Reliability analysis of the Caldicot Levels' flood defence system by using Dutch relaibility method for flood defences

Reliability analysis of the Caldicot Levels flood defence system by using Dutch reliability methods for flood defences F.A. Buijs In the light of the shift to a risk-based safety assessment of flood defences in the UK a project called RASP, Risk Assessment of Strategic Planning, is carried out at among others HRWallingford. Part of this project is to develop a detailed level methodology supporting decision-making at the scale of one flood defence system. In this Msc thesis the reliability-based tools as applied to flood defences in the Netherlands are tested on a flood defence system in the UK, the Caldicot Levels flood defence system, in order to support an evaluation of the appropriateness of these tools as part of the detailed level methodology in RASP. The Caldicot Levels flood defence system is located at the south coast of Wales in the UK. The system borders the Severn Estuary in the south, the river Usk in the west and a distinct line of hills in the north and east. In the Severn estuary one of the largest tidal ranges in the world occurs, a range that varies between 9 and 15 meter. The River Usk is a small river, however the water levels can reach quite high values especially in case of high water levels at the Severn Estuary. The Dutch reliability methods for flood defences mainly consist of two parts. Software called PC-Ring which calculates the probability of failure of a flood defence system. Second, a process that selects the flood defence sections which are most representative of the systems probability of failure. The first step in the reliability analysis is the definition of the system. This step points out the relevant defence length for the calculation of the probability of inundation and the area which suffers consequences in case the flood defence system fails. The second step is to define the systems components, or the defence types that occur in the flood defence system. Part of the second step is to determine the different failure modes which can cause failure of the components. The third step is to select for each failure mode the weak cross sections in the flood defence system which contribute most to the total probability of failure. The last step is gather the data connected to the selected cross sections and perform the calculations with PC-Ring. The results point out the weakest links in the system and which random variables contribute most to the variance of the total probability of failure. Apart from the probability of failure of the present system, the probability of failure of the system with improvements is regarded. The Dutch reliability-based methods for flood defences prove to be suitable for relatively highly engineered systems with a high data availability - compared to UK standards. Adaptations of PC-Ring to the UK site point out that the set-up of the program is not very flexible. However, considering the diverse nature of the flood defences in the UK and the often relatively low data availability, a flexible reliability-based computer program for flood defences will be required in the UK.