An assessment of processes and strategies for management of the Penarth Coast
In 1997, the effects of severe erosion along the Penarth foreshore became apparent when the beach surface fell to critical levels. From consideration of global influences, the physical and cultural environments and anthropogenic activities, including the construction of the Cardiff Bay Barrage, a five-year monitoring programme was devised to assess coastal processes, identify possible causes of the erosion and develop management strategies for protection of the Penarth coast. Approximately 1·5 km of the foreshore, comprising two orientations, NNE and NNW, were surveyed each September and April between 1997 and 2002, to assess summer and winter changes. Results showed in September 1997, sediment transport was southerly in direction whilst from April 2000, there was a consistent return to the traditionally accepted south to north longshore drift; verified by significant differences in longshore gradients. Foreshore analysis provided important regression models representing the variation of the shoreline indicator Mean High Water (MHW) with shoreline position (mean beach level) and gain/loss of beach material. Models represented the temporal variation of MHW and depth of closure with further significant correlation between the two-shoreline indicators (83·85%). A Management Response Indicator (MRI) equation provided a simple tool to rapidly assess the health of Penarth beach, MRI = 17·035 +tan·' (x14- x6)/240, whilst temporal variation of longshore gradients, demonstrated an important inter-relationship between beach evolution and orientation (97·37%). Significant temporal and spatial models identified changes in beach formation adjacent to the Barrage breakwater pre and post construction and showed its influence decreased with distance. A further management tool was developed to monitor beach evolution and morphological change in response to this structure. Analysis of ten-year water and wind data identified a projected mean sea level rise of 0-4mm/year and significant increases in easterly wind components between 1995 and 1997, coinciding with significantly higher extreme water levels. Following correlation of these forcing agents with shoreline indicators, supported by documented beach losses of 350 kg/m2 subsequent to an easterly storm, it was concluded that the unprecedented erosion of Penarth beach was caused by increased wave attack from the northeast and southeast quadrants. Function Analysis justified foreshore management from a development perspective whilst risk analysis supported management strategies. Initial recommendations of on-going monitoring to assess beach health were supported by developed models and underpinned by management responses detailed for each of three defined beach sections. These included beach nourishment, wave wall construction in conjunction with the proposed Penarth Headland Link and groyne removal adjacent to the Barrage.