Investigations on the Trajectory of Large Sandbags in Open Channel Flow

Abstract

Investigations undertaken to understand the mechanics of the motion of large sandbags and to compute their trajectories are reported in this paper, along with the details of the experimental setup and procedures. The motion of sandbags is recorded from the side of a flume by a high-definition charge-coupled device (CCD) camera, and the digital particle tracking velocimetry (DPTV) technique is used to track the motion of the bags. An equation is developed for the normalized maximum horizontal settling distance from the experimental data. It is found that the particle velocity normal to the flow depends mainly on the characteristic diameter of the particle and the Froude number of the flow, whereas the particle velocity in the streamwise direction shows lower dependency on the Froude number. Analysis of the particle tumbling shows that the Magnus force may be neglected for the purpose of modeling the trajectories of sandbags in uniform flows. A model of particle motion is developed by solving the Lagrangian equation numerically. Two approaches to computing the trajectory of sandbags are investigated. The results show that the approach in which the drag coefficient is varied based on the orientation of a particle gives better results than if the drag coefficient is kept constant and is based on the broadside orientation of the particle. The results also show that the change in the drag coefficient may be as low as 28% to as high as 76% based on the orientation of the particle with respect to the flow.