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

Causes and  Human  Impacts  of  the  Seti  River  (Nepal)  Disaster  of   2012   Jeffrey  S.  Kargel,  Lalu  Paudel,  Gregory  Leonard,  Dhananjay  Regmi,  Sharad  Joshi,  Khagendra   Poudel,  Bhabana  Thapa,  Teiji  Watanabe,  and  Monique  Fort   ABSTRACT:   On   May   5,   2012,   a   hyperconcentrated   slurry   flood   in   the   Seti   River   suddenly   burst   forth   onto   a   small   village   and   rural   areas   in   the   valley   upstream   from   Pokhara,   the   second   largest   city   of   Nepal.   The   flood   swept   away   unsuspecting   tourists,   picnickers,   laborers   and   local   residents   of   Kharapani   village.   It   killed   32   people   and   left   another   40   missing,   and   it   displaced   many   more.   The   flood   killed   livestock,   wiped   out   local   livelihoods,   destroyed   temples,   roads,   community   buildings   and   vital   infrastructure   such   as   suspension   bridges,   electric   poles   and   drinking   water   transmission  pipes.  The  disaster,  at  first  seemingly  without  cause,  also  took  a  psychological  toll   on   the   survivors   in   the   affected   villages   and   in   Pokhara,   whose   residents   wonder   if   the   events   could   recur   and   if  they   could   be   the   next   victims.   Satellite   remote   sensing   and   field   investigations   support  the  following  scenario.    A  hazardous  condition  started  by  a  rockfall  blockage,  a  few  weeks   prior   to   the   disaster,   of   the   Seti   River   gorge   and   then   filling   of   the   impoundment   reservoir   by   early   spring   melting   of   the   snowfields   and   glaciers.     A   rock   and   ice   avalanche   from   Annapurna   IV   (~7500  m)  dislodged  the  previous  rockfall  dam  when  the  rock-­‐ice  avalanche  mixture  swept  into   the   reservoir.   A   hyperconcentrated   slurry   flow   then   swept   down   the   Seti   River.     Eyewitness   reports   leading   to   and   during   the   disaster   and   during   recovery   operations   support   this   sequence.   The   geologic   causes   of   the   disaster   pertain   to   the   unique   physiographic   attributes   of   the   upper   Seti  Basin  as  well  as  the  general  tectonic  environment  and  lithologic  makeup  and  glacial  history  of   the  Himalaya,  which  together  have  produced  a  highly  unstable  environment  of  frequent  bedrock   failures,   deep   river   incision   and   river   damming,   deposition   of   vast   amounts   of   unconsolidated   glacigenic  sediment,  and  frequent  mass  movements  and  floods  involving  the  sediment,  bedrock,   ice,  and  impounded  water.  We  also  gathered  information  about  the  human  root  causes  of  the  high   death  toll  and  to  gather  demographic  and  physiographic  data  that  help  to  constrain  scenarios  of   potential   future   disasters   of   similar   types   in   this   area.     The   toll   increased   as   a   result   of   people   inhabiting  unsafe  places  against  existing  land-­‐use/habitation  zoning  restrictions.  Nature  and  the   law,  if  both  had  been  respected  by  Seti  Valley  residents,  would  not  have  caused  a  disaster  of  this   magnitude.   However,   an   even   greater   disaster   could   happen   any   year,   as   we   have   identified   several  possible  modes  of  catastrophic  discharge  of  water  and  sediment  into  the  Seti  River.       1. Introduction On   May   5,   2012,   just   after   9   AM   local   time,   a   tourist   flight   operator—Captain   Alexander   Maximov—was  flying  an  ultralight  Aeroprakt  aircraft  over  the  Seti  River  valley  just  north  of  his   Avia  Club  operating  base  in  Pokhara,  Nepal.    He  observed  a  huge  yellow  cloud  spreading  across   the  upper  part  of  the  basin  (the  Sabche  Cirque);  according  to  our  interviews  of  him,  the  cloud  was   0  

unlike any  meteorological  cloud  or  snow  avalanche  he  had  ever  seen.    He  then  observed  a  muddy   flash  flood  racing  down  the  Seti  River.    During  his  return  to  the  Pokhara  airport,  he  radioed  the   first   eyewitness   report   of   something   devastating   in   progress,   and   this   news   was   broadcast   by   local  radio  stations.    Speculation  holds  that  the  timely  dissemination  of  Captain  Maximov’s  report   may  have  saved  many  lives  in  areas  farther  downstream.    The  upstream  residents  and  tourists,   picnickers,  and  streambed  laborers  were  caught  completely  unprepared,  with  no  warning  at  all,   and   many   perished.     A   preliminary   report   prepared   by   ICIMOD   with   Kargel’s   input   offers   some   details   of   the   disaster   and   the   early   development   and   testing   of   working   hypotheses   (REF).     Captain   Maximov   also   had   wingtip   video   cameras   mounted   on   his   aircraft.   The   aircraft’s   cameras   recorded  the  dust  cloud  as  well  as  the  flood  racing  at  high  speed  down  valley.   On  the  ground,  mayhem  and  death  occurred  as  the  sediment-­‐  and  log-­‐laden  slurry  flow  proceeded   in  a  series  of  pulses  lasting  for  many  hours  cumulatively.  Each  pulse  had  a  peak  flow  lasting  only  a   few   minutes.     Many   of   these   pulses,   starting   with   the   first   one,   were   recorded   on   resident   and   tourist   eyewitnesses’   mobile   phone   video   cameras;   quite   a   few   of   these—many   rather   tragic— were  posted  rapidly  on  Youtube.    The  eyewitness  accounts  and  videos  from  the  aircraft  operator   and  people  on  the  ground—including  media  reports  as  well  as  our  own  interviews  of  survivors— form   a   cornerstone   of   observations   upon   which   the   geomorphologic   process   causes   and   geological  underpinning  of  the  disaster  could  be  reconstructed.      Other  key  observations  include  a   seismic   signal   picked   up   due   to   the   triggering   avalanche,   satellite-­‐based   imaging   before   and   after   the  disaster,  and  helicopter-­‐borne  field  reconnaissance  and  ground-­‐based  field  studies  following   the  disaster.       Our  analysis  of  an  amateur  video  taken  of  the  first  flood  wave  as  it  reached  Pokhara  indicated  a   peak   discharge   of   >1000   m3/s.     It   was   further   estimated—with   wide   uncertainty—that   the   cumulative   flow   volume   emitted   during   more   than   twenty   flood   waves   was   at   least   2   x   106   m3,   perhaps  more  by  a  factor  of  several,  but  not  likely  more  than  107  m3.   The   Seti   River   disaster   could   have   been   just   one   more   in   an   unending   series   of   barely   noted   human  tragedies  in  the  Himalaya  at  the  hands  of  nature  if  it  was  not  for  the  modern  way  that  this   tragedy  was  documented.    From  ubiquitous  mobile  phone  video  cameras  that  almost  everybody   has   these   days   the   event   was   documented   starting   within   minutes   of   its   avalanche   trigger   through  all  the  human  suffering.    Satellite  eyes  in  the  sky  observed  the  scene  shortly  before  and   shortly   after   the   disaster,   aiding   in   the   forensics   search   for   causes.     Ubiquitous   modern   technology—including   mobile   phones—may   be   instrumental   in   development   of   an   SMS   text-­‐ based   warning   system.     Imprudent   habitation   and   improper   usage   of   the   flood   plain   shared   as   much  blame  for  much  of  the  disaster  as  nature  carries.    Tragic  as  this  disaster  was,  it  could  have   been  much  worse  according  to  what  our  field  survey  found.   2. Study  Area     The   Seti   River,   west   Nepal,   originates   from   the   Annapurna   Range   (Tethys   Himalaya)   and   flows   across  the  Higher  Himalaya  and  down  to  the  Lesser  Himalaya  along  the  Pokhara  Valley  (Figure  1).   It  has  a  very  steep  profile  in  the  north  near  the  Annapurna  Range  (Tethys  Himalaya  and  Higher   Himalaya)   and   then   flows   with   a   gentle   profile   in   most   parts   of   its   length   across   the   Lesser   Himalaya,  where  it  flows  within  terraced  clastic  sedimentary  deposits  of  the  Pokhara  Valley.  The   Pokhara   Valley   is   a   result   of   at   least   two   giant   debris-­‐flow   events   in   the   past   (Yamanaka   et   al.,   1982;  Fort,  1987).  One  took  place  12,000  ±  1000  B.P.,  at  the  end  of  the  last  glaciation  (Koirala  and   Rimal,   1996;   Koirala   et   al.,   1996),   and   led   to   the   formation   of   oldest   terrace   of   the   Pokhara   Valley    


known as   the   Ghachok   Formation.   A   second   event   similar   size   and   nature   occurred   750   ±   50   B.P.   (Koirala   et   al.,   1996;   Hanisch   and   Koirala,   2010)   and   resulted   in   the   Pokhara   Formation   (Yamanaka  et  al.  1982,  Fort  1987).    Damming  of  tributaries  of  the  Seti  River  by  the  huge  amount   of   sediments   along   the   main   channel   resulted   in   the   formation   of   a   number   of   lakes   in   the   Pokhara  valley.  The  sediment  source  is  believed  to  be  the  sediments  accumulated  in  the  bowl-­‐like   structure   in   between   the   Annapurna   IV   and   Machhapuchre   mountains.   This   bowl-­‐like   structure   has   been   named   as   the   Sabche   Cirque   (Yamanaka   et   al.   1982,   Fort   1987).     During   the   Pleistocene   late   glacial   maximum,   ice   probably   occupied   the   entire   cirque   basin,   whereas   today   glaciers   have   retreated  to  positions  along  the  cirque  headwall,  where  snow  avalanches,  wind-­‐blown  snow  from   the  Annapurna  peaks,  and  rock  debris  from  the  steep  bedrock  walls  feed  them  at  elevations  lower   than  the  terminations  of  most  other  glaciers  of  the  eastern  and  central  Himalaya.         3. Methodology   Study  was  started  with  the  working  hypothesis  such  as:  i)  GLOF;  ii)  Rockfall-­‐impounded  lake;  iii)   karst  cave  lake;  iv)  The  rock  avalanche/landslide  trigger;  v)  All-­‐of-­‐the-­‐above  approach  with   multiple  water  sources.  Every  hypothesis  was  evaluated  on  the  basis  of  the  field  observation  and   mapping  and  have  led  us  to  the  conclusions.    

Figure  1:  Google  map  showing  the  location  of  Annapurna  IV  and  the  Seti  river  valley     The   field   work   mainly   consists   of   the   geological   mapping   at   1:25000   scale.   Basic   mappable   lithological   units   were   identified   first   and   lithological   boundaries   were   traced   on   the   topographic    


base map.  Geological  traverses  were  made  mainly  on  the  roads,  rivers  and  foot  trails.  Dilute  HCl   was   used   to   identify   the   carbonates.   Structural   data   (strike   and   dip   of   beds   and   foliation)   were   measured   with   the   help   of   geological   compass   and   plotted   on   the   map.   A   cross-­‐section   was   prepared   across   the   mapped   area.   Representative   samples   were   taken   from   the   bed   rock,   sediments  of  the  Annapurna  Formation,  air  fall  dust,  and  sediment  of  the  Seti  River  flood  plain  for   laboratory  analysis.       Flood   inundation   map   was   prepared   using   the   existing   top   sheets   and   the   detail   topographical   survey  at  1  m  contour  level  and  by  using  software  like  HEC-­‐RAS  and  Arc  GIS.     4.  Results   4.1.  Bedrock  geology  of  the  Seti  River  Basin   A   regional   geological   map   and   cross-­‐section   of   the   Seti   River   Basin   covering   the   area   north   of   Pokhara   was   prepared   in   the   field   (Figs.   2   and   3).   The   lithology   of   the   area   was   separated   into   several  mappable  units  based  on  distinctions  of  lithology,  presence  or  absence  of  fossils,  sediment   consolidation,   and   position   within   the   overall   rock   sequence.   The   lithological   boundaries   observed   along   the   accessible   routes   were   extended   to   the   inaccessible   areas   based   on   orientation   of   the   beds   and   foliation   (strike   and   dip).   The   bedrock   of   the   area   can   be   divided   into   three   tectonic   units,   i.e.,   the   Lesser   Himalaya,   Higher   Himalaya   and   the   Tethys   Himalaya   separated   by   major   regional   faults   namely   the   Main   Central   Thrust   (MCT)   and   Annapurna   Detachment   (AD).   Unconsolidated   materials   are   much   younger   and   include   the   Annapurna   formation  (calcareous  silts,  sands,  and  gravels)  and  Recent  glaciers,  moraines,  debris  flows,  and   alluvial   gravels.   As   we   describe   below,   the   2012   disaster   has   a   strong   involvement   from   the   deep   Seti  River  gorge  and  the  high,  steep  peaks  of  the  Annapurna  Range;  these  are  erosional  features   developed  in  the  rocks  of  the  Tethys  Himalaya  and  Higher  Himalaya.    The  disaster  also  involved   sediment  derived  from  the  Quaternary  age  Annapurna  formation.  Hence,  the  disaster  relates  very   strongly   to   the   geology   of   the   Sabche   Cirque   as   well   as   to   both   ancient   and   extant   glaciers   and   glacial  processes.  A  detailed  report  of  the  geology  and  geomorphology  of  the  Seti  Basin,  due  for   submission  to  a  peer-­‐reviewed  journal,  is  in  preparation.     4.2.  Geology  implications  for  the  Seti  Flood  Disaster  of  May  5,  2012  and  future  hazards   The   main   objectives   of   the   present   study   were   to   evaluate   which   of   the   proposed   working   hypotheses  is  supported  by  geological  data  and  to  access  future  hazard  in  the  Pokhara  valley.       4.2.1.  Implication  for  the  karst  formation   Karst   topography   is   formed   in   easily   soluble   rocks   such   as   carbonates   (limestone   and   dolomites)   and   evaporates   (gypsum   and   salt).   It   is   evident   from   the   present   geological   mapping   that   about   7   km   stretch   of   the   Seti   river   flows   across   carbonate   rocks   (marbles,   calc-­‐schists   and   calc-­‐gneisses)   (See   Fig.   2).   These   rocks   are   mainly   composed   of   calcite   (CaCO3).   Karst   topography   is   very   common   in   the   Pokhara   valley   in   the   carbonate   cemented   terraces.   Mahendra   Cave,   Chamere   Cave  and  Gupteshowr    Cave  are  some  examples.  Therefore,  it  is  quite  possible  that  underground   channels   and   caves   are   present   also   in   the   Seti   River   gorge.     This   would   increase   the   volume  


available to   store   water   when   the   gorge   is   dammed   and   hence   increase   the   potential   flood   volumes  when  the  dam  is  broken.     4.2.2.  Implications  for  the  sediment  source  of  the  2012  hyperconcentrated  slurry  flow   Sedimentological  analysis  shows  that  the  samples  from  the  Annapurna  Formation,  airfall  dust  and   Seti   River   flood   plain   deposits   (recent   sediment   and   ancient   terraces)   have   similar   sedimentological  and      

Fig.  2.  Geological  map  of  the  Seti  River  basin  north  of  Pokhara.  A-­‐B  line  of  cross  section  in  Fig.  4.        


Fig. 3.   Simplified,   schematic   geological   cross-­‐section   across   the   Pokhara   valley.   The   line   of   cross-­‐ section  is  shown  in  Fig.  2.  MCT:  Main  Central  Thrust,  AD:  Annapurna  Detachment  (=South  Tibetan   Detachment  System).     compositional   characteristics.   All   the   samples   contain   rock   fragments   composed   of   marble,   limestone,   calc-­‐gneiss   and   calc-­‐schists.   It   indicates   that   they   have   come   from   the   same   source   (provenance).     Our   interpretation   is   that   the   fine   sediment   contained   in   the   slurry   flow   was   derived   from   the   Annapurna   formation,   which   in   turn   represents   glacial   deposits   laid   down   during   the   Pleistocene,   when   a   huge   glacial   ice   mass   occupied   the   Sabche   Cirque.     Extensive   glacial   erosion   took   place,   enlarging   and   giving   form   to   the   Sabche   Cirque   and   also   generating   abundant  rock  debris,  which  accumulated  in  moraines  and  a  thick  debris  cover  on  the  ice.       During  a  climatic  amelioration,  probably  around  13,000  years  ago,  supraglacial  lakes  formed  on   the  debris-­‐covered  glaciers,  and  these  enlarged  and  merged  into  an  enormous  lake  at  least  5  km   across   and   possibly   over   1000   m   deep.     The   lake   was   dammed   at   the   downstream   side   by   a   thick   accumulation   of   ice-­‐cored   debris.     Glaciers   continued   to   flow   into   the   lake   and   transported   boulders,  sand,  and  silt,  and  moraines  collapsed  into  the  lake  as  ice  gradually  retreated,  causing   huge  debris  flows  and  landslide  deposits  in  the  lake.    This  ancient  mass  of  lake  silt  beds,  moraines,   debris  flow  deposits,  and  landslides—mainly  of  glacial  and  glaciolacustrine  origin—are  what  now   comprises  the  Annapurna  formation.    Outbursts  from  the  lake  and/or  the  Annapurna  sediments   produced  the  terraced  deposits  of  the  Pokhara  Valley.    These  outbursts  as  well  as  steadier  glacier   meltwater  discharge  also  eroded  the  deep  gorge  near  the  exit  from  the  Sabche  Cirque.    Erosion  of   the  gorge  was  both  due  to  dissolution  of  carbonates  and  mechanical/hydraulic  erosion  due  to  the   Seti  River  and  its  tributaries.        


Rock debris  eroded  and  transported  by  small  residual  glaciers,  rockfalls,  and  avalanches—such  as   the   rock/ice   avalanche   of   May   5,   2012—continue   to   add   masses   of   sediment   within   the   Sabche   Cirque.  Remnants  of  the  Annapurna  formation  as  well  as  the  younger  glacial  deposits  and  mass   movement   materials   still   tower   in   unstable   relief   and   remain   geomorphologically   very   active,   producing   many   small   debris   flows   and   floods.     Most   of   these   are   contained   within   the   Sabche   Cirque   and   have   no   consequences   to   people   downstream.   The   largest   of   the   floods   and   mass   movements   could   travel   as   far   as   Pokhara   and   thus   have   a   potential   to   produce   tragic   consequences  in  the  Seti  Valley  below,  as  residents  unfortunately  discovered  on  May  5,  2012.       4.2.3.  Implications  for  the  water  source  of  the  2012  hyperconcentrated  slurry  flow   Large   single   events   of   anomalous   monsoon   rainfall   could   produce   up   to   107   m3   or   even   2x107   m3   of  water  runoff,  which  could  be  discharged  in  as  little  as  a  day  from  the  Sabche  Cirque  and  may   produce  peak  flows  of  100-­‐400  m3/s.    The  May  5,  2012  event  was  not  a  monsoonal  event  and  not   due  to  any  kind  of  precipitation  event,  and  anyway  it  produced  brief  but  highly  peaked  discharges   exceeding  1000  m3/s.      The  behavior  of  the  hyperconcentrated  slurry  flood  is  such  that  sudden   release   from   a   natural   water   reservoir   must   have   taken   place.     We   have   considered   four   candidates   for   a   reservoir:   (1)   in   or   on   the   glacier   ice,   (2)   inside   caves   in   the   Annapurna   formation,   (3)   inside   caves   within   the   high-­‐grade   metamorphic   rocks   of   the   Tethyan   Group   or   Higher   Himalayas,   or   (4)   inside   the   gorge.     We   have   found   evidence   that   all   of   these   types   of   spaces   may   exist   in   the   Sabche   Cirque   and   its   outlet   gorge,   any   of   which   could   remain   a   factor   for   future   impoundments   of   water   and   potential   outburst   floods.     However,   six   clues   point   to   the   gorge  as  the  main  reservoir  and  hence  indicate  a  blockage  and  then  a  sudden  unblockage  of  the   gorge   as   the   primary   cause   of   the   outburst.     Next   we   summarize   these   clues   and   what   they   imply   about  the  sequence  of  events  that  led  to  the  disaster.     1.  Water  flow  in  the  Seti  River  was  virtually  cut  off  to  Pokhara  and  the  upstream  villages  in  the   days   and   weeks   before   the   disaster,   according   to   many   eyewitness   reports.     This   implies   a   nearly   complete   stream   blockage.     The   blockage   must   have   been   below   the   point   where   the   major   tributaries  within  the  Sabche  Cirque  join  but  above  the  upstream  villages  where  the  diminished   (almost   zero)   flow   was   observed.     This   constrains   the   point   of   blockage   to   a   short   segment   of   the   Seti   River.     Had   the   blockage   been   above   the   point   where   the   two   major   tributaries   (the   north   branch  and  west  branch)  join  in  the  gorge  area,  perhaps  half  (more  or  less)  of  the  water  would   have   been   blocked   but   much   flow   would   have   continued.       This   first   clue   tends   to   rule   out   the   glacier  ice  and  the  Annapurna  formation  as  hosts  of  the  reservoir,  but  it  allows  the  gorge  and  also   could  allow  any  karstic  cavernous  spaces  connected  to  the  gorge  that  could  have  been  filled  due   to  damming  of  the  gorge.       2.  When  the  Seti  River  was  blocked,  a  trickle  continued  but  changed  color  from  the  usual  white   caused   by   abundant   suspended   rock   flour.   (“Seti”   means   “white,”   so   it   is   the   White   River)     Hence,   water   draining   from   the   glaciers   and   from   the   Annapurna   formation   was   blocked,   leaving   only   small  amounts  of  water  runoff  from  points  below  the  glaciers  and  the  Annapurna  formation.    This   again  points  to  a  blockage  in  the  gorge  area  below  the  Annapurna  formation  and  below  where  the   two  major  tributaries  join.   3.  The  ice/rock  avalanche  from  Annapurna  IV  triggered  the  outburst,  and  to  have  done  so  there   should   be   a   direct   pathway   from   the   avalanche   to   the   reservoir.     The   main   glacial   lakes   and   drained   ice   basins   we   have   observed   (Fig.   Y)   are   west   of   where   the   avalanche   impacted   and   traversed,   and   so   a   glacier   lake   outburst   flood   appears   improbable   as   the   cause   of   the   2012     6    

disaster. We   have   traced   the   route   of   the   avalanche   to   the   gorge,   and   so   again   the   gorge   is   implicated  as  the  most  likely  reservoir.   4.   Satellite   observations   have   definitively   identified   several   discrete   erosional   events   along   the   walls   of   the   gorge.     Our   observations   via   satellite   and   helicopter   have   identified   the   biggest   of   these  as  a  site  of  recurrent  rockfalls  into  the  gorge.       5.  Helicopter-­‐borne  observations  show  a  white  sediment  staining  or  covering  of  the  walls  of  the   gorge  consistent  with  it  having  contained  a  sediment-­‐laden  reservoir  in  the  gorge.     6.   Our   observations   have   indicated   that   the   gorge   volume   is   far   greater   than   we   had   initially   believed,   and   so   it   could   have   contained   enough   water   and   sediment   volume   to   have   explained   the  slurry  flood  disaster.    The  gorge  is  both  wider  in  some  sections  and  far  deeper  (exceeding  500   m)  than  we  had  suspected  at  first.    The  gorge  geometry  is  such  that  a  contained  volume  of  more   than   107   m3   is   possible,   i.e.,   enough   to   explain   the   2012   outburst   flood   volume.       Furthermore,   there   remains   some   speculation   with   some   limited   supporting   observations   that   the   gorge   geometry  may  widen  at  the  bottom  with  karst  cave-­‐like  structures,  which  may  add  to  the  present   estimations   of   contained   volume.     Consequently,   our   earlier   assessment   that   the   flood   water   volume   required   multiple   sources   is   no   longer   a   requirement.     The   gorge   alone—with   or   without   additional   water-­‐filled   karstic   caverns—might   be   sufficient   to   explain   the   flood   volume.     The   rockfall-­‐dammed  gorge  hypothesis  is  thus  the  simplest  and  likeliest  explanation.       4.3.  Implication  for  future  hazards   4.3.1.  Landslide,  rock  fall,  and  rock  avalanche     The  bedrock  in  the  upper  Seti  Basin  are  quite  unstable  due  to  the  steep  slopes  and  high  relief  and   the  known  history  of  large  and  small  mass  movements.  Huge  rock  sliding  along  the  bedding  plane   (plane  failure)  is  quite  common  in  the  area.  Freeze  and  thaw  action  of  water  is  playing  significant   role   in   the   failure   of   slopes,   including   failure   according   to   knickpoint   theory   (REF).     Large   rock   sliding   along   the   gorge   wall   of   the   Seti   River   may   cause   frequent   damming   of   the   Seti   River.   Similar  flash  floods  are  thus  possible  in  the  future  by  the  failure  of  the  landslide  dam.     4.3.2.    Ice  avalanche   Ice  from  a  cornice  was  apparently  involved  in  the  May  5,  2012  avalanche.    Hanging  glaciers  are   also  present  high  on  the  walls  of  the  Sabche  Cirque.        Collapsing  ice  could  impart  enough  energy   to   the   unconsolidated   sediment   of   the   Annapurna   formation   to   generate   a   large   sediment   mass   movement.    An  ice  avalanche  into  a  glacial  lake  could  generate  a  GLOF.       4.3.3.  Debris  flow  hazard  by  liquefaction     The   Annapurna   Formation   is   loose   sediment   of   about   500-­‐600   m   thick   (more   in   some   areas)   covered  in  the  upper  part  by  glaciers  and  kettle  lakes.  In  the  spring  and  summer,  huge  amounts  of   ice  and  snow  are  melted  and  this  melt  water  saturates  the  sediments,  potentially  weakening  the   cohesion   of   the   sediment.   Strong   monsoon   rains   could   likewise   saturate   the   sediment.     Many   debris   flows   are   observed   in   the   Annapurna   formation,   so   we   know   that   sediment   flow   is   a   frequent  occurrence.    The  recent  debris  flows  we  have  observed  are  relatively  small,  and  events   of  that  magnitude  do  not  pose  a  threat.    However,  if  a  strong  earthquake  occurs  at  this  situation,   or  if  a  large  discrete  monsoonal  rain  event  adds  onto  already-­‐saturated  conditions,  the  sediment  


may liquefy  from  a  large  area  and  flow  downstream  and  potentially  cause  a  huge  disaster  in  the   Pokhara  valley.       4.3.4.  Glacier  lake  outburst  flood   We  have  discovered  a  substantial  ice  basin  which  could  impound  several  million  cubic  meters  of   water   if   not   for   the   fact   that   the   basin   is   breached   already.     We   also   see   indications   of   rapid   drainage  downstream  from  it  in  recent  years.    It  would  appear  that  glacier  lake  outburst  floods   (GLOFs)   have   occurred   from   the   glaciers   in   the   Sabche   Cirque,   but   their   hydrograph   peak   discharge   magnitudes   were   apparently   too   small   to   be   noticed   downstream.     We   have   some   concern   about   these   as   possible   future   hazards   if   the   ice   basin   should   become   closed   and   lakes   should   grow   and   a   GLOF   of   larger   magnitude   should   occur   or   if   sediment   from   the   Annapurna   formation  should  be  ingested  into  a  GLOF  and  a  large,  fluid  debris  flow  should  form.    Hence,  there   should  be  satellite-­‐based  or  aircraft-­‐based  monitoring  of  glacial  lakes  in  the  Sabche  Cirque.         5. Socioeconomic/demographic  survey  of  the  disaster’s  impacts     To  investigate  the  socio-­‐economic  status  in  flood-­‐affected  area  of  Seti  river  basin  in  Pokhara  we   carried  out  a  socio-­‐economic  field  survey  in  the  downstream  affected  areas  of  the  Seti  river  basin   from   the   end   of   the   December   2012   to   first   week   of   January   2013.   Similarly,   flood   inundation   mapping  of  Seti  River  was  also  conducted  in  this  period.  The  results  are  summarized  here.     The   most   of   the   riverbank   dwellers   are   migrants   and   have   low   ability   and   low   propensity   to   purchase   safer   lands   for   settlements   in   urban   area.   Since   labor   was   the   main   occupation   of   majority   of   the   affected   population,   their   rent   paying   capacity   would   be   lower.     Therefore,   they   settled   along   the   marginal   public   lands   without   caring   about   the   risk   of   flood   havoc   or   in   a   calculated  gamble  knowing  that  there  is  some  risk.  More  than  90  percent  of  households  had  prior   knowledge   about   probable   risks   in   the   settlement.   They   had   seen   mud   in   the   river   water   without   heavy   rain.   However,   they   were   busy   in   eking   out   their   livelihoods   and   became   less   careful   about   the  risk  and  stayed  at  their  own  dwellings.     The   river   mapping   focuses   on   producing   a   flood   inundation   map,   overlaying   the   map   prepared   from   the   survey   data   so,   that   the   scenario   of   the   Seti   River   and   its   periphery   can   be   visualized,   and   possible   future   risks   to   nearby   areas   can   be   better   assessed.   Ultimately,   the   hazardous   and   vulnerable   zones   by   the   river   are   depicted.   Furthermore,   we   have   suggested   precautions   and   remedies  that  could  be  undertaken  to  mitigate  future  floods,  hyperconcentrated  slurry  flows,  and   debris  flows  in  this  and  nearby  valleys.       We   have   identified   several   villages   that   are   exceptionally   prone   to   being   swept   away   by   floods.    We   have   withheld   naming   them  pending   detailed   study   so   as   to   avoid   a   possible   panic   response   when   our   findings   are   still   preliminary   and   based   on   a   quick   assessment.     In   one   village   more  than  fifty  houses  are  clustered  together  and  about  fifteen  houses  on  the  banks  in  clear  and   immediate  danger  of  being  swept  away  by  possible  floods.  In  another  village  a  school  along  with   the  school  children  and  teachers  as  well  as  residents  are  in  immediate  jeopardy  of  a  flood  due  to  a   GLOF  or  a  monsoon-­‐driven  flood  or  a  landslide;  in  June  2012  already  a  landslide  buried  part  of   that  village  but  amazingly  nobody  was  killed.    Residents  in  these  and  other  villages  can  be  swept   away   at   any   time;   therefore   there   is   an   urgent   need   for   developing   and   implementing   suitable   tools  and  procedures  for  forecasting  and  real-­‐time  warning  of  flash  floods  and  debris  flows.  


A fuller   report   on   these   aspects   of   our   work   is   in   preparation   and   will   be   submitted   to   a   peer-­‐ reviewed  journal.    A  PhD  dissertation  on  this  topic  has  been  initiated.     6.  Recommendations   The  2012  disaster  is  not  likely  to  be  replicated  exactly,  but  something  similar  is  likely.    To  protect   the   Pokhara   residents   from   future   geological   hazards   comparable   to   the   2012   event,   or   potentially  worse,  the  following  are  recommended.   (1) Landslide,  rockfall,  and  debris  flow  mapping  of  the  Seti  River  basin.   (2) Mapping  of  the  landslide  and  rockfall  hazards  (potential  for  future  events)   (3) Studies   of   the   monsoon   precipitation   regime   and   extreme   weather   (temperature   and   precipitation)  with  effects  on  runoff.   (4) Studies  of  the  earthquake  or  precipitation-­‐driven  liquefaction  potential  assessment  of  the   Annapurna  formation.     (5) Modeling   of   flow   peak   discharge/flood   level/inundation   for   floods,   hyperconcentrated   slurries,  and  debris  flows  arising  from  the  Sabche  Cirque.   (6) Design   of   a   warning   system   perhaps   involving   SMS   text   messaging.     The   residents   and   officials   receiving   any   warnings   should   be   trained   in   how   to   respond   if   an   anomalous   situation  is  observed.    Residents  will  have  to  be  a  part  of  the  system,  so  it  will  be  important   to   engage   with   the   residents   by   consulting   with   them   during   the   system   design   and   implementation   and   in   educating   them   on   the   nature   of   the   hazard   environment   and   training  them  in  the  use  iof  the  warning  system.   (7) Discussion   with   city,   district,   and   national   officials   about   land-­‐use   and   demographics   in   relationship  to  the  2012  disaster  and  remaining  hazards.       7. Acknowledgements     This  work  was  supported  by  the  USAID  Climate  Change  Resilient  Development  (CCRD)  Project   (Grant  Number  CCRDCS0009)  and  by  the  NASA/USAID  Science  of  Terra  and  Aqua  Program.       8. References       Fort,   M.,   1987,   Sporadic   morphogenesis   in   a   continental   subduction   setting:   an   example   from   the   Annapurna  Range,  Nepal  Himalaya.Zeitschr.  Geomorphology,  Suppl.  V.  63,  pp.    9-­‐36.   Koirala,   A.   and   Rimal,   L.N.,   1996,   Geological   hazards   in   Pokhara   Valley,   western   Nepal.-­‐J.   Nep.   Geol.  Soc.,  13:  99-­‐108.     Koirala,  A.,  Rimal,  L.N.,  Sikrikar,  S.  M.,  Pradhananga,  U.  B.  and  Pradhan,  P.   M.,   Hanisch,  J.,  Jäger,  S.,   Kerntke,   M.,   1996:   The   engineering   and   environmental   geological   map   of   the   Pokhara   Valley  1:50.000,  DMG/BGR-­‐Project,  Dept.Mines,Geology,  Kathmandu.   Harris,  N.  And  Whalley,  J.,  2001.  Mountain  building.  Block  4.  The  Open  University,  UK,  165p.   Jackson,   M.,   and   R.   Bilham,   1994.  Constraints   on   Himalayan   Deformation   inferred   from   Vertical   Velocity  Fields  in  Nepal  and  Tibet,      J.  Geophys.  Res.,  99(B7),  13897-­‐13912.   Mattauer,   M.,   1989.   Monts   et   Merveilles,   Beautes   et   Richesse   de   la   Geologie.   Hermann   Editeurs   des  Sciences  et  des  Art,  Paris,  267p.      


Shrestha, A.B.,   P.   Mool,   J.   Kargel,   R.B.   Shrestha,   Samjwal   Bajracharya,   Sagar   Bajracharya,   and   D.   Tandukar,   2012,   Quest   to   unravel   the   cause   of   the   Seti   flash   flood,   5   May   2012,   online   report  posted  by  ICIMOD.   Yamanaka,   H.   and   Iwata,   S.,   1982,   River   terraces   along   the   middle   Kali   Gandaki   and   MarsyandiKhola,  Central  Nepal.  J.  Nepal  Geol.  Soc.,  v.  2,  pp.  95-­‐111.    


Dhananjay Regmi: Causes and human impacts of the Seti River (Nepal) disaster of 2012  

On May 5, 2012, a hyperconcentrated slurry flood in the Seti River suddenly burst forth onto a small village and rural areas in the valley u...

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