A conceptual approach for deciphering mega-activity of debris flow fans and polygenetic fans for climate change anticipation

Debris flows are amongst the most dangerous geomorphic processes world-wide. In alpine environments, settlements and infrastructure are commonly located on debris flow fans. An over-proportional increase in debris flow activity has been shown to occur in response to climate warming under transport-limited conditions. Enhanced debris flow activity might not only affect transport volumes on existing fans. It could trigger a phase shift from regular debris flow activity to mega-activity leading to the generation of outsized fans. However, sediment fan formation during phases of exceptionally high debris flow activity remains largely unexplored: While more than 65 outsized sediment fans have been identified throughout the European Alps, only for three of them detailed sub-surface information exists. Although outsized-fan activity may provide invaluable information on the “things to come” in the next decades, the potential of outsized fans as analogues for future debris flow activity is so far unknown. To enhance our understanding of the magnitude, style, and landform-building potential of future debris flow activity, this research project investigates the following research question: What can current and past mega-activity on alpine debris flow fans teach us about future debris flow activity? We will investigate the spatio-temporal patterns of debris flow activity on a small, a medium, and an outsized alpine debris flow fan over decadal, centennial, and millennial timescales. On the small spatio-temporal scales, we will use geomorphic mapping, aerial photography, dendrochronologic dating, and digital elevation model differencing to investigate links between decadal climate change and debris flow activity. On the larger scales, we will combine several methods of near surface geophysics to derive the subsurface architecture of the sediment fans and thus gain insights into their formation history. We will use this information to calibrate a landscape evolution model of outsized fan formation to test the plausibility of the inferred formation history and ensure transferability of the results. This research project systematically deciphers the magnitude and outsized fan building potential of massively changing debris flow activity in the past to anticipate phase shifts in the foreseeable future. Key Words: Debris flow, polygenetic sediment fan architecture, climate change, outsized fan, geophysical tomography