84-10 Bridging the Waterline: Multitemporal Sonar and Mobile Laser Mapping of Channel Change

Abstract:

High-resolution change detection in gravel-bed rivers is commonly divided by the waterline: laser scanning captures exposed banks and bars, whereas sonar resolves the submerged channel. We present a multitemporal workflow integrating mobile laser scanning and multibeam sonar to map channel adjustment along a meandering reach of the Oulanka River in northeastern Finland. Repeat surveys were conducted during open-water periods and georeferenced within a common GNSS-based reference framework. Laser point clouds were classified to separate terrain and vegetation, sonar observations were filtered and corrected for vessel motion and water-level variation, and the datasets were merged into seamless topobathymetric surfaces. Surface differencing was then used to identify erosion and deposition across the channel bed, point bars, banks, and the wet–dry boundary.

Preliminary comparisons reveal spatially coherent patterns of bed-level change that cannot be reconstructed from either sensor alone. Sonar resolves pool, riffle, and thalweg adjustments beneath the water surface, while mobile laser scanning captures bank retreat, bar-surface modification, and exposed sediment storage at finer local detail. The combined approach also exposes important uncertainty sources, including variable point density, acoustic shadowing, vegetation, changing water levels, platform trajectory, and cross-sensor registration. These effects must be incorporated into spatially variable levels of detection before sediment budgets are interpreted.

The Oulanka case demonstrates how repeated, complementary close-range surveys can provide continuous wet–dry observations of gravel-bed river morphodynamics. The workflow supports event-scale change detection, sediment-budget analysis, and evaluation of how floods and seasonal flow variability reorganize channel morphology.

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