Numerical simulation of embankment breach processes (APUNCH)

Introduction

Extensive flood run-offs can be caused by extreme rainfall events in Alpine watersheds or by the collapse of temporary natural dams formed by accumulated sediments. Such extreme events jeopardize the stability of earthen river dyke structures in the lower river reaches.

This modelling approach wants to analyze numerically the most relevant breaching processes of such dyke breach events. Delivering a numerical tool for engineering practice is important to improve prognoses of breach development and the associated flood waves with their large hazard potentials. Such a numerical tool must consider multiple interacting physical processes which makes it a very challenging task. The most relevant processes are the overtopping flow, the dyke surface erosion caused by the stream forces as well as gravitational induced slope collapses of the steepening breach side walls. Furthermore, these slope collapses depend strongly on the internal pore pressure distributions within the dyke structure, which requires additionally a consideration of the internal seepage flow. These modelling approaches are combined here to a multi-physical model and are described briefly in the following.

Hydrodynamic and surface erosion modeling

The 2D shallow water equations and empirical sediment transport formulas are used to simulate the overtopping of the dyke and to analyze the vertical surface erosion caused by the acting stream forces. For this purpose the freely available numerical software BASEMENT (www.basement.ethz.ch) was applied and further developed. Also, a suspended load transport module was integrated to account for suspension transport of fine materials during overtopping.

The model allows the use of unstructured, irregular meshes consisting of triangles and quadrilaterals, which allows adaptations to complex and realistic geometries being an important requirement for many practical engineering applications. In order to guarantee mass conservation of water and sediment masses a new “dual-mesh” approach was developed using two different meshes for the hydraulic and sediment transport modeling.

Seepage flow through dyke body

Slope failure modeling depends on the pore pressures in the embankment in the saturated and unsaturated zones. Therefore a modelling of the 3D Richard’s equation was chosen, which is capable to simulate the flow in the saturated and unsaturated zones of the dyke. Also it is able to model accurately the infiltration of water into the dyke body.

The rather new modelling technique of the Lattice-Boltzmann Method (LBM) was chosen for this purpose. The LBM was recently adapted to solve the Richard’s equation and there is need of further tests of this new and promising approach, which has some advantages like its suitability for complex and changing geometries and its simplicity. A LBM based model for the Richard’s equation was implemented and applied to homogeneous and heterogeneous dykes. It could be successfully validated against analytical, experimental and other numerical results.

Modeling of slope failures

Slope failures, occurring during the dyke overtopping, often contribute significantly to the lateral widening of the breach channel and therefore must be considered in the numerical simulation. For this purpose a new approach applicable on unstructured meshes was developed. This approach bases on the assumption that slope failures of the steep side walls take place if critical side wall angles are exceeded. These side wall angles are estimated as functions of the pore pressures within the dyke body, the water levels in the breach channel and the material properties.

Applications

The described numerical approaches are combined to a coupled multi-physical model. Therefore, they are integrated into the software BASEMENT, which provides an easy-to-use infrastructure and a well documentation and eases the transfer to the engineering practice. The combined model was tested and to some degree validated against small-scale (laboratory) and large-scale (field) cases.