Hydraulics and Morphodynamics of Overtopped Dams with Chimney Filter

Abstract

The present work addresses the influence of the dam’s draining system on the failure by overtopping. It comprehends the critical assessment of four homogeneous dam breach experiments, with different drainage systems: i) two dams had only a toe drain and ii) two dams had a 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. The analysis of the results includes the comparison of the breach hydrograph and the stages of morphological evolution. Key measurements included water elevation in the reservoir, video footage of the evolution of the breach and 3D breach reconstruction, using a depth sensing device (Kinect sensor). Major differences in the breaching mechanisms were observed. In the tests i) the breach growths though headcut erosion and underscouring, associated with mass detachment episodes. In the tests ii) when erosion uncovers the filter, the turbulent flow pulls the sand of the dam body creating a cavity in its location. While the filter erosion evolves inside the dam, breach progression upstream is slow, as the increase of the outflow hydrograph. The cavity originated by the filter erosion induces a fragility on the downstream slope, leading to its collapse by underscouring and mass detachment episodes at relatively small discharges. 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.