Medium timescale stability of tidal mudflats in Bridgwater Bay, Bristol Channel, UK: Influence of tides, waves and climate

摘要

This paper presents the results of an 11-year study into mudflat elevation changes within the intertidal zone at Stert Flats in Bridgwater Bay, Somerset. This site is located in the outer Severn Estuary/inner Bristol Channel which is a macro-hypertidal regime dominated by physical processes, characterized by strong tidal currents, high turbidity and a significant degree of exposure to wind generated waves. Two transects of stakes were installed perpendicular to the coast, extending seawards 300 m from the edge of the saltmarsh onto the mudflats, against which variations in accretion or erosion could be measured. The mudflats themselves consisted of an underlying consolidated clay of Holocene age and a surface veneer of fluid mud and/or mobile sand patches which varied both spatially and temporally. Mudflat development was recorded over both short-term (monthly/seasonal) and medium-term (inter-annual) timescales. The results display a significant degree of scatter over all timescales. Such variability in response may be expected in such a dynamic system where noise can be attributed to a combination of factors such as the mobility of surface fluid mud and sand patches and the migration of the underlying ridge-runnel drainage network. Despite this, the expected short-term variations related to neap-spring tidal conditions and seasonal influences were observed at a number of locations on the transects although these were weakly expressed. The over-riding feature of the profiles is :3 consistent long-term trend of erosion which appears to be masking shorter term trends within the dataset. Viewed over the 11-year period, the changes in mudflat elevation closely match the pattern of the index of the North Atlantic Oscillation (NAO) during the 1990s, suggesting a strong climatic control over mudflat development on a medium-term/decadal scale. Most profiles display a strong erosional trend during the early 1990s when the NAO index was positive. The erosional trend peaked in 1995 at a time during which the values for monthly winter mean significant wave height were notably high. Between 1996 and 2001 the profiles generally record accretion and the data display significant variability. This corresponds with a shift to a strongly negative and then weakly positive NAO index phase. The fact that such a general atmospheric factor correlates so closely with medium timescale elevation change is attributed to relative weakness of biological binding and burrowing at this site, and more-so to the overwhelming dominance of the physical regime, especially the tidal current and the wind-wave regime. Both the background erosional trend and the influence of the index of the NAO in controlling mudflat evolution have important implications relating to coastal management. These are discussed in relation to coastal defence measures, morphological response to major civil engineering projects (e.g. Severn Tidal Power Barrage) and the prospect of climate change, sea-level rise and a possible increase in strength of NAO conditions in the future. (C) 2008 Published by Elsevier Ltd.