Analysis of the model for a 4.40 m flood at the Mopti scale With   a   maximum   water   height   of   4.40   m   at   the   Mopti   scale,   the   1984   flood   was   the   lowest   recorded   since   1922 ,   the   onset of   records.   This   poor   flood   resulted   from   very   poor   rainfall   in   the   upstream   basin   of   the   Niger   and   Bani   rivers,   in   humid   and   sub- humid bioclimates. Meanwhile in the Sahel it was the most severe drought recorded since the beginning of the 20 th  century. , In   Ké   Macina,   when   the   Niger   River   enters   the   Delta,   the   flood   was   higher   by   0.54   m   than   expected   from   the   propagation   model proposed   by   J.P.   Lamagat   for   a   flood   height   of   4.40   m   at   the   Mopti   gauge,   and   this   height   was   reached   twice   (3.96   m   and   3.97   m)   six weeks   apart.   If   the   flood   of   the   Bani   River   observed   at   Sofara   corresponds   to   the   flood   calculated   by   the   model,   the   flood   at   the   exit of   the   Débo   at   Akka   shows   a   very   strong   deficit   with   a   very   short   duration   for   the   propagation   along   the   Mopti   to   Akka   reach   (11 days   versus   39   days).   There   is   therefore   a   strong   imbalance   between   the   flood   on   the   western   branch   of   the   Delta   along   the   Diaka, which was well supplied, and the Eastern branch, the Bani and Niger towards Mopti, poorly supplied. A   series   of   Landsat   images   (25   Oct,   10   Nov,   26   Nov)   was   available   to   analyze   this   exceptional   year. The   image   of   November   10 gives the best information on the extent of open water and the green vegetation across the Delta. The   flooded   areas   calculated   by   the   model   (map   Niv_440)   extended   on   fragmented   basins   with   a   total   area   of   open   water   and green   vegetation   of   423,299   ha.   The   transposition   of   the   model   in   Veg7   -   shape   (Niv_440_Veg7)   -,   after   correction   of   the   residuals linked   to   the   smoothing   of   the   Veg7k3   matrix,   gives   a   floodable   area   of   391,484   ha,   the   figure   retained   for   comparison   with   the Landsat images. The   flooded   area   appears   fragmented,   with      a   block   around   the   Débo-Walado,   poorly   flooded   central   basins,   and   very   little   flood in southern basins such as Yongari-Mangari, Sebera, Bani and Pondori. The   analysis   of   the   Landsat   images   makes   it   possible   to   separate   open   water,   the   dense   green   vegetation   of   the   flooded   plains and   the   light   green   vegetation   of   the   non-flooded   plains   and   togge ”   as   well   as   bare   soils.   Overlaid   on   Veg7   (shape   Veg7_1984),   the flooded areas with or without dense vegetation where further separated  as fully or partly flooded areas. The areas were calculated in two different ways: 1 . by counting pixels on Landsat 2 . by reporting Landsat data on shape Veg 7, resulting in Veg7_1984 If   we   consider   that   the   partly   flooded   areas   are   effectively   flooded   at   50%,   the   flooded   area   assessed   from   the   vegetation   map   in 1984 would be 321,225 ha, close to the 325,453 ha measured by pixel count on Landsat images. The   relatively   higher   flood   of   the   Niger   River   at   Ké   Macina   is   part   of   the   reason   for   the   mismatch   in   the   distribution   of   the   flood between   the   western   and   eastern   parts   of   the   Delta,   but   probably   not   the   only   contributing   factor:   the   peculiar   longitudinal   altitude profile   of   the   Diaka   could   also   be   a   factor.   Indeed,   while   the   bottom   slopes   of   the   Bani   and   Niger   rivers   are   quite   regular   (see graphs:profil_fond), this is not true of the Diaka (see CARIMA : a Mathematical model of the Niger River - SOGREAH, 1984). The   Niger   River   has   a   bottom   slope   of   4.5   cm   /   km   along   200   km   before   it   decreases   to   2   cm   /   km,   50   km   upstream   after   it enters   the   Débo.   The   Bani   river   alternates   reaches   with   slopes   of   4.5   cm   /   km   and   reaches   with   very   low   slopes   (between   1   cm   and 0.8   cm   per   km).   However,   neither   of   them   presents   back   slopes   as   the   Diaka   does,   in   particular   between   km   90   and   126   downstream from   Diafarabé.   These   back   slopes   would   explain   the   great   extent   of   the   Toggere   Koumbe   basin   and   account   in   part   for   the dysfunction of the model with equal flood depth isolines during poor floods such as  that of 1984. The   peculiarity   of   the   altitude   profile   of   the   Diaka,   linked   to   the   transversal   dune   (NW-SE)   extending   along   the   stretch,   Saré Béré-Sildé-Rogonta-Sormé.   This   specificity   is   not   found   east   of   the   basin   where   the   bottom   slope   of   the   Mayo   Dembé   reaches      4.3 cm   /   km   before   it   zeroes   20   km   before   it   joins   the   Débo,   highlighting   the   difference   between   the   better   flooded   northwest   part   of   the Delta, and the poorly flooded south and east parts of the Delta.
Flood    scale   Maximum flood in 1984   Time lag between maximum  in 1984   observed   Modeled*   reach   observed   Modeled*   (m)   dates   (m)   (day)   (day)   Mopti    4.40   19/10   4.40 -   -   -   Ké Macina**   3.97   17/10   3 . 37   Ké Macina - Mopti   2   11   Sofara   3.10   12/10   3 . 05   Sofara - Mopti   4   3   Akka    3.36   30/10   4 . 18   Mopti - Akka   11   39 Pixel counts   Veg7_1984   Open water (ha)   Flooded   vegetation (ha)   Total   (ha)   104 998    220 455   325 453   Fully   flooded   101 695   145 212     Partly   flooded   46 598   102 038     Total Veg7_1984   148 293   247 250   395 886
* With Jean Pierre Lamagat 's  model **The height of 3.96 m is reached on 21/08 then again 3.97 m is reached on 17/10
1-   The   flooded   area   common   to   the   Landsat images         and         the         model         -         shape (Commun_440_1984)   -   extends   over​​   251,891   ha, setting      the      confidence      ratio      of      common area/model   at   63.3%,   a   rather   poor   achievement in   relation   with   the   exceptional   1984   flood,   but also   with   a   change   of   flooding   process   developed further. 2-       The        synthesis        map        -        shape (synthesis_440_1984)    highlights    a    partition    of the    Delta    in    two    regions:    the    southern    and eastern    region    suffering    from    a    strong    flood deficit    with    the    non    flooding    of    the    Pondori, Sebera   basins   and   along   the   Bani   River,      very limited    flooding    of    the    Yongari-Mangari    and Mopti    basins,    and    even    poor    flooding    of    the Konna   basin   attached   to   the   Débo-Walado;   the central   and   north-western   region   benefiting   from much    better    flooding    in    the    Moura,    Sossobé, Kadial,   Kakagnan   and   Toggere   Koumbe   basins, as   well   as   the   western   Walado.   This   better   than expected   flood   with   a   4.40m   height   at   the   Mopti     scale   results   in   particular   in   a   continuous   flooded area    across    the    Kakagnan    and    Kadial    basins, which   are   separated   by   the   model   at   isoline   -2.20 m,   and   the   same   applies   to   the   Toggere   Koumbé and southern Walado basins. There   are   139   593   ha   which   are      floodable according   to   the   model   but   are   not   flooded   on Landsat   -   shape   (inon_   moins_280)   -   ,   while   143 994     ha     areas     flooded     on     Landsat     are     not floodable     according     to     the     model     Niv_440 flooding    logic    -    shape    (inon_plus_1984).    The area      flooded      "in      addition      to      the      model predictions"   represents   36%   of   the   flooded   areas observed on Landsat, which is considerable.
Simplified model for a 4.90m flood in Mopti
Extract     from     the     Landsat     image     of November 17, 1984
Depth   Totally flooded (ha)   Partially flooded (ha)   [ - 0.60]   815   1 204   ] - 0.60 à  - 1.05]   2 015   2 799   ] - 1.05 à  - 1.70]   20 443   40 109   [ - 2.15]   39 726   36 883
A   thorough   examination   of   the   vegetation   units   presenting   a   dense   green   vegetation   on   the   Landsat   images   while,   according   to the   model,   they   should   not   be   flooded   with   a   flood   of   4.40   m   on   the   Mopti   gauge,   reveals   that   many   (over   800   units)   have   a   flood depth   of   -2.15   m   assigned   by   the   model,   very   close   to   the   -2.20   m   deficit   of   the   flood   at   Mopti   in   relation   to   the   reference   flood   of 6.60m.   These   large   units   (416   ha   on   average)   are   binary   mosaics   (B   /   VOR,   B   /   O,   B   /   EOR,   VB   /   VOR,   VB   /   O,   OP   /   VOR,   OP   / EOR,   OP   /   O)   composed   of   a   deeply   flooded   (-2.80   m)   vegetation   association   combined   with   a   fairly   flooded   (-1.50   m)   one.   One   can think   that   the   deepest   flood   association   extends   over   the   majority   of   the   unit   area.   However,   this   will   not   completely   explain   the discrepancy,   as   indicated   by   the   following   table,   since   associations   that   should   not   be   flooded   for   a   4.40m   level   at   Mopti   are   shown   to be flooded on the Landsat images (with depths ranging from -0.60m to 1.70m).
440_1984.gdb  contain two shapes files : - Niv_440 : The raw calculated model -Synthèse_440_1984 with the following items:     sigle   : acronym of vegetal associations         niveau  : level of vegetal association       profond : submersion depth of vegetal associations Value:  1 = water     2 = flooded vegetation Inon  : 1 = ttotally flooded    2 = partially flooded Value and Inon come from the analysis of Landsat Images Synthèse : 1 = commun_440_1984 2 = inon_plus_1984 3= inon_moins_1984   Synthèse 1+2= Veg7_1984 (Landsat)  et 1+3 = Niv_440_Veg7