Implemented in river landscapes, Nature-based Solutions (NbS) have the potential to reduce flood risk, while also playing an important role in restoring many of the ecosystem services that are lost as result of human interventions and global warming. A barrier to succesfull implementation of NbS is the lack of a global and common framework with guidelines for its implementation and evaluation. In response, the IUCN Global Standard for NbS was published. Even though the IUCN Standard has been designed to be applicable to NbS in all sectors and over the entire globe, knowledge on its applicability and usefulness for specific sectors remains limited at present. Therefore, this study aims to identify the challenges that occur in ex-post application of the IUCN Standard to river restoration projects with a focus on flood risk mitigation, and the added value that this application may provide.

At first, a literature study is conducted in which the content of the IUCN Standard is related to twenty-two other assessment frameworks for NbS and compared in-depth to the three most relevant frameworks. These comparisons indicate that that the IUCN Standard has a broad scope of application, provides limited flexibility in assessment to its users and is descriptive. Furthermore, the comparisons reveal that the IUCN Standard can be used as a tool to evaluate the extent to which the essential processes of a NbS, established by the IUCN, have been incorporated in the project (i.e., process-oriented framework). The standard can, however, not be used to evaluate project results, including biophysical and social results.

In the next part of the research, the IUCN Standard is applied to three case studies of river restoration projects with a focus on flood risk mitigation, of which at least two differ significantly in the surface area, position in the catchment, kinetic energy of the river, data accessibility, resources and the type of riverine NbS measures that were implemented. The selected case studies are the Eddleston Water Project, the “Room for the River” Deventer Project and the Missouri River Levee Setback Project. Despite facing challenges in the interpretation of indicators, data accessibility, a relatively coarse scale for evaluation, and tensions between the project objectives and the use of the IUCN Standard, the standard is successfully applied to all case studies. The case study results consist of (1) a total percentage match to the IUCN Standard and a statement on being in adherence to the standard, which may provide credibility to the project processes, (2) the strenghts and weaknesses of a project, which may be used to guide projects and strengthen (future) projects, and (3) a radar chart, which may provide possibilities to compare to and learn from other projects. Based on these results, it can be concluded that, despite of a few challenges, the IUCN Standard is applicable to river restoration projects with a focus on flood risk mitigation, and that application of the standard may provide added value in various ways, although restricted by the limited evaluation of flood risk mitigation.

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.