David Vetsch

Numerical modelling of non-cohesive embankment breach with the dual-mesh approach

The numerical model BASEMENT, based on the two-dimensional shallow-water equations, is applied to breaching processes of non-cohesive earth embankments due to overtopping. The governing equations are solved using an explicit finite-volume method combined with a Godunov-type approach. For spatial discretization, a mass-conserving dual-mesh approach with separate, unstructured meshes for hydrodynamic and sediment computations is developed allowing for accurate terrain representation. The surface erosion is modelled with the Exner and sorting equations for multiple grain classes in combination with empirical bed-load transport formulas and advection–diffusion equations for suspended-load transport. Additionally, the lateral breach widening caused by gravitationally-induced slope failures is considered using a novel algorithm adapted for unstructured grids. The model is successfully applied to two recent laboratory experiments including plane and spatial dike breaches, and a field-scale embankment breach. Detailed comparisons between measured and simulated laboratory spatial breach formations confirm the basic model assumptions.

Numerical embankment breach modelling including seepage flow effects

Abstract The multi-physical processes during embankment breaching due to overtopping are complex and known to depend upon seepage flow through the embankment. A numerical breach model for progressive embankment breaching is proposed, taking the seepage flow into account including its influence on slope stability due to apparent cohesion effects. Overtopping and embankment surface erosion are computed using finite-volume methods applied to the 2D shallow-water and the Exner equations. The seepage flow is modelled using a Lattice–Boltzmann approach solving the 3D Richards’ equation. The modules are executed in a coupled manner with dynamically adapted boundary conditions. The breach side wall failures are considered using a geometrical approach based on critical failure angles. This approach is enhanced to account for apparent cohesion effects by dynamically adapting the critical failure angles as a function of the water saturation in a heuristic way, as a major enhancement compared to previous approaches. The model is successfully applied to a laboratory experiment of a small-scale spatial embankment breach.

Measuring suspended sediments in periglacial reservoirs using water samples, laser in-situ scattering and transmissometry and acoustic Doppler current profiler

ABSTRACT Climate change will impact the water and sediment conveyance into periglacial reservoirs. It is therefore important to understand and forecast future reservoir sedimentation processes with regard to climate change. In the present project, particle size distribution (PSD) and suspended sediment concentrations (SSC) were measured in three reservoirs in the Swiss Alps whose catchment areas are covered by glaciers by at least 40%. The threefold combination of water sample analysis, laser in-situ scattering and transmissometry (LISST) and acoustic Doppler current profiler (ADCP) was applied and the results were compared to each other. The combination of the three measuring techniques was proven suitable for assessing PSD and SSC in periglacial reservoirs. Water sample analysis and LISST records showed that most of the suspended sediments in the reservoir are in the range of clay and silt. SSC was relatively low in the order of 100 mg/l. An increase in both PSD (e.g. median diameter d50) and SSC with increasing reservoir depth could be observed in deep reservoirs. Flow velocities and Signal-to-Noise ratios (SNR) were measured with ADCP. SNR values allowed to study the mixing of inflowing river water and the evolution and decay of turbidity currents. There is evidence of dominant homopycnal flows, whereas stratified flow was restricted to the regions close to the inflow. Flocculation, influence of mica, and organic content could be neglected. This paper presents detailed information about PSD and SSC gained with water sample analysis and LISST measurements. Furthermore, flow field measurements and tracking of mixing by means of ADCP will be illustrated. Finally, application experiences and limitations will be discussed.

Assessment of flow field and sediment flux at alpine desanding facilities

ABSTRACT This paper deals with flow field and sediment flux measurements at alpine desanding facilities. 3D flow velocities and turbidity were recorded and water samples were taken at three alpine desanding facilities. The samples were evaluated regarding suspended sediment concentration (SSC) and particle size distribution (PSD) in the laboratory. SSC was correlated with turbidity and reliable correlations were found for two facilities. The applied instrumentation and methods proved to be appropriate to assess flow field and sediment fluxes. The results show that the flow field is inhomogeneous in large parts of the basins and that the presence of tranquilizing racks has a strong influence on the flow. PSD revealed a refinement of the mean particle size in streamwise direction. The mass-related trapping efficiency of the desanding facilities was estimated based on calculated sediment fluxes and compared to two different trapping efficiency definitions. The results are briefly compared with a current design guideline.