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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 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
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

country profile
Flash floods and debris flows due to glacial lake outburst floodsCountry 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 in bhutan
Flash floods and debris flows due to glacial lake outburst floodsFlash 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

slide4
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

map of glaciers
Flash floods and debris flows due to glacial lake outburst floodsMap of glaciers

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

glacial lakes
Flash floods and debris flows due to glacial lake outburst floodsGlacial lakes

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

potentially dangerous lakes
Flash floods and debris flows due to glacial lake outburst floodsPotentially dangerous lakes

24 lakes – identified based on the condition of lakes, dams, associated mother glaciers and topographic features around the lake

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

GLOFS
glofs9
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
glofs10
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
the 1994 glof
Flash floods and debris flows due to glacial lake outburst floodsThe 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

impacts
Flash floods and debris flows due to glacial lake outburst floodsImpacts

Lugge lake

Thanza village and erosion downstream

Thanza

Tenchey

Lhedi

Tsoju

Tenchey village-sand deposition

Punakha

Wangdue

impacts14
Flash floods and debris flows due to glacial lake outburst floodsImpacts

Lugge lake

Tsoju village-sand deposit

Thanza

Tenchey

Lhedi

Tsoju

Debris deposit below Lhedi village-destroyed pasture land of yak herders

Punakha

Wangdue

impacts15
Flash floods and debris flows due to glacial lake outburst floodsImpacts

Lugge lake

Punakha Dzong 3 days after the flood

Thanza

Tenchey

Lhedi

Tsoju

Damage to Dzongchung- a very sacred temple

Punakha

Wangdue

impacts16
Flash floods and debris flows due to glacial lake outburst floodsImpacts

Lugge lake

Erosion on the right bank of Mochu

Thanza

Tenchey

Lhedi

Tsoju

Logs brought down-scene near Wangdue

Punakha

Wangdue

impacts17
Flash floods and debris flows due to glacial lake outburst floodsImpacts

Lugge lake

Thanza

Tenchey

Lhedi

Tsoju

Punakha

Hydrograph of 7 Oct 1994 GLOF

Peak discharge>2500 m3/s

Wangdue

lunana study
Flash floods and debris flows due to glacial lake outburst floodsLunana study

Lunana

Tarina

1

2

3

Punakha town

Wangdue town

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

stretch 2
Flash floods and debris flows due to glacial lake outburst floodsStretch 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

stretch 3
Flash floods and debris flows due to glacial lake outburst floodsStretch 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