Analysis of the model for a 5.97 m flood The   reference   year   is   2006   with   a   flood   height   of   5.97m   at   the   Mopti   gauge,   which   can   also   be   compared   to   the   year   1996 with 5.95m at  the Mopti gauge (but no images available). In   2006,   the   flood   recorded   at   Ké   Macina   matches   that   of   the   model;   at   Sofara   on   the   Bani   the   flood   is   a   little   higher.   On   the contrary,   the   resulting   flood   at Akka   is   lower   than   expected,   with   above   all   a   very   fast   propagation   speed   which   reduces   the   flooding in Akka. Unfortunately,   the   repeatability   of   the   Landsat   images   in   2006   is   not   satisfactory   and   the   spatial   coverage   is   incomplete. The   only image   198_050   (10/5/2006)   of   the   extreme   southwest   of   the   Delta   is   not   fully   usable   because   it   is   very   noisy;   however,   it   shows   that the flood is already largely declining in south of the Delta while the north is not yet completely flooded and / or vegetated. With   a   flood   height   of   5.97m   at   the   Mopti   gauge,   the   area   potentially   flooded   on   NIV_597      totals   1,192,890   ha. After   integration within   the   limits   of Veg7   and   reintegration   of   errors   related   to   the   smoothing   of   the   matrix,   the   flooded   area   calculated   by   the   model   at 5.97m (shape NIV_597_Veg7) is 1,152,745 ha. On   the   Landsat   images   of   11/23/2006   (plus   the   image   of   Oct.   30   for   the   south   of   the   Delta)   the   flooded   area   within   the   limits   of Veg7   reaches   1,091,847   ha.   -   shape   (VEG7_SF_2006)-.   However,   fully   flooded   or   highly   vegetated   areas   extend   over   887,004   ha while the  partially flooded area extend over 204,843 ha. The   common   area   -   shape   (Commun_597_2006)   -   between   NIV_597_Veg7   and   VEG7_2006   extends   over   987   844   ha.    The   ratio between this common area and the area calculated by the model thus reaches 85.6%. Analysis   of   the   differences   between   the   flooded   area   calculated   by   model   and   observed   on   Landsat   images   (see   the synthesis map: synthèse_597_2006) . Ultimately,   the   synthesis_597-2006   map   shows   the   malfunctions   of   the   model,   with   the   southern   part   of   the   Delta   poorly supplied   in   flood   (in   red   and   pink   on   the   synthesis   map)   and   the   central   and   north   west   Delta   better   flooded   than   expected   (in green   on   the   map).   The   flood   extends   around   the   large   basins   of   the   center   and   north   west   of   the   Delta,   Toggere   Koumbe, Kadial   and   Kakagnan   basins,   as   is   also   highlighted   on   the   map   of   these   large   basins   (Inon_plus_597).   This   “better   than expected”   flood   might   be   related   to   the   peculiar   longitudinal   profile   of   the   Diaka   (as   explained   for   the   4.40   m   flood   analysis   in 1984).

The   classification   of   the   Landsat   image   of   11/23/2006   (tiles 197_050   and   197_049)   with   test   sites   and   maximum   likelihood criteria    (as    described    on    page    42)    highlights    two    types    of vegetation   associations ,    strongly   differentiated,   as   figured   on   the extract located in the northwest of the Delta. 1    –In    dark    green:    a    densely    green    flooded    vegetation, fringed by open water (in blue) 2   –In   light   green:   a   scanty   green   vegetation   observed   in some   places   within   the   Delta   and   on   the   Delta   margins.   These vegetation   units,   which   were   not   flooded   in   November   2006, still   had   notable   green   vegetation   in   late   October   (South)   and late   November.   Yet   these   areas   were   not   included   in   the   flooded areas   observed   on   Landsat   and   compared   with   flooded   areas calculated by the model.

1   -   the   areas   calculated   as   flooded   by the    model    but    which    are    not    flooded    on Landsat    at    the    end    of    November    2006 extend   over   165,492   ha ,    of   which   36,886 ha   are   vegetation   mosaics   of   “ambiguous” flood      status,      while      128,606      ha      are vegetation usually flooded by river flood. The    "ambiguous"    flooded    vegetation associations   (flooded   by   local   run-off   and   / or    combining    floodable    with    unflooded associations)   are   located   in   Peru   Dialoubé, at   the   edge   of   the   Delta   and   on   the   right bank    of    the    Bani,    north    of    the    erg    of Femaye.      They      are      therefore      doubly marginal,   because   of   their   flooding   status and   because   of   their   location   at   the   edge   of the flood plains. The   flood-prone   vegetation   units   that were   not   flooded   in   2006   are   located   partly in   Dialloubé   Peru   but   mainly   extend   in   the south   of   the   Delta.   The   flood   dysfunctions in   the   southwest   of   the   Delta   between   Ké Macina    and    Diafarabé,    and    in    the    south, between   the   right   bank   of   the   Niger   and   the Bani,    already    noted    in    the    analysis    the 6.21m   flood,   are   amplified   in   floods   which only    reach    around    6m    in    height    at    the      Mopti    gauge,    confirming    the    poor    flood provision this "upstream part of the Delta". 2    -    Flooded    areas    according    to    the Landsat    images    of    November    2006    and which   are   not   flooded   by   the   model   extend over approximately 102,845 ha. Distribution   of   fully   or   partly   flooded     areas   on   the   Landsat   images   of   November 2006     and     not     flooded     by     the     model according   to   the   flood   depth   of   vegetation associations.

597_2006.rar

Download