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Investigation of  the  Seti  River  disaster  (May  5,  2012)     and     assessment  of  past  and  future  mountain  hazards  facing   Pokhara,  and  upstream  communities

Dhananjay Regmi  (Himalayan  Research    Center)   Jeffrey  Kargel  and  Gregory  Leonard  (University  of   Arizona)   Lalu  Poudel,  Krishna  KC,  Khagendra  Poudel  and   Anusha  sharma  (Tribhuvan  University,  Nepal)   Teiji  Watanabe  and  Bhabana  Thapa  (Hokkaido   University,  Japan)  


SeV river  outburst  flood  disaster,  May  5,  2012   Annapurna  IV  

Kill zone,  72  dead  and  missing   Hundreds,  perhaps  thousands,   more  would  have  been  killed   if  flood  had  been  2-­‐3  m  deeper  

City of  Pokhara  is  built  on   Holocene/Recent  outburst-­‐     flood/debris-­‐flow  deposits  


Kill zone,  72  dead  and  missing  

Fig. 1: Location of Seti River Flood


Flood Pictures  


Amateur You-­‐tube  video/Google  Earth  analysis   to  assess  flood  discharge  rate  and  volume  

~ Videographer  locaVon  


Flow speed  4.1  m/s  

Flow depth  3.25  m  

Water depth  =  7.75  -­‐  4.5  m  =  3.25  m  (+-­‐  0.8  m)  


Se< river  outburst  flood  discharge  rate   and  volume  calcula<on   •  Flood  discharge  measured  at  Pokhara  from  amateur   You-­‐tube  and  Google-­‐Earth  imagery   •  Flow  speed  4.1  +-­‐  0.6  m/s   •  Flow  depth  3.25  +-­‐  0.8  m   •  Flow  width  87  m  +-­‐  2  m   •  Discharge  rate  =  1160  m^3/s  +-­‐30%   •  High  discharge  probably  lasted  >  200  seconds  (mul<ple   video  analysis)   •  Total  discharge  >  230,000  m^3  (assumes  200  seconds)   •  Equivalent  to  cylinder  124  m  diameter,  19  m  deep  


Se< river  outburst  flood  dischahrge   rate  and  volume  calcula<on   •  Flood  discharge  measured  at  Pokhara  from  amateur   You-­‐tube  and  Google-­‐Earth  imagery   •  Flow  speed  4.1  +-­‐  0.6  m/s   •  Flow  depth  3.25  +-­‐  0.8  m   •  Flow  width  87  m  +-­‐  2  m   •  Discharge  rate  =  1160  m^3/s  +-­‐30%   •  High  discharge  probably  lasted  >  200  seconds  (mul<ple   video  analysis)   •  Total  discharge  >  230,000  m^3  (assumes  200  seconds)   •  Equivalent  to  cylinder  124  m  diameter,  19  m  deep  


Few seconds  before  the  flood  hits  the    Kharapani  


View of  Kharapani  just  a]er  Flood  


The first  two  of  27  waves  of  water  and  hyper  concentrated  slurry  involved   roughly  a  quarter  million  cubic  meters  of  water  each.    The  floods  hit  fast   and  hard  for  those  in  the  way,  and  lasted  a  few  minutes  per  wave,  reaching  over   10  m  deep  at  this  loca<on..  

Survivors lived  on  next  higher  terrace  

Many of  those  killed  lived  on  the  lowest  terrace  


Key ques<ons  to  Address   1.  What  was  the  cause  of  the  Se<  river  disaster?         2.  Is  another  similar  flood  likely?       3.    What  role  might  imprudent  habita<on  have  played   in  raising  the  death  toll?       4.  What  other  types  and  magnitudes  (e.g.,  peak  flows)   of  floods  are  possible  in  the  future?     5.  How  large  a  popula<on  remains  vulnerable?      


Hypothesized ideas   Ø a.  normal  GLOF!     Ø b.  Rockfall-­‐impounded  lake   Ø C.  The  karst  model.     Ø d.  Rock  avalanche/landslide  trigger.     Ø e.  All-­‐of-­‐the-­‐above  (mulCple  sources).    


Normal GLOF  ?  

•  The flood  could  not  have  been  a  usual  type  of   GLOF  (Glacier  Lake  Outburst  Flood),  because   no  such  precursor  lake  existed  in  the  basin.    


Imja Lake  


Small ponds  below  Annapurna  IV  


Rockfall-­‐impounded lake  ?   •  It  was  observed  from  satellite  repeat  imaging,   that  a  modest  rock  fall  occurred  into  the  Se<   gorge  between  2002  and  2008,  and  was   reac<vated  a  few  weeks  prior  to  the  disaster.      

Reac<vated about   few  weeks  before   May  5  disaster  


1. CONDITIONING  EVENT   Rockfall  dam  emplaced  in  gorge,   repeated  episodes,  between  2002-­‐2008,   again  in  2012  


2. CONDITIONING  EVENT   Impoundment  lake  forms  


c. Working  hypothesis  #1:  The  karst   model.     •  The  rock  fall  dam  outburst  model  was  soon   modified,  and    includes  the  possibility  that  not   only  the  gorge  could  contain  a  lake,  but   possibly  karst  caverns  in  those  same  rocks   could  have  been  water  filled  and  may  have   been  dammed.      


Small cave  in  marble  cliff,     about  2  meters  high  


d. Rock  avalanche/landslide  trigger.     Maximov  Brown  Clouds  (  AVIA  CLUB)  

Mr. Maximov  Picture  


Annapurna Himal,  Nepal:    Machapuchre  -­‐  Se<  Gandaki  Basin  

ETM+ pan-­‐321-­‐RGB    (20  April  2012–  15  days  before  the  disaster)  


Annapurna Himal,  Nepal:    Machapuchre  -­‐  SeV  Gandaki  Basin  

Debris avalanche,   ~4  km2   Avalanche   airfall  deposits,   ~10  km2  

Avalanche   source   Ice  +  debris   avalanche,   ~2  km2  

ETM+ pan-­‐321-­‐RGB    (06  May  2012–  1  day  a]er  the  disaster)  


3. INITIATING  EVENT   May  5,  2012,   rock  slide  from   Annapurna  IV,   ~10  M  m3  

Dhananjay Regmi: Seti River disaster and risks facing Pokhara, Nepal (part 1 of 3)  

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

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