Structural Data of Unconsolidated Sediments from Point Clouds on Coastal Cliffs of Mecklenburg-Western Pomerania

Predicting the likelihood of collapses and landslides on the German Baltic Sea coast cliffs requires a wide range of geological, hydrological, and climate data. Point clouds and images from drone surveys constitute a significant part of the data.

The cliffs predominantly consist of Quaternary sediments of glacial origin with highly variable properties, often intricately interwoven. The glacial processes that contributed to these sediments' formation, shaping, and modification left heterogeneous deposits and various glacial-tectonic structures such as joints, shear planes, and oriented stones. These structures are crucial for assessing failure probabilities in cliff areas and are necessary for engineering geological slope stability analysis.

CloudCompare is an open-source software supporting various point cloud analyses. It includes a FACETS plugin for extracting planes from 3D point clouds of rock bodies. The identification of discontinuities has been performed and validated by various authors using the FACETS plugin on hard rock exposures. We are testing the plugin for mapping discontinuities in unconsolidated sediments.

Unconsolidated sediments like glacial till and varved silts reveal glacial discontinuities in cliff exposures. These can be documented in the field but require significant time. In point clouds, facets can be calculated using the plugin in a single step. However, unlike joints measured with a compass, these are always open surfaces on the cliff. While their formation may relate to joint systems, additional factors such as flaking, rolling, erosion, drying, frost wedging, and root growth may contribute to or independently cause the formation of these facets.

We use point clouds generated from drone surveys of three cliff locations. These sites differ significantly in their geological structure and glacial deformation history. The facets are calculated from the point clouds and validated using structural data from engineering geological coastal surveys conducted in the past and our recent fieldwork. The FACETS plugin is suitable for capturing open joint surfaces on cliffs in unconsolidated sediments. However, care must be taken to ensure that the exposure of the steep coastal section does not dominate the measured discontinuity data. Slope-parallel planar surfaces in unconsolidated sediments are not always open joints. Also, shear planes and oriented stones are challenging to detect. Shear planes rarely form open surfaces due to frost wedging, and the long axes of stones cannot be calculated with the plugin method due to their rounding.

The method is well-suited for rapid and reliable documentation of joints. Given the considerable annual coastal retreat of several meters at some locations, the FACETS method makes it possible to create a time series for joints to find potential changes in orientation, dip, and joint density. These structural datasets are particularly valuable for engineering geological slope stability calculations. Specifically, these data could be integrated into training deep learning algorithms as additional features to support the automatic identification of sediments forming the cliffs.