In this thesis, a design of a system of one or multiple flood valley storages in Limburg and an evaluation to what extent such a system can reduce the probability and effects of floods is made. ... Read more

This study investigates how hydraulic and meteorological variables act together to affect dike stability, with emphasis on inner-slope failure during extreme hydrometeorological events. The variables influencing dike stability are first identified through a literature review, after which their correlations are examined and analysed using copula theory. The effect of these correlations is then assessed in more detail by applying a groundwater model developed with Pastas and performing slope stability calculations using the Bishop method.
The analysis focuses on the period from December 2023 to January 2024, when the Netherlands experienced high cumulative rainfall, elevated river discharges, and restricted outflow due to sea storm surges, leading to prolonged high water levels in the IJsselmeer–Markermeer system. The case study is a lakeside dike along Markermeer between Hoorn and Enkhuizen (about 17.8 km), where data availability enables detailed hydrological and geotechnical modelling. Two cross-sections (raai_2 and raai_3) are instrumented with multiple observation wells from phreatic to deep sand layers. Inputs combine hourly lake levels from Krabbersgat Zuid and the nearby Drieban pumping station, hourly precipitation and daily evapotranspiration from Berkhout station, and local groundwater measurements from 10 Nov 2023 to 25 Feb 2025. These data are used to calibrate and validate Pastas groundwater models and to evaluate slope stability for representative hydraulic loading conditions.
The literature indicates that phreatic levels around Markermeer and IJsselmeer are governed by external hydraulic loads, climate, internal soil properties, dike geometry, and local lake dynamics. Using statistical analysis, copula modelling, and time-series groundwater simulations, this study examines how precipitation and lake level jointly influence the phreatic surface within the dike. Results show a moderate positive correlation between cumulative local precipitation and lake water levels. Copula models, particularly the BB8 family, capture asymmetric dependence between rainfall and water level, highlighting an increased likelihood of joint extremes.
For stability evaluation, observed groundwater data were first used as input to D-Stability to compute the factor of safety (FoS) over selected periods. This “dependent” case reflects the real, correlated relationship between precipitation and water level and shows a moderate negative correlation with FoS, meaning increases in either driver reduce stability. An “independent” case is then constructed by generating a new water-level series from the fitted bivariate copula using conditional sampling with rank-exact back-mapping, so that water level is statistically independent of precipitation while preserving the marginal (univariate) distributions. The Pastas model is re-fitted with this synthetic water-level series to produce new groundwater heads, and FoS is recomputed. Under the observed (dependent) case, peak external water levels coincided with prolonged high precipitation, producing higher phreatic levels and a lower minimum FoS (1.745). When the same marginals were used but the drivers were made independent, peak water levels were lower and the minimum FoS improved (1.768). By evaluating the correlation, the dependent case shows stronger negative correlation for both precipitation (-0.67) and water level (-0.49) versus FoS. In the case of independent variable, the correlation between water level and FoS strengthened to −0.85, while the correlation between precipitation and FoS weakened to −0.18.
Based on the previous results, it can be concluded that during the wet season, the correlation between precipitation and water level leads to a more conservative outcome, expressed as a lower factor of safety (FoS) for dike stability. This finding is consistent with real-world conditions, where periods of higher rainfall typically occur together with higher local lake water levels caused by runoff from around the lake, direct rainfall itself, and polder drainage pumping into the lake. This conclusion is based on the assumption that the water level dataset used in this study represents local water level observations, where in reality the actual local water level at this specific dike section may differ slightly depending on wind magnitude and direction. The main recommendation for dike assessment based on this study is that the correlation between precipitation and water level should be explicitly considered in stability analyses, since neglecting this dependence may underestimate phreatic levels within the dike and result in a less conservative estimate of the factor of safety.
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Singapore’s Long Island project aims to protect the East Coast, meet freshwater demands, and support urban development. It involves constructing a freshwater reservoir by closing off part of the sea using three islands and two barrages. The islands, totaling 850 ha, will be used for urban development. The project is currently in its conceptual design phase. Long Island presents several challenges. Singapore’s flood risk policy focuses on raising the platform level, increasing demand for scarce construction materials. The multifunctional nature of Long Island, providing flood protection, freshwater supply, and urban space, complicates design. Uncertainty in future sea level rise (SLR) further challenges sea defense planning and adaptation. This thesis develops a resilient conceptual design for Long Island’s land reclamation, focusing on platform level optimization and sea defense adaptability. Six reclamation variants are proposed, ranging from polder systems to conventional landfills, combined with a caisson or a dike as sea defenses. Sea defenses are designed to accommodate up to 5 m SLR and are integrated into adaptation pathways. Each variant considers reservoir dike design, effective land area, settlements, and polder pumping requirements. Designs are evaluated through capital cost analysis, lifetime cost assessments using Present Value, Multi-Criteria Analysis (MCA), and sensitivity analyses on design parameters, Social Discount Rates (SDRs) and SLR projections. The most cost-effective design combines a platform level of -4 m SHD with either a dike or caisson. This polder approach is technically feasible and reduces reclamation volumes by 80 million m3 and saves 3 billion SGD compared to a 5.1 m SHD design. Sensitivity analyses confirm its robustness under varying assumptions. Both sea defense types are adaptable and have comparable costs, though further research is needed to determine the optimal choice, including geotechnical design and naturebased integration. The MCA did not yield a clear preference due to close value-cost ratios and a lack of stakeholder validation. While technically and economically promising, the polder system’s societal acceptance and integration into Singapore’s urban context require further assessment. Future design phases should address public perception of flood risk, desirability of polder developments, and nature-inclusive coastal environments, supported by stakeholder engagement. Additional research into flood risk, the polder pumping system, and SDRs is recommended to improve the design and inform decision-makers on platform level selection. This thesis provides a technical foundation for Long Island’s next design stages and supports platform level decision-making. It also offers insights for other regions pursuing land reclamation developments, especially where unit rates are high and/or materials are scarce, demonstrating an integral optimization approach focused on multifunctionality and climate resilience. ... Read more

As flood intensities are increasing, Flood Early Warning Systems (FEWS) are becoming more crucial so the right mitigation measures can be taken. This study focused on the Phetchaburi river basin in Thailand, which experiences floods yearly. Currently, precipitation data from rain gauges are used in the FEWS for this region. However, weather radar is also available in Thailand. Weather radar is usually able to capture the spatial variability of precipitation in more detail. Therefore, this study aimed to research the effect of spatially distributed precipitation on flood modelling in the Phetchaburi river basin.

Using HEC-RAS, a hydrodynamic rainfall-runoff model of the Phetchaburi river basin was made. The area of interest was the middle reach of the Phetchaburi river and its two tributaries, lying in-between three reservoir dams upstream and the Phet Diversion Dam downstream. The Phetchaburi river and its tributaries were calibrated using measured dam outflow and water level data. Three types of precipitation data were used as input, namely homogeneous precipitation data and spatially distributed precipitation data obtained from weather radar and from rain gauges.

High infiltration rates were found for the Phetchaburi river basin. When homogeneous precipitation data was used as input, precipitation intensity would be too low, allowing all precipitation to infiltrate into the subsurface. Using homogenous precipitation data as input results in a underestimation of floods. On the contrary, precipitation intensities in spatially distributed precipitation data were high enough to exceed the soil infiltration capacity, leading to surface runoff and floods. When solely looking at the water balance, using precipitation data from weather radar and rain gauges as input lead to similar results. However, when it came to the water levels at specific locations, precipitation data from weather radar performed better. The density of the rain gauge network in the Phetchaburi river basin is too low to capture the spatial variability of the precipitation events in detail. This resulted in floods or the
lack thereof at the wrong locations. Weather radar captures the spatial variability of precipitation in greater detail than rain gauges can. This study showed that using precipitation data from weather radar as input results in more accurate flood modelling in the Phetchaburi river basin. ... Read more

During a failure event on the 2nd of November 2023, non-closure of the spill gates in IJmuiden resulted in sea water inflow into the adjacent water system. This water system consists of the North Sea Canal, Amsterdam-Rhine Canal and multiple side rivers and basins. Considerable research is available on the ability of the complex IJmuiden in discharging the fresh water system and the connected polders. However, research on sea water flowing into the water system is limited. This research focusses on the behavior of a flood wave due to sea water retaining failure of the spill-lock complex in IJmuiden. Several scenarios are modeled in which sea water level, the failure mechanism and the duration of failure are varied. The model is created with the software of HEC-RAS. With the probabilistic analysis in combination with an estimation for the costs of damage, the research concludes with a flood risk estimation for 6 hours of spill failure. ... Read more

Flood risk management has traditionally been centered around economic damages and casualty assessments, providing a basis for preventive measures like levees. However, with increasing climate change-induced weather extremes, a shift toward flood resilience is necessary. This shift involves accepting some level of flooding while enhancing system recovery and damage mitigation. Existing resilience frameworks offer only qualitative insights or rely on significant assumptions, limiting their applicability in objective assessments.

This research focuses on addressing the challenges of quantifying flood resilience, a concept inherently complex due to its multi-layered nature and reliance on diverse perspectives. By incorporating hydrodynamic conditions such as water depth, flow velocity, and momentum, alongside local topography and land use, this study aims to propose metrics that better capture the temporal and spatial dynamics of flood resilience. This enables objective evaluation of mitigation measures, guides resource allocation, and facilitates informed decision-making for engineers and policymakers alike. This could subsequently enhance flood risk frameworks and increase flood safety in The Netherlands.

A methodology for quantifying flood resilience through hydrodynamic modeling is presented in this study. It identifies functionality variables like temporal water depth, temporal impact, number of flooded buildings, and percentage dry area as central to assessing resilience. From these functionalities, resilience metrics are derived, like shock amplitude, arrival time, and residence time.

The methodology is tested through a case study in the Alblasserwaard. The study uses 3Di modeling software to simulate flooding scenarios and evaluate the applicability of resilience measures. This is done through model variations and interventions such as detention basins, moveable barriers, and enhanced pumping capacity. The chosen case study includes diverse land uses, enabling the assessment of resilience across residential, economic, and ecological perspectives.

The metrics of shock rate, residence time, flood arrival time, and flooded utilities provide promising insights into flood resilience. The derivative shock rate evaluates emergency response service capacity, while residence time assesses damage extent and recovery time. Adding indirect hydrodynamic conditions, like nearby flooded roads and utilities, further enhances system understanding in the provided case study. Different perspectives highlight the variability in suitable metrics as well as suitable interventions.

However, some metrics require further research or modification. The depth integrals show potential during shock and recovery, but they lose information when used as a linear metric between time and water depth. The flooded utilities metric provides valuable insights but needs expansion to accurately reflect flooding consequences. The momentum impact metrics are unsuitable for the current model due to limitations in 3Di’s flow velocity calculations.

While the method proved feasible for identifying and comparing
resilience in a specific system, further research is needed to address uncertainties, refine metrics for indirect effects, and test applicability beyond the presented case study. This framework represents a further development toward a comprehensive, objective approach to flood resilience, supporting effective, adaptable flood
management solutions even under the continuous threat of increased climate extremes.
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Accumulation of debris, and subsequent clogging of trash racks upstream of inverted siphons and culverts can pose serious inundation risks. If the upstream water level continues to rise till the banks overflow due to the blockage of the inlet, it can lead to damages and potential casualties in case of floods. Therefore, a thorough understanding of debris accumulation processes and implementation of mitigation measures in design methods is essential, even though, this is not being a common practice currently. While existing research has focused primarily on experimental studies of debris accumulations or retrospective analyses of flood events, this study leveraged datasets from Waterschap Limburg, containing water level and discharge measurements collected on site. To obtain an enhanced understanding of debris accumulation processes at trash racks upstream of inverted siphons. This research analyzed stage-discharge measurements around inverted siphons throughout Limburg to identify debris-induced anomalies in the flow conditions that corresponded to debris extractions. This process served as a foundation for a targeted investigation of these anomalies in the water levels and water level differences across the trash rack at the inverted siphon at Tungelroysebeek. Detection methods were developed to identify instances of debris removal across the dataset. This enabled an analysis of seasonal influences on the frequency of debris accumulation and extractions, where an increased occurrence of removals was found in fall months. Furthermore, Extreme Value Analysis was employed to statistically model the extreme behavior of extraction events, estimating the return periods for days with significant extractions of debris accumulations. This study demonstrated that historical water level and discharge data can be used to detect debris accumulation through stage-discharge analysis and application of identification methods. Insights were acquired on seasonal influences on the debris accumulation process, and on the likelihood of extreme accumulation events. The obtained results are expected to be useful in flood-risk assessment and development of targeted debris management strategies, and can support engineers in decision making for design and mitigation processes. ... Read more

Groundwater extraction could be a crucial driver of subsidence; a phenomenon that has had, and still has, implications for the landscape, infrastructure, and environment. Vitens, a Dutch drinking water company, expects an increase of 10% in drinking water demand in 2040 for the Netherlands. This may intensify Subsidence due to Groundwater Extraction (SGE) in the future. The objective of this research is to quantify the historical and future effects of groundwater extraction on subsidence at Vitens extraction site ”Groenekan” in the Province of Utrecht. In this thesis, a methodology is developed to assess whether Vitens can responsibly expand Groenekan groundwater extraction from 5.0 Mm3/yr to 10.0 Mm3/yr. The groundwater extraction is situated in a rural polder landscape where two subsidence-sensitive soil layers are present: a 1 m thick Holocene peat/clay top layer and a 12 m thick aquitard (Waalre clay) at a depth of -48 m NAP. Under this aquitard, Vitens currently extracts ∼5.0 Mm3/yr of drinking water from the second aquifer. Next, a groundwater model is used to study the lowering of the groundwater level and hydraulic head due to the extraction. Subsequently, the effect of both lowerings has been quantified for four subsidence processes, 1) Biochemical Degradation of Organic Material (BDOM), 2) shrinkage, 3) consolidation, and 4) creep. Groundwater lowering affects all four processes, whereas head lowerings only affect consolidation and creep. Occurred subsidence from the start of the groundwater extraction and for eight future scenarios is calculated with the aid of the groundwater model output, analytical approaches and a 2D subsidence model. The cumulative subsidence of these four processes is labelled as SGE. From 1961 to 2023, the groundwater level at Groenekan is lowered with 33 and 60 cm for 5.0 and 10.0 Mm3/yr extraction discharges respectively. In the second aquifer, hydraulic head lowerings of 1.5 to 2.9 m with a symmetrical influence circle are observed at Groenekan. In a radius of 500 m around Groenekan, significant groundwater level lowerings are found, which enforces compaction processes in the form of BDOM and shrinkage. For the period 1961 to 2023, assuming a 1 m thick clean peat Holocene top layer reveals ∼16 cm of potential BDOM. However, potentially ∼11 or ∼18 cm of shrinkage would occur over the same period assuming a clean clay or organic clay Holocene soil type respectively. Compression of the Holocene and Waalre layers due to consolidation and creep has contributed ∼10 times less to SGE around Groenekan compared to compaction due to BDOM and shrinkage. Within a 500 m radius of Groenekan, SGE is noticeable; beyond that, SGE is negligible. The maximum total subsidence through compression is ∼1.0 cm over a period of 62 years (from the start in 1961 to 2023). Generally, consolidation contributes 75% to the total subsidence due to compression, whereas creep is responsible for 25% of the compression. Sensitivity analyses reveal that the Holocene soil type and the thickness of the soft soils are the most sensitive parameters in calculating the final subsidence. Clean peat soil gives a high potential subsidence rate, whereas sand soils hardly subside. Variations in groundwater level and head lowerings are found to be least sensitive. Within the Groenekan system, the 1 metre Holocene thickness is the limiting factor in the groundwater level lowering, since no additional subsidence effects are found if the groundwater level drops below the Holocene layer. The future scenario of 10.0 Mm3/yr + an extreme climate shows ∼1.2 cm of consolidation and creep in the Holocene and Waalre layers over the period 1961 to 2100 (139 years). The base scenario of 5.0 Mm3/yr gives of ∼0.9 cm of subsidence, which results in ∼0.3 cm of additional subsidence. This extra subsidence value is negligible over a period of 139 years compared to other locations in the Netherlands with dozens of centimeters of subsidence. Though, BDOM and shrinkage can potentially cause centimetres of subsidence in the Holocene on the long-term due to groundwater level lowering. In conclusion, expansion of Groenekan from 5.0 to 10.0 Mm3/yr is considered to be responsible from a subsidence perspective as long as BDOM and shrinkage are exercised with extreme caution. Future studies on the effects of extended droughts and the surface water system on the groundwater level are suggested to better distinguish the total subsidence from SGE. Lastly, it is recommended to Vitens to extract groundwater from deep, confined aquifers with a sandy top layer for maximal SGE mitigation. ... Read more

The increasing demand for stream restoration projects in the Netherlands, driven by legislation, prompted interest in more integrated approaches. TAUW (Technische Adviesbureau van Unie Waterschappen) and WSAM (Waterboard Aa and Maas) collaborated on stream restoration measures for the Lage Raam stream in the Raamvallei, initially using a 1-dimensional model. However, questions arose regarding the suitability of a 2D model for this project.

This research explored the applicability and potential of a 2D hydrological model made in Tygron to provide new insights and outputs for this stream restoration project, including inundation maps, water level fluctuations, and evaluating designed restoration measures. Simultaneously, the study assessed Tygron's applicability for large water systems in the Netherlands by evaluating its underlying settings and parameters.

In the first part of this research, the study demarcated stream restoration measures for the Lage Raam, focusing on redesigning the stream to enhance nature-friendly banks. The Tygron water module was introduced, emphasizing critical simulation setup adjustments such as the rainfall overlay and simulation settings investigated in the initial testcase study. The settings investigated in the testcase were: 'Water level to shorelines', 'Waterline reconstruction', 'Angle stabilizers for partly flooded cells', 'Manning value', 'Grid cell size', and 'Grid/stream placement'.

Results from part 1 indicated that among the six settings tested, only three significantly influenced water level simulations in channels. Variations in Manning values demonstrated a pronounced effect on water height accuracy, with lower values correlating with better simulation outcomes in the testcase. The influence of Manning values was more pronounced in narrower streams, where shallower water depth worsened inaccuracies in the model's backwater effect. Notably, Grid cell size and Grid/stream placement were crucial for achieving accurate outcomes. The optimal grid cell size was found to be 1 by 1 meter or of higher resolution. Additionally, aligning streams parallel to grid cells generally improved results, although the influence of grid placement diminished with increased grid cell count per channel.

The second part introduced the study area, the 'Raamvallei', for case studies 2 and 3, outlining designs for cross sections with swamp areas as restoration measures. Case study 2 validated the Tygron model using measured data from the Raamvallei obtained from WSAM and rain events, testing its suitability and model setup for water systems. Case study 3 implemented TAUW's restoration design to evaluate Tygron's effectiveness of these measures.

The results in part 2 showed that evaluation in a larger watershed scenario (Raamvallei) underscored the model's robustness when configured for extensive water systems. Grid cell size sensitivity analysis highlighted the optimal range (1m x 1m or smaller), lower resolutions causing water loss in the Lage Raam water system, underscoring the resolution’s impact on modeling outcomes. Achieving accurate connectivity between primary, secondary, and tertiary waterways was crucial, requiring iterative adjustments including culvert generation and hydraulic structure calibration. The third case study highlighted challenges in data retrieval and storage due to Tygron's limitations in exporting detailed simulation data over time. However, it also demonstrated Tygron's capability in simulating level fluctuations and flow rates, despite challenges in data analysis.

In conclusion, Tygron was capable of using the explicit Saint-Venant scheme to calculate 2D shallow water equations where it accurately simulated a complex large water system in the Netherlands. Additionally, it could be used for projects such as the Lage Raam to provide insights into stream restoration designs. However, for a model to be successfully used and have results that could be easily understood, some settings were important to look at and some changes in data collection were needed. Future research should encompass diverse test cases to validate Tygron's performance across various scenarios and compare it with other 2D hydrological models for broader applicability insights.

Based on the study's findings, several recommendations were proposed to enhance Tygron's utility in hydrological modeling. These included exploring new data storage approaches to handle extensive datasets more efficiently, optimizing the use of limit areas to simplify model complexity without compromising simulation accuracy, and improving connectivity tools like the culvert generator for seamless integration with external data sources. ... Read more

In July 2021, an extraordinary precipitation led to severe flooding across Europe, particularly affecting South Limburg in the Netherlands, causing significant damage. In response, the Dutch government is seeking methods and new ways to mitigate the effects of future flash floods. Consequently, Waterschap Limburg (Limburg Water Board) initiated a physical experiment in Roermond in May 2023 to assess the effectiveness of movable flood protection barriers under various conditions. TU Delft, Flood Proof Holland, and AccessHub B.V. were involved in establishing a monitoring system that evaluated the stability and performance of these temporary flood barriers.

Although the experiment yielded success and Waterschap Limburg profitably selected appropriate temporary flood barriers to address flooding for the 2023-2024 festive season from Storm Pia, there remain knowledge gaps concerning these barriers. Limited documentation has left some aspects unclear, such as the fundamental physical processes and mechanisms of failure detection. This study aims to provide a thorough understanding of how to monitor physical changes through image processing and identify failure mechanisms by applying the horizontal stability equation. ... Read more

In Dutch polders, numerous structures like bridges, weirs, culverts, and pumping stations have been constructed over centuries to manage water levels. These structures play a crucial role in maintaining water levels within predefined targets. The flat topography of the Dutch landscape combined with the collective impact of head losses, induced by these structures may result in flooding of polders during high runoff scenarios. Over time, culverts and bridges may underperform due to alterations in the water system, increased pressure from climate change, evolved design rules, insufficient maintenance, and shifts in land use.
A challenge is the potential hydraulic underperformance of structures and the need for their premature replacement, which is costly. Waiting until the end of their technical lifespan may contribute to floods. Therefore this thesis focuses on improving existing structures to mitigate the need for replacement, specifically by streamlining inlet and outlet openings to reduce energy losses. This leads to the research question of this thesis: “How can the head loss over existing (too tight) culverts be minimised by adding an inlet or outlet profile and does this lead to a substantial enhancement in the performance of these culverts, providing a practical option to postpone the replacement of underperforming culverts?”
To answer this question, the problem is explored by looking into the fundamentals of energy losses, including entrance losses, friction losses, and exit losses. This gives an understanding of the conditions under which these losses manifest. However, these basic calculations have inherent limitations due to their reliance on predefined coefficients. This renders them inadequate for evaluating the effects of introducing new profiles onto an existing structure. To overcome this, a flume experiment has been performed to verify whether it is possible to measure water level differences for various profiles at the culvert entrance and exit. With a 3D Computational Fluid Dynamics (CFD) model (OpenFOAM), flows around different culverts are simulated. The results of the CFD model are compared to the flume experiment, after which the CFD model is used to simulate a variety of scenarios, with different profiles, culvert dimensions, velocities, and water depths.
As such, this thesis addresses challenges and uncertainties in quantifying head losses in culvert structures through experimental methods and CFD modelling. Experimental setups struggle with controlling all flow-influencing parameters, while CFD modelling offers flexibility but requires careful consideration of uncertainties and limitations. The discussion emphasizes the complexities of comparing experimental and model results, highlighting trade-offs and uncertainties in each approach.
The conclusion answers the central research question, confirming that specific profiles added to culverts can significantly reduce entrance losses up to 65%, thereby lowering headwaters for a constant discharge. The recommendations section outlines possibilities for further research, including optimizing profile dimensions and conducting sensitivity analyses of influential parameters. Practical recommendations involve aligning large-diameter concrete culverts with the socket end in the flow direction and integrating groove or rounded profiles during construction for cost-effective inlet loss reduction.
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Because of climate change, seawater levels are expected to rise considerably, combining this with increased peak river discharges, results in significant strain on inhabited low lying parts of the world. The Rhine-Meuse estuary is a perfect example of this; a densely inhabited region in between the North Sea and multiple rivers.
The Dutch Delta Program researches the effects of climate change for the Netherlands and proposes multiple alternatives to reduce its effects. An alternative comprises of the implementation of large scale hydraulic interventions, in combination with keeping all dikes up to the norm.
Four alternatives are considered, being; A1: Closed seafront and keeping current water level, A2: Closed seafront, allowing an increase in water level, B1: Closable seafront and retaining river discharge distribution and finally B2: Closable seafront with an altered discharge distribution.

This research determines, and compares, the economic efficiency of the aforementioned alternatives considering different spatial planning scenarios. The economic efficiency of an alternative is calculated by summing the resulting benefits of the alternatives and dividing this by the cost of implementation.
A higher economic efficiency indicates that an alternative provides higher value compared to its cost of implementation, this is necessary to help decide if an alternative is worth considering for implementation. Estimating the impact that spatial planning strategies have on the economic efficiency of the alternatives helps in determining if such strategies need to be accounted for when performing efficiency determinations for similar large scale hydraulic measures. The benefit of implementing the alternatives consists of the resulting reduction in dike reinforcement cost and reduction in the region’s flood risk as compared to the baseline strategy. The baseline is the strategy to protect the Netherlands like it has been done i.e. reinforce dikes where the failure probability is close to becoming higher than the norm, combined with mainly utilizing closable flood barriers, also known as the open-closable strategy. All economic factors are converted to present value with a discount rate of 1.6%, alternatives are modeled to be implemented in 2100 and all the alternatives’ effects are considered up until 2200.

This study finds that, amongst the four considered alternatives, B2 appears to be the most economically efficient choice, next to that, alternative A1 has comparable economic efficiency. The efficiency is for a large part a result of the flood risk reduction for unembanked areas, which alternative A1 and B2 specifically, have as an effect. Alternatives A2 and B1 have an economic efficiency far below 1.0 for all spatial planning strategies; thus not worth exploring further based on the considered factors within this research. Spatial planning strategies have a significant influence on economic efficiency; alternative A1 and B2 become around 30% more efficient for the move to unembanked spatial strategy as compared to the densification one. And reversely, the strategy of developing rural land results in all alternatives having an economic efficiency of below 1.0; being cost ineffective.

In this research, three reference locations are used to determine the flood risk reduction of the alternative, the other economic factors are scaled (normalized) to be in proportion with the reference locations. If the flood risk reduction effects of the alternatives would be determined for the whole Rhine-Meuse Estuary the uncertainty of the applied method would be reduced. The flood risk reduction determination relies purely on water levels, their frequencies and corresponding damages, taking into account the alternatives’ effect on outside water level perseverance would lead to more precise flood damage estimations. The unembanked flood risk reduction is the governing factor for the economic efficiency of the alternatives making the (local) protection, of unembanked areas specifically, a possible highly cost effective strategy. This should be explored as a new alternative next to the four in this research, possibly made up out of components of the considered ones. ... Read more

Groundwater extraction has increased significantly in Nepal. In combination with climate change, this might lead to accelerated groundwater resource depletion. No recent research has been done in the assessment of these resources in the Banke district development area in the Terai. This study aims to assess whether the intensification of extractions has led to a depletion of groundwater in the upper aquifer. We investigated six local case studies during the dry season with a sole focus on the upper aquifer. The hydrological fluxes were quantified at every location. Evaporation and recharge were estimated using remote sensing data and locally obtained meteorological data. Domestic extractions, irrigation return flow and irrigation extractions were approximated by fieldwork. The groundwater storage change over the period was estimated using a recorded groundwater table time series and the Water Table Fluctuation method. The net subsurface flow- and percolation were estimated by closing the water balance for every case study. Subsequently, the fluxes at local case studies were extrapolated using Groundwater Response Units. The groundwater table replenishment was estimated using the relation between effective precipitation and groundwater levels over two years. The net extraction from the groundwater was insignificant compared to the contribution of evaporation. The approximate minimum precipitation needed for the monsoon season to recover the shallow aquifers after the dry season was on average comfortably exceeded over the last ten years. Thus, the groundwater resources are currently not depleting due to the intensification of extractions in the upper aquifer. However, the current extractions from deeper aquifers might decrease the pressure in deeper layers increasing percolation. Eventually, this endangers the groundwater resources in the upper aquifer. The limited number of measurements in deeper wells already indicated potential depletion in deeper layers. Further research is therefore recommended in deeper aquifers. ... Read more

The ramifications of plastic pollution on the environment are becoming increasingly serious in various forms throughout the world. In this context, rivers are the most important suppliers of plastics entering the marine environment. However, rivers that contain high loads of plastic waste also directly harm the livelihoods of people living near these rivers. An example is the flooding of urban areas in Indonesia due to clogging of the hydraulic and drainage systems, which is caused by the blockage of local hydraulic structures by plastic debris.
At this moment, there is a lack of knowledge on this accumulation process and its underlying dynamics, since observational and experimental research is lacking. Numerical modeling has proven to be a great tool for expanding experimental research. However, no suitable numerical method has been identified yet to model the plastic accumulation process, since traditional mesh-based CFD numerical methods are expected to be not a viable option, due to their inability to model the individual interaction between plastic particles, critical during this process. A possible solution could be the SPH-DEM method, which is a two-way coupled numerical approach that simulates fluid and debris as discrete particles and elements.
The objective of this report was to find out if SPH-DEM could be a suitable numerical method to model the dynamic processes of the plastic debris accumulation against hydraulic structures. To accomplish this, the first goal was to realistically model a turbulent open-channel flow and the buoyancy of individual plastic debris, which would be validated by experimental research. The second goal was to investigate which are the most important (numerical) parameters affecting the mentioned plastic debris accumulation.
In this report, experimental research was carried out in the form of buoyancy tests and flume tests, and numerical research was carried out in the form of the design of numerical simulations. In the buoyancy tests, the rising velocities of four plastic fragments that differed in size and density were measured, which were released multiple times in a graduated cylinder filled with water. In the flume tests, first the water elevation was measured along the the flume, after which the passing ratio’s and carpet lengths were measured for the four different released fragments for three different gate configurations. Two types of numerical models were designed that represented both types of experimental tests, for which several design choices had to be made to compensate for several physical phenomena, which can’t be directly represented in the model design.
The numerical buoyancy test was validated with the rising velocities obtained from the experimental equivalent. It was discovered, that for relatively low resolution modeled fragments, the rising velocity is heavily influenced by numerical diffusion. The smoothing length was identified as an important numerical parameter, which can compensate this effect. Furthermore, it was discovered that the degree of numerical diffusion is dependent on the depth of the fragment in the water. The numerical flume test was validated with the water elevation obtained from the experimental equivalent. For uniform flows, by adjusting the boundary viscosity coefficient, smooth turbulent velocity profiles could be simulated throughout the flume corresponding to theoretical values. However, no single value of was found in which the velocity profiles of the uniform flow and the validated water elevation of the gradually varied flows were both in agreement with their theoretical values. After validation, fragments were added to numerical flume model. Per fragment type and gate configuration,
four different numerical scenarios were executed, where each scenario was defined by a combination of a certain density ½s and restitution coefficient e. Finally, the best corresponding scenarios were used to simulate mixed fragments released in the flow. It was found that the gate opening height , density and restitution of the plastic fragments have the largest influence on the passing ratio’s , carpet length , carpet shape and carpet stability. Furthermore, it was confirmed that individual fragment interactions play a crucial role in the accumulation process. However, the model is mainly limited by its low resolution and the absence of suitable turbulence models. This means that many forms of fragment behavior seen in the experimental research such of buoyancy, trajectory and individual interactions, which are heavily influenced by turbulence, cannot be sufficiently represented in the numerical model. However, it is shown that by adjusting the density the buoyancy behavior can be partly replicated and by adjusting the restitution coefficient the turbulent individual interactions can be partly replicated. In conclusion it can be stated that SPH-DEM is an interesting option to model the dynamic processes of the accumulation of plastic debris against a sluice gate; however, further improvements in computational power and turbulence models are needed to be more widely applied. ... Read more

In 1972 the Union of Waterboards and the Association of Dutch Municipalities drew up a directive to give waterboards and municipalities a guideline in how to divide tasks and costs in the new situations that were the result of the Pollution of Surface Waters Act, decreed in 1970. At 7 waterboards costs are still settled with (a part of) the municipalities based on the method proposed in the directive. Most waterboards have therefore already abandoned this cost distribution. With the use of several focus points an overview has been created through interviews and document analysis on the current use of the cost distribution based on the directive. Various aspects emerged from the interviews and documents that may play a role in the application of the cost distribution based on the 1972 Directive. In the end, 9 subjects were mentioned or discussed more often that could have a direct or indirect influence on the fact why the cost distribution is (still) applied. These subjects appear to influence whether or not a cost distribution is used on the basis of the 1972 Directive. It could be a correlation of various factors per waterboard, but it could also be a coincidence. All in all, it is clear that the cost distribution based on the 1972 Directive is being applied less and less. The distribution method is also not being reintroduced by water boards and municipalities. ... Read more

Since 2019, Rijkswaterstaat has been applying flexible water level management in the IJsselmeer and Markermeer to make the Dutch water system more robust and future proof. The fixed target water level of -0.20 m NAP during summer is replaced by a flexible water level in which the water level can fluctuate between -0.10 and -0.30 m NAP to enlarge the fresh water supply and to anticipate on weather conditions. In the IJsselmeer region, 15,600 hectares of land is located outside primary levees. These areas are mainly pasturelands, recreation areas, nature areas and some buildings and infrastructure areas. By applying flexible water level management, the probability of high water levels increases and therefore flood risk in areas outside primary levees increases. However, the relation between applying flexible water level management and flood risk has not been investigated in detail yet. This research explores this relation further, on smaller scale as well as for the whole IJsselmeer region.

Flood risk is defined as the yearly probability of exceedance of hydraulic loads (lake water level, wind set up and wave run-up) multiplied by the consequences of inundation and is expressed in euros per year. Risk is a set of scenarios, with each a probability and a consequence, and therefore discretization is necessary. Hydraulic loads are calculated in Hydra-NL, a probabilistic model. By uploading hydraulic loads associated to their exceedance probability in the Waterschadeschatter, the consequences are defined and flood risk can be computed.
Flood risk depends on the lake water level, wind set up and wave run-up. The lake water level and wave run-up are in general higher when flexible water level management is applied and wind set up is lower. This observation is valid for all areas outside primary levees in the IJsselmeer region. About 90% of the total damage comes from water damage in buildings and infrastructure areas, and then especially from residential areas. Since less than 3% of areas outside primary levees consist of buildings and infrastructure areas and less than 10% of buildings and infrastructure areas consist of residential areas, the flood risk is reduced. Moreover, most computations show overestimations. Water levels computed in Hydra-NL are higher than occurred water levels and the upper boundary of -0.10 m NAP is used to calculate effects of flexible water level management. Because both overestimations are used and there are just a few residential areas, most areas located outside primary levees in the IJsselmeer region will hardly suffer from applying flexible water level management. This research shows that interests of areas outside primary levees in the IJsselmeer region and applying flexible water level management go quite well together. Therefore, the IJsselmeer region offers many opportunities.
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Drought and subsidence are two out of several water-related urban climate adaptation challenges many cities in the Netherlands currently face. Drought is expected to increase in frequency and extent due to climate change. Therefore, drought is likely to further pressurize (subsiding) urban areas in the coming decades. Although the impact of drought on soft soils and hence subsidence is described in academic literature, in-practice analyzes are limited. Given the expectation of increased drought impacts, as well as the current urge for new housing developments to combat housing scarcity, this research focuses on in-practice drought impacts on subsidence in two soft-soil urban areas in the Netherlands. This research’s objective is to gain insight into drought-induced subsidence in soft-soil urban areas in the Netherlands, in order to better understand drought impacts on subsidence rates in future housing developments on comparable soils. With a better understanding more appropriate site preparation strategies can be applied. The objective is endeavoured by an in-practice analysis of two study areas: a 90s neighbourhood in Diemen and a recently finished urban area in Kampen. Firstly, variations in relative surface levels, observed by InSAR (2015-2019), are compared for dry and wet periods. Afterwards, this subsidence data is separated based upon local characteristics (soil structure, pavement type, vegetation percentage and vicinity of surface water) to analyze their influence on drought-induced subsidence. Lastly, an expert questionnaire is conducted on suggestions for both mitigation and adaptation strategies. Subsidence is not a linear process in time: surface level movements vary due to the soil’s wetting and drying. The extent of soil compaction in dry periods is found to increase with intenser droughts. The extent of soil swell in wet periods is approximately similar for varying wetness extents. This research found that the severe drought of 2018 thereby caused a soil compaction to such extent it could not be balanced with subsequent winter swell, and hence resulted in approximately 1 to 1.5 millimeter of drought-induced subsidence. Drought-induced subsidence is thus a net result of the (change in) seasonal surface level fluctuation. This net result is assumed to be consequence of extensive groundwater level drops, although the exact share of processes and subsidence mechanisms could not be estimated. Moreover, surface level movements are found to be prone to a lag and difference in duration (hysteresis) in comparison to the start and duration of dry and wet periods. Various local characteristics are found to influence drought-induced subsidence. In general more compressible soils show slightly larger surface level movement between dry and wet periods. The (sand) cover thickness is found to be influential on which subsidence mechanisms are triggered during a drought: a cover thickness resulting in groundwater levels to drop to present clay/ peat layers causes shrinkage and/ or peat oxidation additional to clinch. Unpaved surfaces are found to fluctuate more extensively than paved surfaces, but this does not necessarily result in more irreversible subsidence. Furthermore, abundant vegetation might result in extra irreversible subsidence due to its extensive water usage in dense urban areas. Lastly, in the analyzed soft-soil areas the surface waters seem to influence groundwater levels and hence surface level movements only on short distance. Suggestions on feasible enhancing site preparation strategies are given based upon research results and experts’ opinions. The suggested mitigation strategies consist of two approaches in order to hamper the variations in effective stresses and hence minimize the seasonal surface level fluctuation. The first approach focuses on preventing extensive groundwater level variations: increasing storage and infiltration of water, reversed drainage, building crawl-space free and choosing vegetation types based on their water usage. Additionally, it is suggested to apply a sufficient cover thickness (at raises) if soft soil layers are near the surface. This prevents that future extensive groundwater drops within these layers, and thereby limits (seasonal) shrinkage and/ or peat oxidation. The second approach focuses on reducing the top soil’s weight, via lightweight materials or self-carrying constructions. Adapting to drought-induced subsidence starts with measuring/ monitoring surface level movements in order to analyze spatial and temporal trends. Additionally, drought is to be considered to greater extent in subsidence modelling in order to improve subsidence estimations. This can be done by applying variable or lower groundwater levels in estimations of the (change in) effective stresses, at calculations of consolidation or creep. Lastly, urban utility management should focus on long-term costs via e.g. Life Cycle Analysis, and on overlapping maintenance cycles of surface level raising and e.g. sewer pipe replacements. The most important conclusion derived on urban drought-induced subsidence is that despite individual drought impact on subsidence is limited to 1 to 1.5 millimeters, its seasonal occurrence continuously affects surface levels. Moreover, due to climate change drought is expected to increasingly impact surface levels in Dutch soft-soil urban areas in the coming decades. The suggested strategies mainly hamper variations in soil stresses, via fluctuating groundwater levels, and hence drought impacts on soft soils. These strategies help to limit future soil movements and hence result in more climate adaptable soft-soil urban areas. The focus of this research is on qualitative analyses of in-practice data such that significant processes have been disclosed, rather than statistically verifying the results. Consequently, the research initiates further specific studies on statistical verification of drought-induced subsidence, and moreover topics on its mechanisms; its spatial and temporal variation; its measurement and modelling; the influences of local characteristics hereupon; and the effectiveness of the suggested enhanced site preparation strategies. ... Read more

The Volkerak-Zoommeer is a former estuary located the Rhine Meuse Delta in the southwestern part of the Netherlands. After the completion of the Volkerakdam, Philipsdam and Oesterdam as part of the Delta Works, this estuary was cut of from the Oosterschelde and a fresh water lake was formed in a sea water environment. This new lake became the Volkerak-Zoommeer: a lake that acts as a fresh water supply for the regional water systems of the water boards Hollandse Delta, Scheldestromen and Brabantse Delta as well as providing a tidal free shipping connection between Antwerp and the Rhine. The operational water manager of the lake is Rijkswaterstaat. They manage the water level and water quality by letting fresh water in from the Hollandsch Diep at the Volkeraksluizen and release it at Bath into the Westerschelde. This mechanism enables Rijkswaterstaat to flush the system and adhere to the target levels, as well as managing a maximum chloride concentration of 450 mg Cl/L during the growing season. In 2018, the Netherlands experienced one of the most severe droughts since the beginning of weather measurements started. Low discharges at the Rhine resulted in a decreasing water supply and more salinization in the Rhine Meuse Delta. When the discharge of the Rhine at Lobith reach below 800 m3/s, it is no longer allowed to let in fresh water through the Volkeraksluizen. This could lead to problems in the fresh water supply to the regional system of the water users of the Volkerak-Zoommeer. The question came up whether creating a fresh water buffer by temporarily heightening of the water level could be used to overcome a period with no fresh water supply from the Hollandsch Diep. In autumn 2020, the dynamics of the system were researched during a practical trial. The water level was heightened up to +0.15 m NAP. Next, the inlet was closed and it was measured how long it would take until the water level reached -0.10m NAP with normal flushing operations at Bath. During a second trial, the outlet at Bath was closed too. Based on the insights of these trials, a prototype for a Decision Support System was created. This system is able to give insights in the development of the water level at the Volkerak-Zoommeer according to operational water management decisions. This Decision Support System was used to evaluate the impact of delta and climate scenarios in 2050 and 2085 on the water level of the Volkerak-Zoommeer. The maximum possible duration for a period without a fresh water supply was researched per scenario within the target levels of -0.10 m NAP and + 0.15m NAP. Different flushing regimes at Bath were evaluated. For normal flushing operations, this duration is 5 till 7 days. For flushing every second low tide, this period could be lengthened to 8 to 13 days. When the outlet at Bath is closed, the maximum duration is 20 till 60 days. From the trials, it was found that the chloride concentration will gradually increase in this period. Unfortunately, sufficient insights in the dynamics of the chloride concentration lack at the moment to sufficiently incorporate the chloride concentration in the prototype. From historical data, the maximum duration of a period with discharges lower than 800 m3/s measured at Lobith was 16 days during the growing season and 82 days for all year data. Within the current target levels, this duration cannot be reached. It was therefore researched what initial water level theoretically could be achieved based on historical data. This turned out to be +0.50 m NAP. Here, the Volkerak-Zoommeer was considered as a closed system, neglecting the open connections with the Dintel and the Vliet. With an initial water level of +0.50 m NAP, the duration without fresh water supply could be significantly lengthened. With normal flushing operations, the period can take between 12 and 19 days. For flushing operations every second low tide, this period takes 20 till 34 days. When the outlet at Bath is closed, a period up to 120 days could be bridged. ... Read more

To reduce the flood risk in areas prone to flooding, possible adaptation measures must be investigated. In this thesis a Python-based framework has been developed that can screen flood adaptation measures as an alternative to the existing analytical method of Royal HaskoningDHV. The framework is limited to the costs and effects on the economic flood risk (the benefits) of eleven different measures such as levees, landfills, and flood proofing. First, the existing methods for screening measures were investigated. The resulting strong and missing elements were combined to form the functional requirements of the framework. The underlying script for the framework was elaborated with these requirements. Subsequently, the framework was applied to two case locations, namely Phu Loc (Vietnam) and Waal Eemhaven (Rotterdam, The Netherlands). Because these locations had already been developed by Royal HaskoningDHV, the results could be compared. The existing methods showed that clear results and the possibility for the user to select the measures and location are the main strengths. In addition, an uncertainty analysis and well-organized input table were found to be useful for the framework. The results of the simulations at the case locations were comparable to the results of the existing method of Royal HaskoningDHV for the measures considered. The framework is faster than the existing method and determines the optimal measure(s) and protection level. Therefore, it is a good alternative for screening the eleven considered measures. The framework can be supported with a hydraulic model to include more measures, for example river widening. It is recommended to add an extensive cost database to the framework, so that cost calculations and uncertainty analysis can be performed more easily in future applications. ... Read more

Human interventions in the White Volta River in Ghana increase the flood risk in living spaces of the local communities, causing great challenges in the area. Burkina Faso has built a hydropower dam near the border with Ghana which has a large impact on the discharge extremes of the White Volta River. As a result of poor water management of this dam, water is spilled during the wet season, causing more floods in northern Ghana. Several communities, living near the White Volta River, depend on the river water for their drinking water demand and use the sand in the river bed as construction material. This puts them in a vulnerable position when changes in the natural flow regime of the river occur.

This thesis studies the impact of constructing multiple small dams in the White Volta river in order to decrease the flood risk in villages located close to the river and create an opportunity for controlled sand mining as a result of sedimentation.

Tamale is, with 672.000 inhabitants, the largest city in this area and struggles with the consequences of the floods as well. The drinking water company has trouble meeting the drinking water demand during both the wet and the dry season. When the area is flooded, the water intake point shuts down, and during low water levels, the pumps can get clogged. This clogging occurs even more often as a result of increased illegal sand mining from the river banks by the local communities. This has broadened the river, resulting in lower water levels and a higher turbidity.

A possible solution to ensure the water intake in Tamale and reduce the flood risk in northern Ghana is the construction of multiple small dams in the river bed to flatten the discharge peaks and slow down the water in the White Volta River. At the same time, these dams can create an additional advantage by causing upstream sedimentation that provides a possibility for controlled sand mining in the river bed. To research the effectiveness of this solution, the impact of the dams on flood risk and sedimentation, near seven villages in northern Ghana, was modelled.

A base case of the flood of 2003 was compared to a scenario in which seven dams were implemented at locations close to villages that are often exposed to floods. This was done by building a hydromorphological model of the White Volta River, using the D-HYDRO 1D2D software.
The results of the morphological simulation show that in total, the sedimentation upstream of the dams can fill 81 trucks with sand per day. However, extracting the sediment at the most upstream dams decreases the possibilities for sedimentation near the downstream dams. The results of the 1D2D hydraulic model show that the proposed dams are not able to reduce the flood extent in villages and can even increase inundation depths, resulting in more flood damage.

It is recommended to increase the 2D grid of the hydraulic model in order to receive more realistic backwater curves. ... Read more