Repeat LiDAR measurements of a lake-terminating glacier ice cliff

Abstract

The use of repeat LiDAR measurements to monitor processes of rockfall and landslides is well established in the geosciences, but only a limited number of studies have applied this method to study ice cliff processes and change at the termini of calving glaciers. In this study, the terminus of Fjallsjökull, an outlet glacier in SE Iceland, was scanned using a Riegl VZ-4000, a very-long-range laser scanner which has sufficient sensitivity to obtain laser reflections from ice surfaces and was programmed to acquire data automatically each hour, over a diurnal cycle (13th–14th August 2016). A total of 46 scans were captured across a 1300 m wide section of the terminus from a range of c. 1000 m, generating an average point spacing of 0.15 m. A 300 m wide area of interest was used in subsequent data processing for scans over a 28-hour period. Change analyses show that calving is the dominant mode of ice loss for the subaerial ice cliff, with 70% focussed in the bottom third (11 m) of the ice cliff. Calving events were distributed across the width and height of ice cliff face, but the two largest events (of volumes ~4000 m3 and ~3000 m3) appeared to begin with a small failure (< 10 m3) at the waterline which then propagated up-cliff 30 to 60 minutes later. Precursory deformation of the ice cliff face in the order of 2.00 m h-1 (greater than the average velocity of 0.15 m d-1) and forward rotation from the toe of the ice cliff is also observed prior to failure. Calving event scars are also observed to evolve between scans with smaller failures coalescing with subsequent failures. This study shows that a new generation of terrestrial laser scanners can be used to automatically monitor processes at the termini of calving glaciers at exceptionally high temporal and spatial resolutions.