Publications
Détails
Moeyersons, J. 2003. ‘The topographic thresholds of hillslope incisions in southwestern Rwanda’. Catena 50: 381-400. Elsevier. (PR).
Article dans une revue scientifique / Article dans un périodique
This article presents new quantitative evidence that land use in Rwanda contributes to the
development of hillslope incisions.
Two types of hillslope incisions can be distinguished in southern Rwanda. Incisions of the first
type drain an area depending on the form and extension on the natural topography and geology. The
Runyinya gully (25B) and the Rugabano soil slippage (39B) are two examples. On a logarithmic plot
of critical slope inclination at the incision head versus drainage area towards the incision head, both
incisions lay sensibly to the right of the Montgomery–Dietrich (M-D) envelope. The latter gives the
range of these topographical thresholds for gully and mass-wasting incision in parts of North
America. The Runyinya and Rugabano cases obey the linear equation:
Scr ¼ ðF0:6ÞAðF0:6Þ
where Scr = critical slope gradient (tangent of slope in B) at the gully head or the scar and A= the area
(ha) drained towards the incision head.
Hillslope incisions of the second group rely on a run-on area larger than normal because they are
localised at the ‘outlet’ of artificially runoff-collecting systems like roads, soil conservational contour
trenches, tracks and other linear landscape elements. Such systems often drain a surface much larger
in extension than the natural run-on area to the ‘outlet.’ These hillslope incisions, taking into account
their artificially big drainage area, concentrate more or less along the line:
Scr ¼ ðF0:3ÞAðF0:6Þ
This line is about in the center of the Montgomery–Dietrich envelope. If, however, only the
natural drainage area of these ‘outlet’ incisions is taken into account, all points fall close to the left border or even to the left of the Montgomery–Dietrich envelope. This indicates a much higher
probability for incision in those localities receiving supplementary runoff or interflow from outside the
natural drainage area. In the case of a soil slippage at Rwaza Hill, detailed stability calculations show
that the slope failure should be due to excessive water infiltration into the bottom of a trench. The
digging of the trench provoked an increase in the area drained to the slippage head by a factor of 6.
The phenomenon of ‘forward’ erosion is compatible with the existence of threshold combinations
of slope and drained area. For slopes steeper than 7–8B, the phase of regressive erosion does often
follow the forward incision event with a delay of several years or more.
Finally, the scanty data set now available for Rwanda suggests that the drainage area critical to
hillslope incision on the red-brown ferrallitic soils in Rwanda might be nearly twice as big as those in
North America.