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

Abstract

Abstract. Deforestation is associated with a decrease in slope stability through the alteration of hydrological and geotechnical conditions. As such, deforestation increases landslide activity over short, decadal timescales. However, over longer timescales (0.1–10 Myr) 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. In the past 11 Myr, active continental rifting and tectonic processes have forged two parallel mountainous rift shoulders that are continuously rejuvenated (i.e., actively incised) through knickpoint retreat, enforcing topographic steepening. In order to link deforestation and rejuvenation to landslide erosion, we compiled an inventory of nearly 8000 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. Moreover, to account for the impact of rock strength on both landslide occurrence and knickpoint retreat, we limit our analysis to rock types with threshold angles of 24–28∘. Rejuvenated landscapes were defined as the areas draining towards Lake Kivu or Lake Tanganyika and downstream of retreating knickpoints. We find that shallow landslide erosion rates in these rejuvenated landscapes are roughly 40 % higher than in the surrounding relict landscapes. In contrast, we find that slope exerts a stronger control on landslide erosion in relict landscapes. These two results are reconciled by the observation that landslide erosion generally increases with slope gradient and that the relief is on average steeper in rejuvenated landscapes. The weaker effect of slope steepness on landslide erosion rates in the rejuvenated landscapes could be the result of three factors: the absence of earthquake-induced landslide events in our landslide inventory, a thinner regolith mantle, and a drier climate. More frequent extreme rainfall events in the relict landscapes, and the presence of a thicker regolith, may explain 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 again be a consequence of a thinner regolith mantle and/or 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.

Characteristics and Distribution of Landslides in the Populated Hillslopes of Bujumbura, Burundi

“…In the NTK Rift, erosion rates due to recent shallow landslides have been observed to be higher in the rejuvenated landscapes (i.e., actively incised and more recent) than in the surrounding relict landscapes, upstream of the retreating knickpoints and outside the rift shoulders [38]. This difference is mainly explained by the steeper topography in the rejuvenated landscapes.…”

“…The NTK Rift, in which Bujumbura is located, is situated in the central section of the western branch of the East African Rift (Figure 1a). In this region, tectonic uplift, accompanied with seismic activity and faulting, has initiated landscape rejuvenation through knickpoint retreat, enforcing topographic steepening [16,[36][37][38]. This tectonic setting, combined with a tropical climate that favors the occurrence of intense rainfall events and deep weathering, makes the region a landslide hotspot in Africa [6,16].…”

“…This tectonic setting, combined with a tropical climate that favors the occurrence of intense rainfall events and deep weathering, makes the region a landslide hotspot in Africa [6,16]. It is characterized by a large diversity of landslides of various sizes (up to tens of km 2 ) and ages [17,33,35,38,39]; some being much older than 10,000 years [16]. Most of the recent, often shallow landslides (i.e., being <5 m deep) that occurred over the last decades were triggered by rainfall; and a general link between rainfall seasonality and their occurrence has been recently demonstrated [16,33].…”

“…Landslides in the rejuvenated landscapes are more abundant but smaller. This may be a consequence of the higher levels of seismic activity that fracture and weaken the slope material, and a thinner regolith thickness in the rejuvenated landscapes [38]. Many of the recent landslides also occur along roads.…”

“…Another important human disturbance in the region is associated with deforestation. Depicker et al [38] showed that deforestation in the NTK Rift typically initiates a peak in landslide occurrence that lasts approximately 15 years and increases landslide erosion 2-8 times during that period before it eventually falls back to a level similar to forested conditions. Quarrying of riverbeds for construction material-frequent close to urban centers-is also impacting river systems and consequently landslide activity.…”