Author(s): Silvia Amaral, Maria Teresa Viseu, Rui M. L. Ferreira
Keywords: Earthfill dam failure; Breach Effluent Flow(BEF); Mass detachment episode; 3D embankment reconstruction; Soil geotechnical characterization;
Abstract: Overtopping failure of homogeneous earthen dams was investigated in 6 medium-scale experiments with dams built of compacted sandy soils with high fines content (silty sand (SM) and clayey sand (SC) - according to the USCS). These tests encompassed the variation of the type of soil (fines content) and the soil compaction conditions (relative compaction and water content). The influence of these parameters in the breaching process was generically evaluated, taking as reference, the failure time, the maximum effluent flow and the erosion patterns. In addition, the breach effluent flow (BEF) based on traditional estimates, i.e. mass balance within the reservoir and rating curve of a downstream spillway are presented and analyzed in terms of their response to the relative compaction and water content, as well as to the occurrence of mass detachment episodes. The evolution of the failure erosion was also fully described by a 3D characterization of a failing dam in time instants of interest based on acquisitions of a depth camera, Kinect sensor. It was observed that the breach effluent flow is influenced by the material conditions of the embankment (relative compaction and water content), as well as by the soil itself. Also the 3D reconstruction of the embankment performed in this study showed the breach behaviour to be in line to what had been already observed by several authors in prototype failures (headcutting and underscouring with occasional mass detachment events), fully characterizing the breaching process in overtopped real homogeneous dams, which are mostly composed by materials with higher percentages of fines.
Lastly, no direct relation between the occurrence of mass detachment episodes and immediate changes in the BEF was found, reinforcing the need of characterizing the main variables of the breach flow as well as of the breach morphologic evolution locally.