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Breach growth in cohesive embankments due to overtopping
Zhao, G. (2016). Breach growth in cohesive embankments due to overtopping. PhD Thesis. Delft Academic Press/VSSD: Delft. ISBN 97890-6562-3942. IV, 179 pp.

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  • Zhao, G.

    Breaching is the most frequent form of embankment failure in the world. Due to overtopping, an embankment starts to breach when part of the embankment actually breaks away, leaving an 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 and/or headcut erosion in the embankment. The magnitude and extent of the losses depend highly on the rate of breaching of the embankment, which determines the discharge through the breach and the speed and rate of inundation of the valley or polder. Therefore, modelling of the breach evolution in embankments is of significant interest for damage assessment and risk analysis. It is also important for the development of early warning systems for dike and dam failures and of evacuation plans for people at risk.

    Mathematical breach models have been developed mainly based on empirical methods, physically-based methods and semi-physically-based methods. Empirical models have been developed with probabilistic methods and/or based on case studies. These models can only be applied to similar cases. Semi-physically-based models involve not only empirical data but also include physical processes of breaching. Physically-based models are entirely developed according to the real physical processes of breaching. Generally, empirical models are simple but have a low reliability. Physically-based models are very complex, but predictions are more reliable. In order to develop a physically-based model, the physical processes of breaching require to be exposed. Therefore large-scale experiments are urgently needed to improve and push the breach model development further. Large-scale physical model tests were undertaken in the present study, aimed to increase the understanding of the physical processes, and to provide reliable data for the calibration and validation of breach models.

    The breach flow plays an important role in the embankment breaching process, coupling the hydraulic process and the sediment transport process. During the breaching process, the flow overtops the entire embankment crest and generates the breach channel in the initial phase of the breach development. As the breach further develops, the breach flow goes only through the breach channel due to the decrease of the upstream water level. The breach flow can thus be classified as compound weir flow and weir flow, each having own special characteristics. In a breach, the helicoidal flow accelerates the sediment undermining at the toe of the breach slopes and widens the breach in the lateral direction of the embankment. A triangular hydraulic jump happens when the breach flow changes from supercritical flow into subcritical flow, with a triangular critical area at the toe of the breach. The triangular hydraulic jump works as a driving force to the headcut erosion in the breaching process and the scour hole development at the toe of embankment. According to the hydraulic energy loss in the breach, the discharge coefficients are deducted for both weir flow condition and compound weir flow condition. The resulting discharge coefficients can be used in the calculation of the breach discharge in a breach model.

    Erosion is the result of the interaction between breach flow and embankment material. Surface erosion starts in the initial phase of the breaching process 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 finally deepening the crest level. The breach side slopes are undermined by lateral erosion and the breach widens in lateral direction due to lateral collapses.

    In the present study, five runs of breach experiments were conducted in a relative large laboratory flume. The experimental results clearly expose the hydrodynamic process and the erosion process in the breaching of the cohesive embankment. The breaching starts with the initial erosion of the embankment surface washing away the embankment surface. Due to the surface erosion at the toe of the embankment, the headcut erosion is stimulated on the embankment slope. While headcut migration stimulates the breach to develop in longitudinal direction, the lateral erosion triggers the breach to widen in lateral direction. Three types of erosion (surface erosion, headcut erosion and lateral erosion) contribute to the breach erosion process in the embankment, however, the breach flow is the driving force for the erosion. Sediment deposition in the breaching process, generally ignored in the embankment breaching studies, is also of importance.

    A mathematical model has been developed that couples weir flow and erosion (surface erosion, headcut erosion and lateral erosion). The breaching process is simplified into initial development, deepening development and widening development, corresponding with surface erosion, headcut erosion and lateral erosion, respectively. As the link between flow and embankment material, erosion plays a key role in the embankment breach model. Mathematical descriptions of the headcut migration and the lateral migration rate have been developed to simulate the breaching process in cohesive embankments. The headcut erosion and the lateral erosion are considered to occur in the form of clay blocks instead of in the form of individual clay particles.

    The data of the large-scale breach experiments have been used to calibrate and validate the proposed breach model (headcut migration and lateral migration). The model has also been applied to simulate a laboratory test done in 2005 in the Laboratory for Fluid Mechanics of Delft University of Technology and to the breaching of the Tangjiashan Landslide Barrier (Wenchuan, China, 2008), a breaching event in prototype. It can be concluded that the agreements between the results calculated with the proposed breach model and the measured data are relatively good.

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