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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  

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

Slides for presentation given to High Mountains Adaptation Partnership in Huaraz, Peru on 13 July 2013.

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