Measuring the impact: new insights into flood-borne large wood collisions with river structures using an isolated sensor-unit

Large Wood (LW) transported during floods or channelized mass flows poses a high risk for engineered structures, often leading to significant damage or total failure of the impacted structure. To date little is known about impact magnitudes caused by LW collisions. To better control for such interactions, a better understanding of transport dynamics and impact forces is required. The present laboratory study employs state-of-the-art sensor units installed in scaled logs to capture acceleration data from collisions of waterborne LW with 2 in-stream structures—bridge pier and retention structure—each providing different examples of rigid engineered systems. Through precise measurements of acceleration and impact duration (stopping time), the resultant impact forces of LW collisions can be calculated. Here, for the first time, impact forces were quantified in a scaled stream environment based on the inertial frame of the object causing the impact, rather than the more commonly used instrumented structure approach. High-resolution accelerometer measurements were compared to conventional analytical (force balance) approaches. They revealed the need for accurate inertia measurements to appropriately account for prevailing hydraulic flow conditions and the effects of LW interactions in fluvial environments. Although log velocity and stopping time are crucial parameters for assessing LW impact forces, accurate measurements are still elusive due to limitations in available sensing techniques. By presenting proof-of-concept results, this study contributes to an improved understanding of LW impact forces during floods. Based on these encouraging results, we recommend more sensor-based field studies in future, needed for the design of resilient structures.