An assessment of engineered log jam structures in response to a flood event in an upland gravel-bed river

Engineered log jams (ELJs) are employed to address river restoration goals and a range of river management problems including coarse sediment movement. In the Bowmont Water, a dynamic wandering gravel-bed river in the Scottish Borders, 33 previously untested ELJs primarily designed to capture and store coarse sediment, were installed on a trial basis. Using repeated topographical surveys and field observations, the performance of the ELJs in response to a 5-10 year recurrence interval flood that occurred on the 25 September 2012 was evaluated at two reaches with catchment areas of 28km(2) and 57km(2). Three of the structures were damaged due to scour of surrounding material that exposed the pile anchors and all the timbers of one structure were completely displaced downstream. Sixteen structures induced geomorphic responses and only four induced significant deposition (>0.3m) above that which would occur naturally within the adjacent active gravel bar deposition zones. The placement in gravel bars, minor channel blockage ratio created by the structures and their porous nature limited the hydraulic interference and in turn geomorphic responses. Therefore the ELJ placement goal of increasing sediment storage was not fully met. This study contributes to the empirical evidence base for ELJ performance evaluation of different designs in a range of physiographic settings needed to validate performance and refine design. Using these initial findings and knowledge gained from other studies, recommendations for improving the design and placement strategy are proposed. Copyright (c) 2016 John Wiley & Sons, Ltd.