Evaluation and comparison of the operational Bristol Channel Model storm surge suite

Abstract

Due to its exceptional tidal range, complex geometry, and exposure to flood risk the operational storm surge modelling system for the Bristol Channel (running four times each day as part of the UK Coastal Monitoring and Forecasting service) comprises a number of nested hydrodynamic models. Forecasts for the region are available from the shelf-wide storm surge model, CS3X, as well as from two finer-scale models of the Bristol Channel itself (the Bristol Channel model, BCM, and the Severn River Model, SRM). This report provides for the first time a systematic comparison of the accuracy of the three models when compared against tide gauge observations, for five significant storm surge events in the model archive.

We find that overall the most accurate model predictions for these events (and it should be borne in mind that it is not a large sample size) are from the high-resolution models (BCM and SRM). There is however a large spread of variability and there are instances where the coarser resolution CS3X model has the most skill at particular locations. It is recommended that forecasting methods for the Bristol Channel area should adopt some combination of model outputs in a weighted multi-model ensemble. The good performance of the finer resolution models suggests that they are an important part of the current operational storm surge suite. Any future developments of the UKCMF forecasting system (e.g. based on coupled wave-surge models) should ensure that model performance in the Bristol Channel maintains or improves upon the accuracy described in this report.

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