Comparing overflow and wave-overtopping induced breach initiation mechanisms in an embankment breach experiment
van Damme, M.; Ponsioen, L.; Herrero, M.; Peeters, P. (2016). Comparing overflow and wave-overtopping induced breach initiation mechanisms in an embankment breach experiment, in: Lang, M. et al. (Ed.) 3rd European Conference on Flood Risk Management, 18th - 20th October 2016, Lyon, France (FLOODrisk2016): book of abstracts. pp. 53
As part of the SAFElevee project Delft University of Technology collabored with Flanders Hydraulics Research, and Infram B.V. in the preperation and execution of a full scale embankment breach experiment in November 2015. This breach experiment was performed on an 3.5 m high embankment with a sand core and clay outer layer situated along the tidal river Scheldt in Belgium near Schellebelle. The clay outer layer had a thickness of 0.6 m near the toe of the embankment and 0.3 m near the top. On the landside slope of the embankment a poor quality grass cover was present. During the experiment a wave overtopping simulator and an overflow simulator were used to initiate failure of the grass cover and underlying layers at 4 test sections. During the overtopping experiments the test sections were 4m wide. During the overflow experiments the sections were maximum 2m wide. Both simulators were placed near the top of the waterside slope. The use of the simulators facilitated comparison between the effects of continueous overflow and the effects of intermittent wave overtopping. During the experiment, the discharge, velocities, and water depths were monitored. Intermittently a Leica C10 laser scanner was used to monitor changes in the levee geometry. The paper presents a qualitiative analysis on the results from the experiment. This includes a theoretical analysis of the shear stresses exerted on the landside slope of the embankment during overflow and wave overtopping. The paper furthermore comments on the current state of the art theories describing the resisual strength of grass. Based on the results from the experiment it was noted that current theories describing the residual strength of grass covers subject to overflow are very conservative. The analysis of the theorie and data furthermore show that the failure processes during wave overtopping differ significantly from the failure processes during overflow. In the case of wave overtopping it is insufficient to solely account for the shear stresses i:n determining failure initiation. After the grass cover failed both simulators were used to analyse the breach formation processes. It was noted that the breach formed due to headcut formation and that the headcut formation was maintained when the flow reached the sandy core. A link with the field of dredging engineering is ~ade in the paper to explain the processes.