Morphological patterns at river contractions

Abstract: Sediment transport at river contractions is an important process of engineering concern which might occur when a river encounters a reduction in flow area because of either natural or artificial constraints. This paper focuses on the morphological patterns that are prone to form at and around the constriction of watercourses based on experimental investigations at laboratory scale. Experiments were carried out at the University of Basilicata, Italy, in a 1 m wide and 20 m long rectangular channel. The length of the working section extended up to 16 m, according to 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 contraction 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) also to achieve an equilibrium state. New predictive equations are given on the temporal progress of: the maximum scour depth, the scour hole length, and the axial bed profile with emphasis on the processes of bed aggradation or degradation beyond the contracted region.