Impacts of climate change on debris flow frequency and magnitude in high mountain Asia

Abstract

Debris flows are a mixture of large amounts of sediment and water which are moving at high speeds. These types of mass movements are a common occurrence in High Mountain Asia (HMA) and can be very hazardous for the population. It is expected that climate change will influence the frequency and magnitude of debris flows due to changes in the availability of water and sediment associated with retreating glaciers, mountain greening, and changes in precipitation and temperature. However, there is a considerable amount of uncertainty about what the effects of climate change will be. Some studies have shown that climate change will decrease the frequency and magnitude of debris flows due to drier and warmer conditions. While others have shown an increase in debris flow magnitude and frequency due to shifts in precipitation and increased sediment availability due to weathering and glacier retreat. This study aims to quantify the effect of different parts of climate change in combination with land cover changes for 3 regions in HMA with very different climates: Bagrot valley (Pakistan/Karakoram), the Mustang district (Nepal/Himalaya) and Langtang Valley (Nepal/Himalaya). This study will be looking at transport-limited systems and will be done by using an extended version of the Sediment Cascade model (SedCas), introducing a 3rd hydrological response unit for glaciated areas adding on to the other two (vegetation and bedrock). Overall, it becomes clear that a vegetation increase results in a decrease in debris flow activity. The same is true for glacier decrease. Changes in precipitation extremes have been identified as an important factor and depending on if they increase or decrease, debris flow frequency and magnitude change in the same manner. Temperature increases show a more indirect effect, mostly affecting the amount of snow cover days and the elevation at which there is permanent snow cover present, resulting in increases in debris flow magnitude and frequency due to lack of snow cover. This study shows that changes in these climate factors are not consistent across HMA resulting in Bagrot having an increase in debris flow magnitude and frequency while Langtang sees an overall decrease during the time periods looked at (1951-2022). Mustang has a very large variability in response to climate change, having areas with an increase as well as areas with a decrease. This study gives a better understanding of the effect that the changing climate has on debris flow activity. For future studies it would be of interest to also look at supply-limited systems and at future climate projections to further this understanding. This better understanding can help the population of areas prone to debris flows to better adapt to these hazardous mass movements.