Towards a D.E.M ?
Realizing   a   DEM   (Digital   Elevation   Model)   would   allow   us   to   digitalize   not   only   the   flooded   areas   but   also   the   propagation   of the   flood   throughout   the   Inner   Delta.   The   very   great   complexity   of   the   area   makes   it   very   difficult   to   create   such   a   model   by classical   methods,   such   as   an   exhaustive   survey   of   the   topography   –   whose   cost   would   be   prohibitive   –   or   indirect   methods   using remote   sensing   (passive   imagery,   radar,   etc...).   Because,   on   the   one   hand,   the   area   remains   partly   flooded   even   during   the   dry season,   and,   on   the   other   hand,   the   height   of   the   vegetation   makes   measurements   unreliable   (whereas   significant   differences   in altitudes   may   be   a   matter   of   a   few   centimetres),   such   methods   are   difficult   to   use.   Let   us   bear   in   mind   that   the   Inner   Delta’s   slope, between   Ké   Macina   and   the   Débo   lake,   is   of   only   about   ten   metres   over   200   km,   i.e.   a   global   slope   of   5cm/km;   and   that   the   heights at   which   the   staging   of   the   various   plant   associations   which   point   to   the   profile   of   the   basins   and   channels   takes   place   only   vary over a total of 4 m. We   propose   to   realize   a   DEM   by   relying   on   the   depths   of   water   corresponding   to   the   various   plant   associations.   Contrary   to   the 3-D   model   we   have   used   to   display   the   flooded   areas,   we   intend   this   time   to   give   the   various   basins   and   channels   their   actual altitudes. In order to do so, we wish to use the following method 1 . Each   plant   association   needs   to   be   given   a   specific   water   depth,   as   opposed   to   a   range   of   depths   between   brackets,   as   is currently   the   case.      We   could   have   chosen   the   maximum   depth   for   each   range   (as   we   did   for   the   3-D   model),   or   its   average depth.   Rather,   we   have   decided   to   use   the   IGN   (Institut   Géographique   National/French   National   Geographical   Institute) measures   taken      as   part   of   the   building   of   the   Mathematical   Model   of   the   Niger   river   (about   3,   000   of   them,   with   a   precision of   1   cm).     Those   measurements   were   replaced   in   the   corresponding   plant   associations:   when   they   were   close   to   a   flood   gauge related   to   the   General   levelling,   we   were   thus   able   to   calculate   the   depth   of   the   relevant   plant   associations   and   their   local altitudes. 2 . We   have   used   the   6.60m   flood   as   the   reference   flood   in   the   model   for   the   various   heights   of   water.   We   believe   it   possible   to trace such a flood throughout the Inner Delta and calculate its altitude in every single point. 3 . The internal hydrographical network of the Delta has also been inserted into the model. The   combination   of   those   three   layers   of   information   as   matrices   (depth   of   submersion   of   the   plant   associations,   surface altitude   reached   by   the   reference   flood   of   6.60   m   throughout   the   Delta,   and   internal   hydrographical   network)   should   allow   us   to build such a DEM. We   do   not   claim   that   we   can   offer   a   model   which   exactly   mirrors   the   topography   of   the   Inner   Delta   as   a   topographical   model could,   but   intend   –   through   a   succession   of   approximations   –   to   build   up   a   model   that   will   prove   realistic   enough   to   be   used   in   order to   digitalize   the   way   floods   propagate.      This   model   is   to   be   obtained   through   the   gradual   adjustments   of   two   parameters:   the   water depths of plant associations and the shape of the propagation wave for the reference flood. The   first   attempts   have   proved   encouraging   but   need   to   be   further   improved   and   checked.   One   first   type   of   check   would   be   to digitalize   the   way   water   spread   through   the   Delta   in   past   floods   and   compare   the   results   with   evidence   form   satellite   pictures   of information gathered on the grounds through limnimetric gauges. I n   order   to   do   so,   we   would   like   to   work   with   specialists   of   hydrology   and   digitalization   with   a   command   of   multi-agent systems, so that the model can be better refined before it is made available to the scientific community. If you are interested in such cooperative work, please contact us at