Suspended sediment flux in a muddy mesotidal estuary: tidal and meteorological forcing, and sensitivity to climate change

Data from a 1 year acoustic Doppler profiler deployment in a muddy mesotidal estuary on the UK east coast are analysed to reveal the sensitivity of the suspended sediment dynamics to tidal and meteorological forcing. Flux calculations indicate a small sediment import equivalent to just 1.5% of the gross flood tide transport. Although little confidence can be assigned to either the magnitude or direction of such a small residual when considered in isolation, the inference that the sediment regime is finely balanced is qualitatively supported by the close similarity between flood-tide and ebb-tide SSC values. Singular spectrum analysis of the SSC time series reveals expectedly large contributions to the variance in SSC at intratidal and subtidal (semimonthly and monthly) scales but also picks out intermittent variability that is initially attributed to a combination of nontidal surge and wind stress forcing. Closer examination of the data through cross- correlograms and event-scale analysis implicates meteorological forcing as the major factor. Acting through the resuspension of intertidal mudflat sediments at times of strong westerlies, meteorological forcing is directly implicated in episodic sediment export from the estuary. Thresholding of tide-averaged fluxes using a range of critical wind stress values further indicates that 'tide-dominated' (i.e. low wind stress) and 'wave- dominated' (high wind stress) conditions are associated with sediment import and export. Sediment balance is potentially sensitive to the frequency of high wind stress events, since the associated sediment exports are several times larger than the average import under calm conditions. Intermittent meteorological forcing may thus exert an important control on the sedimentary balance of what are generally thought of as tidally- dominated muddy estuarine systems. These findings may have wider implications for a broader set of estuaries within the southern North Sea and, more generally, suggests that the role of wind climate should not be overlooked in studies of estuary response to environmental change.