5 May: Quest to unravel the cause of the Seti flash flood

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On Saturday 5 May 2012, flash flooding along the Seti River in the Kaski district of north-western Nepal resulted in the death or disappearance of at least 72 people and caused great loss of property including homes, businesses, crops, livestock, and valuable infrastructure. 

On 9 May, a joint team from ICIMOD and the Ministry of Home Affairs, Nepal visited the site. During the visit the team witnessed the damage caused by the flash flood and observed the intensity of the flood. The team noted that the flash flood had deposited a heavy amount of fine sediment and not the larger debris typical of most mountain flash floods. On20 May a team of ICIMOD experts joined Prof. Jeffrey Kargel from the University of Arizona, USA, in an aerial reconnaissance of the Seti headwater area. 

Unfortunately, to date the cause of the flash flood remains unclear. However, several hypotheses have been proposed: 

  • Glacial lake outburst flood (GLOF). This hypothesis was quickly rejected after ICIMOD checked the glacial lake inventory and satellite images and confirmed that sizeable glacial lakes are not present in the headwaters of the Seti River.
  • Landslide dam outburst flood (LDOF). During the aerial observation the ICIMOD team observed that the landslide was smaller than initially thought. The relatively small size of the landslide, combined with the rather narrow river section (gorge) and very steep longitudinal profile of the stream, precludes the possibility of impoundment of a large water body by landslide damming. 
  • Snow/ice/rock avalanche. There is now strong evidence that a large ice or rock avalanche occurred, possibly from the south face of Mount Annapurna IV. However, there is no evidence that the avalanche or the debris run-out reached the river channel, and there is no plausible explanation for the release of large amount of water.
  • Landslide/avalanche cloud fallout into the rockfall-dammed gorge. This hypothesis suggests that while large boulders/debris did not reach the stream channel, fine particles did. However, volumetrically, this mechanism still seems to fail to account for the delivery of a high enough volume of water for the flood. 
  • Outflow from underground karst storage. The fifth hypothesis is the only one that can explain the large amount of water that was observed during the flash flood. Large underground stores of water are very likely to exist in karst formations. However, the only way to prove (or disprove) this hypothesis would be to explore inside the Seti gorge, which would be extremely difficult.

In lack of an easy answer to the real cause of the flash flood, it might be practical to concentrate on downstream risk management for the time being. Ultimately, however, the city of Pokhara will need to understand what happened scientifically in order to understand how future events might scale up in size, or not.