Hydrodynamics of overtopped earth embankments. Consequences for the modelling of dam erosive processes

Rui FERREIRA

Earth embankments breach following sustained overtopping, resulting in an erosion process that may lead to its complete destruction or significant damage. Characterizing the erosion process, including hydraulic erosion, slope instabilities or the detachment of earth blocks requires complex experimental techniques. Modelling the erosion process is paramount to predict the discharge through the breached embankment.
This talk describes experimental techniques to characterize different aspect of flow hydrodynamics over breached dams, aiming to collect evidence to parameterize relations between flow variables and morphological variables. In particular, I describe the secondary currents in the reservoir, the structure of the 3D separation of the flow approaching the breach, the developing boundary layer over the breach crest, the separation of this boundary layer and the resulting vortical cell at the toe of the breach channel, the primary helical flow in the erosion cavity formed by the plunging flow, and the separated flow and helical cell at the rim of erosion cavity. 
These structures participate and influence the erosion rates. At the crest, mostly by determining hydraulic erosion, susceptible to be modelled by a Exner-type equation. But the flow structures at the toe of the breach channel and in the cavity influence also undercutting which is the main driver of mass detachments or slope instability.
I conduct an attempt to parameterize an erosion model of an overtopped dam, implemented in an 1D shallow flow model. The model takes advantage of scale separation and considers a quasi-steady approach for the liquid flow. The erosion model is composed of mass conservation equations for the dam and eroded material and is discretized with a finite-volume upwind scheme. The results show that hydraulic erosion is the main driver of the evolution of the breach discharge. However, the time scales of the discharge hydrograph are influenced by the parametrization of the 3D erosion operator that expresses mass detachments or slope instabilities.