The global economical losses caused by flooding have increased over the last 50 years (IPCC, 2014; UNISDR, 2013). Deltares and HKV are developing new methods to quantify flood risk on a global scale. A Flood Risk Assessment Tool (FIAT) is used to performcalculations. The definition of risk is hazard x exposure x vulnerability. FIAT is able to combine these elements and calculate the Expected Annual Damage (EAD) for each city. Currently two methods are used on a global and continental scale. For Europe a flood risk analysis is performed based on five land uses categories and five depth-damage functions, this method is referred to as the Multiple land use method. On a global scale an analysis is performed based on one land use and one depthdamage function, this method is referred to as the Single land use method. Nootenboom (2015) investigated the difference between the two methods for Europe. He described a good correlation of 0.97. Deltares and HKV want to extend the Multiple land use method to a global scale. A flood risk assessment for Australia is performed with the Multiple land use method. Brisbane, Adelaide, Melbourne, Perth and Sydney are reviewed. Recently derived depth-damage functions by Huizinga (2015) were used. Australian functions are available for residential and commercial land use. For other land use classes European functions are used instead. As a source for flood hazard the PCR-GLOBWB hydrological model is used. Exposure maps for Australia are made with OpenStreetMap (OSM). Metadata from OSM was filtered and processed in such a way five land use classes were formed. A total coverage of 52% was achieved. The areas where no data could be recovered was filled in with ratios of the retrieved data. A sixth layer, named the no-data layer is formed. Visual analysis shows that the assumptions made in the no-data layer are most accurate in densely populated areas. When the results of the Multiple land use method are compared to the Single land use method a correlation of 0.99905 is found. For every city there was a decrease in flood risk. The depth-damage function for Australia is investigated. Typical for the Australian function is the steep shape compared to the European function. A sensitivity research shows that each city reacts different to changes in the depth-damage function. Especially for cities with low inundation depths the risk is strongly influenced by the use of Australian functions. Cities with high inundation depths, such as Brisbane, are far less susceptible for changes in the depth-damage function. The use of European functions for certain land uses classes is therefore acceptable in Brisbane, as long as the dominant residential and no-data layer have continent specific functions.

In the Rhine-Meuse Delta over 100 scour holes are located. Many of these scour holes have dynamic behaviour and are growing in size and depth. A scour hole can lead to an increased probability of flooding. One of the potential hazards of the presence of scour holes is the occurrence of flow slides on the foreshore next to scour holes. Depending on the distance between the dike and the river, a flow slide can affect the water-retaining function of a dike. Flow slide is an indirect failure mechanism and can be used as a scenario in the safety assessment of direct failure mechanisms. In the research, a data-driven method has been developed for the risk assessment of scour holes. The method is elaborated for flooding due to the failure mechanism overtopping. The development of a scour hole is coupled with the safety assessment regarding overtopping for the dike next to a scour hole. In the method, the development of a scour hole and the future dimensions are predicted in a probabilistic way by extrapolation of historically measured bed level data. With the predicted future scour hole dimensions, the probability of occurrence of a flow slide and the probability of several post flow slide profiles is calculated with the methods given in the Wettelijk Beoordelingsinstrumentarium (WBI). Based on the post flow slide profiles, the probability of exceeding the critical overtopping discharge is determined. Combining the probability of exceedance of the critical overtopping discharge with the consequences of a flood gives the flood risk due to the presence of a scour hole. The developed data-driven method is applied to one case study, a scour hole in the river Spui. For this particular scour hole, the flood risk is assessed. A prediction is made for the future dimensions of the scour hole. The flood risk is determined with the probability of occurrence of flow slide, the probability of exceedance of the critical overtopping discharge and the expected consequences of a flood. The assessed flood risk is evaluated and risk mitigation measures are proposed. The economic feasibility of risk mitigation measures is analysed with a cost-benefit analysis.

Risk based economic optimisation of water safety measures is presently being implemented in the more developed countries, but offers potential for developing countries too. With a limited budget, every penny needs to be well spent. It is however questionable if this method fits the circumstances in developing countries. Sufficient data is needed to get reliable results, while developing countries are often data scarce. The sensitivity to uncertainties in the optimisation process due to working with limited data is explored by means of a case study concerning the area of Nyaungdon, Myanmar, which inundates regularly due to the Ayeyarwady River.

The summer and autumn of 2018 showed the negative effects of both low-flow conditions and bed degradation over the last century on the river functions of the Dutch Rhine. These resulted in record-breaking water levels, extreme low navigation depth and subsequently nautical problems. As climate change will increase the inter-annual variability of the Rhine’s discharge pattern, low-flow conditions are likely to occur more often, reinforcing the above-mentioned impacts on nature and navigation. Sediment management is considered as a sustainable way to counteract the bed degradation. To justify an investment in a large scale nourishment, the impact of a nourishment on the river functions has to be compared with the result of a reference situation without nourishment. This study aims to develop a methodology that evaluates the future performance of rivers functions accounting for the autonomous trends (bed level and climate changes) and provides insights in the functional performance and cost effectiveness of river interventions. The methodology enables the assessment of the impact of the trends on the river system without intervention. It appears that the navigational efficiency is decreased enormously by climate change en bed erosion, which will increase transportation costs. Based on these indicative analysis a nourishment is predicted to be cost effective by improving both the navigational efficiency and counteracting dehydration of the floodplains.

The Meuse river facilitates important socio-economic functions. In the light of Integrated River Management (IRM), the question rises whether these functions can be facilitated in future decades (until 2050) as well. The riverbed is expected to degrade over the coming decades, just like in the past century. On top of that, the discharge regime will alter because of climate change. This research studies the impact of bed degradation and altering discharge regimes on the river functioning regarding navigation and flood safety. This is done by linking hydraulic river conditions to functional performance indicators.

Flood defenses are important to keep large parts of the Netherlands dry. These primary flood defences are every twelve years assessed on safety during the Dutch national safety assessment. In 2017, the new safety standards (WBI2017) were introduced. These stricter standards caused that in total 251 km of the primary flood defences was rejected on height, which includes the failure mechanisms wave overtopping and overflow. Traditionally, dike strengthening is required. However, the new Delta Plan gives also spe- cial attention to the implementation of innovative nature-based solutions, for example the integration of shallow foreshores without or with vegetation in the safety assessment. These foreshores can reduce the incoming wave load by dissipating energy through wave breaking, bottom friction and vegetation attenu- ation. (Vegetated) foreshores are also present in front of some river dikes, called floodplains. Probably, 5 to 15 procent of the foreshores in the current reinforcement program (HWBP) is not or conservatively consid- ered in the safety assessment, which entails unnecessary costs (approximately 0.5 - 1 billion euros). In a quickscan of the riverine area several locations are indicated which meet the requirements as promising locations for wave reduction by vegetation. However, despite a renewed focus on building with nature, implementation of nature-based flood protections is hindered by te dike managers because of a knowl- edge gap in the effects of the variable vegetation on wave energy dissipation. The state and functioning of vegetation during design conditions is uncertain. This uncertainty is the most important reason for dike managers to hamper the implementation of a nature-based flood protection. The main objective of this thesis is to determine the reliability of willow vegetation as wave load reduction for river dikes. In order to assess the variability of vegetation on the wave damping, a field test and simulation models are used. The measurement set-up of Deltares consists of a plot of 7 x 7 meters with young willow branches and pressure transducers perpendicular to the river, which measure the (by ships produced) waves in the river Noord. Based on the field measurements, a model is developed and applied to determine the drag coefficient of willow branches. This model is an one-dimensional SWASH model which contains the bathymetry and the willow vegetation, characterised by a density, branch diameter and drag coefficient for three height ranges. These parameters provide the obtained wave attenuation by vegetation. The diameter and density were measured in the field. Therefore, the drag coefficient can be determined by calibrating the outcomes of the numerical model with the field measurements. From the field measurements by Deltares, it can be concluded that willow branches reduce incoming waves with a height higher than 0.20 meter with approximately 4 % per meter willow vegetation width. The wave attenuation depends mainly on the water depth and shape of the willows. The stiff stem of old willows damps waves less than high dense, more flexible branches above the stem, and the wave damping decreases with increasing water depth mainly when branches submerge. The field measurements are used to calibrate the drag coefficient. The 25th and 75th-percent value of the calibrated drag coefficient were calculated on -1 and 5, a large range. A negative value may not be possible, because this means an increase of the wave height by the willow vegetation. This large range in calibrated values and the existence of negative values are caused by the measurement set-up. The plot with willows was too small for accurate measurements and the crates, in which the willow branches were planted, caused distortion of the waves due to the sudden bottom elevation. Because of this, the first sensor at the edge of the crate (at the river side) has probably encouter measurement errors. Third, the angle of the incoming ship waves caused an underestimation of the calculated wave damping capacity of willows. The set-up should be improved to give proper results and conclusions about the drag coefficient of willows. The drag coefficient remains therefore uncertain and gets a large standard deviation in the second part of the research. In this second part, a study is carried out to the uncertainties of implementing wave damping wil- low vegetation. A one dimensional wave model based on the wave energy balance, is created for a dike section at the Heesseltsche Uiterwaarden, alongside the river Waal. Waterboard Rivierenland suggest this floodplain as opportunity for planting wave damping willow vegetation to compensate the required heightening of the dike. In the wave model, various input parameters are required to calculate the wave damping: boundary con- ditions, dike-foreshore parameters and the vegetation parameters. These parameters have all certain stan- dard deviations or variations. First, the effect of these standard deviations on the wave damping is deter- mined, and so the overtopping discharge and required crest level. Second, a thorough analysis is made to exogenous threats which affect the vegetation parameters. For willows this could be diseases, insects, ice, animals, drought, flood, fire and storm. The effects of these threats on the vegetation parameters are estimated by the knowledge and expertise of willow experts and a literature study. It is concluded that from all input parameters with standard deviations, the variations of the vegetation pa- rameters has the largest effect on the obtained wave overtopping. The wave reduction depends mostly on the vegetation height and water depth, which was also concluded by the field measurements. Second im- portant vegetation parameter is the density of the branches or the drag coefficient, if the drag for willows can be lower than 0.4. The most relevant influential threats with a high risk and significant consequence are storms, floodings and droughts. These effects of the standard deviations of input parameters, exogenous threats and annual maintenance on the overtopping discharge (and required crest level) are used for a recommendation about the design and maintenance strategy of willow vegetation as solution. The willow vegetation should ensure enough wave damping during the total lifespan of the reinforcement to comply the safety assessment. Therefore it is recommended to extend the width of the willow vegetation with a buffer zone in which all the vegeta- tion uncertainties can be accommodated. The width of the vegetation plot is therefore the main parameter that should be assessed during inspection. Note that the uncertainties and probabilities of the exogenous threats are assumed. Further research is required to validate these assumed values. To conclude, willows can be used as wave damping vegetation at floodplains along rivers. The branches of willows will reduce the incoming waves significantly if they are not submerged. The height of the willows is therefore the most important vegetation factor and should be assessed during inspections just like the required vegetation width to ensure the wave damping capacity during the total lifespan of the dike reinforcement. The reliability of willows for wave load reduction on river dikes is sufficient if the maintainer apply the design and maintenance principles as recommended.

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.

Regional water systems are being controlled to prevent flood events by different measures, such as water storage, weir management and vegetation maintenance. Vegetation maintenance has a significant effect on flood risk, because the hydraulic roughness decreases after cutting of vegetation. Stream restoration is a project in regional water systems, where floodplains are constructed and weirs are removed to recover the ecological value of streams. In streams with floodplains the vegetation is relatively more important because of smaller water depth. Moreover, climate change increases the flood risk, which increases the urgency to investigate the vegetation maintenance strategy. A vegetation maintenance strategy involves of a cutting frequency, how often and when the vegetation is cut, and a cutting intensity, the percentage of the cross-section that is cut. The aim of this research is to optimize the performance of the vegetation maintenance strategy by consideration of the aspects ‘flood risk’, ‘ecological effects’ and ‘maintenance costs’. The research answers the following question: How can risk-based vegetation maintenance strategy reduce flood risk in a cost-effective way in regional water systems with consideration of ecological effects? A case study, a recently flooded stream in the south of the Netherlands, is used to answer the research question. The performances of nine selected vegetation maintenance strategies are investigated. Dimensionless performance indicators are designed for the three aspects to assess the total performance of each vegetation maintenance strategy. For the aspect ‘maintenance costs’, data from a water board is retrieved and the aspect ‘ecological effects’ is assessed by a literature study. For the aspect ‘flood risk’, several steps are conducted. The vegetation maintenance strategy is translated into a probability distribution function of roughness coefficients. To that end, use is made of roughness functions including vegetation growth curves and roughness coefficients of the stream. Stochastic modelling of the water level by the hydraulic model Sobek 1D is used to examine the influence of vegetation maintenance on the water levels. A consequence model, the Water Damage Estimator, translates the results of the stochastic modelling step into flood risk. The three performances of the aspects are combined into the total performance of the vegetation maintenance strategy. The results of the case study show that the timing of cutting has the largest influence on the flood risk, followed by the cutting frequency. The cutting intensity has the smallest influence on the flood risk. For a high performance of ‘maintenance costs’, a low cutting frequency is necessary. For a high ecological value, pattern cutting and cutting in August is important. In conclusion, for streams with floodplains the optimal vegetation maintenance strategy is ‘cutting of the main channel and parts of the floodplains before summer’. The total performance of the vegetation maintenance strategy can be further optimized by a dynamic maintenance strategy with ‘roughness’ of the stream as maintenance trigger. Hereby, the vegetation maintenance strategy is dependent on the current roughness in spring. Moreover, it is found that the conclusions of the case study can be applied on many other streams in the Netherlands.

The Dutch coast is affected by coastal erosion. In order to limit erosion and prevent inland migration of dunes, sand nourishments have been adopted as a common practice for the maintenance of the coast. The evaluation of the applied coastal erosion policy is achieved by the use of coastal indicators. Increased number of nourishments affects the coastal profiles and coastal indicators are used to describe those changes along the coastal zone. The present work intends to improve the definition of one of the indicators (dune foot position) by proposing a new methodology and to develop a Bayesian network in order to assess the effectiveness of nourishments on the coastal system. Coastal indicators, such as the Momentary Coastline and the dune foot position, are used in the Bayesian network in order to quantify those effects. Available field measurements of the entire Dutch coastline (JARKUS morphometric database) are used in order to develop the methodology of the dune foot position detection. Taking into account the geometry of the profiles, the dune foot position can be detected by calculating the first and second derivatives of the measured points along the profiles. Comparison with visual, in-situ observations, constitute the validation method of the proposed methodology. Root mean square errors, with respect to visual observations, are used to compare the performance of two methods for the dune foot position detection; the proposed methodology and the current dune foot definition, which detects the dune foot at the most seaward crossing of the profile with the level of +3 m NAP. By looking at the performance of the methodologies at different areas, the results are diverse. However, the proposed methodology shows a general improvement of the detection. Moreover, the methodology is believed to be generic and applicable to other dune systems around the world, since it is purely based on the morphology of the profile. A Bayesian modelling approach is used to assess the effectiveness of nourishments on coastal indicators for the Holland coast. The selection of the input and output parameters, the design of the network and the optimization of the structure are the main steps for the development of the model. Once the optimum configuration is chosen and the model is trained, the Bayesian network gives the possibility to investigate the relations between variables, by constraining the input nodes on a specific range of values and assessing changes on the indicators. By applying those constraints it can be found that nourishments have positive effects on the coastal indicators, since larger seaward displacement is observed compared to cases in which no nourishment has been implemented. Moreover, initial erosive trends of the dune foot position are constrained by the implementation of nourishments. In addition, the effects of nourishments at different time scales are investigated. To this end, time horizons of 1, 5 and 10 years after a nourishment implementation are chosen. Positive effects of nourishments are present at all the considered time horizons, with beach nourishment to have an immediate effect on the indicators, whereas shoreface nourishments reach a stronger effect 10 years after the implementation. Finally, the Bayesian inference can be used as a decision support tool for coastal managers, since its user-friendly environment can lead to easy interpretation and use for coastal management purposes. The required sand volume can be estimated in order to achieve a specific magnitude of seaward displacement of the indicators or in order to preserve the coastline at the current state. The estimations concern large spatial scales.