Modelling of tide and surge elevations in the Solent and surrounding waters: The importance of tide–surge interactions

A regional two-dimensional hydrodynamic model using the MIKE-21 software and data from a pre-operational forecasting system of the English Channel is described and applied to the Solent–Southampton Water estuarine system. The regional model was able to predict surge heights with a root mean squared error (RMSE) accuracy of 0.09 m during a three month hindcast in the winter of 2009, comparing closely with accuracy assessments from other independent systems. However, consistent underprediction of tidal harmonic constituent amplitudes was present throughout the region leading to errors in the prediction of the total water level elevations. Despite the complex nature of the Solent tidal regime, interpolation of tidal elevations from harmonic analysis at fixed tide gauge locations was shown to be effective in reducing this uncertainty at gauged and un-gauged sites. The degree to which tide–surge interactions were taking place was examined. Of particular interest was the quantification of the sensitivity of the predicted surge to the levels of uncertainty expected in the prediction of the tide within a complex nearshore region such as the Solent. The tide–surge interaction during three surge events was shown to be greatest in the Western Solent and Southampton Waters regions, where the tidal uncertainty was greatest. Interaction between the tide and surge resulted in a change of up to 0.3 m (11%) in the predicted total peak water level when the surge was added to the harmonic analysis-based tidal prediction. Despite the significant effect of removing the tide–surge interactions, tests indicated that the error in tidal range expected in the regional models tidal prediction altered the prediction of the surge only enough to induce changes in peak total water elevations by up to 0.03 m during an event on 10th March 2008. The findings suggest that the current tidal predictions in complex estuarine systems, such as the Solent, are of high enough quality to reproduce the majority of the tide–surge interactions taking place and that the error in the surge due to uncertainties in the predicted tide are expected to be relatively small.