Geomorphic Process Chains in High-Mountain Regions-A Review and Classification Approach for Natural Hazards Assessment

Populations and infrastructure in high mountain regions are exposed to a wide range of natural hazards, the frequency, magnitude, and location of which are extremely sensitive to climate change. In cases where several hazards can occur simultaneously or where the occurrence of one event will change the disposition of another, assessments need to account for complex process chains. While process chains are widely recognized as a major threat, no systematic analysis has hitherto been undertaken. We therefore establish new understanding on the factors that directly trigger or alter the disposition for subsequent events in the chain and derive a novel classification scheme and parameters to aid natural hazard assessment. Process chains in high mountains are commonly associated with glacier retreat or permafrost degradation. Regional differences exist in the nature and rate of sequencing-some process chains are almost instantaneous, while other linkages are delayed. Process chains involving rapid sequences are difficult to predict, and impacts are often devastating. We demonstrate that process chains are triggered most frequently by progressive failures, being the result of gradual landscape weakening and not due to the occurrence of a distinct process. If fluvial processes are part of the process chain the reach (or mobility) of process chains is increased. Increased mobility can also occur if sediment deposition areas along river channels are activated. As climate changes causes glacial environments to transform into sediment-rich paraglacial and fluvial landscapes, it is expected that the mobility of process chains will increase in the future. Communities and their infrastructure located in mountain regions of the world are particularly susceptible to natural hazards, such as floods and landslides. While these events on their own can be bad enough, when such events occur at the same time or cause a chain-reaction of different processes, consequences can be worse and extend far downstream. In view of this, we have undertaken a first comprehensive review of 51 relevant events occurring across different mountain regions globally to better understand how these process chains develop, and to identify which conditions increase the likelihood and or size of these events. A key finding is that process chains most frequently begin as the result of a mountain slope gradually becoming unstable over time, and in these cases, no other trigger such as an earthquake or unusually wet or hot weather are necessary. Far-reaching process chains occur when rivers are involved and when additional gravel and debris sources are added to the mix. As a consequence of climate warming, icy mountain landscapes are thawing and become more unstable, increasing the risk of such process chains in the future. High mountain hazard process chains commonly are associated with glacier retreat or permafrost degradationProcess chains are triggered most frequently by progressive failure, which is a result of long-lasting landscape weakeningAn important implication of climate change is that we can expect an increase in the reach (horizontal length along the flow path) of process chain events in the future