Comparison of tide-gauge data and a saltmarsh-derived reconstruction of mean sea-level for the Mersey Estuary

Prosser, M.C.; Brown, J.M. ORCID: https://orcid.org/0000-0002-3894-4651; Plater, A.J.; Mills, H.. 2015 Comparison of tide-gauge data and a saltmarsh-derived reconstruction of mean sea-level for the Mersey Estuary. Southampton, National Oceanography Centre, 44pp. (National Oceanography Centre Internal Document, No. 14)

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Abstract/Summary

Using saltmarsh sediment cores, Mills (2011) reconstructed the historic trend of mean sea-level in the Mersey over a period since 1975. The analysis is based on the foraminifera species identified at different levels within the sediment core; each species being associated with a tidal elevation (for example, mean high water neap) identified from present-day vertical distribution of saltmarsh foraminifera at the coring sites. While the reconstruction at Decoy Marsh matched the tide gauge record at Gladstone Dock, the reconstruction at Oglet Bay for the period 1993 and 2003 disagreed. During this period the reconstruction suggested an initial drop in mean tidal level (MTL) of 50 cm followed by a 50 cm rise back to the underlying trend after 2002. Because a local drop in sea-level (SL) is unlikely, and the foraminifera fossils used in the reconstruction are unlikely to have changed their tolerance to inundation, another factor must account for this sea-level anomaly. Here using the 3D hydrodynamic Proudman Oceanographic Laboratory Coastal Ocean modelling System (POLCOMS), the impact of the position of the main estuarine channel and historic sea-level elevations on the tidal dynamics are investigated relative to the conditions in 2008. Changes in the proportion of time that certain elevations at the saltmarsh coring sites are inundated could explain the deviation observed in the reconstruction. Such an effect is hypothesised to occur in response to local changes in the tidal dynamics, i.e. changes in tidal range or asymmetry in tidal elevation. It is found that in response to changes in channel configuration to test the scenario of a northern channel migrating up-estuary through Oglet Bay, a change in inundation characteristics caused by a change in the bank drying phase of the tidal cycle, may well have contributed to the anomalous reconstruction.