Increasing Spatio-Temporal Resolution for Monitoring Alpine Solifluction Using Terrestrial Laser Scanners and 3D Vector Fields

Christoph Holst,Heiner Kuhlmann,Tomislav Medic,Anna Schoch-Baumann,Jannik Janßen,Jan Blöthe,Lothar Schrott,Malte Dercks,Martin Blome,Berit Schmitz

doi:10.3390/rs13061192

Abstract

This article investigates the usage of terrestrial laser scanner (TLS) point clouds for monitoring the gradual movements of soil masses due to freeze–thaw activity and water saturation, commonly referred to as solifluction. Solifluction is a geomorphic process which is characteristic for hillslopes in (high-)mountain areas, primarily alpine periglacial areas and the arctic. The movement can reach millimetre-to-centimetre per year velocities, remaining well below the typical displacement mangitudes of other frequently monitored natural objects, such as landslides and glaciers. Hence, a better understanding of solifluction processes requires increased spatial and temporal resolution with relatively high measurement accuracy. To that end, we developed a workflow for TLS point cloud processing, providing a 3D vector field that can capture soil mass displacement due to solifluction with high fidelity. This is based on the common image-processing techniques of feature detection and tracking. The developed workflow is tested on a study area placed in Hohe Tauern range of the Austrian Alps with a prominent assemblage of solifluction lobes. The derived displacements were compared with the established geomonitoring approach with total station and signalized markers and point cloud deformation monitoring approaches. The comparison indicated that the achieved results were in the same accuracy range as the established methods, with an advantage of notably higher spatial resolution. This improvement allowed for new insights considering the solifluction processes.

Highlights

In the periglacial zones of high-alpine areas, the slow downslope movement of soil, induced by cyclic freezing and thawing, is a prevalent hillslope process
We establish a new workflow for the monitoring of slowly moving landforms with higher spatial and temporal resolution based on the terrestrial laser scanner (TLS) point clouds and 3D vector field representation, with a particular focus on solifluction lobes; We compare this approach with the established point-wise and point-cloud-based approaches, and analyse the advantages and disadvantages, as well as the information they can retrieve about solifluction; We evaluate the spatial distribution and deformation pattern of movement for a particular solifluction lobe
The magnitudes are systematically underestimated, as the iterative closest point (ICP) is sensitive to change detection only in the direction perpendicular to the observed surface. This means that this method is limited to monitoring discrete locations where the land surface normals are parallel with the solifluction direction; the developed feature detection and tracking workflow can be used to derive surface movements with similar measurement accuracy and fidelity to the established point-wise method, without being restricted to pre-determined marker locations, and produces a 3D vector field with high spatial resolution covering the whole study area

Summary

Introduction

In the periglacial zones of high-alpine areas, the slow downslope movement of soil, induced by cyclic freezing and thawing, is a prevalent hillslope process. Summarized under the term solifluction, the downslope movement involves needle-ice-creep, frost-creep, gelifluction and a plug-like deformation that is manifested in lobes, steps, sheets and stripes [1,2]. 1 m per year, the widespread distribution of solifluction processes above the tree line results in a significant contribution to overall mass transport in high-mountain settings [1]. Solifluction lobes are important indicators of past climate conditions. Developing a deeper process understanding of contemporary solifluction processes is vital for palaeoclimatological reconstructions [1,2]. The current trend in many earth and ecological sciences is to increase the spatial and temporal resolution of observations in order to better understand the underlying physical

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Discussion

Conclusion