Large areas of the world are protected by flood defence systems. A common part of flood defence systems is a levee with grass cover, with the primary function to protect hinterland against floods. Waveovertopping might lead to erosion of the grass cover, followed by erosion of the inner slope and potentially levee breaching. The wave overtopping simulator (WOS) was developed for full-scale and insitu testing of the erosional resistance of levees against wave overtopping. The first experiment with this simulator was carried out in 2007 and more followed. However, the data gathered during these overtopping experiments has always been considered on individual experiment scale or in small subsets, exception being the Cumulative Overload method. The experiments proved to be difficult to reproduce, seemingly identical experiments gave different results. The objective of this thesis is to combine WOS data and multiple grass erosion models to make a prediction for the failure of the grass cover. Key features that determine the prediction of failure are the location and moment of failure. Therefore, these are included in the main research question, which is identified as: How can grass erosion models and WOS experiment data be combined in a new method to generate a prediction of the moment and location of the grass cover failure due to wave overtopping? A literature study on grass erosion and grass erosion provides the basis for modelling grass erosion, a review of combination techniques gives insight in combining results for a prediction. A selection of models is discussed and used for setting up a prediction method. These include three flow-based models which are semi-realistic simplified representations and time-independent over a single overtopping event: Cumulative Overload Method, Analytical Grass Erosion Model and Dean Stream Power. These models are, where required, adjusted to model grass cover failure and to comply with identical hydrodynamic input. A fourth model, the Wave Impact Approach, is an impact-based approach. Each model is calibrated on each WOS experiment, creating a set of calibrated models, which function as the set of predictors. The number of predictors equals the number of models times the number of experiments. During calibration, the moment of failure has been traced back using the control list of the WOS, and vice-versa, resulting in a moment during the experiment at which the grass cover failed. Detailed review of all factual reports on WOS experiments and the nature of the grass erosion models highlighted the need for a locationdependent resistance parameter to determine the location of failure. Therefore, calibration was based on a location-depended resistance parameter; critical flow velocity for the flow-based models and critical pressure for the impact based model. This set of predictors has been used to create a set of predictions for each of the five validation experiments, input being the geometry, loading and initial condition of the slope. The initial condition given by a registration of anomalies to the average grass cover. Failure is determined by vote and averaging. After validation, none of the final predictions proved to be fully correct. Meaning that none of the final predictions correctly indicated the location and moment of the first grass cover failure. Despite this, in four of the five validations at least one correct failure location was indicated when considering the specific anomalies. The main shortcoming is concluded to be the prediction of number of waves until grass cover failure. Based on this research and given a certain average grass cover quality, it is concluded that the resistance against grass erosion by wave overtopping is described by resistance against erosion of anomalies. For at least 22 of 28 sections in the dataset, grass cover failure occurred at an anomaly of the average grass cover. No grass cover failures occurred related to the average grass quality. Especially mole activity showed a significant 46% decrease in the averaged calibrated critical flow velocity with relative to that for the average grass cover. This leads to the conclusion that design and safety assessments should include conditions other than the average grass quality. For future research a method is recommended that divides mole activity into classes, based on certain characteristics. Each class distinguishable by unique combination of properties and assigned a resistance against erosion. For design, the probability of occurrence of each class must be determined and combined with the corresponding influence on the resistance against erosion. For assessment, an inventory or representative sample of animal activity must be available to asses if the occurrences of animal activity are within the design requirements.

Large parts of the Netherlands are vulnerable to flooding. Due to the great impact that a flood would have for the Dutch society, The flood safety standards are very strict, which means that the required failure probability of flood defences is very low. To keep the flood risk within acceptable limits of the hinterland, flood embankments are assessed on whether the probability of failure matches the stringent norms using the methods and rules which are stated in the legal assessment instrument (WBI). Within the WBI the probability of failure of the flood embankment equals the sum of the probability of occurrence of individual failure mechanism. This thesis specifically focuses on the failure mechanism: sliding of the landside slope cover due to wave overtopping of clay flood embankments. Due to wave overtopping the relative permeable top layer of the clay embankment becomes saturated. This leads to a shallow subsurface flow parallel to the landside slope cover which is unfavourable for the geotechnical stability of the landside slope cover. The stability assessment of this mechanism is currently assessed with the Edelman & Joustra formula. This formula method is not based on a probabilistic or semi-probabilistic analysis and therefore does not give an failure probability. Instead it uses safety factors. Levee managers therefore believe that the outcome is overly conservative (Waterschappen, et al., 2021). This thesis addresses the question: How can the stability assessment of the Edelman & Joustra formula can be optimized for clay flood embankments in the Netherlands within the legal assessment instrument? To answer this question, new semi-probabilistic safety factors were derived based on a full probabilistic assessment of the Edelman and Joustra formula. The outcomes show that the current approach is not always as conservative as is often assumed. Application of the newly derived safety factor allows levee managers to be more flexible in how to account for the contribution of this failure mechanism to the total levee failure probability. The newly developed method therefore prevents unnecessary rejection and over-dimensioning of flood embankments, so the available resources for flood embankment improvements can be used better.

In this assignment we simulate a two meters wide dike stretch which is subjected to overflow. Three failure mechanisms could occur due to overflow: full erosion of the soil of the dike, macro instability of the levee if too much soil is eroded away, leading to insufficient counterweight on the inside of the levee, and surface slip failure due to reduction of strength of the dike by flow of water in the dike core. Prevention of failure can be achieved by either keeping the water away from the berm surface or holding the soil particles in place while the water flows alongside it. The desired repair is an emergency repair that can placed quickly during a storm, or between two storms to prevent a damaged levee from failing. This is a temporary repair that will later be removed for full reconstruction of the levee. This provides constraints to the installation and repair procedure.
To decide which solution is best, 10 concept solutions are compared based on their score in the Multi Criteria Analysis with 8 weighted criteria. The solution with the highest score is the best solution according to the Multi Criteria Analysis. The selected repair option is to locally cover the damaged area with flexible overlapping sheets of Tyvek®. This is a light and thin material that can easily be transported and cut to size at the location of the repair. These sheets can be easily secured against flow loads using Gripple anchors, while pins are used to keep the Tyvek® in place during windy conditions. An installation plan and cost breakdown was made. Using 3 m wide Tyvek® rolls, the costs would be around 19.19 euros per meter height.
The damaged areas of the levee are covered with Tyvek® which is a waterproof, damp open material. This prevents the penetration of water into the core through the damaged areas. The Tyvek® also prevents the exposed soil from being eroded. Moreover, Tyvek® is a very strong material that won’t be torn off. As there is no strength reduction due to water penetration or further loss of surface material the chance for macro instability is minimized.
First, a design was created, with components dimensioned for a dike stretch in the Hedwigepolder. Subsequently, the design was validated with tests. Including two types of damage. The first was removal of a 2 by 2 m grass layer (0.1 m thick) at the top of the dike and the second was a dug-out step-section across the entire width of the dike with a height difference of 0.5 meters at the toe of the dike. Only the second damage went through the clay layer and into the sand core of dike.
During and after the tests some important observations were made. During all test volumes the plastic sheets were kept in place due to the anchors and pins. Some pins however, came loose due to vibration of the sheets (we presumed the cause of vibration is flow of water between two sheets or turbulence of water). The trench of the lower damage was partly washed away by the water. The erosion of soil in the trench did not result in changing the stability of the top sheet. At the lower damage the plastic sheet was torn off after a tear of 1 m was made with a knife.
Our solution is designed to protect any dike from macro-instability by keeping the water flow away from the berm. This is done by layering multiple sheets of plastic foil over the inner berm, fastened with ground anchors and pins. The solution is a durable and reliable design due to proven waterproof, damp open and UV resistant properties, together with Gripple's demonstrated anchoring capabilities. Another major advantage of our solution is that the repair of the damage is selective, i.e. the solution is applied only to the parts of the levee where damage has occurred. In case of only localized and small damage to the levee, the costs and workload are very minimal compared to other solutions that are applied to the entire surface area of the levee.

This report is not only the final result of the design made for the Infrastructure and Environment Design course (Q4 - Urbanism) but considerably more a representation of the entire learning process and the path towards the goal itself. The aim of this course was to investigate water related urban- and engineering designs in delta metropoles. The international context of the Bayou City of Houston (Texas, USA), its long flooding history, but also the contemporary state of the city made it a very interesting but also challenging task.

In dit onderzoek zal antwoord worden gegeven op de vraag: Moet het falen van gemalen mee worden genomen in de watersysteemtoets? In de 6-jaarlijkse toetsing van de waterschappen wordt bepaald of het watersysteem voldoet aan de door de provincie opgestelde eisen voor wateroverlast. Op dit moment wordt er in deze toetsing geen rekening gehouden met het feit dat gemalen kunnen falen en dit falen invloed kan hebben op de wateroverlast. In dit onderzoek zijn de onbeschikbaarheid van gemalen en de gevolgen van deze onbeschikbaarheid in kaart gebracht. Met deze gegevens is het risico dat gemalen vormen voor het watersysteem bepaald.

Relative sea level rise and climate change represent long-term threats for the ecosystems and flood defence system in the Wadden Sea. Moreover, the flood-dominant tidal basins in the area lead to sedimentation in the navigational channels used by the ferry services that connect the mainland to the Dutch Wadden islands. This report provides flood safety and coastal zone management solutions for the village of Holwerd, in the province of Friesland, Netherlands. High morphological dynamics and biodiversity characterizes this environment. Within this context, the Holwerd municipality, Wetterskip and Rijkswaterstaat were identified as key players to this study from a stakeholder perspective.

Alternative preliminary designs were presented to increase the resilience of the local sea dike and to recommend solutions for improving the ferry connection between Holwerd and the island of Ameland due to current dredging operations considered as excessive and which caused ferry delays. Careful consideration was given to designs due to the Wadden Sea being a UNESCO World Heritage Site and therefore protected under environmental legislation. Therefore, minimal negative environmental and ecological impacts were strived for in design of both the alternative dike designs and ferry connection alternatives. The impact that these alternatives may have on the socio-economy of Holwerd was also assessed where applicable.

Some alternatives were initially eliminated due to fatal flaws and the remaining options were evaluated by means of a multi-criteria analysis. The main criteria considered were the environmental impact, practical implementation, socio-economic impact, safety risk and reliability. Costs were evaluated through a subsequent cost-value assessment. The Wide Green Dike concept was the selected preferred solution as a reinforcement design to withstand extreme wave heights and water levels with a 1000-year return period, including sea level rise projections for the next century. This was achieved by performing an extreme water level analysis and wave transformation using the software Delft3D Wave. The Wide Green Dike concept would therefore ensure that the Holwerd sea dike still sufficiently meets the Dutch flood safety standards in the future. In relation to the navigability of the ferry, channel bend cuts are considered the feasible short-term solution to reduce the sailing time by 10 minutes. The long-term solution, adjusting vessel fleet sizes, potentially decreases the current maintenance dredging volume by 20%. The beneficial use of the dredged material for dike reinforcement and salt marsh development are suggested as integrated Building with Nature solutions to provide additional ecosystem services. Environmental Impact Assessments were conducted to identify potential risks related to each of the preferred alternatives and provide mitigation measures to ensure appropriate project implementation.

In 2010 a large flash flood hit El Arish City, killing at least 6 people and causing a lot of damage. This was not the first time the Sinai was affected by these flash floods, more than 40 flash floods have been recorded in the El Arish watershed in the last 100 years. The enormous amount of water and the flow velocity of these floods are a hazard to its surroundings and to the rehabilitation plan of the Weather Makers. On the other hand the water is scarce in the arid Sinai and the flash floods could contribute to the plan to create a robust water cycle. The underlying goal of this research is to investigate whether a robust water cycle can be created by reducing the flash flood hazard and increasing the available water for the creation of biomass. Measures will have to be taken to start the transition and tackle issues such as drought, unfertile soil, water regulation and to properly address the social benefits to involve the inhabitants of the area. It is therefore essential to assess the hazards of the current and future situation of the Sinai and to apply appropriate measures on the hazardous locations.

                The goal of this research is to propose a strategy to reduce the flash flood hazard, by first identifying hazard prone areas and then propose measures to counter these hazards. This strategy takes into account a scenario on change in catchment hydrology, gives guidance on land use for the region and will propose feasible measures to reduce flash flood hazards. It does not involve the design of the feasible measures nor a cost benefit analysis.

                To assess the hazards in the El Arish watershed, the USACE method is used. The USACE method uses HEC-HMS for the hydrologic modelling and HEC-RAS for the hydraulic modelling. With a DEM sub basins and wadi channels are delineated and measured, precipitation data is collected from the TRMM and analyzed, the retention of the soil is simulated with curve numbers and transmission losses are estimated. The sensitivity of the hydrological model is checked and is used to simulate the 2010 flash flood to compare with imagery of the event. Measures usable in arid areas like the Sinai to reduce flash flood hazards are analyzed on their feasibility and their effects are modelled with the hydrologic model.

The current flash flood hazard is assessed by simulating an extreme event and the region with the highest relative hazard is selected. Measures are proposed based on their applicability and the physical attributes of the surface with a decision tree. The following measures are suggested: spreading dams in flat wadis (<2%), impervious dams in mild sloping wadis (2-5%) and pervious dams on steep sloping wadis (>5%). The effect of these measures combined on the peak discharge is estimated to be 70-75% for an extreme precipitation event. The measures decrease the total- and peak discharge but they also spread the flash flood making the duration of the flow longer. To check whether the suggestions are future-proof a scenario of change in the hydrological cycle is considered by increasing the extreme precipitation with 15%. This causes the peak discharge to increase with 30-35% if no measures would be applied. By applying measures the peak discharge decreases with 60-65%, which is a smaller percentage than without an increase of extreme precipitation (70-75%).

Challenges of the research are that measurement data was lacking to calibrate the conceptual hydrological HEC-HMS model, the large amount of uncertain parameters therefore cause equifinality. The estimated values of the parameters and the results of the simulation are however consistent with literature and gives a good indication on the hazard.

This research shows that the flash flood hazard in the El Arish watershed can be reduced by using feasible measures. The proposed method to reduce the flash flood hazards can contribute to the plan of the Weather Makers to rehabilitate the Sinai by slowing down the flood and to enhance the infiltration. It also creates suitable locations for vegetation by capturing sediment and water this could help to create a robust water cycle.