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Global Change and the World’s Mountains
Perth, Scotland, September 26-30 2010
NATURAL HAZARDS, ROAD CONSTRUCTION, INCREASED
VULNERABILITY IN THE NEPAL HIMALAYAS: WHAT FUTURE
FOR A DEVELOPING COUNTRY?
M. FORT, E. COSSART UMR 8586 PRODIG
NATURAL HAZARDS, ROAD CONSTRUCTION, INCREASED
VULNERABILITY IN THE NEPAL HIMALAYAS: WHAT FUTURE
FOR A DEVELOPING COUNTRY?
M. FORT, E. COSSART UMR 8586 PRODIG
OUTLINE
-Environmental, geographic context
-Methods and functioning of the system
-Study cases along the new Kali Gandaki road
-Concluding remarks
Global Change and the World’s Mountains
Perth, Scotland, September 26-30 2010
Benighat (800m)
Jomosom (2700m)
Annapurna
8091 m
Dhaulagiri
8172 m
Tibet-China
India
Benighat (800m)
Jomosom (2700m)
Annapurna
8091 m
Dhaulagiri
8172 m
Tibet-China
India
Monsoon Pre-
Monsoon
storms
Landslides of varying size, all rainfall triggered
Consequences:
•High relief, steep mountainslopes
•Strong dynamic hillslope-river coupling
=> Slope instabilities, the nature of
which varying with type of substrate
(bedrock vs colluvium), topography, etc.
The Himalayas:
•A collisional range
•Average uplift rate: 6-8 mm a-1
•River incision keeping pace with
uplift
5 cm a-1
Consequences:
•Average uplift rate: 6-8 mm a-1
•High relief, stepp slopes
•River incision keeping pace with uplift
•Slope instabilities
The Himalayas: a collisional range
Road construction: a long,
difficult and dangerous process
5 cm a-1
Landslides distribution in Nepal (1968-2002)
Active landslides, mostly in:
Middle Hills (Lesser Himalaya),
south of the Greater Himalaya,
and along the roads
(ICIMOD)
Active landslides, mostly in: -
Middle Hills (Lesser Himalaya), -
south of the Greater Himalaya, -
and along the roads
(ICIMOD)
Methods to assess
threats to the road
•Extensive fieldwork
•Repeated surveys
•Interactions hillslopes-road-
river, sediments budgets
•Cascading & process-
response system approach
Active landslides
and debris flows
River incision
Threat to the
new road
Old fluvial
material
Bedrock nature
and structure
Old landslide
material
Middle Kali Gandaki
2000 1977
New road and bridges induced development of Beni Ghat
Bazaar more assets increased vulnerability
2008
1998 Landslide (upstream view)
Wedge failure (north
dip + vertical joints)
A landslide-dammed
lake started to build up
rapidly
relative height of
+23m reached in 7
hours
30m
1.1 M m3
Quartzite and chloritoschists of
the upper Lesser Himalaya
1998
2007
dip
+500m
Emplacemento
f the lake
Landslide (downstream view)
Tatopani Village: lower
part under water
Sept. 28 1998, 8 a.m. 9 a.m. 3 p.m., 23 m above the river
Definitive drainage, at the onset
of the next monsoon (June 1999)
15 m drop
First lake
Residual lake depth : 3-to-5 m
Sept. 29 1998, 10 a.m.
Inflow (Q)
estimated to
be 54 m3/s
According to local reports, the lake developed in 7-8 hours
Tatopani landslide
(1998):
1,1 x 106m3
Upstream landslide
dam lake:
1,5 x 106m3
Downstream of
the
landslide dam Bank erosion following
landslide breach (1998)
coarse boulder lag in
the KG channel
Sedimentary archives
reflecting older events
of similar nature
landslide
2000 1978
2007 Changes during the last 3 decades
Original landscape: set of 3 terraces
Channel widening and change in
channel course
Bank undercutting and cleaning off
of former alluvial terraces loss of
cultivated lands and village sites
Threat to the new road
Distally confined debris fan
6 p.m.
Dana village
6.15 p.m.
6.25 p.m.
Debris flow dam
At the confluence with Kali Gandaki
7 p.m.
Next morning
2000
Bank erosion
Cause of Dana debris-
flow ?
• Persistent planar landslide
occasional damming of
the Ghatte khola (during a
few hours or days)
• When the dam fails:
sudden outburst flood
debris flow widening of
river bed potential losses
(crops and cattle)
• Occurrence: once or twice
a year, when very intense
(storm) rainfall.
2000
2007
Very dynamic change of
river bed with time:
Channel widening:
15 m wide in 1974
35 m wide in 2009
Bed surface morphology:
aggradation/incision stages
(short term) in a general
trend of river incision
2009
Bank erosion
(note gabions)
New road open to traffic in Spring
2008, across the Ghatte khola
: undersized bridge
=> management problems and
increased vulnerability for
travellers
42 m2
360 m2
Dec. 2007
The Ghatte khola typifies a
« landslide watershed », with
unsteady landforms and strong
debris inputs to the trunk river
April 2009
New road open to traffic in Spring
2008, across the Ghatte khola
: undersized bridge
=> management problems and
increased vulnerability for
travellers
: management problems and
increased vulnerability for
travellers
42 m2
360 m2
Dec. 2007
The Ghatte khola typifies a
« landslide watershed », with
unsteady landforms and strong
debris inputs to the trunk river
April 2009
TALBAGAR DEBRIS AVALANCHE CONE
23 18
2000
2000
2007
2007 Upstream of debris
avalanche cone
Comments and implications
Himalayas: large potential for landslide and debris flow
events, with possible temporary dams, outburst floods
and related damages
These short term events: most common mode of erosion
and sediment transfer that control sediment fluxes
outward this mountain: « minor » events at the Himalayan
and geological scale history, yet « huge » events for human
beings
Other potential, similar hazards may impact settlements and
infrastructures; they represent a major threat to villages and road along
the full length of the river system
In the future, indirect threats might arise from:
• poor maintenance of the road, hence more
geomorphic hazards => more physical, functional
and economic vulnerabilities to come
• collapse of the tourism economy?
Uncertainties:
• aggravation of natural hazards due to
climate change (monsoon strengthening)?
Thank you for your attention Nilgiri South Peak, 6971 m
Co-author Etienne COSSART For further details, see FORT & al., Geomorphology, in press