Bed morphology changes at river contractions

Contraction scour is a significant problem of engineering concern which might occur when the stream encounters a reduction in flow area because of either natural constraints or hydraulic works. This paper is a part of ongoing investigations by the authors on the spatial and temporal variations in bed morphology at both short and long river contractions based on experimental investigations at laboratory scale. Experiments were carried out at University of Basilicata, Italy, in a 1 m wide and 20 m long rectangular channel. The working section was up to 16 m, depending on the length of the contraction model. Two nearly-uniform sediments were used as mobile bed, sand with median grain size d50=1.7 mm and gravel with d50=9.0 mm. The contrac-tion length was either 0.5,1.0, 2.0 or 3.0 m. Runs were carried out under steady flow and clear-water approach flow conditions. Typically they were of long duration (up to 15 days) to achieve conditions of dynamic equi-librium. New results on the effects of contraction ratio, densimetric Froude number, and relative contraction length are given. Moreover, insights on the spatial and temporal changes in bed morphology are provided with emphasis on the thalweg profile at contracted sections and bed degradation processes downstream of the contracted area.