Overtopping hazard reduction at Churchill Barriers, Scotland

Churchill Barriers, a series of four causeways in Orkney Islands, Scotland were constructed during Second World War and now serve and a critical link for South Ronaldsay and Burray to the Orkney Mainland. Due to their location and design, causeways are exposed to storm waves generated in the North Sea. During coastal storms, some waves are able to overtop the barriers, damaging cars and potentially washing vehicles off the barrier. Due to overtopping risk, the barriers are closed to vehicles around five times per year, with the number of closures increasing. Closures are manually operated by policeman who are sent to barrier. Their responsibility is watching overtopping situation and close the barrier when they it is too dangerous to drive across. This Master Thesis aims to provide insight in the optimal strategy of reducing wave overtopping volumes and frequency of closure of Churchill Barrier No.2, by investigating overtopping volumes of plain and recurved wave walls for various closure frequencies, and exploring tolerable wave overtopping volumes in line with local condition. Because barrier locate in relatively uninhabited area, design closure frequencies are between 0.2 to 5 times per year, which is much higher than normal design. In order to simulate wave overtopping, boundary conditions are required. Since wave conditions at barrier toe are of complete lack, extreme offshore waves are transformed to barrier toe by Numerical Model SWASH. The results of SWASH are used as input of statistic model that can generate wave condition and water level at barrier toe for each design return period. Afterwards, wave overtopping volumes are simulated as a function of no wave wall, present wave wall, plain vertical wave wall with various height, and recurved wave wall with bullnose with various height and bullnose angle. The simulation uses most up-to-date and suitable empirical equations obtained from academic research. Wave volume of present wave is used for evaluating policeman's estimation on closure. It shows that policeman did a good job in estimation. However overtopping splash is a very dynamic process, the evaluation is subjected to many limitations. Wave volumes of plain vertical wave wall and recurved wave wall are used for wave wall design. Various sensitivity analysis is carried out the find the optimal wall height and bullnose shape. It is proved that recurved wall is more effective in wave overtopping reduction than plain wave wall. Afterwards, cost-benefit analysis (CBA) is undertaken to explore the most economical wall design and its return period. CBA concludes that optimal design is recurved wave wall of three years return period. Wave wall is 1.7m high with 70 degree bullnose angle. With this wall, barrier need to be closed only once in every three years. In the end, some recommendations are given to general approach and method to collect raw data and accumulate engineering experience of local overtopping situation, which will contribute to an automatic road gate system in the further.Civil Engineering | Hydraulic Engineerin