Making decisions when future conditions are uncertain is a challenging endeavor. This thesis develops a framework to analyse flood risk and create Dynamic Adaptive Policy Pathways, which can provide insights in the behaviour of flood risk protection measures in many future scenarios. The pathways are used to identify robust measures, dead ends and compare measures. The framework is applied to a case study in the Galveston Bay area.

Low-lying, densely-populated coastal areas across the world are under threat of hurricane-induced floods. This is the case in, among others, the Galveston Bay Area. In response to this threat the USACE (United States Corps of Engineers) recommended a plan to reduce the risk of flooding, through a range of measures to form a resilient coast of Texas. This plan includes a storm surge barrier at Bolivar Roads in Galveston Bay, whilst the other inlet at San Luis Pass remains open for environmental reasons. Leaving the San Luis Pass open still results in a significant rise in the water level of Galveston Bay, so finding a way to close the San Luis Pass is preferred. Therefore, this thesis researches a barrier for the San Luis Pass to contribute to a resilient Texan coast whilst also taking the local natural habitat into account. The methodology combines the concept of 'Building with Nature' and the civil engineering design method. The methodology incorporates the flood safety, ecological values, and socio-economic aspect into the design process. A conventional storm surge barrier does not fulfill all the requirements and criteria for the San Luis Pass. This research thus proposes a new, innovative type of storm surge barrier: The Shade Curtain Barrier. The shade curtain barrier functions by being rolled down in extreme conditions and stored under the bridge under normal conditions. The two main advantages are that no bottom protection is required and the view is hardly disrupted. Concluding, the shade curtain barrier is a promising solution for a storm surge barrier at the San Luis Pass to contribute to a resilient Texas coast.

Flood risk management is the process of analysing, assessing and (if required) reducing flood risks, in terms of economic costs and affected population or loss of life. The analysis is performed through a probabilistic approach in which the factors are represented by a probability distribution function to address uncertainties. However, relevant factors in the analysis, such as mean sea level and the population living in the area, often are deeply uncertain on the longer term, resulting for example from climate change and changing socio-economic conditions. Generally, FRM explores these conditions through scenario analysis. However, the way FRM creates and analyses the scenarios is subjective and leaves open a lot of uncertainty. Often, only a limited set of subjective qualitative scenarios is made, which risks not taking scenarios into account that turn out to be important later. Quantitative scenarios become increasingly uncertain as the time horizon over which these are used lengthens, due to uncertainty in the trend.This research investigates a new addition to flood risk management to support it in dealing with long-term deep uncertainty. It uses robust decision making approaches as an addition to the usual FRM approach, by analysing results for a much larger set of scenarios to increase robustness, both static and flexible, to different scenarios. It combines aspects of dynamic adaptive policy pathways (Haasnoot, Kwakkel, Walker, & ter Maat, 2013) and aspects from robust decision making (Lempert, Popper, & Bankes, 2003), with the FRM approach to create an eight-step iterative approach called Flood Risk Adaptation Pathways, which was later tested in a case study. The approach starts by analysing the current situation and possible futures, after which it determines the difference between those situations and the preferred state. Based on this, possible flood risk reduction measures are designed, and then tested on their performance in a flood risk screening model, to create so called effectiveness regions, which show under which conditions which solutions are effective in reducing the gap between states. Based on the effectiveness regions, pathways of actions over the different possible futures are created. A monitoring plan is then created, which defines the signposts and their trigger values. Finally, the situation is monitored and actions are implemented when required.The Flood Risk Adaptation Pathways approach aims to provide policymakers with the ability to anticipate different futures, by creating actions, which meet the objectives, for a wide range of tested futures. It should allow policymakers to plan the implementation of measures ahead of time. The long-term situation is investigated, including the measures required at that point, and linked to the current situation, including the currently required measures. This should ensure that measures remain implementable by actively assuring the conditions for implementation are not violated, thus reducing negative effects of path-dependency. The link between the long-term measures and the current measures should also allow for an improved cohesion of the sequence of measures, with short-term measures providing a starting point for long-term measures. Finally, it narrows down the measures to the ones highlighted by the approach for later on in the process, when an action needs to be implemented.The approach has been tested in a case study, on Beira, Mozambique, which is prone to severe rainfall and coastal flooding. Furthermore, compound flooding problems arise when high sea water levels coincide with rainfall events. The Flood Risk Reduction Evaluation and Screening model (van Berchum, van Ledden, Jonkman, Timmermans, & van den Broek, 2019) was used to model the current flood risk, as well as the effect of flood risk reduction measures on the flood risk, affected population and construction costs. For this research, the model was adapted to test specific actions over a set of changing conditions, to test their performance for the longer term. The approach was then used in the case study, following the steps (described above). The data was gathered from local sources and reference projects. After the completion of the case study, the approach was reflected upon: The main limitation arising from the case study, is the considerable computation time and time-consuming analysis of the results. Overall, Flood Risk Adaptation Pathways strengthen the FRM approach in dealing with long-term deep uncertainty, and thereby enhance the performance of flood risk reduction strategies under different than expected conditions.

The undertaken thesis work conducts a research study based on the study area — Laakhaven, The Hague, to develop an implementation example of the Adaptation Pathway approach, in order to support long-term adaptive stormwater management planning on urban adaptation measures to mitigate pluvial flooding under the climatic and socio-economic uncertainties.

The methodology is presented in the stepwise procedure to develop adaptation pathways. The core part of this method is expressed as the risk-based approach, which considers the flood risk from the aspects of the probability and the consequence. Different climate and socio-economic scenarios are developed to represent the uncertain environment for policymaking resulting from long-term changes. An urban water balance model is applied to produce the novel empirical performance indicator for the effectiveness of adaptation measures as the critical input to this assessment. Sell-by dates of adaptation actions
are computed based on the assumption that, once a policy action reaches the perspective-based socially acceptable risk, it is said to encounter an adaptation tipping point thus requiring additional interventions. With the computed sell-by dates, the adaptation pathways maps are assembled under certain rules that exclude illogical sequences. Robust adaptation pathways that can succeed over various future scenarios are outlined from the pool of pathways. The developed adaptation pathways map provides the policymakers with a range of possible options. The results indicate the significance of investing in the modular rainwater harvesting devices on private space since it is effective and flexible action
that supports the development of dynamic robust strategies for the long-term adaptive stormwater management planning. The implementation methodology of this case study is theoretically viable and its potential to make a more comprehensive study has been proven. Therefore, it is recommended to take the undertaken study as a starting point and further improve it to find the ultimate answer through sub-selecting preferred pathways.

Additional master thesis

The submerged floating crossing (SFC) considered in this study has demonstrated significant sustainability based on provided assumptions. the SFC has appeared to be a sheltered and sound idea appropriate to satisfy its expected capacities and to tackle given environmental conditions. The structure of the proposed SFC is intended to withstand all functional and environmental loads. The results of worst-case scenarios showed the base SFC design have a sufficient robustness to withstand the presumed significant loads. Some major uncertainties were faced throughout the study, which are manageable by means of modern technologies in civil engineering or some organizational arrangements.

The Clarence River catchment is located in the state of New South Wales (NSW), on the east coast of Australia. The lower Clarence Valley is an area covering approximately 1000 square kilometers and is located on the downstream part of the Clarence River. Due to heavy rainfall, the Clarence River discharge can increase from an average 160 m3/s to 20000 m3/s. As a result, water levels rise significantly leading to severe floods in the Clarence Valley. The main urban areas in this region, Grafton, South Grafton and Maclean, are located in narrowing river bends which makes them particulary vulnerable to flooding during high water levels.

The main goal of this report is to present flood mitigation measures to reduce the impact of flooding in the urban areas of the Clarence Valley, based on the Duthc flood mitigation strategy called 'Room for the River'. Consequently the following research question was formulated:

How can the impact of flooding on the urban areas in the Clarence Valley be reduced by increasing the storage capacity of floodplains?

In order to answer the research question, the following project approach is applied. Six areas were identified, based on a fieldvisit and an extensive preliminary study, to implement flood mitigation measures and assess existing flood defences. Part of these flood defences are the Swan Creek Floodgate and the reinforced concrete levee wall of Maclean, which will be investigated on their performance. A fully calibrated numerical floodmodel provides input for the hydrological analysis. The model represents the current situation in the Valley. Scenarios are created by applying topographic adjustments. The new scenarios are implemented into the numerical model and the effectiveness on flood mitigation in urban areas is assessed by comparing the results of a 5, 20 and 50 year Average Reccurance Interval flood event to the current situation during one of these flood events.

By making use of the proposed floodplains and improving the performance of existing flood defences, the flood defence system of the Clarence Valley can be extended. It can be concluded that it is possible to reduce the impact of flooding in the urban areas of the Clarence Valley by increasing the storage capacity of floodplains around Grafton. Therefore, the usage of a ’Room for the River’ strategy can be a solution to the problems the Clarence Valley is facing, and possibly might be applicable to more flooding-vulnerable areas in Australia.

As population levels protected by large dams have risen in the last decades, accurately assessing flood hazard and risk will become increasingly important for developing and sustaining flood mitigation policies. Recent research has shown that the degree of control and protection offered by retaining structures largely depend on their operation strategies. As such, now, more than ever, it is crucial to be prepared to operate a reservoir under extreme hydrological circumstances, when the consequences of an operational error could be very serious. However, the operation schedules currently use in the engineering practice lack on mechanisms for evaluating and balancing the potential risks associated with storage and release decisions. This results in reservoir flood control strategies that, from the perspective of flood risk mitigation, are distant from the optimal. This graduation project aims to improve existing methods for developing emergency operation schedules by including the concept of risk into the optimization of flood control operations. To address this topic, the research is divided into: (1) development of a methodology for including reservoir operation effects in the risk analysis of dam-reservoir systems; (2) combination of the risk analysis procedure with an optimization algorithm to devise optimal risk-based emergency operation schedules.

The thesis culminates with a case study in which the emergency operations are optimized for the Barker Reservoir system in Houston, Texas. Susceptible to Hurricanes like Hurricane Harvey (2017) and intense precipitation events such as Tax Day (2016), the Barker system presents an operational dilemma requiring trade-offs between released flows and stored volumes. Using the methods developed in this thesis, the flood risk analysis shows that a change in the operational strategy would contribute greatly to reducing the total risk of the system. Under extreme hydrological events, an operation strategy with releases starting at the first stages of the flood event display a reduction of almost a 32% on the total risk of the system as compared with the current operation strategy, including a 40% decrease on the risk associated with the structural failure of the dam. The new operating policy, however, increases the frequency of downstream damages during non-structural low frequency failure scenarios. Therefore, an increase of the downstream channel capacity along Buffalo Bayou and adequate measures to strengthen the dam are further recommended to reduce downstream damaging flooding and diminish the failure probabilities of the structure.

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 climate goals for 2020, that multiple countries in the world signed, are coming closer. Like many other countries, the Netherlands has difficulties reaching their climate goal. A solution came from the Paris agreement in 2015, which sets new goals for 2030, and eventually for the long term in 2050. This time the Netherlands is eager to reach their goal and amongst many other initiatives, a proposition came from TenneT, the country’s national energy operator, to construct an island in the North Sea, functioning as a central “wind connector hub” to connect multiple offshore wind farms and distributing the energy more efficiently over the neighbouring countries. The goal of the project is to propose and analyse a preliminary design for the construction of that artificial island in the North Sea, capable of acting as a central energy hub. An analysis for optimum location for the island was performed based on maximum wind generation, shallow water depths, centrality to the North Sea countries, and environmental restrictions. Of various types of island considered, the reclamation type was chosen for preliminary design because it is the most cost effective for the location’s water depths and the most commonly constructed island type. Following the scope definition, correspondence with TenneT and consultancy with subject experts at TU Delft was made to refine preliminary design outcomes. The preliminary design covers the analysis of available environmental and geotechnical data, safety approach, island shape, zones, elevations, analysis of alternative sea defence structures, building with nature measures, port and terminal design, and preliminary construction plan. The conclusions of this investigation cover practical issues, project risks and uncertainties, and opportunities to reduce costs are discussed.

The Addicks and Barker Reservoirs, built in the forties, are located in Houston and collect precipitation and run-off from upstream areas to reduce flood risks along Buffalo Bayou to protect downtown Houston. During Hurricane Harvey (August 25 - August 30, 2017), the precipitation reached a new record of 910 mm [36.2 inches] in a 4 day period in Houston. The gates of Addicks and Barker Reservoirs were opened during the night of 27-28 August which led to major damages due to downstream flooding. Besides, non-government owned land upstream was flooded due to high water levels in the reservoirs.
In this report, new design water levels for Addicks and Barker Reservoir are calculated based on inflowing discharge into the reservoirs and precipitation directly onto the reservoirs, including data of Hurricane Harvey. These calculated design water levels are compared with the critical water levels calculated based on the failure mechanisms of the dams. This study shows that the original design water level of the dams, based on the Probable Maximum Flood, are 2.83 m and 1.01 m higher than the critical water level for which failure of the dams can occur due to piping for Addicks and Barker Reservoir. However, the maximum allowed water level which is currently maintained by the United State Army Corps of Engineers, is 2.19 m and 2.46 m below the calculated critical water level. During Hurricane Harvey, these maximum allowed water levels were exceeded with 3.46 m and 1.93 m.
The damage of residential properties upstream and downstream of the reservoirs are minimized based on the distribution of excess volume from the inflow of creeks and precipitation onto the reservoirs. The ratio of the amount of volume which should remain upstream of the dams and the volume discharged into the Buffalo Bayou is calculated for every considered event with its duration and return period. The ratio of Addicks Reservoir is the dominant ratio, which should be used for both reservoirs. Run-off alone already produces damage, especially for the 12h and 24h precipitation, so the Addicks and Barker Reservoirs should not release discharge into the Buffalo Bayou for small durations. For events with a longer duration, it would cause less damage to open the outlets of the reservoirs than to keep them closed. However, if the water level in the reservoir exceeds the critical water level for piping, it is advised to discharge more to the downstream area to prevent breaching of the dams. Since the critical water level is reached for approximately 25% of the events at Addicks Reservoir, mitigations against piping should be taken to improve the minimization of damage. For Barker Reservoir, the critical water level is not reached in the optimization. During big events, people living upstream will be more affected by the flooding than people living downstream since this optimization is based on the damage minimization of residential properties.