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
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.
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
Published on Sep 26, 2013
Slides for presentation given to High Mountains Adaptation Partnership in Huaraz, Peru on 13 July 2013.