Dam break over mobile bed: Characterisation ofthe flow by means of pressure distribution and bed shear stress

Ilaria Fent,Mario J Franca,Sandra Soares-Frazão,A Paquier,N Rivière

doi:10.1051/e3sconf/20184005057

Abstract

The study of the pressure distribution within the dam-break flow over mobile bed is very important because it allows to discuss the hydrostatic pressure distribution, which is the main assumption for the validity of the shallow water equations (SWE). Indeed, this assumption is not always valid, especially when the streamlines are not straight and not perpendicular to the gravity acceleration direction. The flow varies abruptly in the first instants of the falling dam-break wave evolution, but it tires towards a stationary motion during time. To better characterize this phenomenon it is important to examine the evolution of each component of velocity vector field and the distribution of the pressure, which is compared to the hydrostatic distribution. Moreover, the present study considers the distribution of pressure linked with the distribution of the shear stress, through the Navier-Stokes equations. The final object of this work is to characterize the dam break experiment over mobile bed by means of forces acting during the event. The dam-break experiment under examination has been performed in a dedicated flume designed at the laboratory of Civil Engineering Department, Université catholique de Louvain. The analysis of this very transient flow it is now possible and more accurate thanks to the application of non-intrusive new techniques, such as PIV.

Highlights

The consequences caused by a dam break are most of the times catastrophic, in particular if the flow induces the movement of sediment along the valley floor
When the flow reaches an almost horizontal direction (Fig. 3b, at time tND = 2.50) the vertical component of the velocity is negligible and we can consider that the shallow water character of the flow is reached
The new dam-break flow experiments over a mobile bed made of coarse uniform sand were conducted with a special focus on the pressure distribution and the shear stress

Summary

Introduction

The consequences caused by a dam break are most of the times catastrophic, in particular if the flow induces the movement of sediment along the valley floor. Aleixo and others [8] investigated the first stages of the dam-break flow in terms of velocity field, shape of the dam-break wave and evolution towards shallow-water (SW) flow over a fixed bed. They claimed that the vertical component of the velocity becomes negligible after the adimensional time tND = 2.5, conforming to the SW assumption [10]. During the first moments of the dam break, the SW hypothesis are not valid and the streamlines inclination and curvature effects must be accounted for [11] In this contest we want to make our contribution, in order to study in detail the passage between the non-SW and SW stadium of the flow, accounting for the distribution of the pressure in space and time. The knowledge of the dam-break flow features has a high degree of interest, in particular because the dam break experimental data are often used as an example of comparison and validation for SW mathematical models

Objectives

Results

Conclusion