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Earth Surface Dynamics An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/esurf-2020-87
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/esurf-2020-87
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  26 Oct 2020

26 Oct 2020

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This preprint is currently under review for the journal ESurf.

Interactions between deforestation, landscape rejuvenation, and shallow landslides in the North Tanganyika – Kivu Rift region, Africa

Arthur Depicker1, Gerard Govers1, Liesbet Jacobs1, Benjamin Campforts2, Judith Uwihirwe3,4, and Olivier Dewitte5 Arthur Depicker et al.
  • 1KU Leuven, Department of Earth and Environmental Sciences, Division of Geography and Tourism (Celestijnenlaan 200E, 3001 Heverlee, Belgium)
  • 2CSDMS, Institute for Arctic and Alpine Research, University of Colorado at Boulder (Boulder, CO, USA)
  • 3University of Rwanda, Department of Soil and Water Management, Faculty of Agricultural Engineering (Street KK 737, Kigali, Rwanda)
  • 4Delft University of Technology, Faculty of Civil Engineering and Geosciences, Department of Water Management (Stevinweg 1, 2628 Delft, The Netherlands)
  • 5Royal Museum for Central Africa, Department of Earth Sciences (Leuvensesteenweg 13, 3080 Tervuren, Belgium)

Abstract. Deforestation increases landslide activity over short, contemporary timescales. However, over longer timescales the location and timing of landsliding is controlled by the interaction between uplift and fluvial incision. Yet, the interaction between (human-induced) deforestation and landscape evolution has hitherto not been explicitly considered. We address this issue in the North Tanganyika-Kivu Rift region (East African Rift). In recent decades, the regional population has grown exponentially and the associated expansion of cultivated and urban land has resulted in widespread deforestation. On a much longer time scale, tectonic uplift has forged two parallel mountainous Rift shoulders that are continuously rejuvenated through knickpoint retreat, enforcing topographic steepening. In order to link deforestation and rejuvenation to landslide erosion, we compiled an inventory of nearly 8,000 recent shallow landslides in Google Earth© imagery from 2000–2019. To accurately calculate landslide erosion rates, we developed a new methodology to remediate inventory biases linked to the spatial and temporal inconsistency of this satellite imagery. We find that erosion rates in rejuvenated landscapes are roughly 40 % higher than in the surrounding relict landscapes, upstream of retreating knickpoints and outside of the Rift shoulders. This difference is due to the generally steeper relief in rejuvenated landscapes which more than compensates for the fact that rejuvenated slopes, when compared to similarly angled slopes in relict zones, often display a somewhat lower landslide erosion rate. These lower rates in the rejuvenated landscapes could be the result of a drier climate, the omission of earthquake-induced landslide events in our landslide inventory, and potentially a smaller regolith stock. More frequent extreme rainfall events in the relict zones, and possibly the presence of a thicker regolith, cause a stronger landslide response to deforestation compared to rejuvenated landscapes. Overall, deforestation initiates a landslide peak that lasts approximately 15 years and increases landslide erosion by a factor 2 to 8. Eventually, landslide erosion in deforested land falls back to a level similar to that observed under forest conditions, most likely due to the depletion of the most unstable regolith. Landslides are not only more abundant in rejuvenated landscapes but are also smaller in size, which may be a consequence of the seismic activity that fractures the bedrock and reduces the minimal critical area for slope failure. With this paper, we highlight the importance of considering the geomorphological context when studying the impact of recent land use changes on landslide activity.

Arthur Depicker et al.

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Short summary
We investigated how shallow landslide occurrence is impacted by deforestation and rifting in the North Tanganyika-Kivu Rift region (Africa). We developed a new approach to calculate landslide erosion rates based on an inventory compiled in biased Google Earth imagery. We find that deforestation increases landslide erosion with a factor 2 to 8 and for a period of roughly 15 years. However, the exact impact of deforestation depends on the geomorphic context of the landscape (rejuvenated/relict).
We investigated how shallow landslide occurrence is impacted by deforestation and rifting in the...
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