A simplified physically based model has been developed to simulate the breaching processes of homogenous and composite earthen embankments owing to overtopping and piping. The breach caused by overtopping flow is approximated as a flat broad-crested weir with a trapezoidal cross section, downstream connected with a vertical drop (headcut) and a straight slope for cohesive and noncohesive homogeneous embankments, respectively. For a composite dam with a clay core, the downstream becomes two straight slopes after the core is exposed. The breach by piping is assumed to be a flat pipe with rectangular cross section until the pipe top collapses and overtopping takes place. Sediment transport and morphology changes on the breach top flat section and downstream slopes and inside the pipe are calculated using a nonequilibrium total-load sediment transport model, whereas the time-averaged headcut migration rate is determined using an empirical formula. Stabilities of the side slope, pipe top, headcut, and ...

The critical shear stress for erosion of sand and mud mixtures is theoretically investigated and expressed as a function of the critical shear stresses of pure sand and mud, mud content, and sand diameter. The proposed formula accounts for different structures and behaviours of the mixtures with low and high mud contents. The critical shear stress of pure mud is related to the solid/void volume ratio, and this relation is extended to the mud component in the mixture to consider the effect of compaction. It is found that the mud dry density, not the mixture dry density, is a direct factor affecting the mixture erosion. The developed formula has been calibrated and tested using four sets of experimental data collected from the literature. It reproduces the variations of the critical shear stress and mud dry density well.