The Netherlands is a country that is being threatened by water, both from the rivers and from the sea. The Dutch have built dikes to keep their lands from inundation. To ensure the strength and stability of these dikes, they are being assessed on the basis of several failure mechanisms. One of these failure mechanisms is Backward Erosion Piping, or piping for short. In piping, the current underneath a dike is strong enough to take soil particles with it. Tests on piping in tidal subsoil were conducted in the summer of 2021, where a pipe was found to have grown at greater depth than expected The occurrence of this deeper piping has rarely been seen before, let alone described. This lack of knowledge poses a potential safety risk, as it may underestimate the vulnerability of certain subsoil configurations. Therefore, the objective of this thesis is to develop a comprehensive understanding of deeper piping and identify the key parameters influencing its formation. To achieve this objective, a definition of deeper piping and its differentiation from conventional piping is established. Sub-mechanisms governing deeper piping are examined by analysing the forces responsible for grain movement and the forces that maintain grain stability. A Finite Element Model of the subsoil is constructed to quantify the driving forces within the subsoil, which, when combined with resisting forces, enables the determination of whether deeper piping can occur in a given subsoil configuration. To investigate the factors contributing to deeper piping, a series of simulations are conducted using this Finite Element Model. By varying the parameter values while keeping other factors constant, the influence of each parameter on the occurrence of deeper piping was examined. The analysis revealed that several key parameters significantly affect deeper piping formation, including cohesion force (P), cohesion anisotropy (?P ), permeability and thickness of the top layer (X0 and 70, respectively), permeability of underlying layer (X1), permeability anisotropy (?X ) and representative grain diameter Q_R]. Also, it was found that the entrance configuration plays a large role in deeper pipe formation. These findings provide valuable insights into the mechanisms underlying deeper piping and enhance our ability to identify subsoil configurations that are prone to this phenomenon. These findings enhance the identification of subsoil configurations prone to deeper piping, thereby improving risk assessment and mitigation strategies associated with this failure mechanism.

Due to climate change, a new water management policy is considered in the Netherlands. The shift towards greater extremes in both wet periods and dry periods requires a more dynamic policy. It is investigated what effect the placement of a permanent water barrier in the Rijnmond has on the water levels in the Rhine branches, and if this can be an opportunity to help the drought prone area in the east of the Netherlands.This study shows that replacing the Maeslantkering with a permanent barrier has no influence on the water levels upstream of Nijmegen in the Waal at average to low Rhine discharges (2000 - 600 m^3/s), and thus has no effect on the water distribution at the Pannerdensche Kop. If the fresh water demand to stop the salt wedge is eliminated, a different water distribution over the Rhine branches is possible, but structural changes at the bifurcation point have to be made in order to achieve this. River water can be used to supply the east of the Netherlands with water. In this study, three options to do so are examined. The most promising options of these is placing a pumping station in the Rhine nearby Lobith and supplying the streams in the area with water, many of which fall dry in summer. From there, the water can be used for irrigation, infiltration, and other fresh water demands, relieving the stress on the groundwater during dry spells. Building a permanent storm surge barrier is not a requirement, but because it increases the navigability of the Waal river, water can be extracted more often with a permanent barrier. A solution to combat drought in the east of the Netherlands seems to be more urgent than a solution to mitigate sea level rise. However, a permanent barrier can help in mitigating the effect of drought on groundwater levels in the east of the Netherlands by allowing for more flexibility in using river water extraction.

The water levels along the dike of Kampen are affected by the magnitude of the discharge on the IJssel and the water level set-up caused by the wind characteristics on the IJsselmeer. The Kampereiland has been assigned as a retention area to reduce the water levels near Kampen during a storm. This area should store excess water during a storm at the IJsselmeer of a magnitude corresponding to an annual exceedance probability of 1/500, to prevent this water from flowing in the direction of Kampen. The Kampereiland is protected by dike trajectory 225 and a part of this trajectory has been made resistant to the overflow of water. According to new insights, the crest of this dike section seems too high and the Kampereiland inundates less frequently. Furthermore, not sure is whether the water level at Kampen benefits from the inundation of the Kampereiland and what the consequences are for surrounding dikes. The Kamperzeedijk is one of these surrounding dikes and protects the city IJsselmuiden, along with the polder behind. The influence of the crest height of this overflow resistant part on the water levels at Kampen and at IJsselmuiden is investigated. The most important assumptions are that dikes do not breach and that the water level at Kampen remains constant after water flows into the Kampereiland. The influence on the water levels at Kampen and at the Kamperzeedijk is evaluated for multiple crest heights of the overflow resistant dike section. Eventually, these water levels are processed with a statistical model to compute multiple water level exceedance frequency lines for each location and for various chosen crest heights of trajectory 225. The results show that higher crest heights of trajectory 225 lead to increased maximum water levels at the Kamperzeedijk and to lower maximum water levels at Kampen, and vice versa. The water levels in front of the overflow resistant dike section remain unaffected. The return periods for overflow at Kampen and IJsselmuiden are derived by comparing the water level exceedance frequency lines to their individual crest heights. Concluded is that the inundation of the Kampereiland does not immediately lead to a load increase in front of the Kamperzeedijk. A delay is observed as a result of the storage capacity of the Kampereiland. As soon as water from the Kampereiland flows into the Ganzendiep, the loads rapidly increase. Furthermore, concluded is that the Kampereiland is capable of reducing the water levels at Kampen. Based on the results from this research, a crest height of 2.7m +NAP leads to equal return periods for overflow at trajectories 10-2 and 11-2. The reduction of the crest height by ten centimeters increases the return period for overflow at Kampen from 30,000 years to 45,000 years. At the Kamperzeedijk, the return period is reduced from 80,000 years to 45,000 years. In reality, these return periods are also affected by other failure mechanisms. A full dike assessment has to elaborate on this. Furthermore, trajectory 225 is assessed whether it meets its safety standard. It would be more legitimate to assess trajectory 225 by explicitly quantifying its consequences on trajectories 10-2 and 11-2

In most current dike assessments only the stationary water levels are investigated in the assessment of the stability of the inner slope, while there are differences for all kind of dikes between the stationary and transient pore water pressures and therefore in the stability. This results in a conservative probability of failure, while determination of a probability of failure should not be conservative but should be as realistic as possible. When time dependency is included in a calculation, an average flood duration is used, while the flood duration is highly variable. The following research question is defined to address the problem: “What is the influence of the flood duration on slope stability and in what degree affects the flood duration the design?” The degree of influence of time dependency on the pore water pressures and slope stability depends on dike characteristics, flood wave characteristics and the delay in failure. The basis for answering the research question is the software SEEP/W to model the time dependent pore water pressures and the software SLOPE/W to calculate the safety factor for the stability of the inner slope. In the research theoretical dike are used and there is focused on the flood waves in the Rhine and Meuse. A correlation analysis is performed to get insight in the contribution of different flood wave shape variables to the safety factor. And a probabilistic analysis is performed using transient and stationary water levels to know the differences in probability of failure between taking the shape of a flood wave into account or not. In both probabilistic analyses is varied in the permeability and the strength of the material; the shape of the flood waves is varied in the transient analysis. In this way the contribution of the flood to the probability of failure can be quantified. Dike characteristics The differences in pore water pressure are especially large for dikes that consist of an impermeable material such as clay. When only the subsoil consists of clay, larger differences are expected than when only the dike body consist of clay. However, large differences in pore water pressures do not necessary lead to large differences in the safety factor. The largest differences in safety factor are obtained when uplifting of the hinterland takes place during the stationary state and/ or during the passage of a flood wave. A transient calculation is therefore most useful for dikes with an aquifer and a thin (thinner than 5 m) weak (low POP values) hinterland. Flood wave characteristics The differences in safety factor during a permanent water level and the passage of a flood wave are large when no stationary conditions are reached during the passage of a flood wave. This is the case for high and short flood waves. Both in the Rhine and Meuse, the amount of short waves (< 7days) is high, which increases the influence of a time dependent calculation. Also, the importance of a time dependent calculation increases when the response to the increased pore water pressures is delayed caused by the permeability of the material. The influence of the height of a flood wave on the stability increases when the soil is permeable. Delay in failure Time dependency causes failure of the embankment to not occur simultaneously with the maximum wave height. The flood wave is decisive for the dike failure, but the permeability and the strength of the dike determines the moment of failure. Influence on design Taking time dependency into account leads to higher safety factors and lower probabilities of failure with exception for dikes that consist completely out of sand. For these types of dikes, the probability of failure and safety factors are the same order of magnitude. This could affect the design, because the dikes are safer when time dependency is considered. The strength of the material is the largest contributor to the distribution of safety factors and therefore to the probability of failure (60-95%). Whereas the contribution of the permeability to the probability of failure is small (2-12%), the variation in the height and duration of a flood wave contribute for 2-20% to the probability of failure. In a permeable dike this contribution is mainly determined by the height of a flood wave, while in an impermeable dike the duration of a flood wave is of importance. Considering the influence of time in stability probabilities of failure, this research proved that probabilities of failure taking the duration into account differ significantly from stationary calculations. It is therefore useful to take time dependency into account when determining the correct safety factor for impermeable dikes, but it is not useful in determining the correct safety factor for permeable dikes, because a stationary calculation is sufficient. In clay dikes it is useful to take the variation in height and duration into account, while for a sand dike it is sufficient to only consider the variation in height of a flood wave. When the variation of the duration of a flood wave is not considered, it is recommended to use a representative duration of a flood wave; that results in the same total probability of failure as when the variation of the duration is included. At Lobith the duration of the representative flood wave varies from 13 - 16 days. At Borgharen the representative duration varies between the 10 – 11 days for different dike types.

Changes in the Earth’s climate system do have consequences for the river discharges. The development of the future climate is uncertain, which causes uncertainty in the future extreme river discharges. For flood protection considerations, it is important to quantify the climate uncertainties and its effects on extreme river discharges. In this study the uncertainty in extreme river discharges due to Climate Change is quantified. The results indicate the uncertainty in discharge return levels given a specific forcing scenario. Subsequently, the discharge return levels of the KNMI climate scenarios are compared with the found results for the forcing scenarios. The study also provides the consequences of the uncertainty in discharge return levels on the design water levels. The results give the uncertainty in the design water level and the additional height in design water level required to take account for this uncertainty.

One of the failure mechanisms for water retaining structures is piping. Piping is an internal erosion
mechanism creating hollow spaces (pipes) underneath, for example, a dike as a result of the transport
of soil particles due to seepage. The formation of pipes can cause collapse of the structure once the
erosion process reaches the outside of the dike. This study focusses specifically on the possibility of
the occurrence of this failure mechanism in the Maasvallei area. The Maasvallei covers the area of the
Maas roughly between the Dutch towns Roermond and Mook. The early signs of piping in the form of sand boils are frequently observed during periods of high water levels in the Dutch rivers. Although the total collapse of a dike in the Netherlands due to piping has
not occurred in the past decades, the frequent observation of the early signs of the piping process
has resulted in the inclusion of the piping failure mechanism in the Dutch legal safety assessment
regulations for water retaining structures. During recent high-water periods in 2011 and 2012 sand boils
were observed along several Dutch rivers except at the dikes along the Maas. The striking absence of
sand boils in the Maasvallei area raised the question if the failure mechanism piping is relevant for this
specific area. In the recent assessment of the Dutch dikes, many of the dikes along the Maas in Limburg were found
to be insufficiently safe against piping. The dilemma then becomes clear: the lack of early signs of
piping contradicts the outcome of the safety assessment. Resources could be saved if the dikes do
not need to be reinforced for piping, however, the safety should not be compromised. Within the thesis this dilemma is studied. The main question that is answered is: Is dike-failure due to piping realistic in the Maasvallei?

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.