For channelized, flood-regulated rivers, morphological changes are avoided as much as possible. Extraordinarily high flows in the past, however, have demonstrated that channelized rivers may also become morphologically active, especially when a discharge exceeds the design discharge, such as in hundred-year floods. However, the morphodynamic potentials and critical flows in such cases have hardly been investigated, and the flood risk to human settlements is therefore poorly understood. The present study aims to analyse the critical flow conditions in Flåmselva, western Norway and the consequences of morphological adjustment on the Froude number from an extraordinary flood event in 2014. Based on a step-backwater modelling approach, three different high-resolution river bathymetries of Flåmselva were investigated: (i) pre-flood, (ii) post-flood and (iii) re-channelized morphology. The results showed that due to the 2014 flood, large parts that were in critical flow conditions in the pre-flood stage (>1), exhibited significantly lower Froude numbers in the post-flood stage. It turned out that the artificially created plane-bed morphologies in flood-regulated channelized rivers can act as drivers for critical flow conditions and that structural as well as non-structural measures should consider non-fluvial, semi-fluvial and fluvial sediment deposits in rivers and floodplains in terms of flood risk management.

中文翻译：

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超高流量期间半冲积河道中的临界流量：对洪水风险管理的影响

对于渠道化、调洪的河流，尽可能避免形态变化。然而，过去异常高的流量表明，渠道化河流也可能在形态上变得活跃，特别是当流量超过设计流量时，例如百年一遇的洪水。然而，在这种情况下，几乎没有研究过形态动力学潜力和临界流量，因此对人类住区的洪水风险知之甚少。本研究旨在分析挪威西部 Flåmselva 的临界水流条件以及形态调整对 2014 年异常洪水事件的 Froude 数的影响。 基于阶梯回水建模方法，三种不同的高分辨率河流水深对 Flåmselva 进行了调查：(i) 洪水前，(ii) 洪水后和 (iii) 重新引导形态。结果表明，由于2014年的洪水，在洪水前阶段（>1）处于临界流量条件的大部分地区，在洪水后阶段表现出显着较低的弗劳德数。事实证明，在受洪水调节的渠道化河流中，人为创造的平面地貌可以作为临界流量条件的驱动因素，结构性和非结构性措施应考虑河流中的非河流、半河流和河流沉积物和洪泛区的洪水风险管理。