Impact of barrages on extreme water levels in the Bristol Channel

The Bristol Channel/Severn Estuary has some of the largest tides in the world with a mean spring tidal range of 12.2 m. On rare occasions, a significant storm surge will coincide with a spring tide and this has potential for very major coastal flooding given the large amount of low-lying land surrounding the estuary and the high population. Due to the high tidal range, numerous proposals have been made to exploit the Severn Estuary for energy extraction. Developing tidal power as a renewable energy resource is currently under consideration in order to meet increased worldwide energy demands and to reduce carbon emissions. The Severn Barrage and the Swansea Bay Lagoon are two options proposed for the Bristol Channel, which have been evaluated in this thesis. A key concern with this technology is that such structures may cause other environmental problems. However, tidal barrages may also have beneficial environmental effects in some areas, such as mitigating coastal flooding from storm surges. In light of this, this study aims to evaluate how the introduction of a tidal barrage would change the susceptibility of storm surge-induced flooding in the Bristol Channel.
In this thesis, the astronomical tides and storm surges are modelled using the open-source hydrodynamic model (ADCIRC DG-SWEM) which solves the two-dimensional shallow water equations. A one-dimensional model is also considered. Both of these models are calibrated and validated with field observations. The models are then modified using an internal barrier boundary to represent the effect on flow of implementing a barrage. During a storm surge event, it is found that to the east of the barrage, the peak water level is significantly reduced by the Severn Barrage. To the west of the barrage, on the ocean side, the presence of the barrage leads to a small increase in the water levels during a surge event. Different operational strategies for the barrage during a storm event are examined, which make a small but perhaps significant difference. The impact of storm surges on power generation is also investigated and is found to be minor. Overall a barrage can help to mitigate the impact of storm surges on coastal flooding. With respect to the Swansea Lagoon, the flooding protection benefits are found to be limited and localised since it is a relatively small project. The resonant response of the Bristol Channel hydrodynamic system is also examined in this thesis. The Severn Barrage is found to cause little change to the resonant response close to the dominant semi-diurnal frequencies and the changes are localised to the device.
The application of a one-dimensional hydrodynamic model can be computationally cheaper and only key hydrodynamic features are captured. Compared with the two-dimensional DG-SWEM, it is found in this study that the tidal characteristics and barrage behaviour can be accurately simulated by a one-dimensional model while discrepancies occur when modelling storm surges. A one-dimensional model is suitable for a preliminary investigation or a qualitative analysis.

Keywords: storm surge
ocean renewable energy
Bristol Channel
tidal barrages
Advisor: Adcock, T