Morphological Patterns at River Contractions

Giuseppe Oliveto,Maria Cristina Marino

doi:10.3390/w11081683

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.

Highlights

The constriction of watercourses may be induced from natural causes, hydraulic structures, or even from river training works
Some experiments were of long duration to assess any conditions of dynamic equilibrium and to allow the development of well-defined bed forms
Some others were of short duration to capture the bed morphology characteristics in the early stages of their temporal evolution

Summary

Introduction

The constriction of watercourses may be induced from natural causes (e.g., landslide debris, development of gravel bars, river junctions), hydraulic structures (like in the emblematic case of bridges), or even from river training works (like in the emblematic case of structures built to obtain greater navigable depths). When the approaching flow is subcritical, the narrowing of rivers involves an increase in flow velocities with possible significant erosional impacts. According to Komura [1] these conditions arise when the ratio of the contraction length, l, to the approaching channel width, B, is greater than 1, whereas greater than 2 according to Webby [2]. In 1934 Straub [4] published a paper on the effects of channel-contraction works upon the regime of movable bed-streams. He suggested a straightforward equation to predict the flow depth in the contracted section based on sediment-discharge and flow-discharge continuity equations

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Conclusion