Hydro- and morphodynamics in curved river reaches – recent results and directions for future research

摘要

Curved river reaches were investigated as an example of river configurationswhere three-dimensional processes prevail. Similar processes occur, forexample, in confluences and bifurcations, or near hydraulic structures suchas bridge piers and abutments. Some important processes were investigated indetail in the laboratory, simulated numerically by means of eddy-resolvingtechniques, and finally parameterized in long-term and large-scalemorphodynamic models. Investigated flow processes include secondary flow,large-scale coherent turbulence structures, shear layers and flow separationat the convex inner bank. Secondary flow causes a redistribution of the flowand a transverse inclination of the riverbed, which favour erosion of theouter bank and meander migration. Secondary flow generates verticalvelocities that impinge on the riverbed, and are known to increase theerosive capacity of the flow. Large-scale turbulent coherent structures alsoincrease the sediment entrainment and transport capacity. Both processes arenot accounted for in sediment transport formulae, which leads to anunderestimation of the bend scour and the erosion of the outer bank.Eddy-resolving numerical models are computationally too expensive to beimplemented in long-term and large-scale morphodynamic models. But theyprovide insight in the flow processes and broaden the investigated parameterspace. Results from laboratory experiments and eddy-resolving numericalmodels were at the basis of the development of a new parameterizationwithout curvature restrictions of secondary flow effects, which isapplicable in long-term and large-scale morphodynamic models. It also led tothe development of a new engineering technique to modify the flow and thebed morphology by means of an air-bubble screen. The rising air bubblesgenerate secondary flow, which redistributes the patterns of flow, boundaryshear stress and sediment transport.