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