Interpreting rockfall activity on an outcrop–talus slope system in the southern Japanese Alps using an integrated survey approach

Abstract Understanding relationships between meteorological conditions and rockfall activity is important for the management of rockfall risks as well as for the estimation of future rockfall activity under climate change. Seasonal activity of rockfall on talus slopes depends on the actual triggers of rockfall, which in turn may vary as well between outcrops because of different rockfall sources and the influence of topography. In this study, we interpret spatial and temporal variations in rockfall activity and related sediment transfer processes in an outcrop-talus slope system in central Japan through monitoring using rock temperature and moisture sensors, sediment traps, and time lapse cameras. In addition, tree-ring analysis has been added to understand possible changes of rockfall activity over longer time periods. Results suggest that the frequency of grouped gravel transport (i.e., soil creep) was higher than that of individual transport (i.e., rockfall and dry ravel) on the talus slope, whereas rockfall likely is the predominant process from the outcrops. Observations from sediment traps showed that coarse gravel is characterized by higher bounce heights and that it travelled longer distances than fine gravel. Rockfall from the outcrops was most active during dormancy of trees (which locally lasts from autumn to early spring) and can be related to freeze-thaw cycles. In contrast, rockfall and other sediment transfer processes were monitored on the talus slope all year round. Secondary movement of gravels on the talus slope, such as soil creep during rainfall events, likely triggers rockfall during times when the outcrops are not producing rockfall (i.e. from late spring to summer). Consequently, our results suggest that the spatio-temporal rockfall activity varies substantially between outcrops and talus slopes and that differences in process behavior should be taken into account in future disaster mitigation measures.