• Energy Research

AbstractWith riverine flooding set to be more frequent in many parts of the world as a result of climate change, the interactions between fluvial morphodynamics and riparian vegetation may depend in part on the sequence of flood events. This paper describes a laboratory study of the geomorphic adjustment of a braided river to sequences of floods across five different strengths of braidplain vegetation. By using alfalfa as a proxy for braidplain vegetation, the differing plant life stages were used to represent the varying strengths of biogeomorphic feedbacks across the floods. Boundary conditions were constrained by sets of experimental runs with both equilibrium sediment loads and deficit loads. Changes in bed topography were monitored and assessed using a detailed digital elevation model, digital imagery and continuous monitoring of the transported sediment. Results demonstrate that in absence of plant colonization, vegetation placed the rivers in a non‐equilibrium condition, in which riparian vegetation encouraged the development of new channels, increased the system channel width and enhanced topographic irregularity, these effects being more noticeable during the low‐flow periods. The morphodynamics was found to be less sensitive to variations in flood discharges as the vegetation influence (strength) increased from minimum to maximum, until vegetation began to die back and the impacts of flood sequences became yet again evident. Although the overall sediment transport rate was reduced under full‐grown vegetation conditions, the presence of the mature plants across the braid bars resulted in the greatest channel scour depths. Results are considered in light of expected changes in flood frequency with climate and likely morphodynamic responses of river systems as a result.

Abstract A three-dimensional tidal turbine simulation based on an oceanographic numerical model has been tested for suspended sediment calculation, particularly in the wake of a standalone tidal turbine. The results suggest a need for further improvement of the model in order to obtain correct predictions of suspension strength of the wake and suspended sediment concentration under the influence of a turbine (compared to measured data). Due to the wide use of FVCOM in coastal applications where turbines are commonly installed, it proves necessary to address this issue. Two approaches with respect to modifying bed shear stress and turbulent mixing calculations in the presence of a turbine are proposed and tested in this research. Using data collected in the laboratory as reference, the turbulent mixing enhancement approach is shown to be effective. A series of tests are carried out to identify the impact of the turbine on suspended sediment transport in its vicinity. The results suggest that the impact is highly dependent upon the sediment grain size.

The interactions between water, sediment and biology in fluvial systems are complex and driven by multiple forcing mechanisms across a range of spatial and temporal scales. In a changing climate, some meteorological drivers are expected to become more extreme with, for example, more prolonged droughts or more frequent flooding. Such environmental changes will potentially have significant consequences for the human populations and ecosystems that are dependent on riverscapes, but our understanding of fluvial system response to external drivers remains incomplete. As a consequence, many of the predictions of the effects of climate change have a large uncertainty that hampers effective management of fluvial environments. Amongst the array of methodological approaches available to scientists and engineers charged with improving that understanding, is physical modelling. Here, we review the role of physical modelling for understanding both biotic and abiotic processes and their interactions in fluvial systems. The approaches currently employed for scaling and representing fluvial processes in physical models are explored, from 1:1 experiments that reproduce processes at real-time or time scales of 10−1-100 years, to analogue models that compress spatial scales to simulate processes over time scales exceeding 102–103 years. An important gap in existing capabilities identified in this study is the representation of fluvial systems over time scales relevant for managing the immediate impacts of global climatic change; 101 – 102 years, the representation of variable forcing (e.g. storms), and the representation of biological processes. Research to fill this knowledge gap is proposed, including examples of how the time scale of study in directly scaled models could be extended and the time scale of landscape models could be compressed in the future, through the use of lightweight sediments, and innovative approaches for representing vegetation and biostabilisation in fluvial environments at condensed time scales, such as small-scale vegetation, plastic plants and polymers. It is argued that by improving physical modelling capabilities and coupling physical and numerical models, it should be possible to improve understanding of the complex interactions and processes induced by variable forcing within fluvial systems over a broader range of time scales. This will enable policymakers and environmental managers to help reduce and mitigate the risks associated with the impacts of climate change in rivers.

This data storage report explains the methodology for a series of experiments conducted as part of the Hydralab Joint Research Activity called RECIPE to investigate the response of physical modelling experiments to climate change drivers. Links are provided to the key datasets collected during the experiments. The experiments are designed to elucidate the influence of flood sequences on braided rivers with erodible banks under different vegetated conditions. The experiments had two specific objectives The experiments were carried out in order to address the following specific objectives: 1. To quantify the impacts of flood sequences on channel morphology with different hydrograph inputs, 2. To determine the role of vegetation growth/age on the impact of flood sequences

© 2017 International Association for Hydro-environment Engineering and Research, Asia Pacific Division. The uptake of tidal stream-turbine (TST) technology lags other renewable energy sources despite the advantages of predictability, stability and increased power output in comparison to wind turbines of the same dimensions. There remains a need to address environmental concerns about the potential impacts of TSTs including the suspension and deposition of bed sediments if TSTs are to be more widely accepted and deployed. Sediment mobilisation and persistent flow vortices will also adversely affect the performance of other TST devices in an array downstream of the wake. The focus of this work is to improve our understanding of the wake recovery structure of a TST to build on the limited field and laboratory data currently available in order better predict the impact of TSTs on flow and sediment transport. Detailed measurements of the wake flow structures generated by scaled TST devices are presented. These results are the first to be derived from the application of high spatial resolution stereoscopic Particle Image Velocimetry (PIV). Two scale model horizontal-axis TSTs were manufactured and deployed in a laboratory flume (11. m long, 1.6. m wide and 0.6. m deep) at different flow speeds and heights above the bed. The results demonstrate greater wake recovery lengths for the rotor design with wider blade tips, despite the higher wake turbulence generated by the blades. Wake recovery is more rapid at the higher flow speed when greater turbulence from the tips is observed, but wake recovery lengths increase when both rotors are positioned closer to the bed.

Abstract A high resolution Computational Flow Dynamics (CFD) numerical model is built based on a laboratory experiment in this research to study impacts of tidal turbines on surface wave dynamics. A reduction of ∼ 3 % in wave height is observed under the influence of a standalone turbine located 0.4 m from the free surface. The artificial wave energy dissipation routine ‘OBSTACLE’ within FVCOM is shown to effectively capture the correct level of wave height reduction, reproducing the CFD results with significantly less computational effort. The turbine simulation system is then applied to a series of test cases to investigate impact of a standalone turbine on bed shear stress. Results suggest an apparent increase in bed stress ( ∼ 7 % ) upstream of the turbine due to the inclusion of surface waves. However, in the immediate wake of the turbine, bed stress is dominated by the presence of the turbine itself, accounting for a ∼ 50 % increase, with waves having a seemingly negligible effect up to 9D (D is the turbine diameter) downstream of the turbine. Beyond this point, the effect of waves on bed shear stress become apparent again. The influence of OBSTACLE on bed stress is also noticeable in the far wake, showing a reduction of ∼ 2 % in wave height.