Dynamics of deep-seated landslides in the tropics: natural and anthropogenic controls

Abstract:

Because they commonly show persistent and long-term motion, slow-moving deep-seated landslides often represent exceptional natural laboratories to study the mechanisms and controls on the dynamics of unstable hillslopes. Here we investigate and compare two deep-seated landslides in the tropical environment of the Kivu Rift (DR Congo). One is a thousand-year-old landslide, that, sited in the rapidly expanding city of Bukavu, experienced rapid and informal urbanisation over the last decades. The second is only a few decades old and has been largely preserved from anthropogenic activity. While these two landslides share similar environmental conditions – at the notable exception of the human activities, we aim at investigating how environmental conditions (climate, geology, tectonics) and external forcings (rainfalls, earthquakes, urbanisation) influenced their activity. We focus over two temporal resolutions: the week-to-week landslide kinematic is detailed using 5 years of temporally dense kinematic data from satellite interferometry and pixel tracking applied on radar, optical satellite and UAS images; the yearly-to-decadal kinematics are analysed using aerial photographs from the ‘50s and ‘70s and high-resolution satellite imagery. We find that seasonal rainfall is the principal regulator of weekly to yearly motion of both landslides, with a very tied relationship between change in pore-water pressure in the slope and landslide kinematic. But our results also highlight the influence of human activity on landslide motion over decadal timescales: we show that urbanisation interfered with the dynamic of the urbanised landslide through a series self-reinforcing feedbacks involving landslide motion, rerouting of water flows and pipes rupture. Such relationship is not surprising, but as hillslopes of the world’s cities are being urbanised at accelerating paces, the observation of such causative mechanisms urges for a better understanding of how anthropogenic activity influences deep-seated landslides. This would ensure the valid evaluation of landslide hazard and the optimisation of mitigation strategies in the area, but also across the other many cities of the tropics where similar environmental and societal conditions are met.