The Beginning of Sediment Transport – A Different Approach R. Aleixo & R. Maia Departamento de Engenharia Civil, Faculdade de Engenharia da Universidade do Porto Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal ABSTRACT: The beginning of motion of particles at the bottom of a water channel is a result of a rather complex interaction between the flow and the particles. Using a simplified bed model it is possible to easily study and isolate the interactions between the flow and the bed and to better define the conditions leading to the beginning of bed particles motion. A theoretical approach that takes into account the different parameters such as support angle and turbulence intensity, is presented and compared with the Shields diagram and experimental results. This study is the first phase of a broader research project in course who aims to better understand and explain the beginning of sediment particles motion. 1 INTRODUCTION Aiming to study the interaction between the flow and the channel bed, a simple model was developed in or- der to allow a proper description of that interaction in terms of its physics. Some assumptions were made and based on the known results of the boundary layer theory. This work summarizes the first stage of a broader research project in course at the Hydraulics Labora- tory of Faculty of Engineering of Porto University, currently focused at the study of the flow around a single particle, aiming to be used in future work to characterize the interaction between particles. 2 CONCEPTUAL MODEL The bed model studied consists on a flat and hori- zontal plate in which a localized two-particles’ width bed, with the full transverse length of the water chan- nel was provided, as depicted in Figure 1. Over this bed of particles, at the channel axis, a single parti- cle was placed. The particles that support that single particle will be from now on referred as supporting particles and the single particle will be designated as test-particle. This model consists in a simplification of one presented by (Nezu and Nakagawa 1993). There are two possible 3D configurations to sup- port the test-particle: a three- or a four-particles bed arrangement. For this analysis the test-particle is as- sumed to be supported by a four particles bed ar- rangement with the centres of the five spheres form- ing a square-based pyramid as shown in Figure 2. It can be shown that the maximum height of the d u y R I D F L P n r Figure 1: Simplified bed model. A test-particle, with the same diameter as the ones of the bed is immersed on a flow with a velocity profile u = u(y). The forces acting on a particle (referred as test-particle) are the weight  P , the drag  D, the lift  L, the buoyancy  I , the normal reaction  R n , the friction force  F r , and d is the particle’s diameter. sphere is h 1t = d/ √ 2 and that the support angle is θ = atan √ 2/2 = 35.26 o (as depicted on Fig 2). For this simple model theoretical hypothesis re- garding the beginning of motion were assumed: 1