No abstract

Large-eddy simulation ͑LES͒ is performed of a curved open-channel flow over topography based on the laboratory experiment by Blanckaert ͓"Topographic steering, flow circulation, velocity redistribution and bed topography in sharp meander bends," Water Resour. Res., doi:10.1029/ 2009WR008303 ͑in press͔͒. In the experiment, the large-scale bed topography had developed to a more or less stationary shape which was prescribed in the LES model as boundary conditions neglecting the small-scale dune forms by means of a straightforward immersed boundary scheme in combination with a simple wall-modeling approach. The small-scale dunes are accounted for in the numerical model by means of parametrization. Sensitivity of the flow to this roughness parametrization is examined by simulating the flow for three different roughness heights. It was found that, notwithstanding the coarse method of representing the dune forms, the qualitative agreement of the experimental results and the LES results is rather good. This good agreement is explained by the minor importance of turbulence stress gradients in the contribution to the transverse and streamwise momentum balance. Moreover, it is found that in the bend the structure of the Reynolds stress tensor shows a tendency toward isotropy which enhances the performance of isotropic eddy viscosity closure models of turbulence. This observation is remarkable since highly anisotropic turbulence might well be expected considering the complex nature of the geometry. Furthermore, the LES results reveal a pronounced recirculation zone near the convex inner bank of the flume due to the shallowness of the flow and strong curvature of the flume. At the interface between the recirculation zone and the main flow, a curved mixing layer is identified as well as strong upwelling flow motion that is accompanied with large production of turbulent kinetic energy.

After validation with experimental data, large-eddy simulation (LES) is used to study in detail the open-channel flow through a curved flume. Based on the LES results, the present paper addresses four issues. Firstly, features of the complex bicellular pattern of the secondary flow, occurring in curved open-channel flows, and its origin are investigated. Secondly, the turbulence characteristics of the flow are studied in detail, incorporating the anisotropy of the turbulence stresses, as well as the distribution of the kinetic energy and the turbulent kinetic energy. Moreover, the implications of the pattern of the production of turbulent kinetic energy is discussed within this context. Thirdly, the distribution of the wall shear stresses at the bottom and sidewalls is computed. Fourthly, the effects of changes in the subgrid-scale model and the boundary conditions are investigated. It turns out that the counter-rotating secondary flow cell near the outer bank is a result of the complex interaction between the spatial distribution of turbulence stresses and centrifugal effects. Moreover, it is found that this outer bank cell forms a region of a local increase of turbulent kinetic energy and of its production. Furthermore, it is shown that the bed shear stresses are amplified in the bend. The distribution of the wall shear stresses is deformed throughout the bend due to curvature. Finally, it is shown that changes in the subgrid-scale model, as well as changes in the boundary conditions, have no strong effect on the results.

Coastal zone management is a multidisciplinary issue that can be assessed from multiple viewpoints such as safety, recreation, ecology and nature. A possible means for properly maintaining the various functions of the coast is sand nourishments along either the shoreface and/or the beach. To assess the influence of sand nourishments, the time-dependency of appropriate indicators of the various coastal functions can be analyzed.HKV has conducted a study that focuses on the probability of failure for dune erosion, as an indicator for safety. For this purpose, the yearly JarKus measurements from the online OpenEarth-database are used. HKV has computed the probability of failure for dune erosion for the entire Dutch coast, from 1965 until 2010, using the dune erosion model DUROS+ and the probabilistic method FORM ('First Order Reliability Method'), as implemented in the package PC-Ring. This approach is closely related to the approach of the project Flood Risks in the Netherlands (FLORIS / VNK).The current paper presents results of these computations, in relation to previous nourishment activities, for the region of NorthernHolland. These gained results can be used to further analyze the effectiveness of nourishments and can hence benefit future nourishment strategies by providing a decision supporting framework.

This paper describes a fully probabilistic safety assessment of the Dutch North Sea coast, in which stochastic properties of both hydraulic loads and strength of the flood defences have been taken into account. The study has led to an overview of failure probabilities along the coast with high spatial resolution. Both dikes and dunes have been considered. Failure probabilities at individual locations have been combined to flooding probabilities per dike ring area. The vast majority of the Dutch coastal defences is quite secure in terms of flooding. This study demonstrates that generally, the Dutch dunes provide a higher degree of safety than the sea dikes. When incorporating the consequences of flooding to the analysis, the calculated flooding probabilities can be used to determine flood risks. The probabilistic method, presented in this paper, enables accurate balancing between avoided flood risks and investments to reinforce the flood defences.