Evaluating the Role of Blockage Ratio and Orientation in the Drag Force Calculation for Large Woody Debris

Large woody debris (LWD) is commonly used in river restoration projects because it provides important habitats. However, when used in urban creeks and rivers, these structures must be designed and installed in a way that balances the needs for flood safety with the goals of ecological restoration. To ensure the stability of LWD during floods, engineers must consider a range of characteristics such as the size of the structure relative to the flow cross-section, the ratio of length to diameter, shape, orientation, flow magnitude, and the overall hydraulic conditions. To properly design and fixate LWD, it is necessary to consider both lift and drag forces in different flow situations. There are several approaches available for calculating these forces, but many of them are often only applicable by simplifying the LWD into an idealized 1D or 2D cylinder case.

The general drag force equation uses an empirical drag coefficient, which is a function of the object's shape, size, and surface roughness, as well as the properties of the fluid and the flow conditions. However, the drag coefficient does not account for factors such as blockage ratio (the ratio of the object's area to the flow cross-section) and orientation (the angle at which the object is oriented relative to the flow direction). These factors can significantly affect the drag force, and their inclusion in the drag force calculation can lead to more accurate predictions.

To evaluate the role of blockage ratio and cylinder orientation, experiments were conducted in a 10 meter long, 79 cm wide glass flume with a rough bed to create a fully turbulent velocity profile. Smooth PVC cylinders, representing the woody structures, were placed in the flume with rotation angles between 0 and 90 degrees relative to the flow. Examined were three cylinders with a ratio of length to diameter from 3.16 to 9.48 and lengths between 20% and 60% of the channel width. A dynamic load cell was used to measure the drag forces on the cylinders in the flow direction, and various subcritical flow conditions with different depths and velocities were examined.