravels.
27.4 The Pokhara Gravels
The Pokhara gravels (Gurung 1970; Yamanaka et al.
1982) are the main component of the basin fill: their
top layer underlies the geomorphological surface upon
which Pokhara town has been built. They extend from
Bharabhure at the mouth of the Seti Khola gorge,
downstream to Dhoban, at the foot of the Mahabharat
Lekh. The total thickness of their accumulation
decreases downstream, from an average thickness of
over 100 m to about 60 m, although local variations are
observed depending on the buried topography. The
sections, well visible from along the Prithvi Narayan
Road across the Seti valley or its tributaries such as the
Bijaypur, reveal the main characteristics of the Pokhara
gravels and their modes of deposition.
The Pokhara gravels consist of a rapid succession of
beds, decimeters or meters in thickness, with flat basal
contacts (Fort and Freytet 1979). The material is mainly
composed of layered, sub-angular to sub-rounded,
mostly calcareous gravels, centimeters to decimeters in
size, embedded in a muddy, calcareous matrix present
in variable proportions (Fig. 27.5). Blocks of a meter or
more in size mostly gneisses (Higher Himalaya) or
quartzites (Lesser Himalaya) can also be found ran-
domly within the whole accumulation package. Blocks
of exceptional size, such as the Bhimsen Kali Boulder
(32 m in diameter) visible on the University campus of
Pokhara (Fig. 27.6), are restricted to the top-most lay-
ers where they are distributed upon the surface of the
Pokhara plain. All these sedimentological characteris-
tics indicate that the Pokhara formation is an alluvial
Fig. 27.3 Major terrace levels south of Pokhara valley,
underlain by the Pokhara gravels. They have buried the older,
indurated gaunda” conglomerates, which are exhumed by
the recent erosion of the Seti River, as observed in the fore-
ground, where potholes and small canyons have developed
(Photo M. Fort)
269
27 The Pokhara Valley: A Product of a Natural Catastrophe
Fig. 27.4 The Phewa Lake, seen from the west, with the
Pokhara valley in the background. This lake is a remnant of a
drowned valley dammed downstream by the catastrophic depo-
sition of the Pokhara gravels. This lake, like many others located
along the edges of the Pokhara valley, is nowadays subjected to
intense siltation by tributary streams (Photo M. Fort)
Fig. 27.5 A 25-m high section of the Pokhara gravels. Noticeable are the flat beds that include random occurrences of big boulders.
This accumulation typically exhibits irregular alternations of debris-flow and mud-flow layers (Photo M. Fort)
fan deposit, transported alternately by muddy flows,
debris flows, and torrential discharge (Fort 1987). The
occurrence of the largest boulders in the final stage of
deposition can be explained as a sorting phenomenon
distinctive of the dense, debris laden, muddy flow of
the Pokhara discharge.
This discharge was first considered to be a product
of glacio-fluvial outwash (Gurung 1970; Hormann 1974;
270
M. Fort
Fort and Freytet 1979), because most of the clasts are
limestones derived from the glaciated cliffs of the
Annapurna Range. However, the gneissic nature of the
largest boulders indicates that materials derived from
the adjacent valley walls located between the glacial
cirques and the Pokhara plain were also incorporated
by this massive discharge. Moreover, the volume of
Pokhara gravels (estimated as >4 km
3
; Fort 1987) and
their dating between 400 and 1,100
14
C years
(Yamanaka et al. 1982; Fort 1987) suggest the occur-
rence of a short-lasting, historical event that led to the
rapid filling of the Pokhara valley by a giant debris
flow and to the damming, and hence flooding of the
adjacent valleys. The present lakes are the relicts of
this exceptional event.
Among the various origins and formation condi-
tions that can explain this sudden, huge supply of both
water and debris, it seems that ice and rock avalanches
and/or landslide-dammed lake outbursts are capable of
liberating the greatest volumes of water and debris
simultaneously, in such a catastrophic way. Hence,
these processes are the most likely causes of the excep-
tional Pokhara gravel discharge. The sources of the
debris are the very steep slopes of the south face of
Annapurna III and IV (Fig. 27.7). The triggering factor
of an event of such a magnitude has probably to be
related to a tectonic-induced instability, i.e., to an
earthquake, the only mechanism capable of bringing
instantaneously a slope into disequilibrium and setting
into motion so huge a quantity of ice and rock material
(Fort 1987).
27.5 Evolutionary History
The Pokhara gravels have buried an irregular topogra-
phy, which includes the relicts of former stages of
Pokhara valley evolution. The evolutionary history of
the valley can be summarized as follows.
The formation of the intermontane basin is the
result of a long-lasting process. As already pointed
out, the basin belongs to the Pahar belt, which is rising
at a slower rate than the Mahabharat Lekh (Lesser
Himalaya) to the south and the Higher Range to the
north. More specifically, it is situated along an anticlinal
Fig. 27.6 The famous Bhim Kali boulder, 3,000 t large, pre-
served on the Pokhara University campus. A local legend tells
that this rock was thrown down from Machapucchare Peak by
the powerful hero Bhim. In fact, it represents the final stage of
Pokhara gravels deposition, and was brought by a competent,
highly muddy flow nourished by coarse debris detached from
the High Himalayan Front. Most of these boulders have nowa-
days disappeared as they have been quarried (Photo M. Fort)
271
27 The Pokhara Valley: A Product of a Natural Catastrophe
structure, mostly carved out into schists with intercala-
tions of quartzite and dolomitic limestone bands. The
distinctive z-shape of the basin and of the Seti Khola
course and its tributaries, together with the arrange-
ment of the surrounding hills, may be interpreted as the
superficial expression of deformation namely fault
scarps induced by the oblique, northward conver-
gence of the Sub-Himalaya underthrusting the Lesser
Himalaya.
The present, large-scale morphology of the valley
is the result of a complex alternation of aggradational
and erosional stages, developed under a tropical,
seasonally contrasted climate. In fact, the Pokhara
valley has experienced several stages of dissection,
separated by brief, yet intense periods of aggradation
(Fig. 27.8). A few patches of perched calcareous
breccia, like those preserved along the Sarankot
ridge, represent early remnants of slope deposits,
spread over an aggradational pediment, presently
reduced to sharp, karstic ridges modeled into towers
by the dissolution of carbonates. After a period of
basin dissection to a depth of 50 m or more, perched
weathered gravels provide evidence of a previous
Seti Khola course. Another period of dissection also
followed which, in the central part of the basin, pen-
etrated lower than the present level of the Seti Khola
river bed (Fig. 27.9).
This evolution was interrupted by the deposition
of the “gaunda (or “Gachok”) conglomerates that
are the first extensive deposits in the Pokhara valley.
They underlie the prominent terrace benches of the
northern part of the valley, near Lachok and Gachok
villages (Fig. 27.8). Their well-rounded gravels and
sand grains, dominated by limestone, are derived
from the south-facing, upper cliffs of the Annapurna
Himal. They were until recently considered to be
glacio-fluvial outwash deposited by the melting of ice
after the last glaciation, but are now explained by
similar, catastrophic processes as were involved in
the formation of the Pokhara gravels. After their
deposition and lithification, these conglomerates
were dissected to form a stepped topography of fluvial
terraces, well preserved north of the valley, upstream
of the Mardi Khola confluence, whereas to the center
and south of the valley, they disappear under the
Pokhara gravels.
The more recent and rapid deposition of the Pokhara
gravels has resulted in burial of the former topography,
disorganization of the entire drainage system of the
valley, and creation of the peripheral lakes. Soon after,
the Seti Khola started incising its bed again to readjust
its longitudinal profile to the base level of the
Mahabharat Range. In the central part of the valley, the
new Seti course, superimposed on the Pokhara gravels,
locally cut through the hard “gaunda” conglomerates
(present beneath the Pokhara gravels), so that it pre-
vented the river to widen its beds, thus leading to the
formation of canyons instead (Fig. 27.4). Elsewhere,
the presence of the loose Pokhara gravels favored the
shift of the Seti River and the subsequent development
of unpaired flights of terraces (Fig. 27.10).
Fig. 27.7 Close-up view of the northern part of the Pokhara
valley, Seti Khola gorge, and the upper glaciated cirque of
Sabche. The terraces developed on the foreground are underlain
by the Pokhara gravels. Upstream the Seti gorge, entrenched
within the High Himalayan gneisses, is particularly narrow and
steep. It channelized the giant, catastrophic debris flow of
Pokhara gravels, originated from the southwest face of
Annapurna IV (Photo M. Fort)
272
M. Fort
Fig. 27.8 Map of the major deposits of the Pokhara valley. The
catastrophic accumulation of the Pokhara gravels buried a dis-
sected topography carved into the older “gaunda” conglomer-
ates. Since the giant debris flow, the Seti River cut through the
Pokhara infill, and found locally the “gaunda” conglomerates
beneath, so it was forced to incise deep canyons
273
Fig. 27.9 Cross section and altitudinal distribution of the
various formations deposited in the Pokhara valley in the last
100,000 years. To the north of the valley, the oldest perched
slope deposits (limestone breccia), the old, deeply weathered
alluvium and the Gaunda-Gachok formations are stepped
above the most recent Pokhara gravels filling, whereas in the
center of the valley, the Pokhara gravels have buried the
Gaunda-Gachok conglomerates. This particular setting clearly
indicates the rising of the High Himalayan Front relative to
the basin
Fig. 27.10 Since the deposition of the Pokhara giant debris
flow, the Seti Khola started incising its bed again at a rate vary-
ing between 20 cm/year upstream and about 10 cm/year down-
stream of the basin. Locally, the presence of loose Pokhara
gravels favored the shift of the Seti River and the development
of many, unpaired terraces (Photo M. Fort)
274 M. Fort
27.6 Conclusions
The catastrophic episode of the Pokhara debris-flow
aggradation serves as a model for the geomorphic evolu-
tion of the High Himalayan Front (Fort 1987; Fort and
Peulvast 1995). It shows how a huge mass of debris almost
instantaneously delivered from the front may temporarily
be stored in an intramontane basin of the Pahar before
transit to the Himalayan foothills. It also demonstrates
how the steepness of the still rising front of the very
Himalaya (“the abode of snow”) is maintained by spo-
radic processes of collapse of the mountain walls involv-
ing a combination of both glacial and seismo-tectonic
factors. This makes the basins and the valley trenched
across the High Himalayan Front, the areas most prone to
unpredictable, catastrophic geomorphological hazards,
and creates a permanent, low-recurrence, but significant
threat for the growing population living in these valleys.
The Author
Monique Fort is a Professor of Geomorphology and
Environmental Sciences, Natural Hazards and Risks, in
the Department of Geography of Paris Diderot – Paris
7 University. She worked extensively in various high
mountains of the world (Alps, Central Asia, and
Himalaya). Her research interests evolved from the
relations of landforms with respect to geological struc-
tures, then to glacial and climatic fluctuations, and
palaeoenvironmental reconstructions. Ongoing field
work includes studies on current instabilities and natural
hazards (large-scale landslides, catastrophic floods) in
the Himalayas and Pamir mountains, floods impacts,
and their prevention in various parts of France. She
published more than 50 peer-reviewed papers. She was
the Vice President of the International Association of
Geomorphologists (2005–2009), and member of the
Commission on Mountain Response to Global Change
of the International Geographical Union (2008–2012).
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... (i) The Ghachok glacio-fluvial conglomerates (composed of all rocks outcropping along the Sabche Cirque cliffs, including the yellowish Larjung limestones; (Colchen et al., 1981), deposition of around 12,000 years ago (Gurung, 1970;Yamanaka et al., 1982;Fort and Freytet, 1983;Fort, 1986Fort, , 1987Koirala and Rimal, 1996;. (ii) The Pokhara gravels (mostly composed of Nilgiri Limestones, or Sombre Formation; Colchen et al., 1981) were deposited by two or three mega-debrisflow events during the medieval ages (Yamanaka et al., 1982;Fort, 1987;Koirala, 1998;Koirala et al.,1998;Fort, 2010;Schwanghart et al., 2016;Stolle et al., 2017). These mega debris-flows were derived from giant mountain-wall collapses in the West face of Annapurna IV (7525 m), East of Sabche Cirque (Fig. 2), and were triggered by mega-earthquakes (Fort, 1986(Fort, , 1987(Fort, , 2010Schwanghart et al., 2016;Stolle et al., 2017). ...
... (ii) The Pokhara gravels (mostly composed of Nilgiri Limestones, or Sombre Formation; Colchen et al., 1981) were deposited by two or three mega-debrisflow events during the medieval ages (Yamanaka et al., 1982;Fort, 1987;Koirala, 1998;Koirala et al.,1998;Fort, 2010;Schwanghart et al., 2016;Stolle et al., 2017). These mega debris-flows were derived from giant mountain-wall collapses in the West face of Annapurna IV (7525 m), East of Sabche Cirque (Fig. 2), and were triggered by mega-earthquakes (Fort, 1986(Fort, , 1987(Fort, , 2010Schwanghart et al., 2016;Stolle et al., 2017). The landforms derived from these two Ghachok and Pokhara Formations are quite different due to their differential resistance to erosion and to their geometric relationship in relation to differential uplift (Hormann, 1974;Yamanaka et al., 1982;Fort, 1987Fort, , 2010. ...
... These mega debris-flows were derived from giant mountain-wall collapses in the West face of Annapurna IV (7525 m), East of Sabche Cirque (Fig. 2), and were triggered by mega-earthquakes (Fort, 1986(Fort, , 1987(Fort, , 2010Schwanghart et al., 2016;Stolle et al., 2017). The landforms derived from these two Ghachok and Pokhara Formations are quite different due to their differential resistance to erosion and to their geometric relationship in relation to differential uplift (Hormann, 1974;Yamanaka et al., 1982;Fort, 1987Fort, , 2010. North of the valley, the indurated Ghachok Formation (locally called "gaunda") forms an upper terrace bounded by steep cliffs; it was progressively entrenched by the Seti River then abruptly and partly filled by the Pokhara gravels, now forming the lower, most recent terraces sets. ...
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The Seti River originates from the Annapurna Massif in the Higher Himalaya of Nepal and flows through the Pokhara valley in the Lesser Himalaya. The Seti River witnessed a disastrous flash flood on May 5th, 2012 causing the death of 72 people, obliterating dozens of homes and damaging infrastructures worth millions of dollars. Despite the 2012 flood event and several warnings by scientists for more yet bigger scale future floods in the Seti valley, fluvial risk is being aggravated by anthropogenic activities such as unplanned human settlement, encroachment of riverbanks, haphazard construction of road, drinking water, and hydropower projects in potential flood hazard areas in addition to the increased impacts of climate change on geological and hydro-metrological hazards as in other parts of Hindu Kush Himalayan Range. Covering some 40-km distance from the Seti headwater (Sabche Cirque) down to Pokhara city, the study is carried out based on hydro-geomorphological mapping, analysis of land-use and land-cover change, hydrological analysis including HEC-RAS modelling, historical archives, and interviews with local people. The study shows a significant change on the land use and land cover of the Seti catchment, mainly the urban/built-up area, which is increased by 405% in 24 years period (1996 to 2020) and by 47% in 7 years period (2013 to 2020). Further the study reveals that anthropogenic activities along the Seti valley have increased fluvial risk and are likely to invite more disasters. From the HEC-RAS analysis, two motor bridges built over Seti River were found to have insufficient freeboard to safely pass the highest flood discharge for 100 years return-period. Instead of relocating people to safer places, the government and local authorities rather seem to have encouraged people to live in the floodplain by providing basic amenities such as drinking water, electricity and access road. Given the context of climate change and Pokhara valley and the Seti catchment being in a high-seismic gap zone, there is a strong possibility of similar flood to the scale of 2012 or even greater in Seti River. Though the fluvial risk can be managed in a sustainable way through the application of functional space concept, i.e., by allowing more space (freedom) for rivers, this economic and environment friendly approach of the fluvial risk management has not been implemented yet in the Seti valley nor in Nepal. Rather the encroachment of floodplains by anthropogenic activities along the Seti valley is on an increasing trend. Many settlements and infrastructures along the valley have been identified vulnerable to hydro-torrential hazards, therefore it is utmost necessity to implement functional space river concept, land use and land plan policy, early warning system and public awareness education in order to mitigate and manage the future impact of fluvial hazards along the Seti valley.
... Source: ©Fort, 2013. the canyons of the seti Khola and its tributaries are among the most intriguing features of the valley (Fort, 2010). Interrupting the long sections of wide floodplain confined by flights of terraces, they occur along limited reaches a few hundred meters to 1 km long and are deeply entrenched (up to 50 m) in a material made of gravel and boulders, cemented together to form a hard, conglomeratic bedrock (the "ghachok Formation, " Fort, 1987), with a lime-rich matrix that is known locally as "gaunda" (hormann, 1974). ...
... several centuries ago, mountain collapses of exceptional magnitude affected the west face of annapurna IV, upheld by nilgiri limestones (Fig. 12.3). these collapses mobilized a volume of debris of ∼4-5 km 3 , the products of which were transported as a megadebris flow (the "pokhara gravel," gurung, 1965), which buried the lower part of the pokhara valley and its former differentiated topography (Fort, 1987(Fort, , 2010; hence, explaining the varying thickness in space of this gravel. the sudden input of such a large volume of debris drastically changed the landforms of the pokhara valley, hiding the initial topography (rocky spurs, conglomeratic alluvial terraces, deep river bed) and transforming the valley into a deadly, coarse debris field, in the form of a wide alluvial fan, with convex-up topography. ...
... representing remnants of the medieval megafloods, which transported thick pokhara gravel that plugged the mouths and lower reaches of the seti Khola tributaries (Fort, 2010;gurung, 2002;schwanghart et al., 2016;stolle et al., 2017), the pokhara valley's seven lakes (phewa, Begnas, rupa, maidi, Khaste, gunde, Dipang) enhance the region's reputation as a natural wonderland. In fact, phewa lake, which is considered to be the second largest lake in nepal, is certainly the most appealing, being the closest to airport and bus stations. ...
... This surface is cut by the meandering Seti Khola shaping terraces of >70 m high relative to their river bed. Steep gorges are intriguing features (Fort, 2010) mostly found in the city center of Pokhara, interrupting the longitudinal profile of the Seti river and its wide flood plain bounded by flights of terraces. These gorges are up to 1,000 m long, 10-25 m wide and deeply entrenched in indurated gravels and boulders forming an older valley infill and conglomeratic bedrock (Ghachok Formation, Fort (1987)). ...
... Obtaining more and slightly younger 14 C ages, Fort (1987) proposed that some 4 km 3 of the Pokhara Formation formed following an earthquake in 1505 AD (Table 3.1). The few samples, their large scatter, high measurement errors (± 100 years; 2σ), and cursory documentation made it difficult, however, to identify a basal age of the Pokhara Formation, leaving both the 1255 and 1505 AD earthquakes as potential candidates for triggering catastrophic sedimentation, if accepting a seismic trigger (Fort, 2010;Hanisch and Koirala, 2010). Schwanghart et al. (2016) re-calibrated all available 14 C ages and added 18 new AMS 14 C ages using several Bayesian priors (Bronk Ramsey, 2009b), and found that three distinct peaks in the pooled age distributions coincided with the timing of nearby M > 8 earthquakes in~1100, 1255, and 1344 AD. ...
... The Annapurna Massif is drained by the Seti Khola ("Khola" = river in Nepali) with a catchment area of ∼1400 km 2 . The river descends from Sabche Cirque in the High Himalayas and traverses the Main Central Thrust (MCT) to the Pokhara Valley in the Lesser Himalaya(Martin et al., 2005;Fort, 2010). Estimated denudation rates in the High Himalayan zone are between 2.0 and 2.7 mm yr −1 , and 0.1 to 0.8 mm yr −1 in the Lesser Himalaya(Robert et al., 2009;Godard et al., 2014). ...
Thesis
Fluvial terraces, floodplains, and alluvial fans are the main landforms to store sediments and to decouple hillslopes from eroding mountain rivers. Such low-relief landforms are also preferred locations for humans to settle in otherwise steep and poorly accessible terrain. Abundant water and sediment as essential sources for buildings and infrastructure make these areas amenable places to live at. Yet valley floors are also prone to rare and catastrophic sedimentation that can overload river systems by abruptly increasing the volume of sediment supply, thus causing massive floodplain aggradation, lateral channel instability, and increased flooding. Some valley-fill sediments should thus record these catastrophic sediment pulses, allowing insights into their timing, magnitude, and consequences. This thesis pursues this theme and focuses on a prominent ~150 km2 valley fill in the Pokhara Valley just south of the Annapurna Massif in central Nepal. The Pokhara Valley is conspicuously broad and gentle compared to the surrounding dissected mountain terrain, and is filled with locally more than 70 m of clastic debris. The area’s main river, Seti Khola, descends from the Annapurna Sabche Cirque at 3500-4500 m asl down to 900 m asl where it incises into this valley fill. Humans began to settle on this extensive fan surface in the 1750’s when the Trans-Himalayan trade route connected the Higher Himalayas, passing Pokhara city, with the subtropical lowlands of the Terai. High and unstable river terraces and steep gorges undermined by fast flowing rivers with highly seasonal (monsoon-driven) discharge, a high earthquake risk, and a growing population make the Pokhara Valley an ideal place to study the recent geological and geomorphic history of its sediments and the implication for natural hazard appraisals. The objective of this thesis is to quantify the timing, the sedimentologic and geomorphic processes as well as the fluvial response to a series of strong sediment pulses. I report diagnostic sedimentary archives, lithofacies of the fan terraces, their geochemical provenance, radiocarbon-age dating and the stratigraphic relationship between them. All these various and independent lines of evidence show consistently that multiple sediment pulses filled the Pokhara Valley in medieval times, most likely in connection with, if not triggered by, strong seismic ground shaking. The geomorphic and sedimentary evidence is consistent with catastrophic fluvial aggradation tied to the timing of three medieval Himalayan earthquakes in ~1100, 1255, and 1344 AD. Sediment provenance and calibrated radiocarbon-age data are the key to distinguish three individual sediment pulses, as these are not evident from their sedimentology alone. I explore various measures of adjustment and fluvial response of the river system following these massive aggradation pulses. By using proxies such as net volumetric erosion, incision and erosion rates, clast provenance on active river banks, geomorphic markers such as re-exhumed tree trunks in growth position, and knickpoint locations in tributary valleys, I estimate the response of the river network in the Pokhara Valley to earthquake disturbance over several centuries. Estimates of the removed volumes since catastrophic valley filling began, require average net sediment yields of up to 4200 t km−2 yr−1 since, rates that are consistent with those reported for Himalayan rivers. The lithological composition of active channel-bed load differs from that of local bedrock material, confirming that rivers have adjusted 30-50% depending on data of different tributary catchments, locally incising with rates of 160-220 mm yr−1. In many tributaries to the Seti Khola, most of the contemporary river loads come from a Higher Himalayan source, thus excluding local hillslopes as sources. This imbalance in sediment provenance emphasizes how the medieval sediment pulses must have rapidly traversed up to 70 km downstream to invade the downstream reaches of the tributaries up to 8 km upstream, thereby blocking the local drainage and thus reinforcing, or locally creating new, floodplain lakes still visible in the landscape today. Understanding the formation, origin, mechanism and geomorphic processes of this valley fill is crucial to understand the landscape evolution and response to catastrophic sediment pulses. Several earthquake-triggered long-runout rock-ice avalanches or catastrophic dam burst in the Higher Himalayas are the only plausible mechanisms to explain both the geomorphic and sedimentary legacy that I document here. In any case, the Pokhara Valley was most likely hit by a cascade of extremely rare processes over some two centuries starting in the early 11th century. Nowhere in the Himalayas do we find valley fills of comparable size and equally well documented depositional history, making the Pokhara Valley one of the most extensively dated valley fill in the Himalayas to date. Judging from the growing record of historic Himalayan earthquakes in Nepal that were traced and dated in fault trenches, this thesis shows that sedimentary archives can be used to directly aid reconstructions and predictions of both earthquake triggers and impacts from a sedimentary-response perspective. The knowledge about the timing, evolution, and response of the Pokhara Valley and its river system to earthquake triggered sediment pulses is important to address the seismic and geomorphic risk for the city of Pokhara. This thesis demonstrates how geomorphic evidence on catastrophic valley infill can help to independently verify paleoseismological fault-trench records and may initiate re-thinking on post-seismic hazard assessments in active mountain regions.
... Nepal in the central Himalayan arc is a typical example area of dynamic slope instabilities. Landslides in this area include very rare gigantic collapse of High Mountains (Ibetsberger 1996), large to moderate failures in the natural mountain slopes (Fort 2010;Gurung et al. 2011;Tiwari et al. 2017) to small shallow slides at the cut slopes (Vuillez et al. 2018). Generally, the deposits of large landslides, occurred in the past, form gentler slopes providing favorable areas for vegetation growth, settlement and cultivation. ...
Article
Kathmandu Kyirong Highway (KKH) is one of the most strategic Sino-Nepal highways. Low-cost mitigation measures are common in Nepalese highways, however, they are not even applied sufficiently to control slope instability since the major part of this highway falls still under the category of feeder road, and thus less resources are made available for its maintenance. It is subjected to frequent landslide events in an annual basis, especially during monsoon season. The Gorkha earthquake, 2015 further mobilized substantial hillslope materials and damaged the road in several locations. The aim of this research is to access the dynamic landslide susceptibility considering pre, co and post seismic mass failures. We mapped 5,349 multi-temporal landslides of 15 years (2004–2018), using high resolution satellite images and field data, and grouped them in aforementioned three time periods. Landslide susceptibility was assessed with the application of ‘certainty factor’ (CF). Seventy percent landslides were used for susceptibility modelling and 30% for validation. The obtained results were evaluated by plotting ‘receiver operative characteristic’ (ROC) curves. The CF performed well with the ‘area under curve’ (AUC) 0.820, 0.875 and 0.817 for the success rates, and 0.809, 0.890 and 0.760 for the prediction rates for respective pre, co and post seismic landslide susceptibility. The accuracy for seismic landslide susceptibility was better than pre and post-quake ones. It might be because of the differences on completeness of the landslide inventory, which might have been possibly done better for the single event based co-seismic landslide mapping in comparison with multitemporal inventories in pre and post-quake situations. The results obtained in this study provide insights on dynamic spatial probability of landslide occurrences in the changing condition of triggering agents. This work can be a good contribution to the methodologies for the evaluation of the dynamic landslide hazard and risk, which will further help to design the efficient mitigation measures along the mountain highways.
... The catchment has a planimetric area of 122.5 km 2 , with elevations ranging from 790 m at the dam outlet to 2480 m on the western limit of the catchment. Pokhara is built on gravels deposited by successive debris, which flows from the southern glacierised slopes of Annapurna III (7555 m) and IV (7525 m) [20][21][22]. There were at least three episodes of sedimentation that occurred between ~700 and ~1700 AD, likely coinciding with three great earthquakes [23,24]. ...
Article
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Phewa Lake is an environmental and socio-economic asset to Nepal and the city of Pokhara. However, the lake area has decreased in recent decades due to sediment influx. The rate of this decline and the areal evolution of Phewa Lake due to artificial damming and sedimentation is disputed in the literature due to the lack of a historical time series. In this paper, we present an analysis of the lake’s evolution from 1926 to 2018 and model the 50-year trajectory of shrinkage. The area of Phewa Lake expanded from 2.44 ± 1.02 km2 in 1926 to a maximum of 4.61 ± 0.07 km2 in 1961. However, the lake area change was poorly constrained prior to a 1957–1958 map. The contemporary lake area was 4.02 ± 0.07 km2 in April 2018, and expands seasonally by ~0.18 km2 due to the summer monsoon. We found no evidence to support a lake area of 10 km2 in 1956–1957, despite frequent reporting of this value in the literature. Based on the rate of areal decline and sediment influx, we estimate the lake will lose 80% of its storage capacity in the next 110–347 years, which will affect recreational use, agricultural irrigation, fishing, and a one-megawatt hydroelectric power facility. Mitigation of lake shrinkage will require addressing landslide activity and sediment transport in the watershed, as well as urban expansion along the shores.
... We focus on the southern Annapurna Massif, where it is drained by the Seti Khola ('Khola' = river in Nepali), with a catchment area of ~1400 km 2 . The river descends from the Sabche Cirque carved into the Higher Himalayan metasediments and traverses in a steep and deeply cut gorge the Main Central Thrust (MCT) before entering the broad Pokhara Valley in the Lesser Himalayas (Martin et al., 2005;Fort, 2010). Estimated denudation rates are 2.0-2.7 mm yr -1 in the Higher Himalayas, and 0.1-0.8 ...
Article
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Mountain rivers respond to strong earthquakes by rapidly aggrading to accommodate excess sediment delivered by co‐seismic landslides. Detailed sediment budgets indicate that rivers need several years to decades to recover from seismic disturbances, depending on how recovery is defined. We examine three principal proxies of river recovery after earthquake‐induced sediment pulses around Pokhara, Nepal's second largest city. Freshly exhumed cohorts of floodplain trees in growth position indicate rapid and pulsed sedimentation that formed a fan covering 150 km2 in a Lesser Himalayan basin with tens of metres of debris between the 11th and 15th centuries AD. Radiocarbon dates of buried trees are consistent with those of nearby valley deposits linked to major medieval earthquakes, such that we can estimate average rates of re‐incision since. We combine high‐resolution digital elevation data, geodetic field surveys, aerial photos, and dated tree trunks to reconstruct geomorphic marker surfaces. The volumes of sediment relative to these surfaces require average net sediment yields of up to 4200 t km–2 yr–1 for the 650 years since the last inferred earthquake‐triggered sediment pulse. The lithological composition of channel‐bed load differs from that of local bedrock, confirming that rivers are still mostly evacuating medieval valley fills, locally incising at rates of up to 0.2 m yr–1. Pronounced knickpoints and epigenetic gorges at tributary junctions further illustrate the protracted fluvial response; only the distal portions of the earthquake‐derived sediment wedges have been cut to near their base. Our results challenge the notion that mountain rivers recover speedily from earthquakes within years to decades. The valley fills around Pokhara show that even highly erosive Himalayan rivers may need more than several centuries to adjust to catastrophic perturbations. Our results motivate some rethinking of post‐seismic hazard appraisals and infrastructural planning in active mountain regions.
Chapter
Catastrophic mass flows (CMF) in the mountain cryosphere are an important geomorphic process that may pose significant hazard to communities and infrastructure in cold mountains; they occur in association with snow, glaciers, and permafrost in environments that are particularly sensitive to changes in thermal regimes and heavy rainfall. CMF form a broad range of cryospheric hazards and include mass movements of glacial ice, rock avalanches, ice-rock avalanches, debris flows, and outburst-generated flows. In some high mountains, avalanches of glacial ice, rock, and ice-rock mixtures may be earthquake-triggered. Broadly, CMF in the mountain cryosphere are characterized by sudden onset, high mean velocity (≥ 5 m/s), and high mobility (i.e., long runout in relation to volume) and generally involve a mixture of earth materials, water, snow, and ice. In some cases, CMF runout may exceed 100 km from source. CMF commonly undergo dramatic process transformation during movement in response to melting of entrained ice and snow, entrainment of additional materials along their path, river-damming effects, and incorporation or displacement of water in the periglacial environment; process complexity, involving long and complex process chains with instantaneous and/or delayed cascades, thus represents a challenge to quantitative hazard assessment. CMF initiate in uninhabited or sparsely populated areas of the mountain cryosphere and frequently descend into denser populated areas where they impact on mountain communities and infrastructure. CMF have been responsible for several notable mountain disasters since 1940 resulting in the deaths of over 15,000 people worldwide. Our focus on an examination of process complexity illuminates an assessment of CMF hazard in ice-affected mountain regions and forms the basis for the development of mitigation strategies based on detection, warning, engineering techniques in source and runout areas, and land-use controls. The precise relationship between the magnitude/frequency of CMF and change in the mountain cryosphere since ca. 1900 remains uncertain.
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After early dissection, which occurred around the Middle Pleistocene (deep lateritic weathering of the related alluvial deposits), the Pokhara valley suffered several sporadic episodes of dissection and filling. The youngest episode is represented by a sudden, widespread, 4 km3 fanglomeratic aggradation that buried a differentiated, terrace-shaped topography, dammed the adjacent valleys and created lakes behind the filling. We interpret it as a brief, catastrophic, probably seismically triggered mass-wasting event, involving both till and ice-rockfall products. The Seti river, actively incising at a rate 10-20 cm/yr, removed half of the original accumulation, dissecting the aggradational surface into more than ten unpaired terraces. This gives an erosion rate around 4 X 106 m3/yr and for the upper Seti catchment including the Pokhara valley, a sediment yield of 3076 m3/yr/km2. the minimum uplift rate of the High Himalayan Front (HHD) relative to the adjacent basin is 0.65 mm/yr. -from Author
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Chronologies for the Siwalik molasse and intermontane basins along the southern margin of the Himalaya and Hindu Kush Ranges constrain the timing and pattern of facies migration and structural disruption of the Indo-Gangetic foredeep. This synthesis indicates that quiescent intervals are punctuated by pulses of rapid deformation as thrusting migrates in a stepwise fashion across the foredeep.
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