River surface sediment grain size and sorting changes after several experimental floods in the lower Spöl River (Switzerland)

Abstract

Purpose Experimental floods released from dams (e-floods) are increasingly used as a management tool to restore dynamics in regulated rivers, yet long-term field evidence of their geomorphic effectiveness remains limited. This study evaluates changes in river bars’ surface grain-size distributions following seven e-floods along the dam-regulated Spöl River, Switzerland, reach influenced by a major tributary. Materials and methods Grain-size data were obtained using field methods, allowing assessment of both event-based and cumulative longer-term sediment responses to e-floods. Surface sediments were surveyed at seven exposed gravel bars before and after each e-flood using Wolman pebble counts, and changes in grain-size percentiles, sorting, and variability were analysed alongside channel morphology derived from drone-based photogrammetry. Results and Discussion Overall, the long-term results indicate coarsening of bar surface across all size fractions, with the strongest responses observed in the finer percentiles. Changes in D16 reached up to 60%, whereas coarser fractions (D84 and D95) exhibited more moderate changes of up to 26%, reflecting selective entrainment and partial mobility of the material after the seven e-floods. Still, some experimental floods caused stronger coarsening (2019 e-flood 1, 2023), while others exhibited a more generalized fining (2025), and in some cases, little to no change (2019 e-flood 2, 2021). Recent fining during 2024–2025 is attributed to human disturbances and reduced e-flood magnitude. In contrast, significant bed coarsening was observed in 2023 despite the absence of a scheduled experimental flood in 2022, potentially reflecting the influence of additional dam flow releases for maintenance purposes that may have preferentially removed finer surface sediments and an enhanced sediment supply from the tributary. Spatially, the largest grain-size adjustments occurred near the tributary and in downstream sections of the study reach. Analysis of grain-size sorting and variability reveals an overall increase in bed heterogeneity. Conclusions These findings highlight the importance of sediment supply, e-flood sequencing, and spatial context in shaping geomorphic responses and provide practical insights for designing effective dam-release flood strategies.

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