UAV-based characterization of the Cuejdel Lake landslide dam (Romania) integrating LiDAR, photogrammetry, geophysics and hydrogeological properties

The fusion of UAV-based LiDAR and RGB surveys with geotechnical, geophysical, and hydrogeological field investigations enables a detailed characterization of the Cuejdel Lake landslide dam and its host landslide. To identify the main landslide features and to map the morphology of the study area, a 3D point cloud was generated from a UAV-based LiDAR survey covering an area of 126 ha on the western slope of the Muncelu Peak. Based on this dataset, a 3D surface model was constructed and textured using RGB imagery from a separate UAV-based photogrammetric survey, revealing the spatial distribution and characteristics of sedimentary facies within the eroded spillway outcrop.

A two-dimensional plane representing the results of an electrical resistivity tomography (ERT) survey across the landslide dam was integrated into the 3D model, allowing sedimentary facies to be linked to distinct resistivity zones. The orientation of intact stratigraphy measured in the field was incorporated and extrapolated until intersecting the ERT plane. In addition, representative facies were sampled for grain-size analysis. Thirteen infiltration tests conducted parallel to the ERT profile provided proxy permeability values that were also integrated into the model.

The investigations reveal that the feature previously interpreted as a single landslide actually consists of two distinct landslides, of which the northern landslide impounded Cuejdel Lake. Facies mapping shows a highly heterogeneous structure composed of large intact flysch blocks embedded in a low-permeability matrix of sand and clayey silt. Despite this heterogeneity, infiltration measurements indicate a relatively uniform permeability within the saturated phreatic zone, with values between 1 × 10⁻⁷ and 1 × 10⁻⁸ m s⁻¹. While facies distributions, laboratory analyses, and resistivity patterns indicate strong internal heterogeneity, the hydraulic behavior of the dam is controlled by the mixture of sand, silt and clay.

This comparatively impermeable structure facilitated rapid lake-level rise and temporary overtopping during the early stage of dam formation. However, geomorphic evidence, water marks on tree trunks, and historical records indicate that this initial overtopping phase was halted by winter-induced lake-level lowering, after which erosion shifted to progressive spillway incision at the landslide toe during the following season.