Spatial and temporal variations in bed morphology at river contractions

This paper deals with 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 d(50) = 1.7 mm and gravel with d(50) = 9 mm. Experiments were carried out under steady flow and clear-water approach flow conditions. Tests were usually 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 characteristics as function of the contraction ratio, densimetric Froude number, relative contraction length, and dimensionless time. A polynomial-type equation for temporal axial scour profiles is also provided. Spatial and temporal observations of the bed morphology are also compared with results from the application of CCHE2D (University of Mississippi) numerical model. The effects of the contraction length are discussed.