River-bed erosion due to changing boundary conditions: performance of a protective measure

Due to the introduction of man-made sediment barriers along a river, the amount of sediment load entering the downstream river reach is different to that leaving the reach, and erosion processes occur downstream of the barrier itself. Designers are often required to take into account the scouring process and to include adequate protective measures against the local scour. This paper addresses the performance of bio-engineering protective measures against the erosion process. In particular, a green carpet, realized with real flexible vegetation, has been used as the protective measure against erosion processes downstream of a rigid bed. Analyses are based on experimental work carried out in a straight channel constructed at the laboratory of the Dipartimento di Ingegneria Civile, Ambientale, Aereospaziale, dei Materiali, Palermo University (Italy)

Bank Erosion and Secondary Circulation in a Meandering Laboratory Flume

AbstractThis paper reports peculiar results of experimental investigation on the secondary circulation motion of flow along a meander wave. Experiments were conducted in a large amplitude meandering laboratory channel for two values of the width-to-depth ratio. Here attention is focalized on how secondary motion affects the bank shear stress distribution, influencing the stability of the outer bank. The analysis essentially highlights that, especially for small width-to-depth ratio, as the channel curvature increases, besides the classical central-region secondary circulation cell a counter-rotating circulation cell forms in the outer-bank region. Such counter-rotating circulation cell allows the bank shear stress to obtain small values at the outer bank

Computation of the regime configuration of a meandering stream

Regime channel formation is a delicate adaptation to the imposed environmental conditions compatible with flow and sediment transport mechanisms. The present paper concerns the computation of the regime configuration of a meandering stream. It is supposed that the channel develops until it reaches the state of “final thermodynamic equilibrium”, where the ratio of the kinetic energy of the flow to its cross-sectional potential energy is minimum. An optimization procedure that allows the analysis of the regime channel formation is presented and it is checked using a real case

Effect of vegetation on Mixing and Dispersion Processes at the apex section of a meander bend

Aquatic vegetation exerts a strong influence on the fluvial ecosystem. Understanding flow characteristics and turbulent structure in the presence of vegetation is especially important with respect to environmental processes as sediment transport and mixing of transported quantities. In the present paper attention is focused on the kinematic and mixing processes in the presence of flexible submerged vegetation on the bed of a curved channel. In particular, the effect of vegetation on the flux of mass distribution and on the transport process at the apex section of a meandering bend is investigated by comparing the distributions of the dispersion coefficient estimated in vegetated areas and in no-vegetated ones

Flow-bed interactions analysis and application of automatic close range digital photogrammetric survey in a laboratory flume

This paper reports on a laboratory study in which the automatic digital photogrammetric survey was applied to derive the high-resolution Digital Surface Model (DSM) of the bed topography, used for the flow-bed interactions analysis, in a large amplitude meandering laboratory flume. The analysis has been conducted with the aid of detailed data of three-dimensional flow field previously collected using the acoustic Doppler velocity profiler DOP2000. The applied surveying procedure has allowed the evaluation of the DSM with a resolution of ±0.5 mm. The detailed DSM has been compared with peculiar maps describing the flow velocity pattern (downstream and the crossstream flows) and the shear velocity distribution along the meandering flume. The comparison has highlighted that high quality topographic data are of crucial importance to evaluate the crosscirculation effect on the redistribution of the velocity and boundary shear stress and, thereby, on the meandering channel evolution

Experimental Analysis of Horizontal Turbulence of Flow over Flat and Deformed Beds

AbstractLaboratory experiments in a straight flume were carried out to examine the evolution of large-scale horizontal turbulent structures under flat-bed and deformed-bed conditions. In this paper, the horizontal turbulence of flow under these conditions is analyzed and compared. The conditioned quadrant method is applied to verify the occurrence of turbulent events. The distributions of horizontal Reynolds shear stress and turbulent kinetic energy are also presented and discussed. Results show the occurrence of an "initial" sequence of horizontal vortices whose average spatial length scales with the channel width. Under deformed-bed conditions, this spatial length does not change

Turbulence in river and maritime hydraulics

Understanding of the role of turbulence in controlling transport processes is of paramount importance for the preservation and protection of aquatic ecosystems, the minimisation of deleterious consequences of anthropogenic activity, and the successful sustainable development of river and maritime areas. In this context, the present Special Issue collects 15 papers which provide a representation of the present understanding of turbulent processes and their effects in river and maritime environments. The presented collection of papers is not exhaustive but it allows for highlighting key priority areas and knowledge gaps in this field of research.</p

Ecological risk assessment of contaminated sediments in a harbour site

During major dredging operations in the harbour of Genoa, one of the largest in Italy, a monitoring study was carried out on the quality of marine sediment output, with a view to identifying possible pollutants engendering environmental and ecological risk. The concentration range of all the pollutants evaluated fell within acceptable limits. The only pollutant with concentrations approaching ecological risk levels was nickel. Differences in concentrations of pollutants were mapped and related to special- ized areas of harbour activity