Author(s): Teresa Alvarez; Solange Valente; Rui Aleixo; Silvia Amaral; Laura Caldeira; Teresa Viseu; Rui M. L. Ferreira
Keywords: Dam breach; Chimney filter; Overtopping; Homogeneous dam
Abstract: The present work addresses the influence of the dam’s draining system on the failure by overtopping. It comprehends the critical assessment of seven homogeneous dam breach experiments, with different drainage systems: i) toe drain only and ii) complete draining system: chimney filter, horizontal drain and toe drain. The tests were performed in identical conditions of embankment body material, reservoir dimensions and inflow. Three dams were equipped only with a toe drain and four were equipped with the complete drainage system. The embankment material was a silty sand with 25% of fines content. The filter and horizontal drain material was a coarse sand with D50 = 0.7 mm. The dams tested were 0.45m height and 1.2m long and were constructed with relative compactions of 90% and a water content of -2.1% of Standard Proctor reference values. The analysis of the results includes the comparison of the breach hydrograph and the stages of morphological evolution. Key measurements include water elevation in the reservoir, video footage of the evolution of the breach and 3D breach reconstruction, using a depth sensing device (Kinect sensor). The 3D reconstruction of the breach required to stop the breaching process, since KINECT sensor is not able to acquire data under the breach outflow. Major differences in the breaching mechanisms were observed. In the tests featuring a dam with only a toe drain, headcut erosion and underscouring were observed, as described in the literature. In the tests where the dam was equipped with a complete drainage system, erosion cavities inside the dam body were observed, as consequence of the filter erosion. However, this is not accompanied by an upstream progression of the breach, through hydraulic erosion. As a consequence, the breach hydrograph increases at a lower rate in an initial stage while the filter undergoes erosion without causing significant underscouring. In a later stage, the dam downstream face has mostly disappeared the breach discharge increases at a higher rate, similarly to the one of the dam without drainage system. A detailed description of the breach erosion process in dams equipped with a complete drainage system is presented. The distinct stages of dam failure are characterized in terms of breach outflow and morphologic evolution. This configures a major novelty in the conceptual model of dam erosion by overtopping that should be incorporated in simulation models. The filter erosion causes a delay in the discharge increase that needs to be accounted in the dam’s safety plans.