1,721,168 research outputs found
Sills and macro-roughness elements as countermeasures for bridge scour
No full textBridge are key infrastructures deserving attention because of their strategic role. In particular, bridges whose piers and foundations are located in water bodies are subjected to complex, localized erosion processes which may lead to undesired consequences, such as the failure of the structure. Therefore, in the last decades many researchers have analyzed the phenomenon, developing and implementing countermeasures that can mitigate the risk of a collapse. Among others, structural countermeasures like riprap, collars, and sacrificial piles have shown to be effective in reducing the maximum scour depth, under certain hydraulic conditions and geometric configurations. However, there are still some countermeasures that require further investigations. Namely, only recently, the effectiveness of transversal structures like sills and gabions have been tested. However, there are still many aspects that remain unclear, like the kinetics of the scour evolution depending on locations of the sills. In addition, macro-roughness elements may be also adopted downstream of bridge piers to protect the channel bed. This paper aims at comparing the effectiveness in controlling scour process of the last two countermeasures and provides interesting insights on the physics of the phenomenon that may be useful for practitioners. Future lines of research are also discussed
Control of Surface Plastic Transport in Natural Streams
No full textIn the aftermath of the worldwide severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic, the disposal of plastic and nonwoven fabric materials originating from personal protective equipment (PPE) into freshwater bodies like lakes and rivers has significantly increased. Surface plastic transport and accumulation have become a relevant source of pollution that adversely affects the quality of water and fluvial ecosystems. Although attempts have been made in the past to study the flow characteristics of plastic at the river surface, an in-depth study of structural configurations that can act as potential plastic traps, facilitating reduction of pollution due to plastic transport, is necessary. This paper investigated the hydrodynamics of surface plastic transport in the presence of several control structures. In addition, useful empirical equations were derived to predict the kinematic and trapping efficiency of the mentioned structures, valid for both straight and curved channels under a variety of hydraulic and geometric conditions. It was found that the Froude number and position of the structure play a dominant role in influencing surface plastic transport mechanism and the overall efficiency of structures in limiting plastic transported downstream
A Methodology to Measure Flow Fields at Bridge Piers in the Presence of Large Wood Debris Accumulation Using Acoustic Doppler Velocimeters
No full textBridge failure due to scour in flooding conditions has been at the center of attention of hydraulic engineers for decades. Large wood debris accumulation on bridge piers has been found as one of the main causes for bridge collapse. The combined action of enhanced scouring at the base of the pier and increased hydrodynamic load is viewed as the main force driving the enhanced failure probability. It is therefore fundamental to understand the distribution of the flow field around the accumulation and in the proximity of the pier, in order to be able to understand the potential acceleration and turbulence enhancement that could help understand what drives the failure and reduces its probability. However, this particular configuration poses physical constraints to flow measurements, as the accumulated mass of logs hinders a direct access to instrumentation directly immersed in water. This paper proposed a new methodology to measure the flow fields in the presence of debris accumulation using an acoustic Doppler velocimeter (ADV). The methodology consists in applying a matrix of rotation on the instantaneous velocity measured in the three dimensions to access locations underneath the debris that could not be accessed using the standard rotation of the debris. The methodology shows the potential to provide accurate measurements in the proximity of the debris and the groove and is able to maintain the statistics of the flow fields in terms of both average velocity and turbulence intensity
Scour around Double-Winged Log Frames under Clear Water Condition
No full textIn-stream low-head hydraulic structures are eco-friendly solutions to maintain the natural aquatic ecosystems. This study focused on scour characteristics around double-winged log-frames under different hydraulic conditions and structure configurations, including stone-reinforced structures. Three types of equilibrium scour morphologies were distinguished for each tested configuration. Generally, two scour formations occur upstream and downstream of the structure, depending on its configuration. Empirical equations are proposed to predict maximum depth and other characteristic lengths of the scour hole and dune. A new equivalent densimetric Froude number was derived by dimensional analysis. The equations were validated with data pertaining to similar structures. Overall, this study shows that double-winged log frames are suitable for creating resting pools for fish species, and can contribute to enhance bank stability. Although the analysis was limited to straight channels, the proposed design equations represent the first quantitative tool to assess scour features for this structure typology
Scour Features due to Inclined and Curved Rock Sills at River Bends
No full textRock-sills are instream low-head structures generally used to stabilize the riverbed. Their presence causes the formation of pools and riffles along the river, thus improving the aquatic habitat and creating resting spots for aquatic species. In this study, the riverbed morphology due to the rock-sill configuration installed at river bends was experimentally analyzed under clear-water condition, with a uniform, granular bed material. Tests were conducted by varying the inclination of sills with respect to the radial direction in three curved bends. Experimental evidence confirmed that the curvature of the channel bend plays a fundamental role, i.e., scour features significantly differed from those occurring in straight channels. Likewise, the inclination of the sill and its shape affect the scour morphology. The presence of the sill and the curvature of the channel caused an asymmetric distribution of the flow in the radial direction, possibly resulting in a shift of the scour hole toward the center. The analysis of experimental data allowed us to provide practitioners with a useful formula to predict the maximum scour depth. Finally, interesting insights on scour dynamics and features at equilibrium are also presented
Analysis of fish migration in correspondence with wood and rock-made instream structures
No full textAnthropic exploitation of soil, forests, and water bodies contributes to the modification of river flow regimes and sediment transport dynamics, thus affecting fish habitats and water quality. In the last decades, the increasing awareness of the consequences of such modifications on natural contexts has stimulated researchers and practitioners to provide more innovative solutions aiming at enhancing river biodiversity. Recently developed eco-friendly approaches for river restoration evidenced the necessity of more careful planning and management of natural resources, including the maintenance and enhancement of fish species' habitats. The present study aims at assessing the interaction of different types of wood- and rock-made instream structures with river flow regimes and habitat requirements of many freshwater fish species, accounting for the influence of downstream scour potholes to facilitate fish species migration and spawning. Experimental tests showed that instream structures cause deep scour holes characterized by low flow velocity zones, thus decreasing the required fish swimming power and potentially providing suitable conditions for the migration of freshwater fish species. Using dimensional analysis, empirical relationships were derived to optimize the longitudinal distance between subsequent instream structures, accounting for the effects of various structure geometries and fish biological characteristics. Nevertheless, further investigation is needed to validate the proposed relationships at prototype scale. This study ultimately contributes at providing tools to design river instream structures favorable to fish migration
Experimental analysis of structures for trapping sars-cov-2-related floating waste in rivers
No full textPersonal protection equipment (PPE, e.g., masks and gloves) related to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic may represent a significant source of riverine plastic pollution. Several studies were conducted to analyze plastic transport in rivers; however, apparently, none of them systematically investigated the efficiency of countermeasures in trapping/stopping floating plastic and nonwoven fabric materials originating from the abovementioned PPE. To fill this gap of knowledge and considering the current importance of the topic, the present paper aims at investigating the efficiency of several structure configurations that can be located in both natural and artificial water bodies. To this end, two different efficiencies were defined, i. e., kinematic (for isolated structures) and trapping (for structures in series). Experimental results evidenced that both the kinematic and the trapping efficiencies increase with the Froude number. We also developed empirical equations, which may be applied for predicting the structure efficiency in limiting plastic transport in rivers
Morphological characteristics of eco-friendly protected basins downstream of block ramps in river bends
No full textBlock ramps are hydraulic structures used to control the sediment transport in rivers and to dissipate flow energy. Moreover, they serve as useful structures to maintain and preserve ecological habitat and to facilitate fish migration. In the past, scour features of both protected and unprotected stilling basins of block ramps have been studied for straight river branches. Sometimes, these structures are placed at channel curves where the flow distribution is characterized by a significant asymmetry, resulting in a scour formation in correspondence with the channel bank, that needs to be carefully predicted and controlled to minimize the risk of a structural failure. Apparently, so far there are no studies focusing on the scour mechanism occurring in protected stilling basins downstream of block ramps in river bends. Hence, this paper aims at investigating the effect of stilling basin protection on the scour process under a combination of different hydraulic conditions, block ramp configurations and locations of the protection structure along the stilling basin. To this end, tests were carried out by placing the block ramp in a curved channel and varying both the flow discharge and the downstream tailwater depth. The downstream stilling basin was protected by introducing a rock sill at various distances from the ramp toe. It has been found that the equilibrium scour morphology mainly depends on hydraulic conditions and it is also influenced by the position of the rock sill in the basin. A classification of different morphological configurations is proposed as well. Finally, an empirical equation is provided to estimate the maximum scour depth under the tested configurations
Theoretical Approach for Shear-Stress Estimation at 2D Equilibrium Scour Holes in Granular Material due to Subvertical Plunging Jets
No full textThe estimation of flow-induced shear stresses acting on the surface of scour holes still represents a challenge for scientists and engineers. From the practical point of view, excessive shear stresses can lead to significant scour depths, resulting eventually in the failure of the structure. From the scientific point of view, detailed knowledge of the shear stresses can yield novel insights for further understanding of scour in particular and of two-phase flows in general. Numerous studies have focused on the interaction between the water flow and a granular bed in order to furnish usable expressions for design and to provide knowledge of the erosive mechanisms. Most of those approaches are empirical, and are characterized by rather significant limitations due to tested conditions. Conversely, only a few studies have derived general theoretical equations for the prediction of the shear stresses based on the phenomenological theory of turbulence. To the best of the authors' knowledge, no works have taken into consideration the effect of the amount of suspended sediment on the value of the shear stress at the dynamic equilibrium configuration. This paper proposes a model based on the conservation of the angular momentum in the turbulent pothole to address those stresses. Novel experimental tests allowed for the validation of the derived equation, which is consistent with accepted theoretical and semitheoretical results
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