Sediment transport at river contractions

River contractions might occur because of either natural controls or hydraulic works. This paper aims to provide new insights on the temporal evolution of the morphological patterns at river contractions based on experiments at laboratory scale. Laboratory tests were performed in a 20 m long straight channel having a rectangular cross section, 1.0 m wide and 1.0 m deep. The erodible bed reach was made by almost uniform sediment consisting of either rough sand or medium gravel. The contraction model was 0.5,1.0, 2.0, or 3.0 m long. Three contraction ratios β=b/B=0.9, 0.8, and 0.7 were considered, with b width of the contracted reach and B width of the approach channel. The experiments were performed in clear-water scour regime and steady flow conditions. Typically, they were of long duration (up to 15 days) to achieve conditions of dynamic equilibrium, but runs of short duration were also carried out to acquire suitable experimental data at the earlier stages of the bed forms development. Compared to previous studies by the author on the same topic, new results are provided on bed morphology characteristics, also considering literature data, and predictive models on the time-dependent development of the bed degradation in and downstream of the contracted area.