Page 1

Glacial Flooding & Disaster Risk Management Knowledge Exchange and Field Training July 11-24, 2013 in Huaraz, Peru

Rates of  Change  on  Spillway  Lake,  Ngozumpa  Glacier,   Nepal   Ulyana  N.  Horodyskyj   University  of  Colorado,  Boulder   Introduction:   Through   a   combination   of   modern   satellite   imagery,   repeat   photography   from   the   1950s,   and   on-­‐going   field   measurements,   we   know   that   glaciers   of   the   Mt.   Everest   and  Cho  Oyu  regions  in  Nepal  have  undergone  and  continue  to  undergo  major  losses   of   ice   volume   (e.g.,   Thompson   et   al.,   2012;   Bolch   et   al.,   2008).   This   volume   loss   occurs  not  so  much  by  loss  of  area,  but  more  from  a  reduction  in  the  thickness  of  the   glacier   at   average   rates   that   locally   can   exceed   1m/year.   The   formation   of   supraglacial   (surface)   lakes   appears   to   be   the   catalyst   for   this   loss;   once   formed,   these   lake   basins   can   grow   and   migrate   by   back   wasting   and   calving   of   surrounding   ice   walls   (e.g.,   Byers   2007;   Sakai   et   al.   2002;   Benn   et   al.   2001;   Benn   et   al.   2000).   Fill   and   drain   events   in   these   basins   throughout   the   summer,   triggered   either   by   monsoonal  precipitation  and/or  englacial  conduit  flooding,  can  accelerate  the  melt   as   more   ice,   previously   covered   up   by   debris,   is   “cleaned   off”   by   the   drainage   of   water,  thus  exposing  more  ice  to  solar  radiation  (Horodyskyj  et  al.,  in  progress).  An   important   concern   is   whether   lake   formation   itself   is   accelerating.   To   this   end,   Spillway   Lake,   a   large   supraglacial   base-­‐level   lake   on   Ngozumpa,   one   of   Nepal’s   longest   glaciers,   (Fig.   1),   has   been   profiled   by   researchers   at   the   University   of   St.   Andrews   and   Swansea   University   (2001/2009).   Interpolated   point   bathymetry   maps,  made  with  a  simple  depth  sounder  (Fig.  2)  revealed  where  parts  of  the  lake   were  deepening,  sometimes  upwards  of  13  m/yr  (e.g.,  Thompson  et  al.  2012).  The   lake   may   eventually   pose   a   flooding   hazard   to   Sherpa   villages   down-­‐glacier,   hence   the  importance  in  quantifying  the  physics  responsible  for  deepening  and  growth.  

Figure 1.  Location  map  for  Ngozumpa  glacier  and  the  terminal  Spillway  Lake.  source:   BBC  (­‐environment-­‐16317090)     Project  Objectives:     As   supraglacial   lakes   deepen,   they   may   eventually   melt   their   way   through   to   rock,   making  the  transition  to  a  proglacial  lake  with  an  active  calving  ice  front,  leading  to   even   more   rapid   ice   loss   (e.g.,   Imja   Lake,   near   Mt.   Everest).   In   certain   cases,   however,   debris   (rocks   and   sediment)   covers   the   lake   floor,   insulating   the   underlying   ice   and   slowing   or   perhaps   reversing   the   process,   leading   to   an   equilibrium   lake   that   may   be   considered   “dead.”   The   bottom   of   Spillway   Lake   is   deepening  in  some  places  but  may  be  partly  debris  covered  elsewhere,  providing  a   unique   opportunity   to   study   the   physics   crucial   to   glacial   demise   as   influenced   by   lake   formation.   Thus,   research   objectives   for   2012/2013   included   (1)   establishing   a   new   bathymetric   map   for   Spillway   through   point   interpolation   (November   2012)   and  (2)  doing  open  water  transects  with  a  side-­‐scan  sonar  system,  to  fill  in  any  gaps   as   well   as   provide   imagery   (e.g.,   hard   vs.   soft   bottoms;   boulders;   ice)   of   the   lake   (May   2013).   In   order   to   quantify   the   physics   of   deepening,   four   locations   (named   Northwest,  Northeast,  Main,  and  Southwest)  in  Spillway  were  chosen  for  long-­‐term   temperature  monitoring  (starting  end  of  May  2013)  of  the  surface  and  bottom.       Preliminary  Results:   Spillway  Depth  and  Area:  A  survey  by  Thompson  et  al.  (published  2012;  survey  work   completed  in  2009)  revealed  the  deepest  parts  of  Spillway  to  be  close  to  27  meters.   Areas   of   deepening   seem   to   be   associated   with   ice   walls,   mainly   south-­‐   and   west-­‐ facing,  which  calve  and  collapse  through  the  melt  season  (Fig.  2).  A  new  (2012)  areal   analysis  reveals  that  the  southern  part  of  the  basin  has  remained  mostly  stable,  with   not  much  expansion.  The  northern  basins  have  lost  area,  especially  in  the  northeast.   What  was  once  open  water  is  now  debris  consisting   of  large  boulders  and  sediment,   due  to  a  partial  drainage  event  (~4  meter  water  level  drop)  sometime  between  the   last   survey   work   and   November   2012.   South-­‐facing   walls   responsible   for   calving  

and deepening   in   2009/2010   have   since   been   covered   up   by   debris   in   2012.   However,  deepening  hotspots  remain,  particularly  in  the  main  basin  of  the  lake.    

Figure   2.   Bathymetry   (depth)   map   of   Spillway   Lake,   from   2009   survey   work   (Thompson   et   al.   2012).   2010   GeoEye   imagery   is   in   the   background,   for   comparison.   Most,   but   not   all,   deep   areas   (between   22-­‐27   meters)   correlate   with   large   ice   walls.   New   bathymetry   and   GeoEye   imagery   (2012),   for   comparison,   to   be   shown   in   July   14th   presentation.     Spillway   Basin   Temperatures:   Four   lake   basins   within   Spillway   were   targeted   for   longer-­‐term   surface   and   bottom   temperature   monitoring:   Northwest   (NW),   with   a   10-­‐m   buoy;   Northeast   (NE),   with   a   15-­‐m   buoy;   Main,   with   a   20-­‐m   buoy;   and   Southwest   (SW,   near   the   outflow),   with   a   5-­‐m   buoy.     Figure   3   shows   results   from   post-­‐thaw  (according  to  local  sources:  May  21,  2013)  and  pre-­‐monsoon.  The  SW  and   NE  basins  have  the  warmest  surface  temperatures  (Fig  3a);  over  the  past  two  years,   they   are   the   basins   that   have   gotten   shallower,   rather   than   deeper.   The   NW   basin   is   influenced  by  nearby  inflow  channels  of  glacial  meltwater,  resulting  in  overall  colder   surface   temperatures.   The   Main   basin,   which   has   been   deepening   significantly   in   recent  years,  is  also  one  of  the  colder  basins.  A  cool-­‐down  of  surface  temperatures  at   the   end   of   May   is   due   to   a   snowfall   event;   all   basins   except   SW   were   affected   by   this   event;  in  the  NE  basin,  bottom  temperatures  were  higher  than  surface  ones  due  to   this.   Significance   is   that   even   if   surface   temperatures   drop,   the   bottom   can   retain   heat  for  melting  any  exposed  ice.       Overall,   bottom   temperatures   (Fig   3b)   reveal   that   the   SW   basin   is   the   warmest,   consistent  with  its  shallow  depth  and  sediment-­‐covered  bottom.  The  Main  basin  is   second   warmest,   at   least   for   a   few   days   post-­‐thaw,   before   dropping   to   the   coldest   temperatures  and  remaining  mostly  isothermal.  This  sudden  drop  may  be  indicative   of  a  subaqueous  calving  event.  An  independent  temperature  probe  was  sent  to  the   bottom  of  all  the  basins.  Unlike  in  the  other  basins,  where  the  probe  got  stuck  in  the  

sediment at   the   bottom,   in   the   Main   basin,   the   probe   came   back   up   with   ice   forming   around   it   and   a   bottom   temperature   of   0.1   deg   C.   Significance   of   this   lies   in   there   being   a   relatively   unprotected   ice   base,   subject   to   more   rapid   melting   and,   hence,   deepening.   Of   all   the   basins,   this   one   shows   the   most   potential   for   significant   deepening.  Two  time-­‐lapse  cameras  are  focused  on  the  region,  to  further  distinguish   sub-­‐aerial  from  subaqueous  calving  events.      



Figure   3.   (a)   Surface   temperature   data   for   the   Main,   SW,   NE   and   NW   basins   of   Spillway  Lake,  Ngozumpa  glacier.  (b)  Bottom  temperature  data  for  the  same  regions.   Data  collection:  May  23  –  June  5,  2013.        

Project Scope:     The  importance  of  this  research  is  two-­‐fold.  First,  the  work  done  here  can  be  applied   to  other  glaciers  across  the  Himalaya  making  the  transition  from  small  supraglacial   “ponds,”  to  large  coalesced  supraglacial  lakes  at  their  termini.  As  glaciers  continue   to   adjust   to   current   climatic   conditions,   these   kinds   of   lakes   will   become   more   commonplace.  Ngozumpa  is  still  in  the  initial  stages  of  forming  a  large  supraglacial   lake,   unlike   Imja   (east   of   Ngozumpa)   and   Tsho   Rolpa   (west   of   Ngozumpa),   which   have  grown  lakes  with  long  fetches  and  active  ice  calving  fronts,  leading  to  further   glacier   demise.   Ngozumpa   presents   an   opportunity   to   observe   this   transition,   as   it   happens,  and  quantify  the  physics  behind  it.  Second,  this  work  matters  to  the  locals   that  live  in  the  vicinity  of  the  glacier  and  down-­‐valley  from  it.  Should  Spillway  Lake   ever  overtop  its  moraine  dam,  it  could  lead  to  a  catastrophic  outburst  flood.  Though   most  villages  are  perched  higher  than  the  river  water  level,  the  potential  exists  for   significant  land  erosion  from  a  large  flood  event.  An  outburst  in  1985,  from  Dig  Tsho   (Langmoche   glacier)   resulted   in   loss   of   life   and   infrastructure   in   down-­‐valley   settlements:  five  people  perished  and  a  $1.5  million  hydroelectric  power  plant  near   Namche,  a  large  Sherpa  town,  was  destroyed  (Bajracharya  et  el.  2007).     Acknowledgements:     The   author   would   like   to   thank   USAID   for   the   individual   climber-­‐scientist   grant   and   the   High   Mountain   Glacial   Watershed   Program   for   providing   funds   for   two   field   seasons  and  scientific  equipment,  without  which  this  project  would  not  be  possible.       References:   Benn.  D.I.,  Wiseman,  S.,  and  C.R.  Warren  (2000),  Rapid  growth  of  a  supraglacial  lake,   Ngozumpa  Glacier,  Khumbu  Himal,  Nepal,  IAHS  Publ.  264,  177-­‐185.     Benn,  D.I.,  Wiseman,  S.,  and  K.A.  Hands  (2001),  Growth  and  drainage  of  supraglacial   lakes  on  the  debris-­‐mantled  Ngozumpa  Glacier,  Khumbu  Himal,  Nepal,  Journal  of   Glaciology,  47,  626-­‐638.   Bolch,  T.,  Buchroithner,  M.,  Pieczonka,  T.  and  A.  Kunert  (2008),  Planimetric  and   volumetric  glacier  changes  in  the  Khumbu  Himal,  Nepal,  since  1962  using  Corona,   Landsat  TM  and  ASTER  data,  Journal  of  Glaciology,  54,  187,  592-­‐600.   Byers,  A.  (2007),  An  assessment  of  contemporary  glacier  fluctuations  in  Nepal’s   Khumbu  Himal  using  repeat  photography,  Himalayan  Journal  of  Sciences,  4(6),  21-­‐ 26.   Horodyskyj,  U.N.,  Breashears,  D.,  Bilham,  R.  (in  progress),  Supraglacial  Lakes   Changes,  Through  the  Lens  of  a  Camera.   Sakai,  A.,  Nakawo,  M.,  and  K.  Fujita  (2002),  Distribution,  characteristics  and  energy   balance  of  ice  cliffs  on  debris-­‐covered  glaciers,  Nepal  Himalaya,  Arctic  and  Antarctic   Alpine  Research,  34,  12-­‐19.  

Thompson, S.,  Benn,  D.I.,  Dennis,  K.,  and  A.  Luckman  (2012),  A  rapidly  growing   moraine-­‐dammed  glacial  lake  on  Ngozumpa  Glacier,  Nepal,  Geomorphology,   10.1016/j.geomorph.2011.08.015.    

Ulyana Horodyskyj: Rates of change on spillway lake, Ngozumpa Glacier, Nepal  

Through a combination of modern satellite imagery, repeat photography from the 1950s, and on-going field measurements, we know that glaciers...

Read more
Read more
Similar to
Popular now
Just for you