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 info@delmasig.com
