Abstract: When modeling the flood routing process in diversion areas with numerical models, the phenomena of scour and collapse at the diversion gate is very serious, which requires the model can adjust the boundary and move the calculation grids accordingly. The traditional constant calculation grid is not suitable to solve the problem of the dynamic change at the calculation boundary, so, in this paper, with the method of dynamic grids, combined with the collapse mechanism of cohesive riverbank in soil mechanics, a two-dimensional (2-D) dike failure plane numerical model with movable grids is established. The model is based on the comparison of calculated and measured data. The model is validated on the Datangwei flood detention basin. The result shows that during the erosion of dike burst flow, the landslip failure to the dike occurs, the flood discharge near the dam burst increases rapidly, which closes to actual experiences. It also shows that with the combination calculation method of the movable grid and collapse mechanism of the cohesive soil river-bank in Soil mechanics, the model simulates reasonably well the broadening process at the diversion gate.