Transport and Trapping of Woody Debris in a River Meander protected with ELJ Groynes

Extraordinary floods, caused by natural and anthropogenic factors, have accelerated river erosion and lateral migration rates in recent years. Therefore, in the lowlands of the Amazon Jungle, collapse of riverbanks and subsequent dragging of logs and dense vegetation near the riverbanks has intensified. Woody debris carried away downstream puts at risk hydraulic structures, villages and even ships. The object of this investigation was to simulate woody debris transport and evaluate trapping efficiency of groynes made up of Engineered Log Jam (ELJ), which protect the external bank of a river meander. Consequently, experimental studies were carried out at the National Hydraulic Laboratory of the National Engineering University (LNH-UNI). Experiments were conducted on a physical model at a 1:60 scale, which represents a 4 km stretch of a Madre de Dios River meander (Perú). Wooden dowels were incorporated into the flow using a manual conveyor belt and their trajectories were recorded. At the end of each test, dowels trapped in between groynes were counted and the percentage of trapped elements was estimated. Discharges corresponding to 25 and 50 years return period events were simulated in order to identify in both cases the trajectories, dragging forces and final disposal of the wooden dowels that were incorporated into the flow. Approximately 8% of the wooden dowels were trapped in the groynes field in the 25-year flow simulation (466 L/s, 12 997 m³/s in prototype) whereas 14 % of the wooden dowels were trapped in the 50-year flow simulation (514 L/s, 14 431 m³/s in prototype). However, when groynes ELJ where fully submerged, wood debris was not trapped at any time. In the experimental phase most of wooden dowels were diverted towards the center of the water course downstream due to flow being redirected by ELJs. Most of the wooden dowels remained at or near the water surface in upstream face of these structures, and the rest of the wooden dowels settled on the embedding zones where they formed a hard and compact mixture with the sediments. In conclusion, the amount of trapped woody debris depends mainly on the centrifugal force exerted by the helical flow and of ELJs number, no matter how much woody debris is in transit. In addition, the low percentage of trapped woody debris does not affect the ELJs permeability and stability. Moreover, woody debris that eventually would settle on the embedding areas would provide resistance and stability to these structures.