Dhananjay Regmi: Seti River disaster and hazards facing Pokhara, Nepal part 2 of 3

4. Rock avalanche with snow ingesVon. 4 M m3 water equivalent possible

Apparent source of the large May 5, 2012 avalanche on Annapurna IV

© J.S. Kargel

Apparent source of the large May 5, 2012 avalanche on Annapurna IV

© J.S. Kargel

300 m length of ridgeline retreat according to ETM+

Apparent source of rock avalanche ~300 m wide X 1300 m high X 30 m ridge recession ~ 107 m3 (from this photo, ETM+, ASTER, and Google Earth).

Mass esVmate ~ 2.5-­‐3.0 X 107 tonnes from volume. Compares to 22 M tons given by David Petley from seismic energy release of this event.

300 m length of ridgeline retreat according to ETM+

Apparent source of rock avalanche ~300 m wide X 1300 m high X 30 m ridge recession ~ 107 m3 (from this photo, ETM+, ASTER, and Google Earth).

Mass esVmate ~ 2.5-­‐3.0 X 107 tonnes from volume. Compares to 22 M tons given by David Petley from seismic energy release of this event.

Picture of Dust Distribu<ons

Dust over vegeta<on

Dust on fallen trees and on gorge

1-­‐5 cm of fine dust covers most of the upper Se< Basin

Rockfall path towards to Gorge

Rockfall path towards to Gorge……

Rockfall path towards to Gorge……

Rockfall path towards to Gorge……

e. Working hypothesis #2: All-­‐of-­‐the-­‐ above (mulCple sources).

• A composite model might work best.

• water produced by fric<onal mel<ng of avalanche snow and ice, and water stored in the rockfall-­‐dammed gorge lake and perhaps in karst each might have contributed to the flood.

• The process-­‐chain of mass movements then was triggered by the large avalanche/landslide.

5. Immense dust cloud, air fall deposiVon and intense avalanche/slope winds

6. Water derived from: -­‐-­‐ Wet snow -­‐-­‐ Supraglacial melt water -­‐-­‐ Avalanche melt water -­‐-­‐ Then added to impounded water in gorge

7. Hyperconcentrated slurry outburst flood

Canyoning and measurement of Gorge

Canyoning and measurement of Gorge Gorge depth is not uniform and it is esVmated between 200 m to 600 m

Gorge width ranges between 50 to 150 m width

Hovering and taking video inside the gorge

Pictures taken by : Stephanie Spray

Damming , materials and topography

Gorge topography h

s to m e e s

f e eli Upper r

ou t s

e h t

s ward o t

g n i s go a

e n i l c tly de h g i l s

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Poten<al water damming limit

Base of the Gorge is almost horizontal, which provides berer space for the accumula<on of water within it.

Avalanche and rock fall on May 5 2012

People noVced the change in hydrology

Damming few weeks before flood

Accumulated water several M m^3

Concluding Remarks • Rock slide dam and accumula<on of water in the gorge has resulted in the forma<on of a natural reservoir before the avalanche event (approx. three weeks before May 5th.)

• Avalanche of rock and ice and the large supply of unconsolidated ancient lake sediments in the gorge created the pressure and resulted on the outburst of the natural reservoir.

• Need to iden<fy the most suitable alarming system to minimize the loses from such events

The city of Pokhara was built on deposits from such floods, including 50-­‐60 m of debris from one flood only 450 years ago.

Vulnerable community on the SeV River floodplain

What Else we Did ?

Detailed geological Mapping

Flood InundaVon Mapping

Socio economic survey

Early warning System