Hydrodynamic Processes due to a Series of Partially Spanning Logjams

Due to human intervention, many rivers worldwide exhibit sediment deficit as well as hydraulic and morphological degradation, resulting in the restoration of river habitat becoming a critical task. In Switzerland, the revised Waters Protection Act demands the restoration of 4,000 eco-morphologically impaired river kilometers by 2090. To meet this target, nature-based solutions such as engineered logjams are being implemented to increase flow heterogeneity and provide shelter and habitat for aquatic organisms. To optimize their design, it is crucial to improve our understanding of the physical and ecological interactions for habitat creation, while also considering the potential flood hazard.In this study, we performed a series of flume experiments on partially spanning logjams positioned in series. The experiments were conducted for different clearance distances between the installed logjams, logjam width, solid volume fraction, and flow Froude number. We analyzed the resulting flow velocity, water depth, and turbulent kinetic energy to quantify the flow heterogeneity and to evaluate the flood hazard.The results demonstrated that the backwater rise upstream of the first logjam was larger compared to the second logjam. Compared to previous experiments with single partially spanning logjams, the presence of the second logjam did not affect the backwater rise upstream of the first logjam. In addition, it was found that the backwater rise increased with increasing logjam width, resulting in a smaller logjam width being more beneficial from a flood hazard perspective. In contrast, a wider logjam led to a higher flow variability and the creation of two distinct flow regions downstream of the logjams. This result highlights the relevance to consider both flow variability and flood hazard aspects to design engineered logjams for river restoration projects.