The debris flows are recognised as a major threat to bridge piers located in mountainous gullies

especially in the areas of the active seismological and hydrologic region in Southwest China. It is important to build the dynamic evolution model and quantitatively describe the dynamic process of debris flow impacting bridge pier. In this study

all of the large-scale laboratory tests are conducted by using large multi-functional debris-flow simulation system. The main objectives of our experiments are to find the correlation between impacting pressures and variable factors including velocity

flow depth

and dimensionless characteristic parameters of fluid mechanics and to obtain the dynamic behaviour of bridge pier under the impact of debris flow. The experimental results and dimensionless analysis show that the dynamic process is mainly controlled by two dimensionless numbers of Froude number (Fr) and Reynolds number (Re). Generally

the dimensionless impact pressure is the function of Fr for the low-viscosity debris flows

while for the high-viscosity debris flows

it is the function of both Re and Fr. Different types of debris flows present significant differences in both the peak impacting pressure and impacting power spectrum. Under the same bulk density

the low-viscosity debris flow has greater impact energy than that of the high-viscosity debris flow. In addition

various types of debris flows are essentially distinguished by the critical Fr line. To provide technical support and scientific basis

we analysed the impact signals of different types of debris flows and discussed the mechanism of impact model.