Engineering Log Jams (ELJs) are often used in rivers and streams for various purposes, e.g., fish habitat and bank protection. Currently, in two-dimensional (2D) hydraulics models such as SRH-2D, they are typically modeled with an extra drag force tern in the governing shallow-water equations. The extra drag force can be through an increased Manning’s n or an additional drag force. This talk concerns the second approach where a drag force coefficient is needed. In practice, there is not much information that is available for the determination of the drag coefficient. In this work, we use three-dimensional computational fluid dynamics (CFD) to simulate the flow through ELJs and directly calculate the drag force, providing useful data for the determination of the drag coefficient. One particular issue in such 3D CFD simulations we encountered is the free surface effect. Under certain conditions, when the free surface is simplified as a shear-free rigid lid, the simulated drag force is consistently lower than that when the free surface is simulated with methods such as Volume of Fluid (VoF). This work investigates the cause of such observation and provides guidance on when the free surface needs to be resolved, not simplified. Flume experiments are also performed to provide calibration data. The drag coefficient formular resulted from this work can be used in 2D models for better design and evaluation of ELJs.
