Dhananjay Regmi: Seti River disaster and risks facing Pokhara, Nepal (part 1 of 3)

Investigation of the Seti River disaster (May 5, 2012) and assessment of past and future mountain hazards facing Pokhara, and upstream communities

Dhananjay Regmi (Himalayan Research Center) Jeffrey Kargel and Gregory Leonard (University of Arizona) Lalu Poudel, Krishna KC, Khagendra Poudel and Anusha sharma (Tribhuvan University, Nepal) Teiji Watanabe and Bhabana Thapa (Hokkaido University, Japan)

SeV river outburst flood disaster, May 5, 2012 Annapurna IV

Kill zone, 72 dead and missing Hundreds, perhaps thousands, more would have been killed if flood had been 2-­‐3 m deeper

City of Pokhara is built on Holocene/Recent outburst-­‐ flood/debris-­‐flow deposits

Kill zone, 72 dead and missing

Fig. 1: Location of Seti River Flood

Flood Pictures

Amateur You-­‐tube video/Google Earth analysis to assess flood discharge rate and volume

~ Videographer locaVon

Flow speed 4.1 m/s

Flow depth 3.25 m

Water depth = 7.75 -­‐ 4.5 m = 3.25 m (+-­‐ 0.8 m)

Se< river outburst flood discharge rate and volume calcula<on • Flood discharge measured at Pokhara from amateur You-­‐tube and Google-­‐Earth imagery • Flow speed 4.1 +-­‐ 0.6 m/s • Flow depth 3.25 +-­‐ 0.8 m • Flow width 87 m +-­‐ 2 m • Discharge rate = 1160 m^3/s +-­‐30% • High discharge probably lasted > 200 seconds (mul<ple video analysis) • Total discharge > 230,000 m^3 (assumes 200 seconds) • Equivalent to cylinder 124 m diameter, 19 m deep

Se< river outburst flood dischahrge rate and volume calcula<on • Flood discharge measured at Pokhara from amateur You-­‐tube and Google-­‐Earth imagery • Flow speed 4.1 +-­‐ 0.6 m/s • Flow depth 3.25 +-­‐ 0.8 m • Flow width 87 m +-­‐ 2 m • Discharge rate = 1160 m^3/s +-­‐30% • High discharge probably lasted > 200 seconds (mul<ple video analysis) • Total discharge > 230,000 m^3 (assumes 200 seconds) • Equivalent to cylinder 124 m diameter, 19 m deep

Few seconds before the flood hits the Kharapani

View of Kharapani just a]er Flood

The first two of 27 waves of water and hyper concentrated slurry involved roughly a quarter million cubic meters of water each. The floods hit fast and hard for those in the way, and lasted a few minutes per wave, reaching over 10 m deep at this loca<on..

Survivors lived on next higher terrace

Many of those killed lived on the lowest terrace

Key ques<ons to Address 1. What was the cause of the Se< river disaster?

2. Is another similar flood likely?

3. What role might imprudent habita<on have played in raising the death toll?

4. What other types and magnitudes (e.g., peak flows) of floods are possible in the future?

5. How large a popula<on remains vulnerable?

Hypothesized ideas Ø a. normal GLOF! Ø b. Rockfall-­‐impounded lake Ø C. The karst model. Ø d. Rock avalanche/landslide trigger. Ø e. All-­‐of-­‐the-­‐above (mulCple sources).

Normal GLOF ?

• The flood could not have been a usual type of GLOF (Glacier Lake Outburst Flood), because no such precursor lake existed in the basin.

Imja Lake

Small ponds below Annapurna IV

Rockfall-­‐impounded lake ?

• It was observed from satellite repeat imaging, that a modest rock fall occurred into the Se< gorge between 2002 and 2008, and was reac<vated a few weeks prior to the disaster.

Reac<vated about few weeks before May 5 disaster

1. CONDITIONING EVENT Rockfall dam emplaced in gorge, repeated episodes, between 2002-­‐2008, again in 2012

2. CONDITIONING EVENT Impoundment lake forms

c. Working hypothesis #1: The karst model.

• The rock fall dam outburst model was soon modified, and includes the possibility that not only the gorge could contain a lake, but possibly karst caverns in those same rocks could have been water filled and may have been dammed.

Small cave in marble cliff, about 2 meters high

d. Rock avalanche/landslide trigger.

Maximov Brown Clouds ( AVIA CLUB)

Mr. Maximov Picture

Annapurna Himal, Nepal: Machapuchre -­‐ Se< Gandaki Basin

ETM+ pan-­‐321-­‐RGB (20 April 2012– 15 days before the disaster)

Annapurna Himal, Nepal: Machapuchre -­‐ SeV Gandaki Basin

Debris avalanche, ~4 km2 Avalanche airfall deposits, ~10 km2

Avalanche source Ice + debris avalanche, ~2 km2

ETM+ pan-­‐321-­‐RGB (06 May 2012– 1 day a]er the disaster)

3. INITIATING EVENT May 5, 2012, rock slide from Annapurna IV, ~10 M m3