Capturing the geomorphological change of a sub-arctic riverbank through unique long term terrestrial laser scanning time series

Riverbank erosion and deposition are fundamental drivers of fluvial geomorphological change yet their long-term interannual dynamics remain poorly quantified at high spatial resolutions, particularly in sub-arctic environments. Existing studies typically rely on short monitoring periods or coarse-resolution remote sensing data, limiting the ability to resolve interannual variability, cumulative change, and the influence of hydroclimatic extremes. This study addresses these limitations by exploiting a unique over a decade long time series of high-resolution terrestrial laser scanning (TLS) point cloud data collected from the Pulmanki River in northern Finland. The main aims of this research are (1) to quantify the decadal riverbank erosion and deposition, and their interannual variability, (2) to investigate the key drivers of the observed geomorphic change, and (3) to evaluate how effectively long-term TLS data can capture these changes and their controlling mechanisms.

The study focuses on a single 18 meters high bank on one compound asymmetric meander bend of the Pulmanki River, where the experimental design was initially established already in 2012. The surface angle of the bank is 36° at the apex and it consists of horizontally bedded fluvio-lacustrine sediments. Annual TLS point cloud data have been collected using Riegl VZ-400i laser scanner, with consistent data acquisition geometry and robust georeferencing using real time kinetic global navigation satellite system (RTK-GNSS) measured reference points. Point cloud data have been acquired from the riverbank during spring and autumn field campaigns, enabling the assessment of both interannual and decadal-scale changes. Point cloud data were collected using multiple scanning locations and merged into a composite point cloud to ensure comprehensive data of the whole riverbank at centimeter-scale resolution. Three-dimensional geomorphic change will be quantified using a direct point cloud to point cloud comparison while accounting for surface orientation and measurement uncertainty. This will enable detection of both gradual bank retreat and episodic mass failures, as well as localized sediment accumulation. Particular emphasis is placed on the uncertainty quantification, including levels of detectable change and the robustness of volumetric erosion and deposition estimates over long monitoring periods. Finally, to investigate and interpret the drivers of the observed geomorphic change and its variability, long-term auxiliary information of river flow characteristics (using acoustic doppler current profiler, ADCP) and water level collected during field surveys together with climatic data (e.g., precipitation and temperature) will be analyzed.