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An experimental campaign (almost 80 tests in a rectangular flume closed by a sluice gate, setting the bed slope at 15° and 20° and adjusting the initial solid volume fraction from 0.1 to 0.5) was performed at the laboratory of the Politecnico di Milano. The aim of the tests was to assess the propagation of a dam-break wave of a hyperconcentrated mixture of water and granular material, and they showed that a stratification in the flow sometimes occurs. Indeed, depending on the bed slope and on the solid volume fraction in the mixture, in the upper part of the debris flow a layer of clear water may be present on top of an homogeneous layer containing both solid and fluid phases. Hence, a correct mathematical modelling of the phenomenon should account for this key feature and treat the mixture as a full two-phase system. Required for this purpose is a criterion for evaluating the saturation degree of the mixture. A dimensional analysis determined that the solid volume fraction and a Froude number are the fundamental parameters governing the dynamics of the level of the surface dividing clear water and fully saturated mixture. The paper also presents the preliminary results of an analysis using an edge detection algorithm in order to obtain the free surface depth and the depth of the fully saturated layer.

To predict debris flow dynamics a numerical model, based on 1D De Saint Venant (SV) equations, was developed. The McCormack-Jameson shock capturing scheme was employed for the solution of the equations, written in a conservative law form. This technique was applied to determine both the propagation and the profile of a two-phase debris flow resulting from the instantaneous and complete collapse of a storage dam.To validate the model, comparisons have been made between its predictions and laboratory measurements concerning flows of water and homogeneous granular mixtures in a uniform geometry flume reproducing dam-break waves. Agreement between computational and experimental results is considered very satisfactory for mature (non-stratified) debris flows, which embrace most real cases. To better predict immature (stratified) flows, the model should be improved in order to feature, in a more realistic way, the distribution of the particles of different size within the mixture.On the whole, the proposed model can easily be extended to channels with arbitrary cross sections for debris flow routing, as well as for solving different problems of unsteady flow in open channels by incorporating the appropriate initial and boundary conditions. Copyright # 2008 John Wiley & Sons, Ltd. Pour valider le modèle, des configurations expérimentales ont été réalisées, à l'aide de moyens de mesures basées sur l'analyse d'images, de manière à pouvoir reproduire la formation et la propagation des ondes de chocs sur fond sec.Le bon accord entre les résultats du modèle et les données expérimentales obtenues permet de confirmer son applicabilité et sa validité pour modéliser le transport solide dans un canal ouvert, ainsi que de donner un aperçu des limites du modèle présenté.

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