Critical flows in semi-alluvial channels during extraordinarily high discharges: Implications for flood risk management

Abstract

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.