Abstract The paper reports on a geodiversity assessment method at the local scale based on spatial multi-criteria analysis (S-MCA). The study area consists of two geomorphosites: Derborence and Illgraben, located in the Swiss Alps and representative of different types of morphodynamics. Apart from glacial and fluvial landforms, the morphology of these two sites is largely due to extreme phenomena (rockslides, torrential processes). Both areas were incorporated in the list of Swiss geosites for their geomorphological interest. The geodiversity assessment criteria were selected and presented on five factor maps: the relief energy (relative height), the landform fragmentation, the landform preservation, the geological setting and the hydrological factor. The factor maps were aggregated using the weighted linear combination (WLC) technique. The accuracy of the final geodiversity maps of Derborence and Illgraben was verified during a field inspection. The final maps were qualitatively assessed in terms of the geodiversity value of specific areas within the selected geomorphosites. The proposed method allows assessing the intrinsic geodiversity differentiation of large geosites, in particular in protected and conserved areas (PCAs). The results can be used for purposes such as the designation of most valuable parts of the area for the preservation of certain abiotic features, spatial planning or tourism management, at least in the alpine fold mountains.

Abstract Flood effectiveness observations imply that two families of processes describe the formation of debris flow volume. One is related to the rainfall–erosion relationship, and can be seen as a gradual process, and one is related to additional geological/geotechnical events, those named hereafter extraordinary events. In order to discuss the hypothesis of coexistence of two modes of volume formation, some methodologies are applied. Firstly, classical approaches consisting in relating volume to catchments characteristics are considered. These approaches raise questions about the quality of the data rather than providing answers concerning the controlling processes. Secondly, we consider statistical approaches (cumulative number of events distribution and cluster analysis) and these suggest the possibility of having two distinct families of processes. However the quantitative evaluation of the threshold differs from the one that could be obtained from the first approach, but they all agree in the sense of the coexistence of two families of events. Thirdly, a conceptual model is built exploring how and why debris flow volume in alpine catchments changes with time. Depending on the initial condition (sediment production), the model shows that large debris flows (i.e. with important volume) are observed in the beginning period, before a steady-state is reached. During this second period debris flow volume such as is observed in the beginning period is not observed again. Integrating the results of the three approaches, two case studies are presented showing: (1) the possibility to observe in a catchment large volumes that will never happen again due to a drastic decrease in the sediment availability, supporting its difference from gradual erosion processes; (2) that following a rejuvenation of the sediment storage (by a rock avalanche) the magnitude–frequency relationship of a torrent can be differentiated into two phases, the beginning one with large and frequent debris flow and a later one with debris flow less intense and frequent, supporting the results of the conceptual model. Although the results obtained cannot identify a clear threshold between the two families of processes, they show that some debris flows can be seen as pulse of sediment differing from that expected from gradual erosion.