Effect of bank height on morphodynamics in a one‐sided widened gravel‐bed river

The use of locally widened river reaches in river restoration is a common approach that aims to enhance morphological heterogeneity and dynamics and therefore increase habitat availability and quality. However, the success of some implemented locally widened river reaches is limited when there is a lack of morphodynamics, which may be related to factors such as sediment supply, channel slope and bank height, reflecting the degree of river bed incision. Therefore, this study examines the morphodynamic development of widened reaches with a moderate longitudinal slope (0.003) in mobile‐bed laboratory experiments. The initial setup consisted of a channel with the adjacent floodplain on one side, measuring 30 channel widths in length and four in width. Sediment supply from upstream corresponded to a rate of 100% of the initial channel transport capacity. Sequences of bed‐forming discharges and larger floods were conducted. Three experimental series were tested in which the bank height was varied from high (0.11 m = water depth at H Q 30 ), over medium (0.07 m = water depth at H Q 2 ), to zero (i.e. no offset between river channel and floodplain). Two‐dimensional hydrodynamic numerical simulations based on the resulting bed topographies added high‐resolution data on bed shear stress. The results show that morphodynamic activity, defined as the areas of erosion and deposition relative to the zones with full sediment transport, remains low for the widening with moderate slope, promoting channel stability across all three experimental series. The high‐offset series experienced lateral erosion, with the main channel shifting into the floodplain and subsequently being constrained by the fixed banks. For the series with medium and no floodplain offset, stabilization in the straight channel was observed. These patterns contrast with the understanding of the influence of sediment supply on the morphodynamic activity for steeper slopes. In previous studies, the morphodynamic activity in a widened reach was high for high sediment supply conditions. Our findings from the experiments with the moderate slope (0.003) suggest that under such conditions, there are additional controlling parameters of the widening process.