Daniel Rebai

Daniel Rebai

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portfolio

publications

Clear Water Scour at Circular Piers: A New Formula Fitting Laboratory Data with Less Than 25% Deviation

Published in Journal of Hydraulic Engineering, 2022

In this paper, we propose a new predictor for the time-dependent, spatially-maximum scour depth at a circular pier in clear-water flow conditions. In spite of a number of approaches used in scour research, a simple predictor based on data correlation still has merit because more insightful studies of the process dynamics lack engineering impact. Furthermore, the simple condition of a circular pier is a reference for several variabilities to be considered afterwards. The present formula is obtained using laboratory data from 30 sources, corresponding to 328 experiments over 66 years. The predictor accounts for 5 dimensionless parameters (pier slenderness, flow intensity. sediment coarseness, sediment uniformity. and time) and has a good predictive ability, largely outperforming that of 27 literature equations. The proposal of a new formula is accompanied by several considerations based on additional parameters and operational conditions, as well as by an analysis of the uncertainty of the computed scour values and recommendations for conservative predictions of the scour depth in engineering practice.

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Experimental Comparison of Inclined Flows with and without Intense Sediment Transport: Flow Resistance and Surface Elevation

Published in Journal of Hydraulic Engineering, 2022

The effect of the intense transport of lightweight sediment without turbulent suspension (Shields numbers of mixture flow from 0.34 to 1.80) on resistance and surface elevation of water flow in an open channel is experimentally examined. Flows over fixed and mobile plane beds, at the same water flowrate and bed slope, are compared. Detailed measurements—including distributions of solids and velocity—permit to identify flow interfaces and determine the role of sediment transport on flow resistance. Two friction factors for mobile bed flows are introduced: one for water only and another one for the mixture. The first seems to be insensitive to the water submergence, while the latter follows the classic formula for the hydraulically rough regime: both increase with respect to the equivalent clear water flow. The water surface elevation seems unaltered by the presence of sediment transport, thus reducing the need for the modelling of the evolution of an erodible bed surface in, at least, some routine applications in fluvial hydraulics.

Traveling or jiggling: Particle motion modes and their relative contribution to bed‐load variables

Published in Journal of Geophysical Research: Earth Surface, 2024

The motion state of a particle is a crucial aspect of sediment transport problems. In this paper, we conceptualized three states: stillness, “transport”, and “non-transport”, considering that not all the particle motions contribute significantly to the mean sediment transport rate. Starting from a data set of bed-load particle tracks obtained from particle tracking velocimetry, we removed the bias from experimental uncertainty and applied one-dimensional, instantaneous, and non-parametric criteria for distinguishing the states. We described the kinematics of particles in transport and non-transport states, presenting some sample trajectories and the distributions of particle velocity and acceleration. While the transport state presents a clear distinction between stream-wise and transverse particle velocity, the non-transport state is related to isotropic particle jiggling, and does not significantly contribute to the bed-load rate. Vice-versa, the particle motions in the non-transport state are relevant for other summary indicators of the transport process, such as the mean number of moving particles and mean particle velocity. We discuss how applying the proposed non-parametric criterion for state separation is beneficial compared to parameter-dependent alternatives available in the literature. Finally, we provide an outlook on possible applications of our concept for the investigation of other sediment transport processes (incipient motion, solid-fluid interface, creeping flow).

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Predictors for Clear-Water and Live-Bed Scour at Circular Piers

Published in Journal of Hydraulic Engineering, 2025

In this work, we propose a new predictor for estimating the local scour at circular piers in live-bed conditions, explicitly accounting for the dependence of the time-averaged equilibrium scour depth on dimensionless factors for pier slenderness, sediment coarseness, sediment gradation, and flow intensity. The empirical model is constructed as the product of four independent subfunctions, a modular formulation that simplifies the normalization of experimental data and enables a clear interpretation of the individual effects of these parameters. The presented equation outperforms other approaches proposed in the literature. We also present a full trend of the equilibrium scour depth as a function of the flow intensity for both clear-water and live-bed regimes (for flow velocity up to 4.5 times the threshold for sediment motion in the unobstructed reach). For the estimation of the scour depth in clear-water condition we slightly modified a predictor that we recently proposed. Our formulae are supported by an extensive data set comprising 1,175 experimental data points, underscoring the robustness and applicability of the proposed model. For uniform sediment, the maximum scour occurs in the clear-water regime. By contrast, in flows with nonuniform sediment, the worst-case scenario occurs in the live-bed regime.

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talks

teaching

Fluid Mechanics

Undergraduate course, Politecnico di Milano, Department of Civil and Environmental Engineering, 2020

Since Fall 2020, I have been serving as a Teaching Assistant for the Fluid Mechanics course. I assisted students with exercises on fluid statics and pipe flow, and also designed a hands-on experiment on orifice flows.

Hydraulics

Undergraduate course, Politecnico di Milano, Department of Civil and Environmental Engineering, 2021

From Spring 2021 to Spring 2025, I served as a Teaching Assistant for the Hydraulics course for Civil and Environmental Engineering students at Politecnico di Milano. My work primarily focused on fluid statics, flow in closed conduits, and open channel flow.