On the Tidal Resonance of the Bristol Channel
- Chanshu Gao (University of Oxford) | Thomas A. A. Adcock (University of Oxford)
- Document ID
- International Society of Offshore and Polar Engineers
- International Journal of Offshore and Polar Engineering
- Publication Date
- June 2017
- Document Type
- Journal Paper
- 177 - 183
- 2017. The International Society of Offshore and Polar Engineers
- numerical modelling, Bristol Channel, shallow-water equations, tidal resonance
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- 28 since 2007
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The Bristol Channel has one of the largest tidal ranges in the world. A key cause for this is the resonance with the dominant semidiurnal tides. In this paper we use numerical simulations to investigate this resonance. We first vary the frequency on the boundary of the model and examine at which frequency the model is excited. Second, we apply a disturbance to the model and analyse the frequency at which it resonates. We examine the sensitivity of these results, finding them sensitive to the bed friction used (with possible implications for energy extraction) but insensitive to small changes in the tidal amplitude on the boundary or the mean-water level.
The Bristol Channel and Severn Estuary constitute one of the largest, semienclosed water basins in the United Kingdom. The Bristol Channel is located in the southwest coast of Great Britain. The Severn Estuary is situated at the upper reaches of the Bristol Channel, which has the second-largest semidiurnal tidal ranges worldwide. The typical mean spring tidal range is 12.2 m, with the high spring tidal range approaching 14 m at the Severn mouth. The large tidal ranges observed in the Bristol Channel and the Severn Estuary are driven by two main mechanisms (Robinson, 1980; Xia et al., 2012; Serhadlıo˘glu, 2014). One is the funnelling effect at the upper reaches of the Bristol Channel due to its wedge-shaped geometry and shallow bathymetry. However, it has long been pointed out by Marmer (1922) that this effect is not enough to produce the observed tidal range. The other mechanism is the quarter wavelength resonance of the Bristol Channel with the incident North Atlantic tidal wave (Fong and Heaps, 1978).
Despite a number of previous model studies having been undertaken for the Bristol Channel, its complex hydrodynamic system is not yet fully understood, particularly given its resonant nature. Resonant systems are typically very sensitive to small changes, and these responses are highly site dependent (Adcock et al., 2015). In this study we seek to improve the understanding of the resonance in the Bristol Channel. A simplified 2-D model has been developed from the model of Serhadlıoglu et al. (2013) to investigate the resonances in the Bristol Channel.
In this paper, the model equations and the model parameters used for the Bristol Channel region are first considered. Then, the model is tested by comparing its results with previous model studies and observations. Two methods have been used to determine the resonant periods of the Bristol Channel. A frequency sweep is used by varying the forcing frequency on the open boundary of the model to find the peak response of the semidiurnal tidal amplitude. Next, the key properties that influence the resonances are investigated. Finally, wind disturbances are applied to examine the oscillation periods of surge response.
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