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Flash floods and debris flow due to glacial lake outburst floods

Flash floods and debris flow due to glacial lake outburst floods Karma Chhophel Hydro-met Services Division Department of Energy Thimphu: Bhutan Country profile Location 26 o 45‘ to -28 o 10‘N 88 o 45’92 o 10‘E Area 38,984 sq.km East-west distance and north south distance

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Flash floods and debris flow due to glacial lake outburst floods

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  1. Flash floods and debris flows due to glacial lake outburst floods Flash floods and debris flow due to glacial lake outburst floods Karma Chhophel Hydro-met Services Division Department of Energy Thimphu: Bhutan

  2. Flash floods and debris flows due to glacial lake outburst floods Country profile • Location • 26o45‘ to -28o10‘N • 88o45’92o10‘E • Area • 38,984 sq.km • East-west distance and north south distance • 340 and 170 km • Elevation range • 100 masl at the border with India to 7500 at the border with China • Topography • The country is mostly rugged and mountainous

  3. Flash floods and debris flows due to glacial lake outburst floods Flash floods in Bhutan • Relevance • Flash floods due to • Glacial lake outbursts • Landslide dam bursts • Intense rainfall during monsoon • Therefore this forecasting workshop is important from the Bhutanese perspective Greater himalayas Middle valleys Southern foothills

  4. Flash floods and debris flows due to glacial lake outburst floods GLOF Landslide Rainfall 7 Oct 1994 Phuentsholing-2000 Sept 2003 rockslide May 2004 Partial failure Lugge lake Volume discharged-18 mcm 10 July 2004 Breach Impact downstream Volume discharged-11 mcm

  5. Flash floods and debris flows due to glacial lake outburst floods Map of glaciers 677 glaciers with an area of 1,316.72 sq. km.

  6. Flash floods and debris flows due to glacial lake outburst floods Glacial lakes 2,674 glacial lakes with an area of 106.80 sq.km.

  7. Flash floods and debris flows due to glacial lake outburst floods Potentially dangerous lakes 24 lakes – identified based on the condition of lakes, dams, associated mother glaciers and topographic features around the lake

  8. Lake creation Global warming Increase temperature leads increase melt Glacier retreat Increase temperature accelerates glacier retreat. Larger area available for storing melt water. On an average glacier retreat in Bhutan between 30-40 m per year Water level rise Rapid change in climatic conditions that increase solar radiation causing rapid melting of ice and snow Intensive precipitation events Decrease in seepage through the moraine to balance inflow Blocking of ice conduits by sedimentation Dead ice weighed down by sediment below the lake bottom which stops subsurface infiltration Flash floods and debris flows due to glacial lake outburst floods GLOFS

  9. Water level rise (Contd..) Blocking of the outlet by an advancing tributary glacier Landslides in the moraine wall Melting of ice from ice-core moraine wall Melting of ice due to subterranean thermal activities Inter/basin subsurface flow of water from one lake to another Dam failure can occur due to Glacial and snow avalanches Landslide and debris flow Blockade of the outlet channel Lake water seepage and piping Rapid drainage from adjacent lake Lateral erosion of moraines Flash floods and debris flows due to glacial lake outburst floods GLOFS

  10. Impact of GLOFs The impact of GLOF events downstream can be devastating in terms of damage to roads, bridges, hydro-power plants, trekking trails, villages, agricultural land, natural vegetation, as well as the loss of lives, property and infrastructure. Much of the damage created during GLOF events is associated with the large amounts of debris that accompany the floodwaters. Damage to settlements and farmland can take place at great distances from the outburst source. Flash floods and debris flows due to glacial lake outburst floods GLOFS

  11. Flash floods and debris flows due to glacial lake outburst floods GLOF events in Bhutan

  12. Flash floods and debris flows due to glacial lake outburst floods The 1994 GLOF • Cause • Partial outburst of Lugge lake • Seepage between ice core and moranic deposit • Melting ice core developed fissures weakening the dam • The dam failed due build up of hydrostatic pressure on the already weakened dam Lugge lake 2 weeks after 7 october GLOF

  13. Flash floods and debris flows due to glacial lake outburst floods Impacts Lugge lake Thanza village and erosion downstream Thanza Tenchey Lhedi Tsoju Tenchey village-sand deposition Punakha Wangdue

  14. Flash floods and debris flows due to glacial lake outburst floods Impacts Lugge lake Tsoju village-sand deposit Thanza Tenchey Lhedi Tsoju Debris deposit below Lhedi village-destroyed pasture land of yak herders Punakha Wangdue

  15. Flash floods and debris flows due to glacial lake outburst floods Impacts Lugge lake Punakha Dzong 3 days after the flood Thanza Tenchey Lhedi Tsoju Damage to Dzongchung- a very sacred temple Punakha Wangdue

  16. Flash floods and debris flows due to glacial lake outburst floods Impacts Lugge lake Erosion on the right bank of Mochu Thanza Tenchey Lhedi Tsoju Logs brought down-scene near Wangdue Punakha Wangdue

  17. Flash floods and debris flows due to glacial lake outburst floods Impacts Lugge lake Thanza Tenchey Lhedi Tsoju Punakha Hydrograph of 7 Oct 1994 GLOF Peak discharge>2500 m3/s Wangdue

  18. Flash floods and debris flows due to glacial lake outburst floods Lunana study Lunana Tarina 1 2 3 Punakha town Wangdue town Geomorphic setting of the pho chhu sections and implications for flood propagation

  19. Flash floods and debris flows due to glacial lake outburst floods Stretch 2 • -River section characterized by narrow, bed rock controlled gorges • Dense vegetation cover and potential landslide activity may cause channel blockage • Last part of section 2- risk of bigger temporary channel blockages during flood events become significant as vegetated river banks develop and channel gradient lowers down to 0.5 to 2%. Lunana 1 Tarina 2 3 Punakha town Wangdue town

  20. Flash floods and debris flows due to glacial lake outburst floods Stretch 3 -Braided river morphology -Depositional cones emerging from lateral valleys confine the river -Sediment depositional section at the junction between pho chhu and mochhu. -Sharp bend at the junction with mo chhu is prone to wooden debris jams, with the potential to cause considerable back water effects Lunana 1 Tarina 2 3 Punakha town Wangdue town

  21. Flash floods and debris flows due to glacial lake outburst floods Thank You

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