Mechanisms, such as flow shear erosion, impinging jet erosion, side slope erosion and cantilever collapse, were discovered in the overtopping breaching process of cohesive levees. The levee breaching flow rates were simulated by a depth-averaged 2-D flow model. The calculated overflow rates can be well expressed by the broad-crested weir flow formula. The deduced discharge coefficient was smaller than that of common broad-crested rectangular weirs.