Experimental investigation of the overtopping failure of a zoned embankment dam

The failure of dams poses an immense risk to settlements and infrastructure throughout the world. As extreme flood events become more likely in many parts of the world, dams become increasingly vulnerable to overtopping. Earthen embankment dams are made of erodible materials, and are therefore susceptible to erosion by an overtopping flow. Erosion of a dam to the point of uncontrolled outflow constitutes a dam breach. The erosive processes that lead to breaching depend on the type of dam. Zoned earthen dams with a mineral core erode differently than homogeneous earthen dams, due to the core material's resistance to entrainment by the overtopping flow. The process of breaching a zoned dam by overtopping has been scarcely observed at laboratory, field, or prototype scales. In this study, we perform a laboratory experiment to investigate the breaching due to overtopping of a zoned earthen embankment dam. A prototype-scale zoned dam was designed based on multiple large zoned earthen dams in Switzerland, with particular focus on the Jonenbach Dam in Affoltern am Albis. The prototype dam, which includes shell, filter, and core zones, was reduced to laboratory scale for experimentation. The laboratory-scale dam was breached by overtopping, and the failure processes of each zone were observed. The breaching process began with formation of a breach channel on the downstream slope of the dam. The breach channel incised downstream of the core, and expanded laterally due to mass slope failures of shell material. The filter material remained temporarily stable after departure of the supporting shell material, due to effects of apparent cohesion, but gradually failed with mass detachments. Erosion of the shell and filter zones left the core unsupported on the downstream side. The forces of water and soil pressure from upstream gradually became too large for the core to resist, causing the core to bulge, crack, and eventually break. Once the core broke, water and shell material from upstream flowed uncontrolled through the breach. Similar morphodynamic and hydrodynamic processes that occurred in this experiment would also be expected to occur during the overtopping failure of a prototype-scale zoned dam. The resistance of the core to the soil and water pressures can be a valuable output for calibration of statics calculations of core stability. Such calculations could be implemented in parametric or numerical dam breach models, for use by engineers to estimate the timing and resulting discharge of zoned earthen dam breaches.