Analysis of the model for a 6.21 m flood The NIV_621 model is compared with the 2001 flood, the characteristics of which are as follows: *   The   model      by   Jean   Pierre   Lamagat,   a   Hydrologist   at   IRD,   dates   from   1983.   It   established   correspondences   between   the   heights   of   the   floods between the main gauges along the Niger River. It was used in particular for the development of the Mathematical Model of the Niger River . Once   again   the   2001   flood   is   quite   particular:   the   flood   level   at   Ké   Macina   matches   a   6.60   m   flood   at   Mopti   and   not   a   6.21   m flood.   The   Bani   flood   at   Sofara   is   slightly   lower   than   expected,   given   the   flood   height   in   Mopti   (about   -   6   cm).   The   flood   height   is   a little   lower   than   expected   in   Akka   at   the   exit   of   the   Débo   lake.   Moreover,   the   observed   duration   of   the   flood   propagation   strongly differs from the model predictions for the Sofara to Mopti and Mopti to Akka stretches. These   peculiarities   partly   explain   the   differences   between   the   analysis   of   model   outputs   and   Landsat   images   for:   October   7, 198_050 (west of the Delta), October 16, 197_051 (south of the Delta) and November 17 for 197_051, 050 and 049 (entire Delta). Flood   NIV_621   takes   the   form   of   a   massive   block   in   the   center   and   north   of   the   Delta,   with   only   a   notch   by   Peru   Dialloubé. The flood   extent   is   more   complex   in   the   south   with   large   flooded   basins   (south   of   Mopti,   Yongari,   Mangari,   Pondori)   soon   tapering between Diafarabé and Ké Macina, upstream of the Delta. The total flooded area calculated by the model is 1,417,331 ha. After   correcting   the   errors   related   to   the   smoothing   of   the   matrix   (both   in   terms   of   additions   and   substractions),   the   potentially flooded   area   of   ​​NIV_621_Veg7   is   1,367,484   ha.   The   relatively   large   gap   due   to   correction   is   explained   by   the   deletion   of   vegetation unit   fragments   whose   depth   range   is   between   -0.05   cm   and   -0.35   cm.   The   flood   depths   between   -0.35   m   and   -0.45   m   are   dubious   for a   6.21m   flood   at   the   Mopti   gauge   (-0.39   m   below   the   reference   height),   as   they   are   intergraded   mosaics   between   levels   -0.10   and   - 0.60   (-0.35   m)   and   between   -0.30   and   -0.60   (-0.45   m).   Parts   of   these   mosaics   were   therefore   likely   to   be   flooded.   The   floodable   area of   ​​1,367,484   ha   should   therefore   be   taken   as   minimum   area,   the   areas   at   level   -0.35   in   the   niv_621   model   extending   over   1672   ha while the areas at -0.45 m were kept within flooded areas. The   total   flooded   area   on   Landsat   (shape VEG7_2001)   reaches   1,426,684   ha,   a   greater   value   than   the   flooded   area   calculated   by the   model   (NIV_621_Veg7)   with,   in   particular,   flooded   areas   in   the   southwest   of   the   Delta   (Diafarabé   region)   sharply   marked   by flood   on   images   of   October   7   and   16   ,   which   no   longer   appear   flooded   on   the   image   of   November   17. The   fully   flooded   or   vegetated areas   cover   1,219,477   ha,   to   which   207,207   partially   flooded   ha   must   be   added.   Out   of      this   total,   11,611   ha   (4,597ha   totally   flooded and   7,014   ha   partially   flooded)   were   flooded   on   October   16   but   did   not   bear   a   marked   green   vegetation   cover   in   November   17. These formations, all located in the Diafarabé region, have a flood depth equal to, or lower than -0.30 m. This   complex   reality   is   reflected   in   the   flooded   area   common   to   the   model   shape   (NIV_621_Veg7)   and   to   Landsat   2001   images     -shape   (VEG7_2001)-.   The   shape   file   (Commun_621_2001)   resulting   from   their   overlay   shows   a   flooded   area   of   ​​ 1,276,476   ha ,   of which 1,154,624 ha  are totally flooded / or vegetated; and 121,852 ha are partially flooded. 1    –    If    partially    flooded    areas    are    included,    the    confidence    ratio    of    the    model    –    the    ratio    between    the    total    surface    of (Com_621_2001)   and   that   of   (Niv_621_Veg7)   –   reaches   93.3% .   If   only   the   totally   flooded/vegetated   areas   are   taken   into   account, the ratio is 84.4% . The actual value must be between these two rates. 2   -   The   comparison   between   (Niv_621_Veg7)   and   (VEG7_2001)   shows   flooded   areas   on   Landsat   which   are   not   flooded   in   the model   shape   (Inon_plus_2001);   and   conversely,   areas   calculated   as   flooded   by   the   model   which   are   not   so   on   Landsat   shape   (Inon_ moins_2001) The shape (synthèse_621_2001) includes (Commun_621_2001, Inon_plus_2001, Inon_ moins_2001) On   the   combined   map,   the   areas   flooded   on   Landsat   and   which   are   not   flooded   by   the   model   all   have   depths   between   -0.05m and   -0.35   m   and   the   majority   are   only   partially   flooded. They   are   mainly   distributed   in   the   Diafarabé   region,   at   the   edges   of Yongari- Mangari,   at   the   edges   of   the   main   flooded   area   which   extends   from   the   west   of   the   Delta   to   the   Peru   Dialloubé.   They   clearly   mark the   influence   of   the   strong   flood   observed   at   Ké   Macina   (+63   cm),   compared   to   the   model   prediction.   This   strong   flood   affects   the Diaka   and   the   Niger   up   to   the   defluent   feeding   the   Yongari,   at   Kouakourou.   Beyond   Kouakourou,   this   influence   is   offset   by   the deficit contributions from the Bani River and becomes negligible. A   significant   part   of   these   areas   (shown   in   black   on   the   map),   all   located   in   the   Diafarabé   region,   were   flooded   on   October   16, but   do   not   bear   a   marked   green   vegetation   cover   marked   on   November   17. All   these   vegetation   units   have   a   maximum   water   depth   of -0.30   m.   The   cause   is   probably   linked   to   a   flood   peak   (between   6   m   and   6.62   m)   which   lasts   only   25   days,   too   short   to   sustainably affect vegetation growth. The   non-flooded   areas   illustrate   two   concomitant   phenomena:   the   poor   water   supply   of   the   Pondori   basin   where   flood   is very   early   and   is   reinforced,   in   2001,   by   the   Bani   flood,   which      is   slightly   in   deficit   and   late.   The   areas   flooded   on   Landsat images   and   which   are   outside   the   areas   calculated   by   the   model   illustrate   the   role   played   by   an   imbalance   in   the   inflows between Niger and Bani. In the case of the 6.60 m flood, the deficit from the Niger at Ké Macina resulted in smaller flooded   areas   in   the   Diafarabé   region   and   the   Moura   plain.   In   the   case   of   2001,   the   higher   flood   in   Ké   Macina   resulted   in a   larger   flooding   in   the   same   region,   although   transient   in   part.   This   imbalance   of   contributions   partly   affects   the   logic   by depth levels developed in Delmasig.
Flood    gauge   Maximum flood in 2001   Time lag between maximum   heights   in 2001  (d ays)   observed   m odeled *   reach   observed   modeled   ( c m )   dates   ( c m )   ( day )   ( day )   Mopti    621   18/10   621.0   -   -   -   Ké Macina    662   22/09   598.9   Ké Macina Mopti   26   24   Sofara   534   27/09   529.4   Sofara Mopti   21   10   Akka    470   15/11   519.7   Mopti Akka   28   43
The   areas   calculated   as   flooded   by   the   model which    are    not    flooded    on    Landsat    represent 89,930   ha   (inon_   moins_2001)   of   which   31,766   ha   extend     over     vegetation     mosaics     which     include vegetation   associations   that   are   first   flooded   by   run- off,   and   58,164   ha   extend   on   vegetation   which   is normally   floodable.The   distribution   by   flood   depth level is given in the following table: Floodable   vegetation   not   flooded   on   Landsat   2001 images (on 58,164 ha) Among   the   areas   not   flooded,   those   first   flooded by   run-off   are   mainly   distributed   on   the   periphery   of the   Delta,   in   Dialloubé   Peru   and   on   the   Chibon,   at the   far   east   of   the   Pondori   basin.   The   flood-prone formations   (in   purple)   are   found   mainly   in   the   south of   Pondori,   and   along   the   right   bank   of   the   Bani, south   of   the   erg   of   Femaye,   and   in   the   high   plains   of the   Sébéra,   whose   flood   is   provided   by   the   Bani,   as well   as   in   Peru   Dialloubé   where   flood   is   provided   by the   Niger,   downstream   from   the   confluence   with   the Bani. However,    the    Landsat    images    also    reveal flooded    and/or    vegetated    areas    which    are    not included   in   the   flooded   areas   calculated   by   the model   for   a   total   of   136,841   ha   (Inon_plus_2001), which    are    distributed    according    to    the    level    of flooding:
Flood depth   levels   Non - flooded areas  (ha)   -   0.45 m   19 184   - 0.60 m   13 929   ] - 0.6 m  à   - 1.50 m]   18 881   ] - 1.50 m  à   - 4 m]   6 170 Status   in 2001   Area (ha)   Fully flooded and/or vegetated   52 897    Partially flooded and/or vegetated   72 317    Flooded the1 6  O ct .with no green vegetation the 1 7 Nov ember   11 627    Total area    136 841