In the past, Galveston Island has suffered from several tropical storms and hurricanes. Some of them have had a tremendous impact on the City of Galveston and its inhabitants. Two recent hurricanes, Ike (2008) and Harvey (2017), caused significant damage and struck the city in different manners. While Hurricane Ike brought about high wind speeds and surge, Hurricane Harvey deposited extreme amounts of precipitation over the island. There is a high probability that the City of Galveston will be struck again by a major hurricane. Hence, research is needed on mitigation measures that reduce the flood vulnerability of Galveston. More specifically, the simultaneous occurrence of surge and extreme precipitation is worth investigating, as currently little is known about the synergy between these aspects. This report elaborates on how the risk of flooding in the City of Galveston can be mitigated, considering the influences of extreme pluvial, coastal and compound flooding. In order to provide adequate mitigation measures, a vulnerability analysis is performed on the City of Galveston using hydraulic modelling software. Furthermore, a stakeholder and system analysis is done for all relevant stakeholders. Their respective interests, influence and interactions are mapped in a power-interest diagram and tube model. In addition, three residential stakeholder focus sessions were organized which provided valuable validation of the model and evaluation criteria for designs. The flood vulnerability of the City of Galveston is not merely limited to a single area. Vulnerability maps and an inventory of critical infrastructure show that Galveston has various bottlenecks scattered around the city. A crucial result of the analysis was that flood risk issues in Galveston can be divided into two aspects: nuisance flooding by regularly occurring precipitation and flooding due to hurricanes. This distinction is reflected in the proposed mitigation measures, as they require a fundamentally different approach. While damage caused by nuisance flooding can be fully prevented with the proposed measures, damage resulting from hurricanes can at best be mitigated. A comprehensive plan containing preliminary measures for both flooding scenarios is proposed for the City of Galveston. As part of this integrated plan, thirteen projects are defined which are elaborated in this report. Proposed measures to prevent damage originating from nuisance flooding include retention and infiltration of stormwater, discharge by pumps and raising frontier roads. Measures that mitigate damage due to hurricanes include breakwaters, retractable barriers and shelters for vertical evacuation. The authors recommend that more stakeholders are actively involved in interactive design sessions to make the plan more inclusive. Furthermore, for more accurate designs a probabilistic approach is preferred to the deterministic approach used in this report. In addition, more work is needed to elaborate on the design proposals as presented in this report.

The Netherlands has always been threatened by water. To protect flood-prone areas flood defenses have been constructed. To guarantee that they keep meeting the requirements systematic assessments take place. Since 2017, these assessments are performed according to the ‘Wettelijke beoordelingsinstrumentarium’ resulting in a safety judgment category (I_V, which is the highest safety category, to VII_V, the lowest safety category). The category depends on the difference between the norm and the actual failure probability of the assessed dike trajectory. To determine this failure probability each applicable failure mechanism is assessed. However, the assessment of piping at hydraulic structures does not result in a failure probability. The objective of this thesis is to find a method that can be used to obtain a failure probability for this failure mode. The current assessment strategy for piping at hydraulic structures is based on a comparison between the critical difference in hydraulic head and the occurring difference in hydraulic head over the structure. This strategy has only two possible (non-probabilistic) assessment outcomes, II_V and V_V (instead of the entire range of outcomes for probabilistic analyses). Currently, flood defense assessment is mainly based on the application of fault trees. Another possible method is to use event trees to quantify the probability of failure due to piping. Both methods clarify and relate all sub-events before failure happens, both qualitative and quantitative. The event tree method is chosen because an event tree can describe multiple consequences and is not limited to Boolean operators. To test the probabilistic event tree method a case study of the Marksluis chamber lock was conducted. In the event tree approach the piping failure mode is divided in three main phases: A) geotechnical part, B) emergency response and C) remaining strength of the structure. Three different event tree variants have been designed. Variant 1 (shown in the picture) is rather extensive and serves solely qualitative purposes while variant 2 and 3 are more concise and have quantification of the failure probability as goal. The geotechnical part is quantified using Monte Carlo and FORM analyses. However, the other two phases cannot be quantified with a model. The remaining strength of the structure is quantified by expert knowledge and it can be concluded that the upper limit of the conditional probabilities of structural failure and breach formation is 0.1. For the Marksluis the difference between the norm and the actual probability of failure due to piping is more than a factor 30. This leads to assessment category I_V. By using an event tree approach the probability of failure of different types of hydraulic structures due to piping can be quantified. This quantification is based on both models and expert knowledge. Adjustment of the WBI 2017 assessment procedure is useful for failure modes that cannot be assessed in a probabilistic manner yet. By implementing an event tree approach it becomes easier to think of what sub-events must be assessed and how this can be done in a probabilistic manner.