Maximum Impact Force of Woody Debris on Floodplain Structures

Abstract

We collided woody debris (i.e., logs) with structures using flume and test basin laboratory facilities to investigate the maximum impact force that floodplain structures are exposed to from floating woody debris. The tests also investigated the influence of collision geometry, determined by the debris orientation on impact, on the maximum impact forces. We reviewed the three approaches that represent the existing guidance for estimating maximum impact forces. Each approach estimates the maximum impact force based on the debris velocity and mass. We show that all the existing approaches can be derived from a single-degree-of-freedom model of the collision and can be considered to be equivalent. The laboratory data show that the maximum impact force was associated with a log striking a rigid structure with its end. Oblique and eccentric collisions reduced the maximum impact load in a predictable and consistent manner. The approach we refer to as “contact stiffness,” a linear, one-degree-of-freedom model with no damping, was able to reproduce the laboratory results over the entire range of data, with an effective contact stiffness of 2.4 MN/m.