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

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


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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


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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


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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


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Map of glaciers

677 glaciers with an area of 1,316.72 sq. km.


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Glacial lakes

2,674 glacial lakes with an area of 106.80 sq.km.


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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


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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


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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


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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.


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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.


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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


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Impacts Lugge lake

Thanza

TencheyTsojuLhedi

Punakha

Wangdue

Thanza village and erosion downstream

Tenchey village-sand deposition


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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


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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


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Impacts Lugge lake

Thanza

TencheyTsojuLhedi

Punakha

Wangdue

Erosion on the right bank of Mochu

Logs brought down-scene near Wangdue


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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


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Lunana study

TarinaLunana

Punakha town

Wangdue town

Geomorphic setting of the pho chhu sections and implications for flood propagation

1

2

3


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Stretch 2

TarinaLunana

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%.


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Stretch 3

TarinaLunana

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


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Thank You

Documents

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