Karma: Artificial lowering of lake water level in Thorthormi Tso, Bhutan

Ar$ficial lowering of lake water level in Thorthormi Tso

Karma Department of Geology and Mines Thimphu

GLOF History in Bhutan • Before 1950: No informa$on, but evidence

exists Puna Tsang Chu Basin • 1957: Along Pho Chu • 1960: Along Pho Chu • 1968: No informa$on but damage on infrastructure downstream • 1994: Pho Chu

-­‐ 17 lives lost

Loca$on PROJECT SITE China

INDIA

Rapstreng Tso Bechung Glacier

Thorthormi Glacier

Lugge Tso

Puna Tsang Chu Basin

Iden$fica$on of GLOF Threats • Rapid Expansion of supra glacial ponds

1993, SPOT

Recent Changes on Thorthormi Glacier

2012

• Size of glacier and future glacial lake

3.42 sq.Km

• Weakened barrier by 1994 GLOF from Lugge

1994 GLOF

• Seepage from the lakes

Seepage point

• Ac$ve slide on the moraine barrier

Moraine Barrier

Rapstreng Tso

Thorthormi Tso

Results in the thinning of the moraine barrier between Rapstreng and Thorthormi Tso

• Rocks and Snow Avalanches Steep mountain slope in the accumula$on area of Thorthormi Glacier

Ice fall and Avalanches are common phenomenon

Moraine Barrier

• Ice cored moraine barrier between Rapstreng and Thorthormi

What is the Risk?

• Moraine barrier failure

X

Thorthormi overtopping into Rapstreng

Water level difference between Rapstreng and Thorthormi is 74m

Worst case scenario -­‐-­‐-­‐-­‐GLOF

GLOF

OVER TOPPING

(53 million cubic meters of water)

Why Mi$ga$on Measure?

Project Site (Lunana) Puna Tsang Chu I (1200 MW) Baso Chu (66 MW) Puna Tsang Chu II (1020 MW)

Sunkosh Project (2560 MW)

Why mi$ga$on Measures?

Khuruthang Town

• Seflements and agricultural land

• Important historical monuments

• Exis$ng infrastructures and future development ac$vi$es.

Khuruthang Bridge -­‐ Hydropower -­‐ Townships

Punakha Dzong

Baso Chu Hydro Power

How do we go about?

• Ar$ficial lowering of lake water level

• Hazard zona$on in the downstream

• Installa$on of Early Warning System

Method to lower lake water level • Manually construc$ng spillway (Deepening and widening of natural outlet channel)

• Siphoning

• Pumping

Op$ons for lowering the lake water level • Siphoning ruled out Ø Unknown rate of in recharge Ø Temporary Ø Reduc$on in suc$on head Ø Higher eleva$on difference in the riser limb Ø Workability of siphons.

Op$ons for lowering the lake water level • Pumping has been ruled out as we require 800cum/hr which requires heavy equipment.

• Pumps were not func$oning properly.

Best Op$on: Manual construc$on of spillway Main Lake

Subsidiary Lake I Subsidiary lake II Main Lake

Subsidiary Lake I

Project Site 1

40 m Project Site 2

Subsidiary Lake I Outlet Direc$on of water flow

Project Site 3

The Project • Design for 4 years with 4 months of working dura$on each year (2008-­‐2011)

• 360 workers

• USD 4.23 million +RGOB Co-­‐financing

Ar#ficial lowering of Thorthormi: Objec#ve 5 m

Goal: Bring down the lake level by 5m Volume: 17,100,000 m3

Revisi$ng earlier technical mi$ga$on plan & Site op$ons SL I SL II

Subsidiary lake I Subsidiary lake II

Selec$on  criteria  Criteria Length

Present outlet 400 m

Earth volume 34209.80 m3 (2% bed slope) Disposal site Subsidiary lakes Effectiveness More effective for lowering (5m cutting) water table Stability Needs concerns stabilization

Old outlet 340 m 93897.68m3 Either side of the channel Entail cutting 7 m moraine ridge Needs stabilization

Subsurface informa$on R4R R5R R6R

R1R

R10R

R11R R9R R8R

Result of Resis$vity Survey

Site Map Main Lake

Project Site 1 Subsidiary Lake I Project Site 2

40 m Subsidiary lake II Project Site 3

Engineering Design

Engineering Safety Aspects • • • •

• •

Down culng of 1.67 m per year Bed slope not to exceed 2% (1:50) Side slope not to exceed 1:1.5 (V:H) 10 m channel bed width(Finished width) Boulder riprap to be constructed at the end Slope treatment to be carried out on outer slope of subsidiary lake II

Approach & Methodology • Transfer plan on the ground • Simultaneous excava$on all along channel • Water to be released on daily basis star$ng from subsidiary lake II • Silent explosives for breaking large boulders • Max puller for pulling larger boulders • Finer materials to be used for water diversion • Reliable Communica$on System

Methodology

Excava$on works

Water Diversion

Lake Level Reduc$on 2009 (m) 2010(m)

2011(m) 2012(m) Total(m)

0.86

1.37

1.45

1.36

5.04

Subsidiary Lake I 0.85

1.20

-­‐

1.61

3.66

Subsidiary Lake II 0.82

2.30

1.10

0.86

5.08

Main Lake

Technical Uncertain$es: Ice at the site of excava$on

Expected problems with ice • Ice lens with seepage beneath Moraine wall 5 m Ice Lense Lake Water seepage

• Fast mel$ng

Moraine wall Water flow

Lake Ice body Very large level difference between the lakes

Lake

Constraints • Unpredictable weather system -­‐ working period -­‐ work output

• Medical problems -­‐ High al$tude sickness -­‐ Evacua$on

• Transporta$on of materials -­‐ coopera$on from the local authori$es

• Recrui$ng & managing large workforce -­‐ worker turn out -­‐ management issues at the site