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Flash floods and debris flows due to glacial lake outburst floods
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Flash floods and debris flow due to glacial lake outburst floods
Karma Chhophel
Hydro-met Services Division
Department of Energy
Thimphu: Bhutan
Flash floods and debris flows due to glacial lake outburst floods
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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
Flash floods and debris flows due to glacial lake outburst floods
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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
Flash floods and debris flows due to glacial lake outburst floods
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RainfallGLOF Landslide
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
Flash floods and debris flows due to glacial lake outburst floods
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Map of glaciers
677 glaciers with an area of 1,316.72 sq. km.
Flash floods and debris flows due to glacial lake outburst floods
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Glacial lakes
2,674 glacial lakes with an area of 106.80 sq.km.
Flash floods and debris flows due to glacial lake outburst floods
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Potentially dangerous lakes
24 lakes – identified based on the condition of lakes, dams, associated mother glaciers and topographic features around the lake
Flash floods and debris flows due to glacial lake outburst floods
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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
Flash floods and debris flows due to glacial lake outburst floods
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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
Flash floods and debris flows due to glacial lake outburst floods
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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.
Flash floods and debris flows due to glacial lake outburst floods
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GLOF events in BhutanYear Cause Impact
Before 1950 Not known There 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.
1957 Bursting of Tarina lake This 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.
1960 Bursting of some lakes in eastern Lunana
. The flood destroyed part of Punakha Dzong. The flood is said to have lasted for 5 days.
1994 Partial 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.
Flash floods and debris flows due to glacial lake outburst floods
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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
Flash floods and debris flows due to glacial lake outburst floods
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Impacts Lugge lake
Thanza
TencheyTsojuLhedi
Punakha
Wangdue
Thanza village and erosion downstream
Tenchey village-sand deposition
Flash floods and debris flows due to glacial lake outburst floods
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Impacts Lugge lake
Thanza
TencheyTsojuLhedi
Punakha
Wangdue
Tsoju village-sand deposit
Debris deposit below Lhedi village-destroyed pasture land of yak herders
Flash floods and debris flows due to glacial lake outburst floods
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Impacts Lugge lake
Thanza
TencheyTsojuLhedi
Punakha
Wangdue
Punakha Dzong 3 days after the flood
Damage to Dzongchung- a very sacred temple
Flash floods and debris flows due to glacial lake outburst floods
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Impacts Lugge lake
Thanza
TencheyTsojuLhedi
Punakha
Wangdue
Erosion on the right bank of Mochu
Logs brought down-scene near Wangdue
Flash floods and debris flows due to glacial lake outburst floods
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Impacts Lugge lake
Thanza
TencheyTsojuLhedi
Punakha
Wangdue
0
500
1000
1500
2000
2500
3000
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
Dis
cha
rge
(m
3/s)
Hydrograph of 7 Oct 1994 GLOF
Peak discharge>2500 m3/s
Flash floods and debris flows due to glacial lake outburst floods
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Lunana study
TarinaLunana
Punakha town
Wangdue town
Geomorphic setting of the pho chhu sections and implications for flood propagation
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2
3
Flash floods and debris flows due to glacial lake outburst floods
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Stretch 2
TarinaLunana
Punakha town
Wangdue town
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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%.
Flash floods and debris flows due to glacial lake outburst floods
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Stretch 3
TarinaLunana
Punakha town
Wangdue town
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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
Flash floods and debris flows due to glacial lake outburst floods
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Thank You
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