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.  

Glacial Flooding & Disaster Risk ManagementKnowledge Exchange and Field Training

July 11-24, 2013 in Huaraz, PeruHighMountains.org/workshop/peru-2013

 Figure  1.  Location  map  for  Ngozumpa  glacier  and  the  terminal  Spillway  Lake.  source:  BBC  (http://www.bbc.co.uk/news/science-­‐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.      

A.      

B.      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.