How to establish operational recommendations to ...

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Should the inner shore of the Arcachon Bay (Gironde, France) be depolderised .... counting the number of walkers, making an inventory of hunting grounds, data ...
How   to   establish   operational   recommendations   to   manage   sea  flooding  risk  on  a  reclaimed  coast?     Should  the  inner  shore  of  the  Arcachon  Bay  (Gironde,  France)  be  depolderised   or  should  the  dykes  be  maintained  ?   Goeldner-­‐Gianella  Lydie1,  Bertrand  Frédéric2   1.  Géographie,  UMR  8586  PRODIG,  Université  Paris  1.        191  rue  Saint  Jacques  75  005  PARIS.   Lydie.Goeldner@univ-­‐paris1.fr   2.  Géographie,  UMR  8586  PRODIG,  Université  Paris  4.        191  rue  Saint  Jacques  75  005  PARIS.   frederic.bertrand@paris-­‐sorbonne.fr         ABSTRACT.   The   BARCASUB   program   dealt   with   coastal   flooding   risk   in   the   Arcachon   Bay   (France)   and   with   the  

possible   management   of   this   risk   through   a   soft   defense   measure   :   "de-­‐polderisation".   The   article   doesn't   describe  the  whole  aspects  of  the  program.  It  only  presents  the  operational  methodology  developed  to  establish   proposals   of   flooding   risk   management,   such   as   de-­‐polderisation   in   particular,   compared   to   doing   nothing,   conserving   the   dikes   or   reinforcing   them.   We   detail   the   steps   that   we   have   followed   :   establishing   firstly   a   gradient  of  eligibility  for  depolderisation  between  the  management  units  of  the  studied  polders  (2)  and  doing   then,  for   the   most   eligible   sectors,   a   cost-­‐benefit  analysis  of  each  possible  method  of  flooding  risk  management   (3),   in   order   to   obtain   distinguished   management   recommendations   for   each   polder   (4).   This   methodology   seems   reproducible,   provided   a   strong   interdisciplinary   work   is   conducted   to   obtain   both   physical   and   social   data  or  quantitative  and  qualitative  data,  all  needed  for  the  evaluation.       RESUME.   Le   programme   BARCASUB   a   porté   sur   le   risque   de   submersion   marine   à   l'intérieur   du   bassin   d'Arcachon   (France)   et   sur   la   gestion   potentielle   de   ce   risque   par   une   mesure   souple   de   défense   :   la   "dépoldérisation".   La   communication   n'évoque   pas   la   totalité   des   aspects   traités   dans   le   programme,   mais   expose   la   méthode   opérationnelle   que   nous   avons   développée   pour   établir   des   préconisations   de   gestion   du   risque   de   submersion   passant   en   particulier   par   la   dépoldérisation,   que   nous   avons   comparée   au   laisser-­‐faire,   à   la   conservation   des   digues   ou   à   leur   rehaussement.   Nous   détaillons   les   étapes   que   nous   avons   suivies   :   établissement  d'un  gradient  d'éligibilité  à  la  dépoldérisation  entre  les  casiers  des  polders  étudiés  (2)  et  analyse   coûts/avantages   de   chaque   mode   possible   de   gestion   du   risque   pour   les   casiers   les   plus   éligibles   (3),   afin   d'aboutir  à  des  préconisations  différenciées  de  gestion  selon  les  polders  étudiés  (4).  Cette  méthodologie  paraît   reproductible,  à  condition  de  travailler  en  forte  interdisciplinarité  pour  obtenir  des  informations  tant  physiques   et  sociales  que  quantitatives  et  qualitatives,  toutes  nécessaires  à  l'évaluation.     KEYWORDS  :  risk  management,  marine  flooding,  de-­‐polderisation,  assessment,  issues,  exposure,  costs.     MOTS-­‐CLÉS  :  gestion  du  risque,  submersion  marine,  dépoldérisation,  évaluation,  enjeux,  exposition,  coûts.

 

 

1.  Introduction   The  BARCASUB  Research  Programme  (2010-­‐13)  dealt  with  the  risk  of  sea-­‐flooding  in  the  Arcachon   Bay  (France)  and  potential  management  of  this  risk  through  use  of  the  "depolderisation"  technique   (Bertrand   and   Goeldner-­‐Gianella,   2013).   This   project   brought   five   teams   together   –   the   research   laboratories   PRODIG,   LGP,   EPOC,   the   BRGM   and   the   EA   PRISM   –   and   four   local   managers   or   stakeholders.   Traditionally   in   France,   the   risk   management   techniques   and   sea-­‐flooding   prevention   have   consisted   of   heavy   defences   taking   the   form   of   embankments   or   reinforcement   of   the   dykes   that  protect  the  polders.  There  is  another,  more  flexible,  method  of  risk  management  consisting  in   reopening   the   polders   to   the   sea,   by   using   locks   or   creating   breaches   in   the   dykes,   or   even   dismantling   a   dyke.   This   depolderisation   generally   translates   in   the   rapid   restoration   of   saltmarsh,   through   the   effects   of   sedimentation.   In   North   America,   as   well   as   in   north-­‐western   Europe,   such   natural   environments   are   perceived   ever   more   frequently   as   infrastructure   in   terms   of   defence   against   sea   incursion   (French,   2006)   and   of   adaptation   to   climate   change.   Increased   attention   is   thus   being   paid   to   these   saltmarshes,   their   extent   and   plant   density   being   positive   related   to   the   attenuation  of  the  energy  of  the  swell  and  height  of  the  waves,  as  well  as  with  the  stabilisation  of  the   coastline (Shepard  et  al.,  2011).  It  should  therefore  be  possible,  where  the  saltmarsh  is  sufficiently   extensive,   to   limit   the   build-­‐up   of   dykes.   This   solution   should   be   of   great   economic   interest   in   the   context  of  climate  change,  as  the  British  have  well  understood  (Goeldner-­‐Gianella,  2013).     If   depolderisation   is   not   mentioned   or   implemented   to   any   great   extent   in   France,   the   current   scientific   and   political   context   would   seem   to   be   increasingly   favourable   to   its   implementation.   Consequently,   BARCASUB   was   created   following   a   consultation   launched   nationally   by   the   Conservatoire  du  littoral  at  the  beginning  of  the  twenty-­‐first  century  concerning  the  adaptation  of  its   properties   to   climate   change.   It   was   storm   Xynthia   that   highlighted   the   dangers   of   sea   flooding   in   France  and  this  translated  into  the  publication  of  a  national  coastline  management  strategy  (2012)   that   evoked   the   mobility   of   the   coastline   and   more   flexible   management   techniques,   such   as   the   restoration   of   the   ecosystems.   This   strategy   is   also   the   basis   for   experimentation   with   strategic   withdrawal  at  pilot  sites.  Finally,  these  events  promoted  the  development  of  an  operational  debate   about   depolderisation,   especially   along   the   coastline   of   the   reclaimed   Bas-­‐Champs   of   Picardy   (Bawedin,   2013).   At   regional   level,   the   authorities   in   Aquitaine   (such   as   the   "GIP   Littoral   aquitain")   who   are   very   aware   of   the   coastal   risks,   launched   a   regional   management   strategy   for   the   coastal   strip   and   are   participating   in   the   national   strategic   withdrawal   experiment.   Finally,   on   a   local   level,   the   Arcachon   Bay   has   been   hit   in   recent   years   by   numerous   storms,   resulting   of   accidental   depolderisations.  The  owner  of  the  sites  affected,  the  Conservatoire  du  littoral,  is  interested  in  this   innovative  management  method  since  it  finds  itself  having  to  deal  with  the  increasing  costs  of  dyke   maintenance  (Clus-­‐Auby  et  al.,  2006).  Finally  the  current  context  of  having  to  establish  the  first  local   sea-­‐flooding  risk  prevention  plans  around  the  Arcachon  Bay  has  proved  very  sensitive.   The  BARCASUB  project  team  wondered  whether  it  were  physically  possible  to  use  depolderisation   as  a  risk  management  method  to  prevent  sea  flooding  in  Arcachon  Bay  and  whether  this  would  be   considered  socially  acceptable  and  economically  advantageous.  Our  paper  will  not  discuss  all  of  the   aspects   dealt   with   under   this   programme   –   such   as   an   assessment   of   the   local   sea-­‐flooding   hazard   and   the   inland   expansion   of   submersions,   the   role   played   by   a   depolderised   marsh   ecosystem   in   attenuating  the  risk  of  sea  flooding  and  social  protest  against  sea-­‐flooding  and  depolderisation  (see   Goeldner-­‐Gianella  et  al.,  2013).  We  shall  discuss  here  the  operational  method  we  developed  to  make   recommendations   for   managing   the   risk   of   flooding   from   the   sea,   in   combination   with   possible   depolderisation.   The   method   was   devised   for   three   polders   on   the   south-­‐eastern   shore   of   the    

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Arcachon  Bay  (Certes,  Graveyron,  Malprat)  which  were  originally  built  in  the  eighteenth  century  for   fish-­‐farming   purposes.   The   Conservatoire   du   Littoral   acquired   these   polders   starting   from   1984,   to   preserve  their  natural  and  cultural  heritage  and  open  them  to  the  public.  The  cost  of  repairing  the   dykes   as   a   result   of   the   storms   and   flooding   in   1999,   2009   (Klaus)   and   2010   (Xynthia)   exceeded   €700,000   (Bertrand   and   Goeldner-­‐Gianella,   2013)   and   could   further   increase   in   the   future   due   to   increased  submersion  of  these  polders  (Anselme  et  al.,  2008).  Consequently,  the  Conservatoire  has   contemplated  whether  to  allow  the  sea  to  flood  some  of  the  polders,  or  at  least  to  stop  maintaining   some   of   the   dykes.   Thus,   it   recently   abandoned   to   the   sea   the   "Pointes"   of   the   polders   of   Graveyron   and   Malprat   that   had   accidentally   been   breached.   The   method   presented   here   concerns   the   three   above-­‐mentioned   polders,   which   are   broken   down   into   internal   "basins"   ringed   by   low   dykes   or   channels  and  edged  by  dykes  of  soil  widely  seeded  with  ripraps  and  sometimes  separated  from  the   sea  by  a  saltmarsh  dotted  with  hunting  lakes  (figures  1  and  2).   2.  Establishing  a  scale  of  eligibility  for  depolderisation  in  view  of  the  risk  of  sea-­‐flooding     In  order  to  establish  management  recommendations  for  the  three  polders  in  question,  we  initially   constructed   a   scale   of   eligibility   for   depolderisation   in   view   of   the   flooding   risk   to   the   internal   “management   units”   for   these   polders   (units   determined   in   the   polder   management   plans).   A   dual   assessment   –   that   of   the   factors   involved   in   exposure   to   sea-­‐flooding   hazard   (2.1)   and   that   of   the   issues  constituted  by  each  polder  (2.2.)  –  made  it  possible  to  create  this  scale  (2.3).   2.1.   Assessing   the   factors   of   exposure   to   sea-­‐flooding   hazard   and   the   degree   of   exposure   of   management  units     In   assessing   the   factors   of   exposure   to   sea-­‐flooding   hazard   that   would   involve   damaging   the   polders   and   their   management   units,   we   took   into   account   the   natural   sea   defence   infrastructure,   i.e.,   the   saltmarshes   alongside   the   dykes,   as   well   as   the   condition   of   the   artificial   defence   infrastructure,  i.e.  the  condition  of  the  dykes  themselves,  and  finally  the  topography  of  the  basins  in   relation   to   their   degree   of   submergibility.   Based   on   very   carefully   spatialised   information   scaled   to   the  size  of  the  management  units  or  sections  of  dykes,  we  assessed  the  respective  importance  of  the   mentioned   exposure   factors:   factor   absent   or   only   faintly   present   (marked   0),   with   an   average   presence  (marked  1)  or  strongly  present  (marked  2).     The  first  damage  factor,  by  order  of  intervention  in  the  course  of  flooding,  concerns  the  condition   of  the  saltmarsh  in  view  of  the  generalised  morphological  disintegration,  largely  due  to  the  creation   of  the  hunting  lakes.  Two  indicators  made  it  possible  to  define  the  condition  of  the  saltmarsh  and  the   degree   of   anthropic   pressure   exercised   on   this   environment,   despite   the   regulation   of   hunting   in   the   marine   public   domain:   the   relative   area   of   the   hunting-­‐lakes   in   proportion   to   the   total   area   of   the   saltmarsh  –  reduced  to  the  linear  measurement  of  the  frontal  dyke  for  each  unit  –  and  the  speed  at   which   the   edge   of   the   saltmarsh   retracted   between   2000   and   2009   also   compared   with   the   adjacent   management   unit.   The   second   factor   involved   in   the   progress   of   flooding   –   through   overflowing,   rising   higher   than   or   breaching   the   dykes   –   was   their   state   of   repair.   Irregular   maintenance   of   sea   dykes   in   fact   explains   why   the   crest   of   the   dykes   are   all   built   to   different   heights,   as   is   shown   by   topographical  elevations  and  LIDAR  data.  Such  irregularities  can  produce  increased  effect  of  overflow   at  the  lowest  points  of  each  crest,  thus  favouring  the  acceleration  of  waves  crashing  against  the  inner   side  of  the  dyke  and  the  tearing  out  of  entire  sections  of  dyke,  which  in  itself  is  liable  to  breaching,   especially   in   sections   that   are   susceptible   to   external   erosion.   The   state   of   the   dykes   was   thus    

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differentiated   on   the   basis   of   the   average   height   of   the   sections   of   each   unit.   The   last   factor   to   be   considered   is   the   topography   of   the   management   units   on   which   flooding   is   propagated.   The   conversion   to   MNT   of   the   LIDAR   data   makes   it   possible   to   determine   the   height   reached   by   the   potential  flooding,  resulting  from  the  propagation  of  the  tidal  wave  that  moves  closer  and  closer  to   the   inside   of   the   polders   as   the   water   levels   reach   4.69   m   IGN   -­‐   an   extreme   water   level   combined   with  wave  and  wind  conditions  of  storm  Klaus  and  a  tide  with  a  coefficient  of  95  taking  account  of  a   sea  level  of  +0.25  cm  based  on  the  worst  case  scenario  adopted  by  the  ONERC  in  2030.   The   addition   of   the   three   exposure   factors   provides,   for   each   management   unit,   a   degree   of   exposure  to  submersion,  whether  it  be  slight  (marked  between  0  and  2),  average  (3  or  4)  or  high  (5   or  6).  The  sectors  the  most  exposed  to  submersion  can  thus  be  found  in  the  parts  downstream  of  the   polders,  especially  at  their  Pointes  corresponding  to  Branne  and  Brèche  units  (fig.  1).  

  Figure  1.  Factors  and  degrees  of  exposure  to  flooding  hazard  in  the  polders  of  Certes  and  Graveyron   2.2.  Assessment  of  the  issues  present  in  the  polders  and  their  extent  per  management  unit     In  a  second  stage,  an  assessment  of  the  issues  present  in  the  three  polders  was  operated  on  the   basis  of  distribution  into  major  categories,  namely,  the  production  factors  (inland  fishing  and  cattle-­‐ rearing),  recreational  facilities  (hunting  from  a  hide,  rambling  on  the  dykes)  and  heritage  sites  (flora   and   bird   life,   sites   of   outstanding   natural   beauty).   In   the   case   of   Malprat,   there   is   also   an   environmental  health  factor  involving  the  control  of  mosquitoes  that  can  be  added  to  the  previous   three  factors.  This  assessment  is  based  on  different  sources,  such  as  interviews  with  the  operators,   counting  the  number  of  walkers,  making  an  inventory  of  hunting  grounds,  data  for  the  management   plans,   etc.).   Certain   factors   are   absent   from   the   polders,   but   are   present   on   the   dykes   or   in   their    

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vicinity  and  have  been  associated  with  adjacent  management  unit.  For  instance,  hunting  and  walking   have  been  associated  with  the  neighbouring  units  since  these  activities  can  rely  or  affect  the  dykes.   Based  on  carefully  spatialised  information  in  relation  to  the  management  units  or  sections  of  dyke,   we   assessed   the   respective   importance   of   each   issue,   using   the   same   notation   principles   as   previously   (from   0   to   2).   After   adding   up   these   marks,   each   management   unit   achieved   a   set   of   factors  considered  to  be  low  (a  mark  of  <  4),  average  (a  mark  of  4  to  6)  or  high  (a  mark  higher  than  or   equal   to   7).   Here   again,   the   strongest   factors   were   concentrated   in   the   parts   below   the   polders,   especially  the  Branne  and  Brèche  Pointes  (figure  2).  

  Figure  2.  Types  and  extent  of  the  issues  exposed  to  a  sea-­‐flooding  risk  at  Certes  and  Graveyron   2.3.  Establishing  an  eligibility  scale  for  depolderisation  to  deal  with  the  risk  of  sea-­‐flooding   The  eligibility  scale  for  depolderisation  was  obtained  by  cross-­‐matching  the  results  from  this  dual   assessment,   applying   an   algebraic   sum   and   subtracting   for   each   management   unit   the   total   value   allocated   to   the   issues   in   the   total   value   assigned   to   the   exposure   to   hazard,   in   such   a   way   as   to   obtain   a   scale   of   "eligibility   for   depolderisation"   (marked   from   -­‐17   to   +4).   We   consider   a   management  unit  to  be  more  eligible  for  depolderisation  if  its  exposure  to  hazard  exceeds  the  value   of  the  issues  present.  Positive  values  or  those  slightly  less  than  0  thus  translate  into  high  eligibility,   since   they   represent   greater   risks   or   lesser   issues.   The   Brèche   unit   at   Graveyron   is   a   case   in   point,   since  the  exposure  is  considerable  and  the  risk  issues  are  low  due  to  an  accidental  opening  to  the  sea   that  has  been  there  for  many  years.  The  Grandes  Plaines  unit  at  Certes,  on  the  other  hand,  illustrates   a  situation  in  which  the  exposure  is  less  than  the  scope  of  the  issues  (which  are  heritage-­‐based  and   above   all   recreational)   and   thus   the   unit   would   appear   on   the   face   of   it   to   be   less   eligible.   This    

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eligibility   scale   for   depolderisation   has   helped   us   to   find   basins   which   deserve   a   cost-­‐benefit   analysis   that  is  more  detailed  of  all  the  possible  risk  management  scenarios.   3.  Creating  a  cost-­‐benefit  analysis  for  management  units  eligible  for  depolderisation   In   the   case   of   the   management   units   that   are   most   eligible   for   depolderisation,   a   cost-­‐benefit   analysis   was   performed,   taking   account   of   the   costs   of   all   the   possible   risk   management   methods   for   these  units  (3.1.),  the  way  in  which  the  issues  developed  depending  on  the  method  chosen  (3.2.)  and   the  potential  for  reconstituting  a  saltmarsh  if  the  site  were  to  be  depolderised  (3.3.).   3.1.  Costs  of  the  various  risk  management  methods  with  or  without  depolderisation   An  assessment  of  the  cost  of  the  various  risk  management  methods  has  been  difficult  to  conduct,   due  particularly  to  the  fact  that  there  is  extreme  variability  in  the  cost  of  ripraps  which  depends  on   both   their   availability   (changing   according   to   the   weather   or   economic   circumstances)   and   their   quality.   The   results   obtained   for   Arcachon   Bay   circa.   2010   are   therefore   hard   to   use   for   generalisation   purposes.   The   dykes   should   be   built   up   by   one   metre   at   Malprat   and   by   1.30   m   at   Certes   and   Graveyron   to   reach   a   height   of   4.70   m   which   would   be   only   slightly   higher   than   the   highest  water  level  of  flooding  by  2030  as  defined  above.  The  cost  of  dyke  reinforcement  would  be   less  than  that  of  heightening  them,  not  necessarily  in  linear  metres  but  due  to  the  generalisation  of   the   extension   process   which   would   also   involve   changing   the   level   of   thirty   or   so   tidal   gates.   Depolderisation  of  a  management  unit  –  which  at  first  sight  might  appear  to  be  less  costly  –  would   only   really   be   cheaper   on   condition   that   it   would   not   be   necessary   to   build   an   additional   rear   protective  dyke  for  the  other  management  units.  Furthermore,  depolderisation  using  gates  would  be   more   costly   than   depolderisation   through   creating   a   breach   without   riprap.   The   costs   need   to   be   estimated  case  by  case  however,  depending  on  the  configuration  of  the  defence  system  chosen  and   the  length  of  the  front  and  rear  dykes  to  be  repaired,  heightened  or  built.   3.2.  Changes  to  the  issues  involved,  especially  in  cases  of  depolderisation   The  changes  in  issues  based  on  the  management  methods  chosen  also  deserve  to  be  examined.   Certain  activities  appeared  to  us  to  be  better  suited  to  depolderisation  than  others,  such  as  walking,   for  which  one  can  imagine  maintaining  footbridges  installed  over  the  breaches,  or  hunting  for  which   the  cabins  and  the  lakes  could  be  moved  –  an   option  currently  studied  in  the  Bas-­‐Champs  of  Picardy.   While   certain   factors   will   inevitably   change   in   the   case   of   depolderisation   (decline   of   inland   fishing   and   heritage   factors),   animal   husbandry   would   be   transformable   without   great   loss   of   value.   To   move   from   cattle   grazing   on   wet   meadows   to   sheep   rearing   on   saltmarshes   would   be   conceivable   and  viable  as  far  as  the  farmers  of  today  are  concerned  as  long  as  a  herd  of  60  cows  and  a  flock  of   about   100   ewes   could   be   kept.   At   high   tide,   the   sheep   would   shelter   on   the   higher   ground   of   the   saltmarsh  and  in  the  case  of  flooding  they  would  be  sheltered  by  the  polders.   3.3.  Assessment  of  the  potential  for  the  reconstitution  of  a  saltmarsh  in  a  case  of  depolderisation   The   choice   of   a   submersion   management   method   also   depends   on   the   effective   potential   for   reconstituting  the  saltmarsh.  In  order  to  establish  this  potential  for  the  recolonisation  by  vegetation   of   a   depolderised   area,   F.   Bertrand   used   as   the   basis,   the   follow-­‐up   performed   since   2003   in   the   sectors   that   were   accidentally   depolderised,   in   such   a   way   as   to   determine   the   current   ecological    

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gradient  of  the  expanding  saltmarsh.  The  lower  limit  adopted  is  that  of  the  continuous  plant  ground   cover  in  which  cover  with  vegetation  exceeds  75%,  corresponding  to  the  lower  limit  of  a  saltmarsh   that  is  firmly  established.  The  upper  limit  is  that  of  the  limit  of  the  plant  growth  in  the  highest  part  of   the   saltmarsh.   When   these   values   –   which   are   different   for   each   polder   –   are   known,   it   will   be   possible   to   determine   the   extent   of   the   area   that   could   be   recovered   by   a   saltmarsh   in   a   case   of   depolderisation.  The  results  thus  show  a  strong  potential  for  a  saltmarsh  reconstitution  of  virtually   the  entire  areas  of  these  three  polders.   4.  Proposals  for  Arcachon  Bay  and  conditions  for  reproduction   The  taking  account  of  the  financial  (3.1),  socio-­‐economic  (3.2)  and  bio-­‐geographical  (3.3)  criteria   has   made   it   possible   to   arrive   at   the   creation   of   ten   operational   datasheets   combining   the   results   for   management   units   that   would   initially   appear   to   be   eligible   for   depolderisation,   and   resulting   in   recommendations.  BARCASUB  will  deliver  its  various  recommendations  fir  each  polder  (4.1).  We  also   consider  this  operational  methodology  to  be  reproducible  under  certain  conditions  (4.2).   4.1.  Differentiated  recommendations  for  the  three  polders   At   the   conclusion   of   this   analysis,   we   should   like   to   discuss   the   best   management   options   proposed  for  each  polder.  At  Certes,  due  to  the  cost,  which  would  be  lower  than  the  current  cost  of   maintaining   the   dykes,   depolderisation   by   creating   an   unstabilised   breach   at   Branne   Pointe,   with   the   construction   of   a   short   rear   dyke,   would   seem   to   be   the   most   desirable   solution.   This   would   have   little  impact  overall  on  the  current  issues  (hunting,  walking,  animal  husbandry)  and  would  deal  with   the  high  exposure  to  sea-­‐flooding  hazard.    

  Figure  3.  Example  of  the  operational  datasheet  proposed  for  the  western  part  of  the  Certes  polder    

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While   depolderisation   by   means   of   a   tidal   gate,   being   considered   for   the   Grandes   Plaines   unit,   seems  expensive  to  us  in  view  of  the  complete  absence  of  impact  due  to  hazard  exposure  (fig.  1),  it   could  nevertheless  be  contemplated  in  the  context  of  the  extension  of  the  recent  reopening  of  the   Cabanot   unit,   which   in   this   case   involves   objectives   that   are   not   defensive   but   solely   ecological.   At   Graveyron,  due  to  the  high  cost  of  depolderisation  and  the  extent  of  the  issues  to  be  preserved  on   and  at  the  foot  of  the  dykes  (walking  and  hunting),  we  merely  propose  retaining  the  current  breach   (which   equates   with   accepting   the   accidental   depolderisation   of   the   Brèche   unit),   without   additional   reinforcement  of  the  rear  dyke  but  with  preservation  of  the  ruined  dykes  and  the  high  plants  present   in   this   recolonised   area   (Tamaris,   Baccharis).   At   Malprat,   considering   the   average   level   of   issues   other   than   those   related   to   hunting   and   the   proven   potential   for   plant   recolonisation,   depolderisation   by   an   accidental   break   of   the   two   rear   units   would   appear   to   be   a   desirable   extension  of  the  accidental  depolderisation  that  has  already  occurred  for  the  Pointe.   4.2.  Scientific  and  operational  conditions  of  reproducibility  of  the  assessment   This   operational   methodology,   described   in   greater   detail   in   the   final   report   of   the   BARCASUB   project  (Bertrand  and  Goeldner-­‐Gianella,  2013)  would  appear  to  be  replicable  on  condition  that  the   information   was   necessary   for   performing   successive   assessments   relative   to   the   levels   of   submersion  and  their  modelling,  the  state  of  the  natural  and  artificial  infrastructures,  the  local  cost   of  dyke  construction  and  maintenance,  knowledge  of  the  current  issues  and  the  potential  changes  to   the   restored   saltmarshes.   This   information   was   first   acquired   by   BARCASUB   through   fruitful   cooperation  between  scientists  and  managers,  as  well  as  between  exact  sciences  and  humanities.   Acknowledgements:  We  thank  our  partners  for  their  financial  support  (MEDDE  (Liteau),  Conseil  Gé-­‐ néral  de  Gironde,  Conseil  Régional  d'Aquitaine)  as  well  as  the  Conservatoire  du  littoral  and  the  SIBA.   References   Anselme   B.,   Durand   P.,   Goeldner-­‐Gianella   L.,   Bertrand   F.   (2008).   Impacts   de   l’élévation   du   niveau   marin   sur   l’évolution  future  d’un  marais  maritime  endigué  (bassin  d’Arcachon,  France).  VertigO,  vol.  8/1254.     Bawedin   V.   (2013).   L'acceptation   de   l'élément   marin   dans   la   gestion   du   trait   de   côte   :   une   nouvelle   gouvernance  face  au  risque  de  submersion  ?  Annales  de  géographie,  vol.  692,  p.  422-­‐444.   Bertrand  F.,  Goeldner-­‐Gianella  L.  (2013).  BARCASUB.  La  submersion  marine  et  ses  impacts  environnementaux  et   sociaux   dans   le   Bassin   d’Arcachon   (France)   :   est-­‐il   possible,   acceptable   et   avantageux   de   gérer   ce   risque   par   la  dépoldérisation  ?  Rapport  final  pour  le  programme  Liteau,  juillet  2013,  198  p.   Clus-­‐Auby  C.,  Paskoff  R.,  Verger  F.  (2006).  Le  patrimoine  foncier  du  Conservatoire  du  littoral  et  le  climatique:   scénarios  d'évolution  par  érosion  et  submersion.  Annales  de  géographie,  vol.  648,  p.  115-­‐132.   French   P.W.   (2006).   Managed   realignment   –   the   developing   story   of   a   comparatively   new   approach   to   soft   engineering.  Estuarine,  Coastal  and  Shelf  Science,  n°  67,  p.  409-­‐423.   Goeldner-­‐Gianella  L.  (2013).  Dépoldériser  en  Europe  occidentale.  Pour  une  géographie  et  une  gestion  intégrées   du  littoral,  Publications  de  la  Sorbonne,  Paris,  350  p.   Goeldner-­‐Gianella   L.,   Bertrand   F.,   Pratlong   F.,   Gauthier-­‐Gaillard   S.   (2013).   Submersion   marine   et   dépoldérisation:   le   poids   des   représentations   sociales   et   des   pratiques   locales   dans   la   gestion   du   risque   littoral.  Espace,  Populations,  Sociétés,  vol.  1-­‐2,  p.  193-­‐209.   Shepard  C.C.,  Crain  C.M.  &  Beck  M.W.  (2011).  The  Protective  Role  of  Coastal  Marshes:  A  Systematic  Review  and   Meta-­‐analysis,  http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0027374.  

 

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