Near-field flow downstream of a barrage: Experiments, 3-D CFD and depth-averaged modelling

Tidal schemes such as the Mersey Barrage are being considered for UK energy generation, because barrage electricity generation is renewable, reliable and predictable and creates no toxic by-products. A key problem perceived to arise from barrages is the detrimental environmental impact, which may include changes in water levels, which affects species like wading birds; altered water turbidity, which affects fish and plant species; and changes to sediment drift patterns, potentially causing sediment or pollution trapping. Some of these, however, may be positive outcomes of the barrage. In order to assess the accuracy of two-dimensional, depth-averaged models that are commonly used for large-scale modelling, flume experiments and 3-D computations have been undertaken to compare with depth-averaged modelling. The 2-D model was found to be inaccurate in predicting the close-to-surface flow patterns close to the barrage, however further from the barrage the model accurately predicted the approximate velocity magnitude at the surface. The 3-D model accurately predicted the velocity profiles at the depths and distances from the barrage that were analysed, with a few exceptions. Both models need altering and refining in order to obtain reasonable results for the friction velocity and bed shear stress, and this will be a further development of this project. Copyright © 2011 by ASME.