Abstract：A theoretical method based on Hertz contact theory is presented for calculation of impact force of outrunner block in debris flow. Special attentions are given to the elastoplastic deformation of structures. Hertz contact theory is modified using perfectly elastoplastic Thornton hypothesis to consider the elastoplastic deformation of structures. In the simulation of the impact of outrunner blocks in debris flow and counter structures，the structure is simplified as a still plane and the block is simulated by a particle with a certain velocity. So the impact force can be expressed by a simple function. Based on the modified Hertz contact theory and the simplified simulation of impact，a new method is approved to calculate the impact force of outrunner blocks in debris flow on structures. Based on a limited parametric study，it is shown that the impact causes plastic deformation on concrete structures at a low velocity. The impact force considering the elastoplastic deformation of structures can be far less than the elastic result. This new method matches the practice better.
