Geomorphic Diversity Loss Following Post-Flood Interventions

Extreme flood events naturally act as drivers of geomorphic heterogeneity, creating complex channel-floodplain systems characterized by diverse bedforms, bank erosion features, and sediment splays. However, the subsequent phase of flood recovery often involves rapid and extensive anthropogenic interventions that counteract these natural processes. This study evaluates the loss of geomorphological diversity in montane streams of the Opava River Basin (Czechia) by analyzing the conflict between natural recovery and technical river management.

The research methodology employs a multi-temporal approach combining field investigation and remote sensing. While systematic geomorphological field mapping was conducted at two key stages—immediately following the 2024 flood to record the "pristine" impact and one year later to assess the final state—UAV photogrammetric campaigns were executed repeatedly throughout the post-flood year. This high-frequency monitoring provided multiple temporal windows, allowing us to track the precise sequence of changes and distinguish between gradual natural adjustments and abrupt anthropogenic modifications.

The analysis of this time-series data reveals a significant trajectory of channel simplification:

• Erasure of Complexity: The repeated UAV models document how initial flood-created features (cut banks, gravel bars) were systematically removed by engineering works. In reaches subjected to heavy machinery, geomorphic diversity was reduced by up to 100%.
• Dynamics of Intervention: The multiple time windows highlighted that the most severe loss of diversity often occurred weeks or months after the flood event itself, during the "recovery" phase. Moreover, this loss of diversity was significantly stronger in proximity to habited areas compared to natural river reaches.
• Impact of Intensity: We identified a direct correlation between the intensity of technical adjustments and the degree of channel homogenization. While "soft" interventions allowed for the partial preservation of flood-induced forms, heavy engineering works resulted in the complete artificial straightening of the thalweg.

The study demonstrates that high-resolution UAV monitoring is essential for capturing the transient states of river recovery. The findings suggest that current post-flood protocols often prioritize rapid hydraulic streamlining at the expense of ecological integrity, effectively "resetting" the river's geomorphic value to a pre-flood, or even simpler, state.