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1

On Flow-Altering Countermeasures for Scour at Vertical-Wall Abutment

Radice A., Lauva O.

Archives of Hydro-Engineering and Environmental Mechanics

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2012

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Vol. 59, nr 3-4

137-153

EN

Results are presented for clear-water scour experiments at a vertical-wall abutment where the obstacle was modified with slots or roughening elements as flow-altering countermeasures against the erosion process. The laboratory campaign comprehended an initial experiment with an unprotected obstacle, two experiments with slots above and beneath the non-scoured bed level, respectively, and one experiment with a roughened abutment. The repeatability of the experiments was checked and found satisfactory. The measured data set consisted of (i) scour depth with time; (ii) geometry of the erosion hole; and (iii) sediment motion pattern at several evolution stages of the process. A novel feature of the work was the attempt to combine evidence on the scour depth and that on sediment motion so as to shed light on the mechanism of scour reduction by the countermeasures tested. It is argued that this strategy might furnish guidelines for future extensive investigations of scour countermeasures, aimed at finding optimal design solutions.

2

Scale-based statistical analysis of sediment fluxes

Campagnol J., Radice A., Ballio F.

Acta Geophysica

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2012

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Vol. 60, no. 6

1744-1777

EN

The flux of sediments over a line perpendicular to the main flow direction was measured during experiments of weak one-dimensional bed load. The standard definition of solid discharge through a boundary is a straightforward issue, yet the dependence of resulting values on the spatial and temporal scales used as a support for measurement is not. In this work, first- and second-order statistics of sediment transport rates were analyzed as scale-dependent quantities. The spatial scales used were significantly larger than the particle size, while the temporal scales covered a two-orders-of-magnitude range enabling the physical time scales of the single particles to be appreciated. In addition, the relationship between sediment fluxes, process intermittency and particle interarrival times was investigated. Proper knowledge of the scaledependence of statistical properties of sediment transport fluxes may allow for adequate design of measuring campaigns (both in the laboratory and field) and for sound interpretation of data from multiple sources.

3

On integrated sediment transport modelling for flash events in mountain environments

Radice A., Giorgetti E., Brambilla D., Longoni L., Papini M.

Acta Geophysica

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2012

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Vol. 60, no. 1

191-213

EN

Sediment production and transport in mountain basins during short-term, intense events depend on a variety of processes. Available models typically consider a limited portion of the phenomenological chain, frequently either sediment supply or solid transport along the waterways. On the other hand, proper depiction of on-site processes requires ability to model all the process stages and suitable integration between different models. In this manuscript, an integrated modelling is attempted for small catchments in Italian Alpine foothills. The integrated approach has involved: (i) hydrologic estimation of peak discharge, (ii) evaluation of the volumetric sediment supply into the stream, and (iii) computation of the morphologic evolution of the river bed. The results are discussed focussing on: (i) the feasibility of a joint modelling like the one presented, in the light of all the limitations imposed by the different nature of hillslope-devoted and river-devoted models; and (ii) the sensitivity of the obtained results to some parameters, for an assessment of result reliability.

4

Gravity- and Turbulence-Dominated Sediment Motion in the Clear-Water Scour Process at a Vertical-Wall Abutment in Pressurized Flow

Radice A., Ballio F., Tran C. K.

Archives of Hydro-Engineering and Environmental Mechanics

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2010

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Vol. 57, nr 1

43-59

EN

Results on the motion of sediment particles on the bottom of the erosion hole are shown for a clear-water scour experiment with a vertical-wall abutment. The paper presents an investigation of particle kinematics starting from the division of the grain instantaneous movements into two populations, namely the "turbulence-dominated" events (those in which the particle motion is triggered by the turbulent flow field) and the "gravity-dominated" events (those in which the particles slide along the slopes of the scour hole due to geotechnical instability). Attention is focused on the well developed stages of the erosion process. For such experimental times, the action of the principal vortex system is particularly evident because the latter is not much stretched along the direction of the mean flow deviation, thanks to the increased flowing area. At the same time, the temporal unsteadiness of the vortices lets a bimodal behaviour of the sediments to emerge. A relevant difference has been found between the dynamics of gravity-dominated and turbulence-dominated events. In addition, it was found that the presence of geotechnical effects in the erosion hole may significantly alter the scour rate. Potential implications of the present results for the modelling of local scour processes are described.

5

Statistics and characteristic scales for bed load in a channel flow with sidewall effects

Radice A., Ballio F., Nikora V.

Acta Geophysica

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2010

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Vol. 58, no. 6

1072-1093

EN

Results are presented for a long-duration sediment transport experiment with a plane bed. High-frequency time series of sediment concentration, sediment velocity and solid discharge per unit width were measured using image analysis of video records. A range of transport intensities from a single experiment has been analysed considering transport at different distances from the sidewalls as an independent variable. First and second order temporal statistics of the spatially-averaged values of the transport parameters are presented, which are in agreement with previous studies. A refined statistical analysis of the sediment concentration dynamics, reflecting bed load process, is also given. The characteristic scales of the sediment concentration dynamics are evaluated and analyzed in conjunction with those of the near-bed and bulk flow fields.

6

Double-average characteristics of sediment motion in one-dimensional bed load

Radice A., Ballio F.

Acta Geophysica

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2008

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Vol. 56, no. 3

654-668

EN

The sediment transport process on a flat bed was investigated experimentally, with reference to the relationship between the average solid discharge and the concentration and velocity of the moving grains. The instantaneous values of the quantities were measured and, therefore, it was possible to quantify the contribution of the temporal fluctuations of concentration and velocity to the resulting average sediment transport rate. Recognizing that the sediment transport process is an episodic phenomenon, an intermittency factor was defined and its contribution to the solid discharge, typically implicit in earlier formulations of the sediment flux, was highlighted. Conceptual analyses of the spatial scale dependence of the quantities were also made.

7

Grain Kinematics in Weak Linear Transport

Ballio F., Radice A.

Archives of Hydro-Engineering and Environmental Mechanics

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2007

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Vol. 54, nr 3

223-242

EN

Some preliminary results of measurements of one-dimensional sediment transport on a flat bed are presented. Image processing was applied to measure the time evolution of the areal concentration and the velocity of transported grains in a series of bed-load tests with values of the bed shear stress up to twice the threshold for incipient motion. The concentration and velocity data were used to compute the time evolution of the solid discharge per unit width. It was found, as expected, that the sediment transport is an episodic phenomenon, particularly at low shear stress; visualization of the moving particles allows recognition of the existence of longitudinal streaks, that can be more clearly observed when the sediment rate is weaker. The first and second order statistics of sediment concentration and velocity and solid discharge have been analyzed; the mean and standard deviation of the quantities increase as the transport intensity increases, while the coefficient of variation decreases. The ranges of variation of all the time-averaged quantities with the mean sediment rate are narrower if the zero values of the samples are disregarded prior to calculating the statistics; the coefficient of variation is almost constant. The solid discharge data samples were analyzed in the amplitude domain, by calculating the Cumulative Frequency Distributions; it was found that the shape of the CFDs computed with reference to the non-zero values of the sediment rate alone is self-similar regardless of the transport intensity.