Natural hazards evolution in a context of climate evolution and infrastructure development: the Kali Gandaki valley case, West-Central Nepal.

Highest geomorphic activity in central Nepal is mostly driven by monsoon rainfall, yet the recent development of infrastructure has increased this activity and the risks for the locals and travelers. Our aim is to illustrate recent cascading hazards and their interactions with, and impacts on, socio-economic development along an important road corridor. We focus on the middle Kali Gandaki valley reach, within the High Himalayan Crystalline Series HHC where the river deeply incised and the topography is characterized by steep hillslopes and high relief.  This 20-25 km long reach experiences strong monsoon rainfall enhanced by orographic effects, with rainfall rates >2000 mm/a. In the last years between 2018 to 2021 the monsoon season was very strong and experienced several strong and long lasting rainstorm events with amplified catastrophic events such as debris flows, landsliding and river activities. On the basis of repeated field surveys, satellite images (Pléiades, Sentinel and Planet) analysis, Global Precipitation Measurement (GPM) data, UAV, river flow seismic noise records, we observed that once destabilized, hillslopes and steep, small tributary catchments evolved very rapidly during the years, all the more since road constructions for the upgrading to a 2-lane road contributed to destabilization of the hillslopes. This rapid disequilibrium has several consequences. (1) First it reworked old colluvium deposits, including old landslide material, old glacial and/or fluvial alluvium and related lacustrine deposits, hence revealing a former, complex paleo-topography of this deep valley (as observed north of Ghasa, along the Kahiku khola and Kali Gandaki). (2) Second, in providing looser material, it has accelerated the cascading system and transfer of sediments into the main Kali Gandaki River, as shown in the Rupse site, famous for its waterfall, and that was destroyed by a debris flood (July 20, 2020) generated by intense rainfall that triggered landslides in the upper catchment, with impacts at the junction with Kali Gandaki (destruction of road, bridge, settlements). Similarly, the Thaplyang site, active since 2014, was repeatedly affected by strong rainfall since 2018, with progressive erosion of an old landslide material – the active area increased from 9100 m² (March 2018) to 9600 m² (Oct. 2018) and 32300 m² (Nov 2021) – hence threatening small settlements upstream. (3) Third, the repeated disasters (river bank collapses and settlements destruction; traffic obstruction) affect the tourism economy and development along this major link between south China and north India. Further work, including SAR analyses, is ongoing to better quantify the overall sediment exported volumes and the impacts of this changing geomorphology on future infrastructure development and settlements.