1. Flash floods and debris flows due to glacial lake outburst floods 1 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 2 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 ­ 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 3 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 4 Rainfall GLOFLandslide Lugge lake 7 Oct 1994 Sept 2003 rockslide 10 July 2004 Breach May 2004 Partial failure Impact downstream Volume discharged-18 mcm Volume discharged-11 mcm Phuentsholing-2000

5. Flash floods and debris flows due to glacial lake outburst floods 5 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 6 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 7 Potentially dangerous lakes 24 lakes – identified based on the condition of lakes, dams, associated mother glaciers and topographic features around the lake

8. Flash floods and debris flows due to glacial lake outburst floods 8 GLOFS 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

9. Flash floods and debris flows due to glacial lake outburst floods 9 GLOFS 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

10. Flash floods and debris flows due to glacial lake outburst floods 10 GLOFS 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.

11. Flash floods and debris flows due to glacial lake outburst floods 11 GLOF events in Bhutan YearCauseImpact Before 1950Not knownThere is no information, but a large number of missing end moraines in many of the glaciers in the high Himalayas of Bhutan indicate that GLOF events had taken place in the past. 1957Bursting of Tarina lakeThis GLOF event affected the Punakha-Wangdue valley, which destroyed part of Punakha Dzong. Gansser (1970) attributed this flood to the outburst from Tarina Tsho in Lunana. 1960Bursting of some lakes in eastern Lunana. The flood destroyed part of Punakha Dzong. The flood is said to have lasted for 5 days. 1994Partial burst of Lugge lake The only GLOF event properly documented is the one that occurred on 7 October 1994. From a survey conducted on 20-23 Oct 1994, it was found that 17 lives were lost, 91 households were affected, 12 houses damaged, 5 water mills washed away, 816 acres of dray land 965 acres of pasture land were either washed away or covered with sand and silt, 16 yaks were carried away, 36 cowsheds and a full year’s manure washed away, 6 tonnes of food grains lost, 2838 pieces of wooden shingles and 68 wooden beams washed away, 4 bridges washed away, 2 chortens destroyed and the temple at Tsojug was badly damaged.

12. Flash floods and debris flows due to glacial lake outburst floods 12 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 13 Impacts Lugge lake Thanza Tenchey TsojuLhedi Punakha Wangdue Thanza village and erosion downstream Tenchey village-sand deposition

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

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

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

17. Flash floods and debris flows due to glacial lake outburst floods 17 Impacts Lugge lake Thanza Tenchey TsojuLhedi Punakha Wangdue Hydrograph of 7 Oct 1994 GLOF Peak discharge>2500 m 3 /s

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

19. Flash floods and debris flows due to glacial lake outburst floods 19 Stretch 2 Tarina Lunana Punakha town Wangdue town 1 2 3 -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%.

20. Flash floods and debris flows due to glacial lake outburst floods 20 Stretch 3 Tarina Lunana Punakha town Wangdue town 1 2 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

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