1,721,013 research outputs found
Analisi dell'interazione dei moti sub-superficiali nelle rampe in massi in presenza del bacino di dissipazione
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Influence of large woody debris on sediment scour at bridge piers
No full textLarge woody debris (LWD) reduces the flow area, deviate the flow and increases the velocity in correspondence of the bridge pier, therefore increases the maximum scour hole depth and accelerates sediment removal. Logs and drifts accumulated on bridge piers are of different dimensions. According to logs characteristics and river morphology, drift accumulations can either extend downstream the bridge pier or they can accumulate totally upstream. This paper aims to analyze the effect of drift accumulation planimetry on bridge pier scour. The experimental investigation has been carried out at the PITLAB hydraulic centre of Civil Engineering Department, University of Pisa, Italy. Drift accumulation was characterized by different relative longitudinal lengths, flow area occlusions, length of longitudinal drift and downstream planimetrical positions relative to the pier center. The experimental investigation has been carried out in clear-water conditions. Several pier sizes, channel widths and sediment materials have been tested. Maximum scour hole in presence of drift accumulation have been compared to the maximum scour hole for an isolated pier. Finally, data were compared with previous literature findings, which highlight the effect of the downstream extension of drift accumulation on bridge pier scour. New relationships have been proposed to predict the effect of drift accumulation on bridge pier scour, both in terms of relative maximum scour and temporal scour evolution
Self-Aeration and Friction over Rock Chutes in Uniform Flow Conditions
No full textInteraction between the free surface and the bed material in flow over rock chutes under macroroughness conditions leads to a high air entrainment into the flow. The note reports on an experimental study about air diffusion features in the flow over a long rock chute. Air concentration profiles and water depths over a uniform bed material were measured. An empirical equation for the average air concentration in macroroughness condition for steep slopes is proposed. A new Darcy-Weisbach equivalent friction factor for long chutes as a function of the slope and the relative equivalent depth has also been found
Influence of Wood Debris Accumulation on Bridge Pier Scour
No full textThis note deals with the influence of debris accumulation on scour around bridge piers. Clear-water experiments in different hydraulic conditions have been carried out with three wood debris shapes: rectangular, triangular, and cylindrical. A wide range of debris thickness and width were studied in order to determine their influence on the maximum scour hole depth temporal evolution. The ratio of the pier diameter to the channel width was varied between 0.05 and 0.12 with total bridge contractions up to 20%. A proposed relation presents a simple design procedure to predict the increase in scour depth, which mainly depends on the flow contraction due to the debris accumulation
Scour and hydraulic jump downstream of block ramps in expanding stilling basins
No full textThe scour process downstream of block ramps is strongly affected by both the stilling basin geometry and the hydraulic conditions. Particular effects include the ramp slope and the geometry of the stilling basin. The aim of this experimental study is to analyze these effects on both the scour mechanism and the hydraulic jump of symmetrically expanding stilling basins, for different hydraulic and geometric conditions. The analysis of the scour phenomenon is fundamental for a correct stilling basin design. The scour hole downstream of a block ramp can have a prominent ecological function as it forms a natural pool for fish. Experiments were conducted for various tailwater levels, ramp slopes and granulometries of the bed material. A qualitative description of the scour hole process is given and simple relationships were derived to predict its main hydraulic and geometrical parameters
Temporal scour evolution at bridge piers: effect of wood debris roughness and porosity
No full textLarge wood debris transported by floods affects the scour morphology at bridge piers, thus increasing the bridge failure potential. The characteristic size and shape of the riparian vegetation includes various roughness and permeability conditions of the debris surface. The interaction between two-dimensional flow and rough debris accumulations increases the shear stress, the turbulence and consequently affects the scour evolution process at bridge piers. An experimental study on the bridge pier clear-water scour evolution in the presence of wood debris was conducted at the PITLAB research centre, University of Pisa, Italy. A debris accumulation is characterized by roughness, shape and porosity. Flow intensities range from 65 to 100% of the threshold velocity and included up to 18% of the total flow area. Flow depths were varied from 2.67 to 5.67 times the pier diameter. The effects of wood debris roughness and porosity were analysed in terms of scour temporal evolution and scour morphology
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