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