Coastal Hazard from Waves, Tides and Surges

Abstract

Flooding and coastal hazards are an increasing threat to coastal communities worldwide, ranging from small islands and developing states to extremely wealthy and increasingly urbanised areas. In the UK, the increase in extreme sea level events recorded in recent years, and coastal hazards have become the top priorities of the most recent national risk register, with regards to natural hazards. One important driver of coastal flooding is the interaction
between waves, tides and surges, which can lead to inundation and over-topping, overflow or, in rarer cases, to breaching of coastal defences. A better understanding of this process is required. In this research this is studied through the use of numerical modelling. The newly developed high resolution coupled model UKC4 is applied to the case study of winter 2013/2014 with the aim to provide an improved understanding of how waves, tides and surges interaction impacts upon the formation of extreme coastal total water level (TWL) around the British Isles during the extreme winter 2013/2014.
The objectives of this research are to understand (1) if, and how effectively, the fully coupled configuration of the UKC4 model is able to resolve the TWL’s main components, (2) what this high resolution model shows with regards to the spatial distribution of the TWLs main components during storms around the British Isles, and (3) whether extreme wave events duration has any impact over the formation of extreme TWLs at the coast.
Results show that the fully coupled UKC4 model is resolving the TWL's main components well, and is suitable for the purpose of this research. This work shows that coupling is an extremely useful tool to study feedback interactions, even though some uncertainties remain in the comparison with observations where fitting the reference level of the model to geodetic references for operational application still proves difficult. The absolute values of SSH and tide error when compared to both TGs and satellites is of the order of decimeters (particularly at peaks), while the correlation is on average at least above 0.98. Waves are also compared to buoys at multiple sites, showing a RMSE of 0.38m on average, and a correlation of 0.94. The simulation of the winter 2013/2014 is used to understand what the distribution of the main TWL components is at the coast, showing that wave influence is dominant over the Atlantic facing coasts, while surges are more important in the north west of England, and in the relatively shallower regions of the south east of England and the Bristol Channel. Tides are also dominant in the Bristol Channel, where the morphology of the Channel amplifies the tidal amplitude, as well as in the north west of England and the English Channel. This leads to considerations about threats faced by different coastal regions being linked to either of these components rather than the TWL per se, and how coastal hazard may change in future. Results also show that in macrotidal regions the simultaneous extreme Hs and residual surges tend to occur over the flood tide, however it was possible for extreme waves, surges and tides to concur at any coastal point. This led to question whether the formation of extreme TWL is mostly related to physical processes such as the wave-tide-surge interaction or if the storm length is a more important driver as it should lead to more chances of extreme events occurring at high water. Results show that while storm duration plays a role in the formation of high coastal water levels, this is not the only parameter to consider. There is a need for a joint metric system to assess coastal hazards, incorporating the contribution of waves, tides and surges to the TWL of a region, along with characteristics of storms such as track, intensity and duration. These results can benefit coastal management and forecasting as they improve our understanding of some of the interactions driving the formation of extreme TWLs in different areas of the British Isles, with methods that can be re-applied to any coastal region.

Date of Award	1 Sept 2022
Original language	English
Awarding Institution	Bangor University
Supervisor	Mattias Green (Supervisor)