Beach Response to a Total Exclusion Barrage: Cardiff Bay, South Wales, UK

The regeneration of 1100 ha of derelict industrial land to the south of Cardiff included the construction of the 1.1-km-long Cardiff Bay Barrage (completed November 1999), which impounded two major South Wales rivers. A 160 m groyned beach, composed of four groynes and three bays, adjacent to and seaward of the barrage breakwater was monitored between September 1997 and September 2002 to assess pre- and postconstruction beach evolution. Overall, mean beach levels increased throughout the five-year period, resulting in a net gain of beach covering equivalent to 818.8 m3 (1800 tonnes). After barrage completion, both longshore and cross-shore gradients became less volatile and increased beach levels in the bay nearest the breakwater, prevented tidal action and erosion at the cliff toe. This was a significant change from initial conditions that was verified by parametric and nonparametric tests at the 99% confidence level. Regression analysis determined that there were significant temporal relationships. Spatial analysis identified two highly significant longshore trends with respect to distance from the breakwater and showed that this influence decreased with distance. More than 72 m from the breakwater, the regression equation (R2 = 96%) modelled a trend of falling beach levels caused by net sediment transport. Conversely, from 72 m to the breakwater, beach levels increased at a rate greater than twice the fall in the previous section (R2 = 92%). These trends were further supported by significantly greater level differences across the second groyne. Impacts of temporal and spatial trends, especially subsequent to barrage completion, were evidenced by the change in beach morphology and similar contour orientation in all three bays. Models were developed and proposed as management tools to identify potential changes in coastal processes, as well as the rate of change of the barrage's upstream influence with respect to net sediment flow. Groyne removal was suggested to provide recreational beach space