Geomorphic impact and assessment of flexible barriers using multi-temporal LiDAR data: The Portainé mountain catchment (Pyrenees)

Multi-temporal digital elevation models (DEMs) obtained from airborne LiDAR surveys are widely used to detect geomorphic changes in time and quantify sediment budgets. However, they have been rarely applied to study the geomorphic impact of engineering structures in mountain settings. In this study, we assessed the influence and behavior of flexible sediment retention barriers in the Portainé catchment (Spanish Pyrenees), using three LiDAR data sets (2009, 2011 and 2016) that covered a 7-year period. Densely forested mountainous areas present some limitations for reliable DEM analysis due to spatial variabilities in data precision, accuracy and point density. A new methodological approach for robust uncertainty analysis along channels, based on changes in cross-sectional elevations, was used to discriminate noise from real geomorphic changes. The obtained results indicated that erosion occurs along most reaches covering a large area, whereas deposition is localized in specific areas such as those upstream of sediment retention barriers and in the debris cone. Despite the presence of 15 flexible sediment retention barriers, the channels presented net degradation during both 2009-2011 and 2011-2016, with 2838 and 147 m3 of material exported from the basin, respectively. For the same periods, the barriers retained 33% and 25% of the total deposition (up to 1300 m3 per barrier), respectively, but also induced lateral and downstream incision, the latter reaching 703 m3 for a single barrier. We detected a horizontal displacement of the net of up to 1.2 m in filled barriers, resulting from net flexion. The interference of the natural river evolution by defense measures has resulted in a complex erosion-deposition pattern. The presented methods show high potential for the hydrogeomorphic study of mountain catchments, especially for a high-resolution assessment of flexible barriers or other engineering structures in remote areas.