Geomorphologic analysis method using ASTER GDEM v2 digital elevation model over the tropical rainforest : implementation to lakes problematic in Sangha National Park (Central African Republic)

1. Geomorphologic analysis method using ASTER GDEM v2
digital elevation model over the tropical rainforest :
implementation to lakes problematic in
Sangha National Park (Central African Republic)
Gilles LEROUGE and Thierry ROUSSELIN
Geo212 – 25, rue Jean Dolent – 75014 Paris – www.geo212.fr
gilles.lerouge@geo212.fr - thierry.rousselin@geo212.fr

2. Location:
Northwest of Congo Basin
Rainforest
Tri-National Park of Sangha
(UNESCO – World Heritage of
Humanity)
South of Bayanga
Central African Republic
A geomorphological analysis
conducted by Geo212
within the scientific program
« Sangha Biodiversity on Pygmy land -
Insects of the World in Central African
Republic » (2011 -2012)

3. Context:
 Dense and Fairly intact
rainforest
 Presence of multiple small
lakes
 No topographic or
geological mapping
Goals :
 Contribute to a better
understanding of the
environmental distribution of
the fauna (insects, lizards,
frogs…)
 To draw up the river system
map
 Understand the river system
organisation (location /
distribution / communication)
An old hypothesis :
Through photointerpretation, geometries of some of the
lakes were interpreted in the littérature as sinkholes of a
paleo cryptokarst above continental sands

4. Our program is one of the very few
scientific exploration programs to have
penetrated this forest for more than a
century
Unfavorable for geological field
observations but favorable for
geomorphological interpretation :
 Homogeneous forest with native plant species, trees over 70 m high.
 A canopy still intact, undisturbed by human activity, maintaining the
underlying relief influences.

5. First step of the analysis :
River system restitution and
geomorphological
interpretation
Choice of SPOT5 HRG image
(2008/10/23) :
 Resolution (2,5m) fits the
scale of the objects
Choice of a photo-
interpretation approach:
 Geometric accuracy
 Avoids any artifact related to
DEM automatic extraction
River system:
 Covered by forest
 Low density
 Sharp contrasts of vegetation
 Open valley little incised

6. Sangha watershed
(a tributary of Congo River)
Four main flow directions :
 SSE (Sangha and main
tributaries)
 SW (upstream part of the
flow)
 E or ESE (in the north-east)
 NW (in the north and north-
east)
The lakes are located in an
arc interfluve area

7. Remarkable river system
geometries :
 Opposite curved lines,
under folded structure
influences
 Straight lines with angular
joints, under faulted
structure influences.
 General flow direction shift
in the south, under tilting
block influence

8. Second step of the analysis :
Geomorphological
interpretation of ASTER
GDEM v2 data :
Choice of DEM :
 Planimetric resolution (30
m) and elevation accuracy
(20 m) at the scale of
elevation changes searched
into the canopy,
 Less smooth reliefs
(essential to detect breaks
of slope)
 Fewer artifacts or holes
 Global coverage
 Economic choice

9. DEM analysis principles in a
rainforest context :
 Identify and reproduce
breaks of slope disrupting
the canopy with geological
meaning (lithological
contrasts …),
 Forget height variations
related to species and
wetlands (requires a canopy
analysis on the SPOT5
image),
 Only the most important
altitude contrasts are
detected (smoothing effect,
vertical resolution)

10. To the West, Sangha river
(wide valley, asymmetrical)
 From north to south, 360 m
to 340 m gradient,
 High reliefs (450 m) located
3 km from the river on the
right bank ,
 Left bank side very open
(altitude 450 m are 20 km
from the river)
To the East, relief over 700 m
 NNW-SSE morphological
axis
 Increasing altitudes to the
north-east
 Arched valley,
 Diverging slopes
North and South
 Valleys and ridges straight
ENE-WSW
To the South
 Slopes tilted to the SE

11. Break of slope
Break of slope geometries :
 Regionally oriented NNW-
SSE
 Morphological structures
closing to the north
 Elliptical structure in the
center
 Chevrons between talwegs
Dips interpretation :
 Periclinal closure
 Coombs and valley

12. Break of slope
Folded structures :
 Major fold axis : NNW-SSE
 Asymmetrical folds
succession
(synform / antiform,
long limb / short limb)
 Curved axis on the
western limb.
Overall structure :
Polyharmonic fold
Light inverted relief

13. Break of slope
Faults
Straight and narrow valleys
Sections with angular joints
ENE-WSW orientation:
 2 corridors to the north
and to the south
 Associated orientations
E-W,
NW-SE
NNE-SSW
NW-SE orientation
Intersects the lakes area

14. Tilting direction
Movement direction of
fault
Morphostructural
interpretation
 Folds
Intersected to the NW
and to the SE by fault
corridor,
Intersected by NW-SE
faults
 ENE-WSW shear zones
 Tilted southern block to
the SE along the shear zone

15. Third step of the analysis :
Lakes distribution
interpretation
 On the polyharmonic fold
western limb
 In the interfluve area
 Distributed in a light
arched depression
 Lakes do not have always
a circular form, often
elongated form
 No communication
observed between the
lakes

16. Structurally controlled lakes :
 In an anticline coomb
 According to the fold axis,
shifted by faults
 Divided by faults into small
diamond-shaped basins,
isolated by topographic
scarps
 Outside of the coomb, lakes
are located just under the
break of slope (anaclinal
side)
 In faulted consequent valley

17. Flow directions and
connections between lakes
 Lake 1 directly connected
to the river system through
a consequent valley
 In the northern part,
possible connections
between the lakes into the
same cataclinal coomb.
 In the southern part,
opening of the coomb on
the southern side and flow
in this opportunity direction
 On the western side, direct
connections to the river
system by consequent
valleys
Drainage area
Flow direction

18. Conclusions
This example of morphostructural analysis over the Sangha
rainforest demonstrates, over a complex and out of the way place,
the opportunity to :
 Establish an accurate structural pattern,
 Explain the river system logic,
 Guide the field reconnaissance work.
Using ASTER GDEM v2, with its global coverage, this approach can
be applied everywhere with an accuracy coherent with the sought
after geological objects.

19. Geo212 – 25, rue Jean Dolent – 75014 Paris – www.geo212.fr
gilles.lerouge@geo212.fr - thierry.rousselin@geo212.fr