Experimental study of debris flow impact forces on bridge piers

Debris flow destroying piers is a common destructive form of bridge under the impact of debris flow. To investigate the magnitude of debris flow impact forces on bridge piers, we adjusted the contents of clay, sand, gravel, and water to generate debris flows with different rheological properties and densities. Two types of pier scale models with circular and square cross section were impacted in debris flow trough by using the above debris flows to comprehensively investigate the relationship between rheological characteristics, flow velocity, pier shape and impact force. The results show that the obtained debris flow materials have distinct rheological properties which can be easily measured through a rotational viscometer and represented by Newtonian fluid or Bingham fluid. The velocity of debris flow can be calculated by the Manning equation, and the roughness coefficient and the viscosity of debris flow in the equation satisfy a power function relationship. In the same cases, the impact forces on a round pier and on a square pier are significantly different. Generally, the drag coefficient of impact force on a round pier is much larger than that on a square pier. Because using non-Newtonian fluid Reynolds number (Re) can comprehensively represent the debris flow's rheological properties and velocities, the drag coefficient of the round pier be expressed as a function of Re. However, there exists this function for the square pier. For the convenient application in engineering, the drag coefficient of a round pier can be selected as 2.3 and 0.9 for viscous debris flow and sub-viscous debris flow, respectively. For a round pier, the drag coefficients are 2.6 and 1.9 for viscous debris flow and sub-viscous debris flow, respectively.