Worldwide, cities rely on the proper functioning of critical infrastructures (CIs) such as electricity, telecommunication, water supply and transportation. Failure of those infrastructures can lead to significant and long-lasting impacts, even far beyond the flooded areas due to cascading effects. Local authorities are eager to take action to reduce flood risk and strive to increase the resilience of their communities. However, CI are often not considered in flood risk assessments. One of the reasons is that CI operators do not share their CI data and internal risk assessments. Therefore, an integral view on flood risk is lacking and risks may be unidentified or underestimated. To overcome this limitation, in this paper we propose an integrated framework for flood risk assessment of urban critical infrastructures (UCIs) for local authorities, which is based on publicly available and field-surveyed CI data. The proposed framework supports cities to carry out cross-sectoral risk screenings on urban district level to evaluate the need for in-depth risk assessments and risk dialogues with CI operators.

Assessing dike safety is of key interest to societies in low-lying areas, but results can be implausible when, for example, they contradict the observed performance of the dike. To improve credibility, load monitoring data can be incorporated using reliability updating techniques. This paper investigated the role of load variations in reliability updating and assessing credible failure probabilities. It was found that the impact of reliability updating increases when load variations are small, as a large contribution to failure probabilities comes from relatively frequent load levels, of which the conditional failure probabilities are reduced most through reliability updating. Moreover, a credibility check was introduced for dikes that have been stable for decades, where load levels with return periods of up to 10 years are not expected to contribute more than 50% to the failure probability, indicating an imbalance between load variation and strength uncertainty. This imbalance occurs when the inverse gradient of the fragility curve exceeds 1.5 times the decimate height of the load. Many Dutch dikes, including canal dikes and dikes along the large lakes and delta regions, have small decimate heights. For these dikes, strength uncertainties must be sufficiently small to obtain credible failure probability estimates.

Objective of this paper is to study how reliability standards, expressed as probabilities of dike segment failure, can be practically updated to improve opportunities for risk-based dike design and planning. The approach to assess the economic optimal flood probability, used by the Dutch Delta Committee (1958, in this paper referred to as Van Dantzig), is adapted to reflect time-dependent effects of a.o. climate change and subsidence. Furthermore, the approach is adapted to reflect overtopping instead of overflow and it is extended to include reinforcements over time. A comparison of the results of the Adapted Van Dantzig approach with the economic optimal probabilities used as input for the recently formalised Dutch standards (2017) is performed for 73 dike segments in the Netherlands, showing good agreement. Following the Adapted Van Dantzig approach, an analytical relation is developed for economic optimal design horizons, dependent on the dike design, and characteristics of load, investment, climate effect, and economic growth. Finally, a dynamic and simple-to-use approach is developed to enable updating of the economic optimal reliability based on a proposed design and investment planning. This can serve to consider whether an existing reliability standard still fits adequately or needs updating.

In absence of sufficient data, structured expert judgment is a suitable method to estimate uncertain quantities. While such methods are well established for individual variables, eliciting their dependence in a structured manner is a less explored field of research. We tested the performance of experts in constructing and quantifying a nonparametric Bayesian network, describing the correlation between river tributary discharges. Specialized software was provided to assist the experts. Expert performance was investigated using the dependence calibration score (a correlation matrix distance metric) and the likelihood of the joint distribution. Desirable properties of the dependence calibration score were investigated theoretically. Individual expert judgments were combined based on performance into a group opinion aka decision maker. All experts were able to create and quantify a correlation matrix between 10 variables that resembled the correlations between observed discharges well. The decision makers performed similarly to the best expert. Based on the metrics investigated, it mattered little which expert opinions and with what weight were combined in a decision maker. This is partly because all experts performed well. Adding a bad performing expert increased the positive effect of performance-based weighting, underscoring the importance of developing scoring rules for dependence elicitation. The overall results are promising: Aided by specialized graphical software, the experts in this study were able to quickly create and quantify dependence structures.

Internal erosion is a significant cause of failure in dams, levees and other hydraulic structures. This article studies the time-dependent reliability of such structures under Backward Erosion Piping (BEP), a form of internal erosion in the foundation. First, a physics-based time-dependent piping failure model is presented. Second, a time-variant reliability analysis method is presented which allows to quantify how the reliability evolves over the years due to cumulative pipe growth over multiple flood events. Finally, these models are used to study the importance of time-dependence for reliability estimates of flood defenses in The Netherlands. The findings show that, particularly in coastal areas, incorporating time-dependence significantly reduces the computed failure probability. Reductions vary widely, ranging from a factor of 5 to more than (Formula presented.) depending on flood duration and levee properties. Therefore, reliability estimates for levees can be improved by incorporating time-dependent pipe development in the BEP failure model, and thereby contribute to avoiding unnecessary reinforcements.

The condition of flood defence revetments is influenced by many different degradation processes such as animal burrowing, rutting and growth of weeds. Many of these processes are shock-based rather than progressive continuous. As shocks can cause a drop in performance, this means that the condition of a revetment can suddenly decrease, meaning that revetments can have significant initial damage at the beginning of a storm. Combined with the limited detection probability of common visual inspections of flood defences, this can have a significant influence on the reliability of flood defence systems, something typically not considered in reliability analysis. In this paper we study the reliability of a flood defence system subject to shock-based degradation. Various maintenance concepts are compared for a case study of a riverine flood defence of 20 kilometres length. This demonstrates that the current maintenance concept is insufficient to satisfy the reliability requirements for failure of the revetment. Overall, the joint influence of degradation and the existing maintenance concept leads to a 20 times higher failure probability estimate compared to a typical assessment without these aspects. Next, we demonstrate that both additional inspections, and targeted interventions to reduce the impact of for instance animal burrowing, can significantly reduce total cost and improve robustness of the considered flood defence system.

Flood events are expected to increase in their frequency and severity, which results in higher flood risk without additional adaptation measures. The information gained from flood risk models is essential in effective disaster risk management. However, vulnerability estimations are often a large driver of uncertainty, and flood damage is rarely estimated due to a lack of empirical damage data from flood events. This study uses a unique data set with experienced damages and the implementation of flood damage mitigation (FDM) measures on the household level, collected after the flood event in the Netherlands in 2021. Flood damage models that control for several hazard, exposure, and vulnerability indicators are estimated and allow for additional input in flood risk models. Previous estimates of the effectiveness of FDM measures are prone to a selection bias, as households that do, and do not implement FDM measures systematically differ in their risk profiles. By using an instrumental variable-estimation, this study overcomes this selection bias and finds significant reductions in flood damage due to FDM measures. These reductions can be incorporated in multivariate flood vulnerability estimations, which indicate that FDM measures significantly reduce flood damage. Providing information on flood hazard, as well as implementing early warning systems, is crucial for ensuring effective flood risk management.

A system of dikes in flood-prone areas continuously requires measures to mitigate changes such as ageing and climate change. Planning costly measures requires proper insight into system risk effects. Especially in a riverine dike system, the risk contributions of individual assets to the system flood risks are not independent, because reinforcement of a dike upstream increases the risks downstream. Tactical plans define the planning of consecutive measures to implement a flood risk reduction strategy, which may take decades. They may differ due to choices such as a prioritization metric, planning conditions and budget. In this study, a method is developed to compare different tactics to prioritize and plan measures in interdependent systems of dikes to reduce risks most effectively and efficiently. A case study meant as a proof of concept was carried out for the reinforcement of about 500 km of dikes along the Rhine River branches in the Netherlands. We studied the effects of 12 different tactical plans on the aggregated risks over time. The economic risks differ by up to about 40%, and the risks on victims differ by up to 70 %. We conclude that tactical planning and corresponding decisions are important for reduction of time-aggregated flood risks.

Risk-informed decision-making needs methods for assessing flood risk, and often the flooding probability and the consequences of floods (such as economic damage and fatalities) play an important role in the risk assessment. Also, measures to reduce the flood risk need to be formulated, in order to make a risk-informed decision, whether to accept the risk or to implement measures to reduce or to transfer the risk. These measures can be technical (for example, storm surge barriers) but can also be in the area of spatial planning or crises management. The outcome of such a process might be formalized in a “safety standards,” which can be used for the design of measures. These standards show remarkable differences between the United States, the United Kingdom, and the Netherlands.