Transport and Trapping of Woody Debris in a River Meander Protected with Elj Groynes.

Abstract

Exploratory tests were conducted to observe the effects of incoming Large Wood Debris (LWD) on Engineered Log Jams (ELJ) groynes’ stability and permeability and the capacity of an ELJ groyne field, built on the outer bank of a river meander, to trap LWD and establish new habitat. An existing experimental facility, where a 4.2 km curved section of the Amazonian river was modeled at a 1/60 scale, was used to simulate transport and retention patterns of LWD. Bed material was sand, slope was 0.022 per one thousandth and flow was subcritical in all tests. Radius of curvature to width ratio was approximately 2. Wood dowels of different sizes were incorporated into the flow using a manual conveyor belt and trajectories were recorded with video cameras mounted on a drone and on both sides of the testing flume. When ELJ groynes were fully submerged at peak flows, no simulated logs were captured by the ELJ groyne field. When ELJ groynes remained unsubmerged throughout the tests, with flows representing 25 and 50-year-floods, between 8 and 14 % of the simulated logs were trapped mostly near the upstream face of the ELJ groynes. Therefore, an ELJ groyne field placed on the outer bank of a curved section holds potential for supply new material over time for rehabilitating erosion control structures and forming new riverine habitats. In addition, LWD did not appear to affect ELJ groynes stability or permeability, at least in laboratory tests.