Erosion and Deposition around Porous Engineered Log Jams: Flume Experiments and Improved Predictive Formulas

Abstract

Wood reintroduction into rivers, such as the construction of engineered log jams (ELJs), is increasingly popular in river restoration. ELJs are widely used because they mimic natural hydraulics and are relatively easy to construct. One key characteristic of ELJs is their porosity and resulting leaky nature. Based upon previous work, this study further examines the effects of ELJ porosity on flow and sediment transport. Flume experiments were performed over a wide range of porosity values and two ELJ placements (center and side of channel). Detailed flow and scour were measured and analyzed. To better quantify the porous nature of ELJs, two porosity definitions were proposed: the surface porosity and the volumetric porosity. Based on the two porosity definitions, a general formula for equilibrium scour depth has been developed, which captures the trend and reduces scatter in the data. The formulas were verified with data from a real river restoration project. The temporal evolution of scour depth was further analyzed based on a scaling analysis and a saturation growth curve was proposed.