What triggered Nepal flash floods? Scientists trace disaster to massive Himalayan avalanche

The devastating flash floods that swept through Nepalrsquos BhotekoshiTrishuli corridor on August 26 were triggered by a massive icerock avalanche in the high Himalayas rather than conventional heavy rainfall according to a preliminary report by Nepalese scientistsThe report commissioned by the Nepal Environment Society NES was released on SundayThe findings come as the death toll from the disaster rose to 903 with authorities continuing to recover bodies from affected areasMassive icerock avalanche triggered floodAccording to the study a huge mass of ice rock and debris fell from an altitude of around 5200 metres in the LangtangLirung area of Rasuwa and entered the Lhende RiverThe resulting surge travelled downstream through the Bhotekoshi River and into the Trishuli creating a powerful debrisladen flood that devastated towns and villages across northern and central Nepal reported PTIThe study concluded that the disaster was not caused by a glacial lake outburst flood GLOFResearchers analysed satellite imagery hydrological and meteorological data terrain characteristics and changes in elevation and slope along the river system They found no evidence of a GLOFIcerock mass travelled at 167 kmphAccording to PTI the report said the icerock mass began falling from the northern part of LangtangLirung at around 837 am on August 26 It descended steep terrain from about 5200 metres before reaching the Lhende River at around 2930 metresSatellite imagery showed major geographical changes across nearly 10 square kilometres while a glacier section covering approximately 056 square kilometres had broken awayThe icerock mass travelled around 22 kilometres to Rasuwagadhi in just over seven minutes reaching an estimated average speed of 463 metres per second or approximately 167 kmph according to the studyIts speed subsequently dropped to around 73 kmph between Rasuwagadhi and Betrabati and to approximately 22 kmph further downstream Despite slowing the flow continued carrying large quantities of rock sediment and soil PTI reportedNo evidence of exceptionally heavy rainfallAs per PTI the study also found no evidence of exceptionally heavy rainfall in the upper watershed on the day of the disaster weakening the assumption that extreme rainfall or a cloudburst alone caused the floodsResearchers noted that river levels and flow rates in the Rasuwagadhi and Syafrubesi areas had actually declined several hours before the flood arrivedAt Rasuwagadhi the average water level fell between 5 am and 8 am while the flow at Syafrubesi began declining at around 7 amFurther investigation neededDespite identifying the icerock avalanche as the likely trigger experts said further field investigations are required to establish the precise geological mechanism behind the eventResearchers also said more work is needed to determine the contribution of melting ice and whether the river was temporarily blocked upstream before the sudden surgeHydropower projects severely affectedThe disaster also caused extensive damage to Nepalrsquos hydropower infrastructureCiting preliminary data from the Nepal Electricity Authority the study said 13 hydropower projects and one solar project were affected taking around 431 MW of generation capacity out of the systemThe study citing a preliminary government assessment estimated the overall economic losses at around Rs 3 trillion although detailed sectorwise assessments are still underway PTI reportedWith PTI inputs