Background

This study explores morphologic changes in the Ahr River, Germany, caused by the high-energy flood in mid-July 2021. This study aims to assess the flood's significance as a morphological driver using the Lateral Mobility Index (LMI), as well as the impact of infrastructure on morphodynamics in terms of the formation of mud deposits and stagnant water pools, considering three focus areas of about 1 to 5 km river length. The three focus areas cover differences in the valley morphology as well as near natural and anthropogenically affected sections.

Results

The LMI is derived from orthophotos from 1998 to 2022, and based on the surface area of the old and the new channel. It describes the sum of new and abandoned channel area in relation to the previous channel area. For the 2021 high-energy event, an increased LMI suggests an increasing change in river course and morphology. Post-flood, the LMI in focus areas 1 (furthest upstream, LMI of 1.5) and 3 (at the Rhine confluence, LMI of 3.2) surpasses the historic LMI by a factor of about 3. Focus area 3 exhibits the highest activity, while focus area 2 (LMI between 0.25 and 0.70) the lowest. The area of mud deposits and stagnant water pools increased immediately after the flood, especially in focus area 3. Over the following months, initial post-flood mud deposits not only washed away but also rebuilt to some degree later on.

Conclusions

The LMI enabled to quantitatively identify a rare and significant morphological disruption in the Ahr River’s lateral development in three focus areas, and thus, serves as additional proof that the flood of mid-July 2021 was a high-energy event impacting the local river geomorphology. Infrastructure impacted the formation of mud deposits and stagnant water zones during and after the flood, with additional influence from factors such as driftwood and valley width. The data suggested a phase of resuspension of sediments after the flood, likely due to cleanup and ad hoc river management, highlighting the need for continued river monitoring.

中文翻译：

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评估 2021 年 7 月中旬的高能洪水作为德国阿尔河谷人为开发的形态驱动因素与基础设施的相互作用

背景

本研究探讨了 2021 年 7 月中旬高能洪水引起的德国阿尔河的形态变化。本研究旨在利用横向流动指数 (LMI) 以及考虑到河流长度约 1 至 5 公里的三个重点区域，基础设施对泥浆沉积物和死水池形成方面的形态动力学的影响。这三个重点区域涵盖了山谷形态的差异以及接近自然和人为影响的部分。

结果

LMI源自1998年至2022年的正射影像，并基于新旧河道的表面积。它描述了新的和废弃的通道面积相对于先前通道面积的总和。对于 2021 年高能事件，LMI 的增加表明河道和形态的变化越来越大。洪水后，重点区域 1（最远上游，LMI 为 1.5）和重点区域 3（莱茵河交汇处，LMI 为 3.2）的 LMI 超过历史 LMI 约 3 倍。重点区域 3 表现出最高的活动，而焦点区域 2（LMI 在 0.25 和 0.70 之间）最低。洪水过后，泥浆沉积物和积水池的面积立即增加，特别是在重点区域3。在接下来的几个月里，最初的洪水后泥浆沉积物不仅被冲走，而且后来又在一定程度上得到了重建。

结论

LMI 能够定量识别三个重点区域中阿尔河横向发展中罕见且显着的形态破坏，从而进一步证明 2021 年 7 月中旬的洪水是影响当地河流地貌的高能事件。基础设施影响了洪水期间和洪水之后泥浆沉积物和死水区的形成，此外还受到浮木和山谷宽度等因素的影响。数据表明，洪水后沉积物出现了一个重新悬浮的阶段，这可能是由于清理和临时河流管理造成的，这凸显了持续河流监测的必要性。