Experimental and numerical investigation of the bed morphology evolution in river contractions

This paper focuses on the bed morphology evolution at river contractions based on laboratory experiments and 2D numerical models. Experiments were carried out at University of Basilicata, Italy, in a 1 m wide and 20 m long rectangular channel. Two nearly-uniform sediments were used as mobile bed, sand with median grain size d50=1.7 mm and gravel with d50=9 mm. Contraction model was either 0.5 or 1.0 or 2.0 m long. Experiments were carried out under steady flow and clear-water approach flow conditions. Several tests were of long duration to achieve conditions of quasi-equilibrium and deep scour holes as well. Based on the experimental data empirical models are proposed to predict the main bed features for long contractions as function of contraction ratio, densimetric Froude number, relative contraction length, and dimensionless time. Spatial and temporal observations of bed morphology are also compared to results from the application of CCHE2D (University of Mississippi) numerical model. Effects of contraction length and average adaptation length are discussed.