A complete rockfall inventory across twelve orders of magnitude.

Understanding the magnitude-frequency relationship of rock falls is one of the most important issues for both geomorphologists assessing sediment budgets as well as public stakeholders evaluating rock fall hazards. Multi-temporal Terrestrial Laser Scanning (TLS) surveys, or more general LiDAR, is often applied to produce rock fall inventories of event magnitudes and their frequency. However, LiDAR-based rock fall inventories systematically miss or underestimate both ends of the magnitude bandwidth.

Here we present the first attempt of a complete rock fall inventory including the full spectrum of magnitudes, ranging from fragmental rock falls (cm³) to Bergsturz-sized events (10⁶ m³). We combine rock fall inventories derived from multi-temporal TLS campaigns over six years, rock fall collectors and the historic record in a previously intensely investigated study area (Reintal, German Alps). We investigate which factors – such as structural geology, systematic sampling limitations or different rock fall processes – can lead to possible misinterpretation of rock fall inventories regarding geomorphic systems.

The study shows that (i) LiDAR-based rock fall inventories do not cover the full spectrum of rock fall magnitudes due to their limitations in temporal and spatial resolution, (ii) structural geological features control the magnitude/frequency relation beyond the roll-over of these inventories and (iii) taking fragmentation as well as a clear distinction between rock fall processes into account when analysing rock fall inventories is crucial.