Transcript Flash floods and debris flow due to glacial lake outburst floods Karma Chhophel

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
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Flash floods and debris flows due to
glacial lake outburst floods
Country profile
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Location
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Area
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26o45‘ to -28o10‘N
88o45’92o10‘E
38,984 sq.km
East-west distance and north
south distance
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340 and 170 km
Elevation range
- 100 masl at the border with
India to 7500 at the border
with China
Topography
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The country is mostly rugged
and mountainous
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Flash floods and debris flows due to
glacial lake outburst floods
Flash floods in Bhutan
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Relevance
- Flash floods due to
• Glacial lake
outbursts
• Landslide dam
bursts
• Intense rainfall
during monsoon
Greater himalayas
Middle valleys
Southern foothills
Therefore this forecasting
workshop is important
from the Bhutanese
perspective
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Flash floods and debris flows due to
glacial lake outburst floods
GLOF
Landslide
7 Oct 1994
Sept 2003
Rainfall
Phuentsholing-2000
rockslide
Lugge
lake
Volume
discharged-18 mcm
May 2004
Partial failure
10 July 2004
Breach
Impact downstream
Volume
discharged-11 mcm
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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.
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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.
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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
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Flash floods and debris flows due to
glacial lake outburst floods
GLOFS
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Lake creation
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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
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Flash floods and debris flows due to
glacial lake outburst floods
GLOFS
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Water level rise (Contd..)
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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
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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
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Flash floods and debris flows due to
glacial lake outburst floods
GLOFS
• Impact of GLOFs
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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.
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Flash floods and debris flows due to
glacial lake outburst floods
GLOF events in Bhutan
Year
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 . The flood destroyed part of Punakha Dzong. The flood is
said to have lasted for 5 days.
in eastern Lunana
1994
Partial burst of Lugge The only GLOF event properly documented is the one
lake
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
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Flash floods and debris flows due to
glacial lake outburst floods
The 1994 GLOF
• Cause
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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
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Flash floods and debris flows due to
glacial lake outburst floods
Impacts
Lugge
lake
Thanza
Tenchey
Tsoju
Thanza village and erosion
downstream
Lhedi
Tenchey village-sand deposition
Punakha
Wangdue
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Flash floods and debris flows due to
glacial lake outburst floods
Impacts
Lugge
lake
Thanza
Tenchey
Tsoju
Tsoju village-sand deposit
Lhedi
Debris deposit below Lhedi villagedestroyed pasture land of yak
herders
Punakha
Wangdue
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Flash floods and debris flows due to
glacial lake outburst floods
Impacts
Lugge
lake
Thanza
Tenchey
Tsoju
Punakha Dzong 3 days after the
flood
Lhedi
Damage to Dzongchung- a very
sacred temple
Punakha
Wangdue
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Flash floods and debris flows due to
glacial lake outburst floods
Impacts
Lugge
lake
Thanza
Tenchey
Tsoju
Erosion on the right bank of Mochu
Lhedi
Logs brought down-scene near
Wangdue
Punakha
Wangdue
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Flash floods and debris flows due to
glacial lake outburst floods
Impacts
Lugge
lake
Thanza
Tenchey
Tsoju
3000
Discharge (m3/s)
2500
Lhedi
2000
1500
1000
500
0
Jan Feb Mar Apr May Jun
Jul
Aug Sep Oct Nov Dec
Month
Hydrograph of 7 Oct 1994 GLOF
Punakha
Peak discharge>2500 m3/s
Wangdue
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Flash floods and debris flows due to
glacial lake outburst floods
Lunana study
Lunana
Tarina
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2
3
Punakha town
Wangdue town
Geomorphic setting of the pho chhu sections and implications for flood
propagation
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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
Tarina
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2
3
Punakha town
Wangdue town
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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
Lunana
Tarina
-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
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2
3
Punakha town
Wangdue town
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Flash floods and debris flows due to
glacial lake outburst floods
Thank You
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