Breaching of Homogeneous Dams With Internal Drainage System

The present work addresses the influence of internal drainage systems on failure by overtopping of earth dams. It is based on data collected in six homogeneous dam breach laboratory experiments. Different drainage systems were tested: (a) toe drain only and (b) complete internal drainage system—chimney filter, horizontal blanket and toe drain. The tests were performed with the same embankment body material, reservoir dimensions and inflow conditions but different relative compactions. We analyze the breach hydrograph and the stages of dam breaching. The failure of dams with complete and fully functional drainage systems exhibits marked differences, the most conspicuous of all being the longer duration of the initial erosion stage. This is accompanied by a lower increase rate of its breach discharge. We argue that this is the combined result of the upstream migration of the locus of hydraulic control and of a higher resistance to erosion of the downstream shell, kept unsaturated by the drainage system. Hydraulic control moves upstream, to a less morphologically active cross‐section, as a result of channel deepening once the chimney drain is uncovered and eroded. Once the erosion merges with the cavity where the chimney filter was, marking the end of the initial erosion stage, the rate of erosion increases and becomes comparable to that of dams with toe drain only. Our argument about the longer duration of the initial stage is not applicable to latter stages—the total time before full collapse is not necessarily increased.