River dikes in the eastern part of the Netherlands are characterized by a relatively low daily water table. In the unsaturated zone above the water table, a negative pore pressure (matric suction) is present that binds the soil particles together. Suction can be especially large in soils with small grains, such as clay, and increases the strength of a soil due to higher effective stress. High suction in soils has been proven to significantly increase the stability of dikes. However, due to water level variations and other climatic influences such as precipitation and evapotranspiration, suction in a dike will vary over the year and throughout the cross section. The research described in this report aims at evaluating whether there are parts of a cross-section of a dike where soil suction remains throughout a high water event in a river such that an increase in the strength of the soil can be used in stability analyses. The analysis is centered around a case study of an existing dike in the east of the Netherlands and uses time-dependent unsaturated groundwater flow models to simulate the spatially and temporally varying suction for a number of different initial conditions and climatic influences defined by scenarios. The results show that it is very unlikely that suction in the dike will be lost during the summer months. Due to high evapotranspiration, the water content in the dike is low, which results in a lower water table during a high water event and less infiltration in the dike occurs during precipitation. During winter, the probability that suction is lost throughout the dike cross-section is larger and will occur after heavy precipitation with a long duration where the return period is 1 – 10 years. Although in most scenarios some suction will remain present, the amount of suction during the winter is generally low, between 0.5 and 1 meter, which results in an apparent cohesion of 2 – 4 kPa. A simple stability calculation illustrates that the increase in strength in the dike due to suction does not result in a significant increase in the stability of the dike during high water conditions.

Fieldwork Hydraulic Engineering’ is a course given at Delft University of Technology for the MSc Hydraulic Engineering. In collaboration with local experts Boyan Savov and Traian Marin, a team of 8 students guided by Mark Voorendt was sent to investigate the local conditions at Asparuhovo beach in Varna, Bulgaria, for purely educational purposes. Before 2019, Asparuhovo beach used to have a stable coastline with some seasonal variations. However in 2019, the Karantinata port was constructed and disturbed the equilibrium state of the beach. Rapid sedimentation occurred near the port and the port entrance. Due to this excessive sedimentation near the port entrance, the fishing port has lost almost all of its intended functionality, as minimum water depths in the port entrance approach 0.3m. The port was originally designed for larger fishing boats, which are currently not able to enter and making the port lose functionality. It is yet unknown how this sedimentation trend is formed with the construction of the fishing port. The main objective of the research was to examine the current sedimentation near Karantinata port by executing a measuring campaign during the Hydraulic Fieldwork and by setting up a 5 year monitoring program for the marine environment. By doing so, the processes which lead to sedimentation can be understood and a model can be made. With this model, adjustments to the port layout can be examined which are potentially needed for the port to operate at full functionality. To tackle these problems, the students performed the fieldwork. With 2 days of beach and foreshore measurements the research question was assessed. With the acquired data of the system, supported by additional lab sieving analysis, data processing and modelling in Delft, the students formed theories on the origin of the sedimentation problem. With these insights, recommendations for the area can be suggested. The measurements are performed on multiple locations and at each location multiple variables were researched. For Asparuhovo beach and foreshore these are the bathymetry, waterline position, wave climate, beach profile, sediment characteristics and ecology. At the fishing port Karantinata these are the port characteristics such as functions, planning and infrastructure, port entrance, bathymetry and breakwater design. At the Asparuhovo breakwater it is the top protection layer, damage assessment and measures of improvement. At Veteran beach this is the soil samples for grain size distribution. At Martsiana quarry the length to thickness ratio, blockiness were researched, as well as the diameter to check if potentially suitable for breakwater material. With this information, a preliminary model in Delft3D has been set up with the land boundaries, grid and bathymetry file. With knowledge of the coastal processes and the processed data, potential causes of sedimentation are speculated on. It is unlikely that such large amounts of sediment are coming from outside of Asparuhovo beach system, as there are no sediment rich rivers nearby, the sedimentation occurred in a very short period of two years and the sediment would mostly not be able to cross the deep navigation channel as it would settle due to lower flow velocities. It is expected that large parts of the settled sediment near the port entrance is from the beach itself. This is also more likely due to the two closed boundaries of the beach, the Asparuhovo breakwater and the Karantinata port. This was checked by analyzing the grain size diameter compared to other locations at the beach and looking at the waterline developments. It was found that the grain size at the middle of the beach was 1.8 mm and at the port entrance between 0.2 and 0.3 mm. It was suspected that the fine sediments of the middle of the beach are eroded and deposited at the port entrance as the sediment can settle at the Southeastern part behind the port breakwater due to sheltered conditions. The mechanisms that could have induced this are: rip currents, longshore currents and the different wave patterns. To examine and validate these findings, a monitoring plan for the coming 5 years is proposed. This is very important to create an understanding of the systems parameters and behaviour. Without monitoring, adjustments to the port cannot be tested in a correctly calibrated model. The parameters which need continuous measurement are the wave parameters, sea level measurements and visual beach observations. Biannual measurements are needed for currents, hydrographic works, visual observations with a drone, bathymetry and sediment parameters. Before the port is fully operational again, the port entrance needs dredging. This can be done in this time span of 5 years to ensure the passage of fishing boats. After dredging a short survey of the area needs to be performed to incorporate the changes into the model. These are the bathymetric survey, visual observations and sediment samples all around the dredging area. There are three potential solutions incorporated in the report, which can be modelled with the findings of the monitoring plan. The first potential solution to make the port fully operational again is relocating the port entrance with a curved breakwater stretching into the sea. The second solution is a combination of the entrance relocation and water flow through the port. The third is a blocking groyne stretching from Asparuhovo beach into the sea, blocking the sedimentation going into the port entrance. For all these potential solutions, dredging works are needed to reensure the required water depth for the vessel draught. The least costly and most promising is the relocation of the port entrance design. It can be noted that continuous dredging is not a sustainable solution as the sedimentation keeps occurring near the port entrance as the hydrodynamic conditions will not change. It is recommended to first find the source of the settled sediment by comparing control volumes of sediment on the beach over the years. Then a model should be set up of the Asparuhovo beach and foreshore and Karantinata port to give insights in the processes. This model needs to be validated and calibrated with input from a monitoring campaign. With a working model, the causes of sedimentation can be found and further research can be done whether the potential solutions are appropriate.

Extreme value analyses (EVA) are often used to determine the frequency of extreme events. The length of the available observations is an important aspect when performing EVA. It is generally known that more available data results in better estimates with less uncertainties. The main objective of this research report was to assess what the influence of the length of the observations is when inferring rare events. This was done by first analyzing the sensitivity of inferred return levels from synthetic data from three known distributions. Also, three case studies were analyzed to observe the sensitivity of inferred return levels and return periods from the observations. The results of the analyses were that a larger sample size generally leads to a higher confidence in the estimates of inferred return levels from synthetic data. However, there will always remain some uncertainty associated with the estimates. The confidence in the inferred return levels from observations also generally increase for an increasing sample size. However, this can not always be observed and other aspects can be more dominant than an increasing sample size. No correlation could be observed between the sample size and the inferred return periods. The conclusion of the research was that, while there is a positive correlation between the sample size and the confidence in the estimates, there will always remain some uncertainties. It is therefore important to always communicate the uncertainties associated with estimates.