Holocene tides of The Wash, U.K.: The influence of water-depth and coastline-shape changes on the record of sea-level change

Abstract

Tidal simulations are conducted using a numerical model to investigate the relative influence of water depth and coastline shape changes on the Holocene tides of The Wash. Tidal input to this model is obtained by running a nested series of models commencing with a model of the northeast Atlantic. Results of both types of simulations are compared with those of other simulations incorporating all the palaeogeographic information available on both past water depths and coastline shapes around The Wash. The model employs six harmonic tidal constituents to obtain mean high water of spring tides (MHWST) altitudes. The Wash was a depositional embayment during the Holocene and results of model simulations are compared with sea-level indicators in the stratigraphy which record past MHWST altitudes. Tidal simulations for water depths reduced by 2, 5 and 10 m compared with present sea levels generally give lower tidal amplitudes around The Wash than found at present. MHWST altitudes simulating the coastline shape for 3000, 4000 and 5000 yrs B.P. utilising present water depths show similar patterns of tidal changes to those obtained from the palaeogeographic simulations. However, reduced water-depth tidal simulations accord more closely with the palaeogeographic simulation results in terms of the magnitude of changes to MHWST altitudes than the palaeocoastline simulations. This is due to the fact that the rise in mean sea level during the Holocene is far greater than any change in the amplitude of tides in The Wash. Palaeogeographic simulation model results for MHWST agree, within experimental errors, with sea-level curve altitudes. Reasons for discrepancies include inaccuracies in model parameters or measurement errors in obtaining MHWST altitudes from the stratigraphy. Overall, tidal heights increase with sea-level rise towards the present day. Alterations to coastline shape and non-uniform water-depth changes (for example, due to sediment movements or coastal defence schemes), must be taken into consideration when assessing changes to the tidal regime for different sea levels in coastal embayments in order to determine both the pattern and magnitude of tidal changes. This applies to both past and possible future tidal simulations.