Modelling tidal changes within the wash and Morecambe bay during the Holocene

Abstract

Palaeotidal changes are one of the least known factors of the sea-level record variation at the local scale (Shennan, 1986a; Devoy, 1987). This thesis extends knowledge of tidal alterations with sea-level change by means of an approach integrating numerical tidal models with geological stratigraphic data recording former tidal heights. The last 10,000 years (the Holocene period) were chosen for study due to the sedimentary sequence available recording sea-level changes. Two macro-tidal embayments, the Wash and Morecambe Bay, are examined for palaeotidal changes by running a series of seven numerical tidal models from the scale of the north-east Atlantic to that of the bays. In order to obtain results to the required resolution to carry out the work, two new tidal models were developed for the Wash. Tidal model simulations for lowering of sea depths from current bathymetric values without coastline shape changes showed reductions of a maximum of 10% of the sea-level reductions in the bays. Changes to tidal altitudes were not so great for alterations to coastal shape alone, where no modification of present sea depth values was included. A combination of sea depth and coastline changes used in the reconstruction of former tidal height patterns within the embayments showed differences corresponding broadly to the variations in altitude of sea-level index points within the Wash Fenlands, although altitudinal differences are within the model error band for tidal predictions. For Morecambe Bay, however, tidal inundation does not occur to altitudes predicted by sea-level index points and it is suggested, following Tooley (1978, 1987), that neotectonic movements may well have influenced the Holocene sea-level record in this area. Better palaeogeographic data are required for more accurate palaeotidal simulations in embayments. Sediment compaction is also identified as an area requiring further research in the attempt to explain altitudinal variation of sea-level index points within local areas and so enable regional comparisons of sea-level change to be made.