Himalaya mass-wasting: impacts of the monsoon, extreme tectonic and climatic forcing, and road construction

In mountainous regions, mass-wasting processes dominate landscape evolution and pose serious risk to life and socioeconomic development. In the Nepal Himalayas, annual rates of mass-wasting are primarily driven by the Asia Summer Monsoon (ASM), the strength of which is highly sensitive to changing global climate.  However, whilst relationships between precipitation intensity and suspended fluvial sediment flux in the Himalaya are well described, a longer-term empirical relationship between ASM strength and total mass-wasting volume has remained elusive. Here, we use a new 30-year landslide inventory for central-eastern Nepal to quantify an empirical relationship between ASM strength and total mass-wasting volume. As well as providing insight into how Himalaya hillslope denudation rates might change under possible future ASM strength scenarios, these data allow a quantification of how background rates of ASM-triggered mass-wasting have been perturbed by extreme climatic and tectonic forcing (e.g. earthquakes, storms) and anthropogenic activity (e.g. road building).

We find a strong exponential relationship (R² = 0.66 – 0.88) between total ASM precipitation and total ASM-triggered mass-wasting volume, suggesting that relatively small changes in ASM strength can lead to significant increases in mass-wasting.  This relationship also allows the calculation of a climate normalised annual rate of mass-wasting for the study region between 1988 and 2018. This normalised rate reveals several years (1993, 2002, 2015 – 2018) with mass-wasting rates perturbed significantly above the rates expected given the ASM strength. We find that the perturbations in 1993 and 2002 correlate with the occurrence of extreme cloud outburst or flood events, resulting in above-expected mass-wasting equivalent to that caused by 3.5 average ASM seasons. By contrast, the 2015 – 2018 perturbation is more complex. We interpret the perturbation in 2015 as being caused by landscape preconditioning associated with the M_w 7.9 Gorkha earthquake, which caused above-expected ASM-triggered mass-wasting equivalent to that caused by 2.0 average ASM seasons. However, the increased mass wasting across the period 2016 – 2018 is actually found to be the result of a sudden increase in road-construction, with mass-wasting due to road-tipping in this period equivalent to that caused by 2.6 average ASM seasons. These results show that, in the Himalayas, extreme events and human activity can cause significant hillslope denudation above that expected from background ASM-driven mass-wasting.