Re-analysis of the December 1981 storm surge event in the Bristol Channel using the current operational tide-surge model suite

Abstract

The Bristol Channel is an area of complex hydrodynamics which include a very large tidal range, strong currents, extensive inter-tidal areas and river inputs, all of which contribute to make predicting storm surges difficult.

The highest water levels experienced in the Bristol Channel in a century occurred during the storm of 13th December 1981 when severe flooding was experienced along the north Somerset coast. This was due to the passage of a secondary depression which tracked unusually far south for the time of year (Figure 1), producing strong westerly winds in the southern Celtic Sea estimated to be in excess of 30 ms-1 (Proctor & Flather, 1989). The meteorological situation was complex and rapidly changing. Also the passage of the storm coincided with tidal high water in the Bristol Channel during a period of spring tides.

At the time of this event, the operational storm surge modelling system comprised solely of the original 35km continental shelf model (CSM) which was implemented in 1978. This is shown in Figure 2. CSM had only two boundary tidal constituents (M2 and S2), was forced by an atmospheric model of resolution 100km and produced surge forecasts only twice a day (0000UTC and 1200UTC). This combination of atmospheric and surge forecast models failed to provide adequate warnings of the expected levels in this region. Analysis of the performance (Proctor & Flather, 1989), suggested that the most significant factor was that the direction of the modelled winds was incorrect at the critical time leading up to high water in the Bristol Channel.

The purpose of this study is to quantify any improvement in the forecast that might be achieved if there was a repeat of this event (i.e. how well would the present, much- improved, operational forecasting system deal with a similar weather event). To facilitate this study, the atmospheric model forcing for December 1981 was obtained from reanalyses performed by the European Centre for Medium Range Weather Forecasting (ECMWF).

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