E-proceedings of the 38 th IAHR World Congress September 1-6, 2019, Panama City, Panama doi:10.3850/38WC092019-5554 1375 HYDRODYNAMIC EROSION IN OVERTOPPING BREACH OF COHESIVE EMBANKMENTS GENSHENG ZHAO (1) , PATRIK PEETERS (2) & PAUL VISSER (3) (1) Nanjing Hydraulic Research Institute, No. 225, Guangzhou Road, Nanjing, China gszhao@nhri.cn (1,2,3) Department of Hydraulic Engineering, Delft University of Technology, Stevingweg 1, 2628CN, Delft, the Netherlands g.zhao@tudelft.nl; p.j.visser@tudelft.nl (2) Flanders Hydraulics Research, Berchemlei 115, 2140 Antwerp, Belgium patrik.peeters@mow.vlaanderen.be ABSTRACT Embankment breaching is a composite process coupled by hydraulic processes and sediment transport processes. Erosion is the link in the interaction between breach flow and embankment material. Surface erosion starts in the initial breach phase and triggers the initial damage of the embankment. As the surface erosion develops completely, the headcut erosion leads the breaching process by cutting the embankment slope and deepening the crest level finally. The helicoidal erosion undermines the side slopes of the breach and widens the breach in lateral direction by triggering the breach side slope to collapse. In order to reduce the scale impacts in the lab and defects from the prototype experiments in the field, there were 5 runs of large-scale sediment (sandy clay) models (2m high, 3m long) conducted in the flume (3m×3m×60m) to investigate the cohesive embankment breaching mechanism. The breach hydrodynamic processes (discharge, water level, velocity) were measured in the experiments and topography changes were recorded with 3D scanner every 5 minutes. The results for each runs of the breach experiments were good validations of the 3 types of erosion (surface erosion, headcut erosion and helicoidal erosion) in the cohesive embankment breach. Keywords: Erosion; overtopping; breach; cohesive embankments. 1 INTRODUCTION Breaching is the most frequent form of embankment failure, which has a composite process with uncertain initiation and formation. Due to overtopping and/or piping, an embankment starts to breach when part of the embankment actually breaks away, leaving a large opening for water to flood the land protected by the embankment. A breach can be a sudden or gradual failure that is caused by surface erosion, headcut erosion and lateral erosion in the embankment. Erosion is the interaction link between breach flow and embankment material. Surface erosion starts in the initial breach phase and triggers the initial damage of the embankment. As the surface erosion develops completely, the headcut erosion leads the breaching process by cutting the embankment slope and deepening the crest level finally. The breach side slopes are undermined by the lateral erosion and their collapses widen the breach in lateral direction. For cohesive embankments, the breaching process starts with surface erosion, but the breach takes place due to headcut erosion. After the surface erosion in the beginning phase, the headcut typically starts at the toe of the embankment and then advances upslope until the crest of the embankment is reached. In some cases, a series of stair-step headcut forms on the downslope face of the embankment. The action is similar to that described by Dodge (1988) for model testing of embankment overtopping, which is related with headcut initiation and headcut advance by hydrodynamic and geotechnical mass wasting. According to Ralston (1987), Fread (1988) and Zhu et al. (2006), the headcut erosion plays a significant role in the breaching process in cohesive embankments. The mechanism of headcut erosion, however, still needs further understanding. A variety of breaching experiments have been conducted in the past (Zhu et al., 2004), most of which generally focused on the breaching process. As part of the breaching process, the mechanism of the headcut erosion is insufficiently understood to describe the breaching process and to simulate it with mathematical models. The lateral erosion results in the embankment widening in transversal direction. Due to the undermining of the helicoidal flow, the breach slopes lose the balance and collapse in the form of blocks. The lateral erosion stimulates the breaching process by increasing the area of breach channel, which increases the breaching discharge dramatically. The lateral migration of breach determines the breach embankment development in transversal direction and the change of breaching discharge.