Analysis of the model for a 5.10 m flood The   reference   for   a   5.10   m   flood   at   the   Mopti   gauge   is   the   1990   flood.   Landsat   images   are   available for October 1, October 10 and November 27. The   1990   flood   is   very   peculiar,   with   higher   flood   than   expected   for   a   flood   height   of   5.10   m   at   the   Mopti   gauge   in   Sofara   and Ké   Macina   but   with   an   atypical   time   lag   between   the   Niger   and   Bani   floods   and   a   bimodal   flood   at   Sofara.   The   flood   at   Akka   is significantly lower than expected with 5.10 m at Mopti. According   to   J.P.   Lamagat's   model,   a   5.10   m   flood   at   Mopti   should   correspond   with   4.24   m   in   Ké   Macina,   while   4.41   m   were actually   recorded,   a   level   corresponding   to   a   5.30   m   flood   height   at   Mopti.   Similarly   the   flood   height   at   Sofara   should   be   3.89   m while   4.28   m   were   actually   recorded,   corresponding   to   a   flood   height   close   to   5.40   m   at   Mopti.   The   low   flood   height   at   Mopti   in 1990   is   explained   by   the   bimodal   flood   at   Sofara   and   by   the   very   unusual   flood   peak   offset   between   Sofara   and   Mopti   which   also explains the abnormally low value in Akka. The   flooded   area   calculated   by   the   model   for   5.10   m   -   shape   (Niv_510)   -   extends   over   ​​747,325   ha,   which   overestimates   the   area actually   flooded   quite   significantly.   This   is   explained   in   particular   by   the   overflows   of   the   matrix   at   level   55,   which   precisely correspond   to   the   5.10   m   flood   height. After   integration   into   Veg7,   the   shape   file   Niv_510_Veg7   shows   a   flooded   area   reaching   ​​664 827   ha,   the   value   retained   for   comparison   with   Landsat.   This   value   is   very   close   to   that   obtained   by   directly   taking   into   account   on Veg7 only the vegetation associations whose flood depth is below -1.50 m (664 953 ha). The   map   above   (left)   figures   the   flooded   areas   according   to   the   model   for   a   water   height   of   5.10   m   at   the   Mopti   gauge.   While for   a   flood   of   5.97   m,   the   Delta   still   presented   a   continuous   body   of   flood   water,   for   one   of   5.10   m   the   body   is   fragmented, suggesting   the   large   deep   basins   of   the   Delta   (see   page   49).   The   map   also   shows   areas   with   scanty   flooding,   in   particular   the   upper Niger River downstream from Ké Macina as well as downstream along the Niger and Diaka separation, and also the southern Delta. The   map   drawn   from   the   analysis   of   Landsat   images   shows   flooded/vegetated   areas   extending   over   ​​623,138   ha,   rather   close   to the   flooded   area   calculated   by      the   Niv_510   model   (664,827ha)   but   with   a   somewhat   different   distribution,   as   shown   in   the   synthesis map    -    shape    (synthesis_510_1990)    -    which    groups    together    the    shape    files    (commun_1990_510,    inon_plus_1990    and    inon_ moins_1990) The   comparison   of   the   flooded   areas   calculated   by   the   model   with   flooded   areas   observed   in   the   Landsat   images   leads   to   the following remarks: 1   - The   flooded   and   heavily   vegetated   area   common   to   the   ​​Landsat   images   and   the   flooded   area   calculated   by   the   model   extends over   496,775   ha .   Thus   the   ratio   of   common   area   /   flooded   area   of   ​​the   model   reaches   74.7% ,   showing   that   confidence   in   the   model calculations decreases with the lower floods. 2   -   The   summary   map   figures   (in   red)   areas   calculated   by   the   model   as   floodable   but   not   flooded   on   Landsat   extending   over 167,534   ha    ;   and,   conversely,   areas   flooded   on   Landsat   which   are   not   flooded   according   to   the   model      figure   (in   purple   and   mauve)   : they extend over 126,101  ha 3   -   The   effectively   non-flooded   areas   (in   red)   are   located   in   four   places:   on   the   right   bank   of   the   Niger   river   upstream   from Mopti,   confirming   the   poor   flood   water   provision   of   this   region   of   the   Delta   with   the   flood   almost   disappearing   from   the   Pondori basin;   the   south   of   the Ténenkou   Basin;   the   surroundings   of   the   Peru   Dialloubé   and   the   northwest   of   the   Delta   in   connection   with   the lower than expected flood at Akka at the exit of the Débo lake. 4 - The areas totally or partially flooded on Landsat and not flooded according to the model are mainly located: • In   the   MB2   vegetation   mosaic   units   along   the   Niger   and   Bani,   which   according   to   the   model   have   a   flood   depth   of   -1.40m,   and are   therefore   not   flooded   at   5.10   m;   yet   in   reality,   they   are   composed   of   banks   at   the   0   m   level   and   a   series   of   levees   and channels whose flood depth ranges as low as - 2.80 m; • In   and   around   the   Toggere   Coumbe,   Kakagnan   and   Kadial   basins,   highlighting   the   influence   of   the   peculiar   longitudinal   profile of the Diaka defluent and the limits of its influence in relation to the Débo lake (see details on page 47).

510_1990.gdb contain two shape files - Niv_510 : The raw calculated model -Synthèse_510_1990 with the following items:     sigle  : acronym of vegetal associations          niveau :  level of vegetal association profond   :  submersion depth of vegetal associations         Val_rast : 0 = non flooded                 1 = totally flooded                 2 = partially flooded Val_rast come from the analysis of Landsat Images Synthèse : 1 = commun_510_1990 2 = inon_plus_1990 3= inon_moins_1990

510_1990(EN).rar

Download