3D computational modeling of stream How resistance due to large woody debris

The study of Large Woody Debris (LWD) is very important in the stream restoration practice. Due to the effects of the irregular geometry of LWD and bathymetry, the stream flow around LWD is complex and turbulent. This paper presents a numerical model to deal with the turbulent flow around LWD with complex geometry, and calculates the flow resistance due to LWD in the streams, with the aim to quantify the momentum extraction from the main flow. In order to simplify the case, the LWD used in this study is made of small trees without leaves manufactured by 3D printing technique. They are placed in a small Hume, mimicking the LWD in real stream with an approximately rectangular cross-section. In the computational model, free surface is captured due to its importance when the LWD is partially submerged. The simulated free surface is comparable with the surface elevation data measured by a point gauge in the flume. The drag force due to the tree is calculated by the integral of stress as well as the ID momentum equation for different LWD angles. The velocity coefficient and the water depth coefficient are used to correct and improve the estimation. The comparison between these two methods suggests there might be substantial error in force when the LWD has complex geometry. The simulation results also indicate that the trees with angles to streamwise larger than 45° cause high velocity in nearbank region downstream, while smaller angles result in large slow flow regions in the wake.