Infrastructure is at risk to climate uncertainty due to a combination of long life spans, complexity of the systems it is embedded in, and the high investment costs often necessary. Current infrastructure planning approaches lose efficacy under deep uncertainty, necessitating new approaches that function better under conditions where the future cannot be predicted. The approaches that attempt to deal with this are also called Decision Making under Deep Uncertainty (DMDU). Bangladesh, the Netherlands, and New Zealand have all already adopted the use of these DMDU approaches in their delta protection guidance. One popular DMDU technique is Robust Decision Making (RDM). RDM can be seen as a computational extension of scenario planning, where proposed plans are tested against every potential combination of uncertainties. The Deep South Challenge (DSC), a New Zealand based research institute is looking into using RDM on a regional scale to discover vulnerabilities and identify robust strategies based on them. One of the test cases is in Helensville, where a Wastewater Treatment Plant (WTP) serving a small community is located in the middle of a floodplain. The WTP discharges its effluent into a strongly tidally influenced river, which drains the entire watershed and flows past large tidal flats into a dynamic estuarine environment. In order to identify potential vulnerabilities in the system, robust decision making uses a vulnerability analysis. This consists of a scenario discovery and global sensitivity analysis which sample through every combination of uncertainties to characterize the vulnerabilities of the system. In order to facilitate this, usually simple conceptual models are used due to the high number of runs necessary. However, these types of models can oversimplify complex physical processes and topography. These complicating factors are all present at the current case site selected by the DSC. This research investigates whether the added computational demand of a complex model is worth it compared to a simple conceptual model. To do this, two models are selected and forced for the same event. They are then compared on predicted system behavior, identified vulnerabilities, and potential policy advice. From a larger selection, the FLORES and SFINCS models were chosen. FLORES uses a simple hydrological balance to calculate the water level in the subbasins for each timestep. SFINCS is a reduced physics solver which only uses the Local Inertial Equations (LIE). Both models are forced by a compound rainfall and stormtide event for a storm with a 24 hour duration, for which they were calibrated and validated using the results of previous modeling efforts in the region. After the calibration and validation, a sensitivity analysis and scenario discovery were run for both models. The results of the sensitivity analysis show similar model behavior between FLORES and SFINCS. The upstream part of the model domain is only sensitive to rainfall, while the downstream part is mostly sensitive to storm surge and mean sea level, and to a lesser degree to tidal amplitude. This downstream part includes the wastewater treatment plant. Compared to SFINCS, the outcomes for FLORES on average overestimated water levels at the WTP. This is most likely due to the lack of flood attenuation taken into account by FLORES compared to SFINCS. The scenario discovery showed similar results for each model’s box describing 73% of the outcomes where failure occurs. Both models had the same three thresholds: storm surge, mean sea level, and tidal amplitude. The main difference between the boxes was the storm surge threshold being 21 centimeters lower for FLORES compared to SFINCS, indicating FLORES overestimates the water level at the WTP. The results of the scenario discovery also showed a linear relationship between these three factors. From this relationship, it is possible to see that, keeping all else similar, and with the same high tidal amplitude and storm surge, for SFINCS the plant only starts flooding when a mean sea level of at least 0.4 meters is reached, while for FLORES this is 0.25 meters. Using a RCP4.5 emissions scenario, a mean sea level of 0.25 meters will be reached in 20-30 years, and a mean sea level of 0.4 meters in 50 years. The proposed policy options for both SFINCS and FLORES would be to mitigate storm surge as long as possible, since the water level at the WTP is most sensitive to this factor. Once this is no longer possible, the WTP should be relocated. The results of SFINCS indicate this relocation is necessary later than for FLORES. These results show that while the behaviors exhibited by both models is relatively similar, the small differences in accuracy affect which are most likely due to the lack of flood attenuation taken into account for FLORES lead to a different timing of proposed adaptations. This leads to reason that while a conceptual model such as FLORES works well to identify important factors within the system, a more accurate model such as SFINCS can be more helpful once timing of adaptation becomes important. Further recommendations are to repeat this research for more models, further calibrate and validate the models, and to include scenario discovery methods that better deal with the found linear relation.

Urban areas of coastal cities are increasingly susceptible to the consequences of climate change. In particular, the threat of increasing floods due to sea level rise and heavier rainfall is growing. Cartagena de Indias in Colombia is such a coastal city, where flooding is a significant issue, with floods becoming more frequent in recent decades. Arcadis, together with the ConAgua consortium, is currently investigating and designing mitigation strategies for the city’s water-related issues. The thesis focuses on defining and quantifying the sources of urban flood problems and assessing the impact of potential mitigation measures for reducing these flooding problems.

The study consists of four parts:
1. Identifying the main causes of floods using a data analysis;
2. Quantifying these findings with a numerical D-Hydro model;
3. Investigating suitable kind of mitigation measures and quantifying the impact of the best feasible mitigation options;
4. Evaluating the investigated mitigation options based on several criteria with the help of a multicriteria analysis.
Parts 1 and 2 focus on understanding the system. Parts 3 and 4 of the study focus on finding the best option to reduce floods.

The data analysis allows the development of a system description identifying the main causes of flooding. The analysis reveals that the limited capacity of the drainage system, high topographic variations, and extreme rainfall are the main causes of flooding in Cartagena. Extreme rainfall flows quickly from steep upstream areas to milder slope lower-lying areas, particularly those at the edge of the Ciénaga de la Virgen. It is hypothesised that downstream water level plays a role in the accumulation of runoff water, thereby influencing the frequency and intensity of floods in the neighbourhoods at the southern edge of the Ciénaga de la Virgen, especially with a relative sea level increase of approximately half a meter factored in. This hypothesis is checked by quantifying the impact of different scenarios with a numerical model.

A numerical D-Hydro model is used to quantify the impact of different meteorological conditions and downstream water levels on floods. The model’s simulation results show that rainfall intensity is a more dominant factor in flood generation than downstream water levels. Even with a return period of two years, rainfall causes significant flooding of almost 25% in the study area. The model also indicates an increasing influence of tidal conditions due to relative sea level rise. Still, the increased influence is mainly visible in the borders of the Ciénaga de la Virgen with minimal residential impact. These conclusions highlight the need for mitigation efforts to focus primarily on improving the areas influenced by the rainfall, especially for short return period rainfall events.

Various flood mitigation measures are considered for the southern border of the Ciénaga de la Virgen to reduce floods caused by the dominant intense rainfall. Wetland creation and evaporation measures are ruled out due to the dominant influence of rainfall-induced floods and the inability to prevent floods caused by extreme rainfall events respectively. After investigating the scale required, considering feasibility, the three types of measures that are converted into mitigation options are:
• Mitigation Option 1: Increasing channel dimensions,
• Mitigation Option 2: Increasing infiltration,
• Mitigation Option 3: Constructing retention areas.
These mitigation options are assessed for their impact on reducing flooded areas and flood levels. The three mitigation options are evaluated to recommend the best-preferred mitigation design. This evaluation employs a Multi-Criteria Analysis (MCA)...

Flood risks and its consequences become more and more challenging and are demanding for the currently present dike systems and hydraulic structures in the Netherlands. This automatically raises the question for the possibility of adjustable temporary deployable flood defences that work autonomously. Nowadays it is more likely that a standard earthen dike will not be possible everywhere, due to site-specific characteristics such as available space or protected townscape. At these specific locations, a self-closing flood barrier can offer a solution, in view of minimally affecting the spatial quality with a reliable solution. The goal of this design study is to gain insight in the feasibility of applying a self-closing flood barrier in areas where spatial quality plays an important role and where the hydraulic boundary conditions demand for heavier structures. With the increase of densely built areas nowadays, the concept of an autonomously closing barrier is promising and an interesting alternative to consider.
In this thesis the objective is to develop a customised design of an adaptive self-closing flood barrier in the upper river region of the Netherlands. The city of Arcen in this case study is characterised by its cultural-historical values and its strong connection to the Meuse. This makes it difficult to construct conventional earthen dikes.
For the design purpose software with statistical data such as Hydra-NL is used to provide site characteristics, hydraulic and geotechnical boundary conditions and literature for information on currently applied barrier types and drive mechanisms. Furthermore, the design process has been completed with the help of the Design Guide Hydraulic Structures of Rijkswaterstaat and the Eurocode, from which a design originated in which stability and strength requirements have been met. The design resulted in a floating flat barrier of steel, which does not involve a complex design and is in line with integrating the structure in the area with minimal hindrance. In the design the focus lies on the functionality and the structural integrity with an in-depth look at adaptability and integrability.
The result shows that the design of a floating flat barrier is feasible as a self-closing structure in the city of Arcen. However, because of the location of the structure with the requirement of no townscape obstructing elements, a heavy support structure is needed for a design water level of 2.5 m above the ground surface. Furthermore, the applied boundary conditions leads to a large floater, even though the structure is slender. In order to develop a complete design, it is recommended to make a detailed design on the concrete foundation, to do a cost analysis on the construction and materials and investigate how a certain structure is received by the residents of Arcen. This will gain insight in areas to optimise the design furthermore.

This dissertation investigates the role of structured expert judgment in quantifying uncertainties central to flood risk assessments, particularly for engineered flood defense systems. Flood risk in the Netherlands involves rare events with high consequences that cannot adequately be quantified with empirical data alone. As an alternative to the traditional approach of physics-based modeling, this research explores the use of expert judgment to address these uncertainties. Through the Classical Model (Cooke's Method), expert estimates of uncertainty are evaluated and combined with the aim of improving the credibility of failure probability estimates. Four key research questions are explored: 1) the performance of statistical tests and distributions in the Classical Model, 2) the accuracy of expert estimates for different types of uncertainties related to flood defense safety, 3) the integration of expert judgment into a Bayesian framework to reduce uncertainty in hydrological extremes, and 4) the quantification of statistical dependence through expert judgment. The findings show that while structured expert judgment effectively addresses certain types of uncertainty and dependence, its accuracy depends on the nature of the variables and the methods used to process expert data. This research demonstrates several methods to incorporate expert judgment in flood risk modeling, offering insights, tools, and recommendations for future studies and practitioners.@en

The sea level is rising, possibly by one meter, by the end of this century. The Netherlands is preparing for this scenario through Kennisprogramma Zeespiegelstijging (KP-ZSS). One of the goals is to determine the hydraulic effects of sea level rise (SLR) on the current system. For the flood risk analysis within
KP-ZSS, the current strength of the flood defences needs to be described. A set of typology-defined fragility curves based on river dikes in river areas describes the current strength of all the primary flood defences in the Netherlands. However, it is unknown if these fragility curves are applicable to sea dikes
due to shorter high water and differences in soil parameters in sea/tidal areas. The research described in this thesis aims to evaluate if site-specific fragility curves for sea dikes result in significantly different predictions of future reinforcement cost for sea dikes compared to the typology-defined fragility curves. The analysis is centred around three dike sections in trajectories 29-3, 30-3 and 32-4 in the Western Scheldt. Initially, an evaluation is conducted on the fragility curves related to the geotechnical failure mechanisms of macro-instability and backward erosion piping. Subsequently, the height requirement is taken into account during the cost calculation. The typology-defined fragility curves underestimate the strength of macro-stability in two out of three
cases with a factor 104 and factor 102, while underestimating it for one section with a factor 105. For the piping failure mechanism the typology-definde fragility curves overestimate the strength in two out of the three cases with a factor 103 and factor 102, but underestimates it for one section with a factor 103.
The pre-overburden pressure appeared to be the most important factor influencing the failure probability of the macro-stability failure mechanism, with higher occurring values for the sections in the Western
Scheldt. The failure mechanism of piping was influenced most by the hydraulic conductivity, where the
encountered soil in the Western Scheldt consisted of finer soils with lower hydraulic conductivity.
From the results, it can be concluded that the use of site-specific fragility curves resulted in a decrease of 13% in net present value, averaging over all SLR scenarios for an analysis until 2200. Considering the most probable SLR scenario resulted in a 12% decrease. If the height requirement is included in the cost calculation, the use of site-specific fragility curves results in a 7% decrease in net present value on average and a 12% decrease for SLR scenario low. For the low SLR scenario, the cost of relocating the road infrastructure in and around the expansion zone of the dike is dominant. With increasing SLR scenarios, the increase in crest height becomes the most important factor, with the revetment the dominating factor cost-wise, leading to minimal differences in reinforcement cost between the fragility
curve approaches.

The Dutch Rhine-Meuse delta is expected to require many dike reinforcements on a short and long term, due to (accelerated) sea level rise. Average damage in unembanked areas will increase too, and a costly replacement of the Maeslantkering is expected after 2070. Delta21 is a project that aims to address these challenges related to flood protection, while also providing hydraulic energy storage and opportunities for valuable nature development.
Delta21 consists primarily of a large (salt)water storage lake attached to the Tweede Maasvlakte. It is connected to the North Sea with a pump-turbine station. On the southern side, a closable storm surge barrier spans the remaining gap to the island of Goeree Overflakkee. Under normal circumstances the lake functions as a hydraulic battery, using the pumps and turbines to store or generate electricity as needed. However, when water levels threaten to exceed NAP + 2.5 m at Dordrecht, the barrier closes and an upstream spillway into the lake is opened. To determine the viability of the project in the context of flood protection, the question arises: “To what extent is Delta21 capable of providing a cost-effective alternative to the current flood protection policy in the Rhine-Meuse delta?”
To answer this question, a one-dimensional hydraulic model is constructed with a detailed and accurate schemati-zation of Delta21. Results are processed into exceedance frequencies for a system with and without Delta21, to obtain reductions in water levels at normative frequencies. Furthermore, a sensitivity analysis is performed to de-scribe how various designs and configurations affect the magnitude of these reductions. Other than Delta21, no system-changing interventions are included in the scope of study.
Three strategies are formulated in which Delta21 potentially reduces costs, and evaluated using the model’s results:
1 Extending the lifetime of the Europoortkering before it needs to be replaced.
2 Obviate dike reinforcements.
3 Reduce the average yearly damage of unembanked areas.
Approach 1 is shown to be ineffective. Delta21 does not achieve changes in the Europoortkering’s closure frequency or failure probability per closure. Neither does Delta21 effectively mitigate a failure in the region where water level exceedance frequencies are dominated by the Europoortkering’s failure probability.
Approach 2 is far more successful, accounting for 96% of the total cost reductions. Reductions in water levels at normative frequencies are translated to reduced failure probabilities of dike segments using fragility curves, which potentially yields a positive reassessment. Delta21 achieves this and obviates dike reinforcement for 41 km by 2050, and 150 km more by 2100. These lengths comprise respectively 33% and 60% of the total considered dike lengths that would need reinforcement. The net present value of obviated reinforcement costs is €752 million, with a 90% confidence interval of [€220 million, €2,821 million]. This large interval is due to large uncertainties in reinforcement cost estimations. Delta21 most effectively obviates reinforcements in trajectories with stricter norms and closer proximity to the storage dominated region. Water levels at normative frequencies are very sensitive to the opera-tional control (i.e. closure criterion) of Delta21. An additional nominal cost reduction of ca. €278 million can be achieved when a criterion of NAP + 2.5 m at Dordrecht is maintained in the future. The sensitivity is far weaker to varying dimensions or capacities of Delta21, which are generally unnecessarily large during illustrative conditions.
Approach 3 provides the additional 4% contribution to total cost reductions. The only considered area is Dordrecht, due to its unique high economic value in low lying unembanked locations. Damage profiles are integrated with changes exceedance frequency curves to obtain the average yearly damage. This is reduced by approximately €53,000 per year now, and grows to €1.36 million per year in the future climate scenario (2100). The decrease in relation to the current system is about 42%, but the absolute value rises greatly with sea level rise. Implementation of lower closure criteria variations can yield an additional +20% now, and +10% in the future scenario.
The total net present value of cost-reductions by Delta21 is €783 million, with a 90% confidence interval of [€251 million, €2,852 million]. This covers about 20% of the total construction costs of €3.7 billion, and is insufficient to make Delta21 viable alone. However, not all costs are related to flood protection exclusively. If the components required for energy storage are viable on their own, merely the additional costs of the spillway and storm surge barrier have to be included in the cost-benefit analysis. Operational costs, which heavily depend on the operational control, must also be added. Smaller design dimensions or pump capacity of Delta21 has been shown to be just as effective, and would further cut costs. Additional savings beyond 2100 are plausible, but outside of this research’s scope. Furthermore, less people displacement or flooding of unembanked areas may achieve additional societal value, but is difficult to quantify. More research will have to indicate whether attributing only specific costs, including additional value sources, and estimating reinforcement cost more accurately ultimately lead to Delta21 being fea-sible in the context of flood protection.

Where water flows, prosperity follows.

Many prosperous areas around the world are situated near large bodies of water such as oceans, seas, and rivers. These bodies of water play a vital role in enhancing the prosperity of societies through various means. They provide convenient access to transportation, food, recreation, and many other benefits. However it is not only prosperity that follows, but also risk. This risk, defined as the product of probabilities and outcomes, is an inherent price that has to be paid to enjoy the social and economic fruits that are provided by these oceans, seas or rivers. As whenever one is close to a body of water, there exists a probability of a devastating outcome: A flood.

In order to effectively minimize and handle risks, the field of Flood Risk Management has naturally evolved. Within this field numerous decisions need to be made, such as determining an acceptable probability of flooding for a particular area or deciding when to evacuate if a flood is imminent. These decisions involve individuals who process information and make judgments under risk. However, it has been observed that the decision-making process is vulnerable to the influence of risk preferences. The St. Petersburg Paradox provides an illustrative example of how risk preferences can affect behavior, as individuals are only willing to pay a limited amount of money for an expected outcome of an infinite sum. By considering risk preferences, researchers were able to explain the observed mismatch between the expected value and the willingness-to-pay. This example raises the question of whether similar discrepancies between willingness to pay and expected outcomes can also arise in flood risk management, and whether the inclusion of these risk preferences can aid in the decision-making process under risk.


This research looks at ways of incorporating risk preferences into the field of Flood Risk Management. A stated preferences method is used to uncover the risk preferences of individuals for flood risk related problems, consisting of a series of flood related choice problems. The subsequent results are fitted to several commonly used risk preference models, which consist of a utility- and probability weighting function. A modified version of Prospect Theory by Tversky and Kahneman (1992) is found to best describe the risk preferences of individuals towards flood risk related problems. This research shows that individuals have similar risk preferences for flood risk related choice problems as for general (behavioural) economic choice problems. The found utility function shows that individuals have a diminishing sensitivity for outcomes that are larger in magnitude, with a greater diminishing effect for positive outcomes than for negative outcomes. Additionally, it became apparent that individuals experience flood-related losses around 1.5 times more intensely than equal gains, aligning with the current understanding of behavioural economics. The identified probability weighting function indicates that individuals overestimate small probabilities, perceiving them as greater than their actual value, while simultaneously underestimating large probabilities. The point where overestimation switches to underestimation occurs around probabilities of 0.3, in line with the results found in behavioural economics. Special attention was given to probabilities between one in a hundred thousand and one in a hundred where, instead of assuming a functional form for the probability weighting function, an additional elicitation was performed. This elicitation revealed that individuals perceive probabilities below one in a hundred as largely the same. Suggesting that people are unable to distinguish between very small probabilities, such as one in a thousand and one in ten thousand. This finding carries significant implications for the perceived level of safety in flood risk management, considering that flood safety standards typically fall below one in a hundred.

The found risk preferences are subsequently used to reevaluate the Statistical Value of Life (VOSL), previously found to be around €6.7 mln. by by de Blaeij (2003) and Bockarjova et al. (2009). Based on the found risk preferences and answers to the choice problems a new value of €11.8 mln. is found. Adjusted for inflation this value is around 1.18 to 1.33 higher than the previous found values. This research further looked at the influence of the additional risk premium in the Dutch discount rate for infrastructure projects and its effect on the safety standard in flood protections. Including a positive risk premium, which increases the overall discount rate, leads to a decrease in safety standards. This is the logical result of discounting future benefits in the form of reduced risk, while the incurred costs are borne in the present and are therefore not discounted. This decrease in safety standard is indicative of a risk seeking approach, which contradicts the risk-averse nature of the risk premium in the discount rate. To resolve this contradiction and adopt a risk-averse approach, several options for incorporating the risk-averse premium in the discount rate are proposed.

All the previous insights are used in a case study, which consists of the reevaluation of Dutch safety standards for all dike sections along rivers and coasts. The evaluation of optimal flooding probabilities for the Local Individual Risk (LIR) and the Social Cost-Benefit Analysis (SCBA) is modified to include the found risk preferences, along with the adjusted values for the discount rate and VOSL. The resulting criteria are then used to reassess the optimal classified flooding probabilities for various dike sections in the Netherlands. This adjusted evaluation shows that the ratio between the leading principles in the flood safety standards remains largely the same, but the optimal flood probabilities associated with these principles do change. Generally, these probabilities are reduced by approximately one order of magnitude when the best estimates found in this research are included, leading to a stricter safety standard for flooding. An analysis is conducted to examine the sensitivity of the different changes. This analysis demonstrates that the inclusion of risk preferences has the most significant impact on the optimal flooding probability for both the LIR and SCBA, with the probability weighting function exerting the most influence. When the best estimates for the adjusted evaluation are used, the additional costs for each resident of the Netherlands that are the result of risk aversion amount to €37 per year.

This research is concluded with the remark that the insights found in behavioural economics are useable and of value in the field of flood risk management. People tend to show the same degree of risk aversion when presented with an uncertain choice about flooding as they do when they are presented with an uncertain choice about economics, such as the problem posed in the St. Petersburg Paradox. Taking these risk preferences into account can help to better distribute the scarce resources such as time and money to where they are of most utility to society.

Land subsidence poses significant damage risks to residential real estate, including pile rot, differential settlements, pluvial flood risk, and dewatering risk amounting tens of thousands of euros. In the Netherlands, the number of affected houses has surpassed one million and is estimated to reach two million, accounting for a quarter of all houses in the country. Without an action perspective homeowners are left to their own devices which this research shows can lead to postponing action until risks materialize. To mitigate these risks, multiple adaptation strategies are available. This thesis focuses on determining the cost-effectiveness of two general pathways for residential real estate constructed with either a wooden pile foundation or a shallow foundation: renovating existing houses or replacing them entirely. The study also investigates whether incorporating additional measures aimed at improving overall benefits such as increasing the amount of living space or transitioning towards a more sustainable house with energy label A, is cost-effective by comparing the costs and benefits associated with the adaptation strategies. When the benefits outperform the costs the strategy is labelled favourable. Through a comprehensive analysis, this research provides valuable insights into economically viable approaches for addressing land subsidence-related risks in residential real estate in relation to the location specific characteristics. It proves additional measures are required to make either renovation or replacement a cost-effective adaption strategy. Two options are shown to be cost-effective. Firstly, renovation including additional investments to
increase square meters of living space and transitioning to an energy performance A-label. Secondly, replacement including densification by building back more square meters of living space, increasing the amount of houses or a combination of the two. Both require large investments resembling around 75% to 150% of the current housing price respectively. Increasing the amount of square meters even further improves the result however this increasingly affects the character of the neighbourhood and its social composition. With these results, this thesis hence provides an action perspective to homeowners and stakeholders including policy makers and financial institutions by means of a solution space to address
land subsidence in an economically favourable way, whilst simultaneously reflecting on the differences in implications for the parties involved.

Climate change and its impacts have necessitated a shift from relying solely on mitigation measures to implementing effective adaptation strategies. The financial requirements for adaptation, particularly in developing countries, have increased substantially. The annual projected needs for adaptation have nearly quadrupled over the past decade and are anticipated to reach significant amounts by 2030 and 2050. Sea level rise is considered a critical threat, and allocating financial resources efficiently becomes challenging due to the uncertain nature of SLR. Adaptive pathway planning, which allows for flexible decision-making over time, offers a promising approach to develop flood risk reduction strategies that can adapt to changing circumstances. However, evaluating the effectiveness of adaptive pathways requires accounting for uncertainty, using multiple evaluation metrics, and considering the value of time. To address these requirements, a new framework for evaluating flood risk strategies was established, incorporating Monte Carlo analysis to incorporate uncertainty and several evaluation metrics including NPV, BCR, EAC, POL, and CV. The framework emphasized the importance of decision moments, where new measures are required, and the balance between flexibility and costs. A conceptual case study involving seven strategies was conducted to test the framework, and sensitivity tests were performed to assess its robustness. The results demonstrated the framework's effectiveness in capturing uncertainty and the value of time, providing a solid foundation for further research. However, certain aspects of adaptive pathway planning, such as preventing future lock-ins, were not quantified in this analysis. Future research should focus on refining and validating the framework through real-world and more complex case studies, evaluating the performance of adaptive pathways, and incorporating various stochastic variables. Additionally, a comprehensive assessment of the entire flood system, encompassing both pluvial and fluvial flood risks, is crucial. Once refined, the framework has the potential to serve as a valuable evaluation tool, enabling the comparison of static and robust strategies against more flexible adaptive strategies, and facilitating decision-making in flood risk management.

The study explores the potential impact of accelerated sea level rise on coastal flood risk in the Netherlands, particularly focusing on the Plan B NL2200 approach. Sea levels are expected to rise faster than initially thought due to factors like Antarctica's melting ice. The Sea Level Rise Knowledge Program in the Netherlands investigates consequences and mitigation strategies, including the categories 'protect-open,' 'protect-closed,' 'advance,' and 'accommodate.' The research compares the effectiveness of multiple spatial adaptation strategies, with a specific emphasis on the Plan B NL2200 approach, which envisions a Netherlands without dikes.

The study analyzes various spatial adaptation strategies, while focusing on extreme sea level rise and storm surge conditions, using a probabilistic model. The first part evaluates different spatial adaptation strategies in an idealized coastal polder, considering factors like spatial impact and flood risk mitigation. Findings suggest that strategies involving complete dike reinforcement are effective in reducing flood risk. The second part applies these strategies to the real case study of Walcheren, comparing damage, casualties, and affected people. The results indicate that the plain dike-ring strategy with an increased safety standard (P2) is the most cost-effective for preserving Walcheren entirely.

Regarding the Plan B NL2200 approach, the study concludes that, for Walcheren, it does not perform as the best strategy. The primary dike-ring strategy with tightened safety standards is more effective due to lower costs, reduced risks, and no land relinquishment. The study recommends maintaining the current strategy for reinforcing primary dikes while enhancing safety standards to protect Walcheren, including Middelburg and Vlissingen. However, the findings may not directly apply to other Dutch coastal areas due to location-specific factors, urging further research, including sensitivity analyses, exploration of additional spatial adaptation strategies, and consideration of societal aspects related to the Plan B approach.

This research addresses how Ricanau Mofo, a low-lying village in Surinam, can become a more water-adaptive and sustainable village, while it faces land erosion, river bank erosion, changing rainfall patterns and sea level rise. It is urgent to intervene, as these issues are expected to increase in occurrence due to climate change. Constraints and limitations that are important to take into account are cultural preservation, maintaining accessibility to the Cottica River, the limited availability of financial resources and the need for a low-maintenance intervention.

Three strategies are proposed. The first focuses on addressing land erosion. Planting vegetation on critically eroding areas is a short term measure, while the long-term involves constructing footpaths with drainage channels. This not only mitigates soil erosion, but also regulates water and is relatively cost efficient. The second strategy targets the river bank erosion, which includes wooden bulkheads with vegetation and stones for short term implementation. For the long term, a river bank protection system with groynes is designed, to break waves, slow down the stream velocity and in time causes land gain. As the long term plans require external financial aid, a business case is set up and shared with the captain of Ricanau Mofo, STEORR, the District Commissioner and the Ministry of Public Works. The third strategy addresses water damage in the urban environment. Short-term it consists of providing building guidelines of where to build more water adaptive, and how. This is placed on an informational board in the village. The long-term contains a flood early warning system and recommended equipment.

This project's significance lies in identifying interventions that are effective against erosion and water damage while being locally implementable in the rural areas of Surinam. It can be seen as a pilot project that is scalable to other villages along the Cottica River or in the whole of Surinam. However, there are limitations to the project. The most important is the lack of data quantity and data quality. This caused implications for dimensioning the interventions and their financial impact. Another limitation is that the project does not create ‘dry feet’ for the village; it creates a way of mitigating water damage while living next to the Cottica River. In addition, there is a limitation in the financing of follow-up projects. Therefore, a business case is also being delivered to the Ministry of Public Works, the District Commissioner of Marowijne South-West and the captain of Ricanau Mofo. They can use it to apply for funds from international organisations and include it in future policy plans.

To summarise, Ricanau Mofo can become more water adaptive by regulating how to build and where, by continuing the prototype of the bulkheads by planting more vegetation and, by requesting financial aid for the river bank protection long term. As Figure 0.2 shows, it not only contributes as a report, but also in a tangible form of a prototype and in educational information boards in the local language to enhance the continuity and help the village.

This thesis evaluates flood insurance solutions for the Netherlands, focusing on the repercussions of primary flood defence failures. Recognizing the amplified need for a solid safety system due to climate change, the study uses a comprehensive approach, including literature review, expert opinion, and numerical modelling, to assess existing insurance systems and suggest novel ones. It begins by identifying seven insurance failure mechanisms, vital for appraising potential solutions. International practices are examined for context, and experts from various sectors provide quantitative analysis of these mechanisms on reimbursement schemes. The study proposes seven solutions, narrows them down to four, and constructs an insurance premium model reflecting Dutch housing risks to evaluate the financial implications for homeowners. It then scrutinizes each solution's framework, efficacy, and financial impact. A final reflection considers the preferences of key stakeholders—government, individuals, insurers—and recommends four tailored flood insurance solutions for the Netherlands. The thesis aims to guide policy formulation and enhance the collective understanding of flood insurance management.

Under growing pressures of climate change, it is vital for our society to implement cost-effective reinforcements in a flood defence system. This thesis proposes a method for optimising reinforcements in a flood defence system with multiple lines of defence under sea level rise.

After overtopping, the most common failure mechanism in the world that leads to a dike breach is the instability of the inner slope. Two stability calculations are required for the semi-probabilistic assessment of inner slope stability. One calculation is without significant overtopping and the other calculation is with significant overtopping whereby the dike is saturated. The semi-probabilistic assessment method with overtopping is prescribed in a KPR factsheet, but is not verified by calibration calculations or probabilistic calculations. After performing semi-probabilistic and probabilistic calculations at Jaarsveld-Vreeswijk (JAV) in the Netherlands, there is an inconsistency between the semi-probabilistic and probabilistic estimates of the reliability index for inner slope stability with overtopping. Semi-probabilistic and probabilistic calculations have been performed to gain more insight in the inconsistency between the semi-probabilistic and probabilistic estimates of the reliability index for inner slope stability with overtopping. Based on the results, it is recommended to perform a calibration study for inner slope stability with overtopping to obtain a better estimate of the failure probability based on semi-probabilistic calculations.

In the summer of 2021 severe flooding occurred in Belgium, Germany and The Netherlands. High damages have been estimated in the Geul catchment, leading up to 250 million euro. Flood risk management in The Netherlands is mostly focussed on reducing the probability of flooding and in lesser extent on damage mitigation. Especially in areas such as the Geul catchment, reducing the probability of flooding through flood defences is difficult to achieve. Also taking climate change and urban development into consideration, it is challenging to meet flood safety standards in the (near) future, as the frequency and intensity of flooding is expected to increase. Under these circumstances a shift is observed in scientific and policy discourses towards a flood risk reduction strategy that includes resilience, in which the capacity to resist, mitigate and recover has a larger role. As a consequence, there is an increasing interest to include private actors in the strategy by promoting private flood mitigation measures. However, it is difficult to quantify the effects of these private measures and little research in The Netherlands has been performed. This thesis investigates which improvements can be made to the flood risk reduction strategy in the Geul catchment, thereby increasing resilience through private flood mitigation measures. Thereto a method to include and asses the effect of the measures in a risk based approach is derived.

An international literature study about flood resilience and society provided two applicable private mitigation measures and their damage reductive effect. Dry-proofing types of measures are defined as measures that prevent water from entering an object and can reduce damages between 60%-100% up to an inundation depth of 1 meter. Wet-proofing measures reduce damages and recovery time, while allowing water inside the object, by flood adapted use or structural measures. The maximum damage reduction is 53% up to an inundation depth of 2 meter. Investigation of the Protection Motivation Theory provided important factors that improve the uptake of these measures, by identifying connections between awareness, preparedness and self-reliance. The two main findings are that the communication strategy should focus on the relative risk reduction effect of private measures and should provide guidance on implementation to motivate private actors to initiate action. Five interviews were used as a reference to these findings. To describe the relative effect of measures, they are modelled in a publicly available risk assessment method (SSM-2017), based on hazard, exposure and vulnerability. The effect of the measures is calculated for two scenarios, that describe at which locations the measures are implemented. In the first scenario, top risk locations are identified and measures are applied. This is representative for high awareness and object specific guidance. The second scenario randomly distributes measures over the flood prone area and represents the current approach without specific guidance. For the modelling of measures and scenarios, an automatic procedure using a Python script is developed, which is applicable in The Netherlands in combination with SSM-2017.

The results showthat both dry-proofing andwet-proofing houses and shops are applicable in the Geul catchment. At many locations measures have a positive benefit-cost ratio, indicating it is worthwhile to implement them compared to a situation without measures. More importantly, it is found that high awareness of flood risk or object specific guidance increases the effectiveness and cost-efficiency significantly. For residential objects, an optimum was found to implement measures at the top 40% locations most at risk. The achieved risk reduction was 47% and the benefit-cost ratio 1.3-2.2 for dryproofing. At this coverage the benefit-cost ratio ofmeasures is a factor 2 higher compared to a situation without specific guidance. Location specific investigation revealed that the inundation depth is the main driver for determining the most effective measure. Wet-proofing is cost-efficient in a smaller number of areas at risk for inundation depths higher than 1 meter. For most areas dry-proofing is more cost-efficient as the expected inundation depths in the catchment are mostly below 1 meter.

The research demonstrates the potential of privatemeasures with a significant reduction of flood risk. Given the limitations to reduce the probability of flooding in the Geul catchment, the measures are highly interesting. The method itself was able to determine the type of measure thatwas most effective with confidence. However, based on uncertainties in the model, the benefit-cost ratios have to be interpreted carefully. An extension of this research with a higher resolution of input data can address this uncertainty. Current approaches and communication to motivate private measures were found to be ineffective, as they mainly focus on the type of measures and costs instead of the effect and implementation. It is suggested to improve this by explicitly including the before mentioned factors and bymaking use of social network on the spatial scale of a neighbourhood.

Flood risk models are frequently used to analyse the climate- and socio-economic-driven impact of flooding hazards. However, model validation is rarely done adequately due to the rare occurrence of floods and even less frequent reporting of corresponding damages.

In this research, validation is defined as the process of ensuring that a model performs within a range of accuracy and precision, satisfactory for its intended use. To guide experts in their validation efforts, a four-phased framework is developed to validate flood-event damage estimations, created with hazard x exposure x vulnerability models.

The framework was applied to two damage estimates created by the Global Flood Risk Tool (GFRT). 1) For damage caused by the Limburg 2021 river flood (The Netherlands - Europe) and 2), for damage caused by a 2019 hurricane-induced coastal flood in Beira (Mozambique - Africa). For the Limburg case, total direct damage was determined at 349,4 million euro. An initial model overestimation of 34% was caused primarily by a large exaggeration of exposed agricultural surface area, and significant modelling errors of linear infrastructure. Furthermore, an uncertainty range was quantified between 271,8 (-23%) and 388,2 million euro (+11%) due to uncertainty in residential assets (across all three model parameters) and an uncertain exposure parameter of agricultural assets.

To create additional damage estimates for verification, a Structured Expert Judgement (SEJ) experiment was executed with ten flood-damage experts. Due to the high experiment cost and low expert-informativeness, the method is currently not advised as a validation approach. In situations with limited data, experts may still be a relevant information source.

For Beira, damage was determined at 8,1 million US dollar. The model underestimated damage by 82% due to errors in infrastructure, industrial, and commercial assets. Besides, overestimations were found for informal residential- and agricultural assets. The estimate ranges between 5,2 (-36%) and 13,2 million US dollar (+62%). This range excludes uncertainties at port and industrial assets, as insufficient information was available. Contrary to the Limburg case study, insights from the plausibility assessment were too uncertain for quantification, thus the validated estimate is based on damage- and construction cost data. Novel techniques were used to disaggregate the compound damage data, such as comparing wind and flood vulnerability curves and applying employee-based estimations.


The significantly altered damage estimate for both case studies demonstrates the usefulness of the framework. However two main limitations remain: first, lacking information on direct damage to critical infrastructure hinders validation.
Second, additional detail in data is required to allow parameter calibration that increases accuracy across multiple flooding scenarios. Therefore, the main recommendation for future research is to increase the detail in damage data reporting so that parameter calibration is supported. This may be done by increasing the spatial resolution of reported damages or adding additional variables such as inundation depth in reports.

Major flooding occurred on the Meuse River in 1993, 1995 and 2021, causing significant damage to regions in NL, BE, DE. The flood events of 1993 and 1995 occurred in the winter, while that of 2021 occurred in the summer. There are large differences in the characteristics of these floods separated by nearly three decades; not only the seasons differ, but the topography of the river bed and the shape of the flood wave have also changed. Since 1995 dikes and flood-routing measures have been constructed along the river. Although all these projects are not yet completed, the flood event of July 2021 indicates that the Meuse projects are producing the expected results. This research aims to understand the effect of the shape of the flood wave during a high-discharge event, the impact of the different types of flood protection along the river and the interaction between the main river and its tributaries. The main research question is: what are the hydrodynamic differences between the high-waters levels on the Meuse in Limburg in 1993, 1995, and 2021, and how can these differences be explained?
The first step to complete this research was to gather the needed information. The second step is to identify the differences between the different flood events and assessing the impact of the different flood protection measures. The third step is to assess the impact of the different flood protection measures.
After an analysis comparing inundation maps, five flood prone areas were identified and investigated, namely: Maastricht, Bunde and Meerssen, Maaseik-Roosteren, Roermond and Venlo. Flood wave characteristics play an important role: the width illustrates the duration of the event, while the height indicates the maximum water level that can be expected.
Theoretical predictions of flood wave behaviour derived by scenario-based simulations with hydraulic models (i.e., report of peak attenuation, evaluation of the Grensmaas and the tool Box of building blocks) were compared with measurement data, water levels, discharges and inundation maps in order to assess if intended effect of certain measures correspond with the measured effect of the flood events of 2021.
The flood event of July 2021, the amount of water already in the river system, before the heavy rainfall in the southern part of the Netherlands, Ahr-Eifel region in Germany, Luxemburg and Eastern part of Belgium, was only approximately 200 m3/s instead of the 600-700 m3/s in 1993 and 1995. This means that there was a large buffer, before the water level increased to dangerous levels. Secondly, the short duration of the flood event in 2021 compared to 1993 and 1995 the increased role of peak attenuation. The flood routing-measures have helped to increase this effectivity due to the fact that more water could be stored in floodplains, high-water channels and retention areas.
For this event it can be concluded that the duration of the flood event in combination with the already executed flood-routing measures have prevented further flooding.

The flooding of the River Geul in July 2021 was an eyeopener for the society, the government and many others involved. Besides the major structural damage as a consequence of the flooding, the emotional damage was and still is massive. Therefore, the need for measures to reduce the impact of these extreme events is bigger than ever. Many people had to temporarily leave their house and companies had to shut down. One thing became clear: the impact of comparable or even worse flooding has to be lowered, and one way to achieve that is by implementing measures in the River Geul which reduce the flood risk. The main goal of this research is to find out the impact that certain measures have on the consequences of flooding of the River Geul. Therefore, the main research question is: What is the impact of hydraulic or hydrological measures on the consequences of extreme future flooding of the River Geul? An important aspect is to consider the multidimensional impact of each measure. This can be either social, economic, or ecological impact due to flooding and the implementation of the measure itself.

The evaluation of the impact of measures is done by using different methods. First of all, an existing SOBEK model is used to simulate two extreme events: July 2021 (recurrence interval ≈ 1/100 - 1/1000 per year) and a stress test (recurrence interval ≈ 1/10.000 per year) which simulates 60% larger discharge and precipitation amounts than July 2021. Based on the bottlenecks in the River Geul, which are either bridges, weirs, culverts or orifices, measures are defined and simulated in SOBEK. A distinction is made between possible measures down- and upstream of Valkenburg and in Valkenburg, dependent on the expected effect of the measure. All the measures are simulated for both July 2021 and the stress test. The measures that significantly reduce the flooding depth and local water level nearby urban areas, which follows from the longitudinal cross-sections of the maximum simulated water level, are rated as effective. The effective measures are also evaluated with a damage victim module (SSM2017) to calculate the reduction of damage and victims, but no cost-benefit analysis is performed. Finally, expert judgement is used to estimate the costs of the effective measures. Social impact and the impact on Natura 2000 area is also examined here.

Eleven measures were considered, and four of them showed effective results and therefore their impact is compared. The effective measures are: widen the river downstream Valkenburg, bypass through Valkenburg performed as a tunnel (in-situ or drilled) or channel, remove obstacles in the River Geul along Valkenburg and enlarge the current basins in the River Geul. For the flooding of July 2021, the optimization of current basins in the River Geul scored by far the worst on damage reduction and costs. River widening and removing obstacles both reduce the amount of damage by 6 percent, while the costs are at least a factor of four lower compared to the bypass (dependent on the type of bypass). However, the bypass reduces the damage by at least 34 percent. The most expensive bypass is the drilled tunnel, but it comes with less social impact, because barely any buildings have to be removed in the city centre of Valkenburg (assuming that the subsoil in Valkenburg is suitable for a drilled tunnel). For the stress test, the damage reduction of the effective measures was not significant compared to July 2021. Thus, for extremer events the measures have no longer a considerable impact. This is not an argument to not implement measures, because the probability that a similar event as the stress test recurs is at least a factor of ten lower compared to July 2021.

Depending on the preferences of decision makers the choice for a measure can be made. Based on this research and looking at the flooding of July 2021, a bypass in Valkenburg reduces the damage along the River Geul the most with approximately one-third. For a bypass performed as a drilled tunnel the social impact is limited as maximum five buildings have to be removed but a cost estimate of 112 - 207 million euros is associated with this choice. River widening downstream Valkenburg or removing obstacles in Valkenburg comes with lower costs of 4 - 23 million euros in combination with no social impact but this results in a damage reduction of only 6 percent.

In mid-July 2021, heavy precipitation caused flooding for multiple tributaries of the Meuse and Rhine rivers which resulted in many deaths and exceptional damage. The Ahr in Germany, the Vesdre in Belgium and the Geul in the Netherlands suffered the greatest damage in each country. The first step in reducing such high damage from international, minor catchments is to build an understanding of the flood event.

The goal of this study was to analyse how extreme several hydrological aspects of the flood event of July 13-16 were for the Ahr, Vesdre and Geul. Analysing the extremity of an event requires a description of that event which helps with building knowledge. Its extremity places that knowledge into context. A flood with a 10-year return period will require a different approach than one with a 1000-year return period to balance the risk reduction and required effort. Three international catchments have been selected as case studies to allow an in-depth analysis of catchments and compliment current research on the flood event with a transboundary perspective.

The hydrological aspects considered for this study concern the forcing, flow mechanisms and response. For the forcing and response, a data analysis has been performed to study the extremity of the origin, magnitude, spatial and temporal variation of the precipitation, as well as the magnitude of the flow. Characteristics of both were used to determine the flood type. The data analysis on flow magnitude and seasonality has been supplemented by historical information to give more insight. The flow mechanisms have been derived from two models with varying structure and spatial variation by analysing parameter values and internal states.

The origin of the precipitation is considered as extreme since the cold core low, a low pressure area with a cold centre, was accompanied by several factors in a rare combination that likely contributed to the high precipitation amounts. The precipitation amount of July 14, the wettest day of the event, was of extreme magnitude according to the return periods of over 1000 years for several gauges. Although there was a large spatial variation in precipitation and thus their return periods, the spatial pattern showed a positive correlation of medium strength with the climatic pattern and is thus not considered as extreme. The temporal variation of the event is also not extreme. So during the event, only the amount of precipitation was extreme, but the precipitation amount in the preceding month was also extreme.

The peak discharges have been estimated to be around 990, 600 and 90 $[m^3/s]$ at the downstream gauges of the Ahr, Vesdre and Geul. The higher peak discharges coincide with sharper hydrographs and a stronger relief and impermeable soils. The return periods of the peak discharges were over 1000 years. Such extreme return periods are not surprising as such discharges had never been measured before. However, historical reconstructions show that similar flood events have occurred at least once for the Ahr. The assumed rare occurrence of summer floods in the regions was also debunked by historical information as 21 to 44\% of the historical floods occurred during the summer half-year. Another assumption by reports on the flood event was the flood type identification of a flash flood. The large spatial and temporal scale, as well as lag times for downstream gauges of 13 to 24 hours, do not resemble a flash flood, but rather a long precipitation event with a fast response.

Two models, a lumped, simple model based on the HBV model and a distributed more complex model based on the SBM model, have been calibrated for several years preceding the flood event, resulting in a good performance. Their performance for the flood event was low due to a strong underestimation of the peak discharge. This underestimation was resolved by a re-calibration. The re-calibrated parameters and internal states of the models show an increase in fast responding mechanisms, up to unprecedented magnitudes. The SBM model includes more processes and shows a rare amount of overland flow due to saturation excess. The fast response mechanisms, both in magnitude and relative runoff contribution, were unprecedented and can be considered as extreme.

The magnitudes of the precipitation, flow mechanisms and discharge were extreme for the 2021 flood event. The extreme flows were possible due to fast flow mechanisms, most likely overland flow due to saturation excess, which can be linked to the extreme pre-conditions. The strong relief and impermeable soils also seem to play a role in this extreme response. All catchments are capable of a fast and strong runoff response and although the forcing of the event was extreme, a new flood event in the future can not be excluded and so a higher level of preparedness and protection is required.

This study should be considered as an exploratory first step for action on the flood event. In order to move forward and increase the protection of the region, more detailed research is required. Such research can focus on several areas. Varying the study regions could be helpful for specific purpose. Several examples are an analysis of the entire region for a precipitation analysis, one catchment study for flood measures, or a catchment comparison for detecting hydrological influences. The anthropogenic influences can also provide more insight and opportunities and could be included in these future researches. The use of models was limited in this research but their further development is essential for flood risk management. Uncertainty is another topic that deserves more attention. An important factor in decreasing the uncertainty is to construct robust and extensive measurement infrastructure. Nevertheless, without more international cooperation and data accessibility the additional data will not reach its full potential. International cooperation also provides more value to the previous recommendations. This research shows that the transboundary catchments of the case study can be compared and give more insights to the overall behaviour.

Adequate flood protection is important to many countries, but especially so to the Netherlands. With a large share of its population centers located below sea level, in so-called polders, the need for flood protection systems quickly becomes apparent. This need is even more pressing with the rise of sea levels and the increase in river discharge variability due to the onset of climate change. To future proof themselves, the Netherlands needs to maintain and strengthen their flood defences. Especially precar- ious is the situation for polders, which are low-laying areas protected by one or more dikes. From a technical perspective, the feasibility of the polder system has been proven to withstand the expected water level rise as result of climate change for at least 2 to 3 meters sea level rise, shown by Kok et al. (2008). However, research into the economic perspective on the feasibility of the polder concept has been less extensive.

To more accurately determine the economic viability of the Dutch polder concept, additions to the research of Eijgenraam are proposed that fill in the current knowledge gaps. The result of this research is a mathematical framework for the optimization of dike reinforcements in two dimensions, the lifetime of the structure and the crest height increase of the dike. The framework consist of a discounted Cost-Benefit Analysis with a financial constraint and a constraint on the maximum allowable time before reinforcement is needed. This framework contains stochastic elements in it’s parameters and a stochastic model for the discount rate. The derived framework was subsequently used to analyse two case studies based on regions in the Netherlands. The two case studies were based on the dikering of IJsselmonde (dikering 17) and the dikering of Walcheren (dikering 29). The results were determined for the two climate scenario’s posed by the IPCC and KNMI by means of a Monte Carlo simulation.

This research has opened up the possibility to compare alternatives over different time periods and re- inforcement measures with different constraints and stochastic parameters, adding to the work done by Eijgenraam. As such, a more risk-informed discussion on the general viability of the polderconcept can be had, ultimately resulting in a more informed decision on the future of the polders in the Netherlands.