Abstract ID: 3.12863 | Accepted as Talk | Poster | TBA | TBA

Debris flows are fast-moving masses of rock, soil, and water, which occur in mountain areas all over the world. Debris flows achieve maximum discharges that are many times greater than those associated with floods and are therefore often hazardous to people and infrastructure. Climate change is altering the high mountain landscapes, exposing loose material as glaciers retreat and affecting slope stability by changes in freeze-thaw cycles and permafrost degradation. However, while these changes can increase the water availability in mountain catchments, increasing the total sediment yield, changing land cover such as greening may also favor a reduction of runoff generation and sediment supply and, therefore, reduce sediment yield activity. The combined impact of glacier retreat together with other changes in the high mountain landscape on sediment transport and debris flows processes is not yet fully understood. We aim to quantify the impact of the change of land cover (glacier retreat and mountain greening) on the water availability, debris flow activity and sediment yield in both transport-limited and supply-limited upland catchments. We address it by extending the sediment cascade model (SedCas), expanding the available hydrological response units to bedrock, vegetated and glaciated parts of the catchment and conceptualizing different sediment recharge regimes to mimic supply-limited catchments. We find that in the case study of transport-limited catchments, from 1950 to 2022, glacier retreat decreased the water supply, therefore the potential total sediment yield and the total number of potential events also decreased. An increase in the vegetation cover enhanced the effect further, by increasing the soil storage and limiting the peaks in water discharge. For supply-limited catchments, the inter-annual distribution of sediment recharge affects the seasonality of debris flow and flood events. Unless sediment recharge is constant throughout the year, sediment storage empties toward the end of monsoon season, limiting the number of debris flows towards the second half of the year. Our findings shed light on the debris flow and flood hazard in the data-scarce areas of HMA and highlight the importance of considering regional climate and land cover conditions for hazard assessment in addition to region-wide estimation of glacier retreat.