sangha 2012 biodiversity in pigmy land - presentation at geomorphology 2013
TRANSCRIPT
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 – [email protected] - [email protected]
Location:
Northwest of Congo Basin
Rainforest
Tri-National Park of Sangha (UNESCO – World Heritage of Humanity)
South of BayangaCentral 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)
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
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.
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
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
Sangh
a
Ndoki
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
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
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)
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
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
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
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
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
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
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
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
ConclusionsThis 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.
Geo212 – 25, rue Jean Dolent – 75014 Paris – [email protected] - [email protected]