The importance of monitoring interval for rockfall magnitude-frequency and sequence characterisation
Abstract
Rockfalls commonly exhibit power law size-frequency distributions. The mechanisms that drive these distributions, and the spatial and temporal scales over which they vary, remain to be fully understood. Key to this is detecting the smallest and most frequent rockfalls, which are often underrepresented in inventories based on the resolution of monitoring relative to their occurrence. A challenge arises when contiguous events in a single monitoring interval are recorded as one, producing ambiguity in event location, timing, volume, and frequency. Approaches to defining measurement intervals that minimise these uncertainties are therefore of value. To address this, we use an hourly dataset of 8987 point clouds that describe changes to a cliff experiencing frequent rockfalls over a 10-month period. When monitoring with long intervals, here ≥ 12 h, the probability distribution of measured rockfall volumes remains apparently constant. Below this interval, increasingly short intervals result in a nonlinear increase in the number and a reduction in the volume of observed rockfalls. Predicting individual block size based on rock mass structure alone is therefore problematic over short timescales, with rockfall geometry only converging with that of the rock mass over longer monitoring intervals. Although magnitude-frequency distributions aggregate behaviour across the cliff, the pattern within our observations also holds for rockfall sequences prior to the largest events in our inventory. For these, we identify recurrent patterns of rockfall from the failing mass, alongside deformation within the footprint of the ultimate failure prior to collapse. Our data imply that small rockfalls cannot be considered independent in space or time, with the patterns we reveal indicating a progressive failure mechanism. Without further monitoring and characterisation of these events, identifying rockfall triggers and accurately constraining the hazard they pose is likely to remain difficult.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMNH034..08W
- Keywords:
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- 1810 Debris flow and landslides;
- HYDROLOGY;
- 1826 Geomorphology: hillslope;
- HYDROLOGY;
- 4302 Geological;
- NATURAL HAZARDS;
- 4313 Extreme events;
- NATURAL HAZARDS