It is observed that the bed level development of the Pannerden Canal (PC) and the Waal changed since the peak flows of 1993, 1995 and 1998. The erosion rate of the Pannerden Canal decreased greatly just downstream of the bifurcation while the erosion rate of the Waal stayed more or less constant. This leads to an increasing depth of the Waal relative to the Pannerden Canal and therefore the Waal attracts more discharge. The model results from this study confirm that without anthropogenic interventions in the future, the Waal discharge fraction keeps increasing. The shift in discharge partitioning towards the Waal is an undesired development of the system as it causes problems for navigability, ecology, freshwater distribution and flood safety. Currently there is a lack of knowledge on how interventions can be used to mitigate the gradual change in discharge partitioning at the Pannerdense Kop (PK). This study aims at filling the knowledge gap by considering several interventions studying their effect on the discharge partitioning at the PK and on the Rhine branches surrounding the PK.

Firstly, an inventory of interventions which could possibly increase the Waal discharge fraction is made. The resulting selection of interventions is: A) a side channel from the Boven-Rijn towards the PC, B) fully opening the weirs in the NR-Lek, C) widening on the IJssel, D) directly steering the sediment partitioning at the PK and E) dredging in the PC and dumping in the Waal. These interventions are implemented in a 1D-morphological model of the Dutch Rhine branches.

It is found that the side channel from the Boven-Rijn to the PC (A) is not capable of significantly influencing the discharge partitioning at the PK. The backwater effects due to the water addition counteract the added discharge to the PC.

Furthermore, it is found that for the low Lobith discharges opening the weirs in the NR-Lek (B) can reduce the Waal discharge fraction below the initial value from the reference case for the whole simulated period of 60 years. The opening of the weirs does however lead to significant relative decrease in the IJssel discharge fraction which is unwanted regarding the freshwater supply of the IJsselmeer. For low Lobith discharges the widening on the IJssel (C) initially leads to a significant reduction in Waal discharge fraction of 2.4%. The effectivity reduces throughout the simulation because of aggradation along the widened reach. For high flows the opening of the weirs and the IJssel widening are less effective. The final relative Waal discharge fraction decrease is respectively 1.1% and 0.7%.

The results of the directly steering the sediment partitioning (D) are similar for high and low flows. The water level is not changed initially and therefore initially the discharge partitioning is not affected. Throughout the simulation the slope of the Waal adjusts to the increased sediment supply and the slope of the PC adjusts to the decreased sediment supply. For both flows the final relative decrease in Waal discharge fraction is 1.8%.

Dredging and dumping (E) shows similar effects on the bed level and discharge partitioning as steering the sediment partitioning at the PK. Dredging and dumping also does not alter the water level and the discharge partitioning initially. The relative bed level increase of the Waal is somewhat higher for the dredging and dumping simulation than for the simulation in which the sediment partitioning at the PK is steered. Therefore also the relative decrease in Waal discharge fraction is higher with 2.4% for the low Lobith flows and 2.2% for the high Lobith flows.

Currently the IRM program aims at stopping further skewing of the discharge partitioning of the PK. None of the interventions from this study is capable of realising this on its own. Therefore multiple river interventions will have to be combined to be able to reach the ambitions of IRM. The knowledge on the effect of the interventions on the discharge partitioning at the PK and the bed levels of the Dutch Rhine branches can be used as a basis for further study towards to combined effects of river interventions on the bed level development of the Dutch Rhine branches and the discharge partitioning at the PK.

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.

Sediment management is necessary to keep the Waal navigable. Sediment management in the Waal is carried out by both dredging and plowing of the river bed. By analysing historic data available at Rijkswaterstaat, the relation between the hydrograph and the amount of required dredging and plowing is investigated. Also the influence of the hydrograph on dredging and plowing locations are investigated. The locations of dredging and plowing are placed into context of the river geometry. The different results for dredging and plowing are compared to each other and substantiated.

River bars are large-scale bedforms formed by the local deposition of sediments, the length of which scales with the channel width and the height with the water depth. They create suitable habitats for aquatic fauna and riparian vegetation, making them useful in river restoration projects. Understanding the dynamics of river bar development is required for a proper design of the restoration project and for developing a sustainable management scheme. The channel half-width-to-depth ratio is the key controlling parameter for the formation of alternate bars. Existing stability analyses define a resonance point and critical width-to-depth ratio, marking the transition between the bar regimes. Seasonal variations in the river discharge and the propagation of flood waves make the half-width-to-depth ratio a time-dependent parameter, varying over periods of days to months. This research aims to create a better understanding of the development of alternate bars in varying discharge conditions. For that, insight into the timescale of development of the bars is necessary to link this with the timescales associated with natural discharge variability. A straight river channel is modelled in Delft3D, with non-erodible banks to which a fixed perturbation is added, to trigger the formation of free bars and to which hybrid bars become fixed. In the first set of simulations, the upstream boundary condition is a constant discharge, with magnitudes between 50 and 1250 m$^3$/s, corresponding to a half-width-to-depth ratio between 77 and 8. The different types of alternate bars and the time to develop the bars are analysed from this constant-discharge analysis. The damping length and wavelength of the hybrid bars showed good agreement with the values determined from the theory of Struiksma et al. (1985). Free bars became suppressed when a hybrid bar pattern developed, showing that hybrid and free bars do not coexist locally. The resonance point is clearly obtained and assigned to a half-width-to-depth ratio of 37-38. Contrary to existing stability analyses, the critical half-width-to-depth ratio is marked by a gradual transition at half-width-to-depth ratios from 11 to 20 instead of a single value. In this transition, free bars develop with intervals of a few months. A clear difference in the development of hybrid bars and free bars results in a shorter timescale of development for free bars than for hybrid bars, which is in line with laboratory experiments (Fujita and Muramoto 1985; Crosato et al 2010). Also, in the mathematical derivations for describing the two types of bars, the distinction between an initial response (free bars, Colombini, Seminara, and Tubino (1987)) and a final stable bed (hybrid bars, Struiksma et al. (1985)) is considered. The numerical results show that free bars develop fastly when the bed becomes unstable, in the case of a relatively flat bed. The associated timescale shows little variation over the different simulations and stays around 20 days, which is in the same order of magnitude as the timescale related to seasonal variation. A pattern of hybrid bars develops in downstream direction, starting at the fixed perturbation. Based on the definition of the damping length by Struiksma et al. (1985), a theoretical expression is defined, which gives a good fit to the time at which a pattern of hybrid bars has developed. The transition of alternate bars around resonant conditions is modelled using a steadily increasing and decreasing discharge. The distinction between the timescales of hybrid and free bars results in a different response of the bars to a varying discharge. The first response of the bed in the transition from subresonant to superresonant conditions is the adaptation of free bars towards the new bar regime. A pattern of hybrid bars follows, developing in downstream direction. The reversed transition is governed by a migration of the hybrid bar pattern, making room for the damped bar pattern. The difference in the transitions between rising and falling discharge in combination with a bed adaptation after a temporal in- or decreased discharge led to a type of hysteresis in the transition around the riverbed's resonance point, which was not earlier detected. Hysteresis is ascribed to the increased bed stability when a hybrid bar pattern has developed, suppressing the formation of free bars in the transition from superresonant to subresonant conditions and to the time-lags associated with lower discharge conditions. This thesis assesses the timescales of alternate bars and the transition around the resonance point. The numerical model is able to assess both free and hybrid bars and therefore gives valuable information in the difference of their development. The difference is linked to the (linear) theories describing the bars and applied for answering the research question, showing the development of free bars should be largely influenced by discharge variation due to seasonal variation. The varying discharge simulations provided insights into the response and transition of alternate bars around resonance conditions, leading to a type of hysteresis which was not detected earlier. Recommendations are made towards increasing the river geometry complexity and discharge variation and the assessment of the transition around critical conditions to verify analyses around critical conditions.

Inland flooding along with its casualties is expected to increase. Climate change, urban expansion on floodplains, degraded ecosystems due to human interventions, lack of effective emergency flood risk management plans contribute to flood intensification and risk exposure. The heretofore flood mitigation practices seem to stop short of both mitigating flood risk and providing further economic, social, and environmental benefits. Instead, practices co-working with nature and its physical processes are constantly gaining ground.
Nature-based solutions (NBS) are measures encompassing the ‘co-operation with nature’ approach; mitigating fluvial flood risk while being cost-effective, resource-efficient, and providing numerous environmental, social, and economic benefits. Current progress is working on strengthening the evidence-base of NBS projects through multiple on-going projects. Such evidence would lay the foundation for the mainstreaming and transferability of the NBS practice that will, eventually, lead to its upscaling.
To this end, there is need for holistic and coherent organisation of all the NBS information and approaches acquired in practice. Therefore, several frameworks have been developed in an attempt to map the NBS design and implementation and/or evaluate them using indicators. Contribution of NBS to sustainable development has also been established since the social, economic and environmental sectors, on which NBS have an impact, constitute the so-called “triple bottom line” of the sustainability concept.
Given the unequivocal link between NBS and sustainability, a growing interest recognizes the need to explore a NBS connection to the Sustainable Development Goals (SDGs), as set by the United Nations Organization. Additionally, since NBS seek to complement conventional infrastructure, their engineering characteristics should also be assessed.
Therefore, this research project develops a framework for assessing the NBS performance for fluvial flood mitigation. The framework adopts an integrated approach, using indicators for evaluative and benchmarking purposes. Novel aspects of the framework are the direct link between the NBS for fluvial flood mitigation and the SDGs and the introduction of technical assessment indicators. Ultimately, the NBS framework is meant to assist experts in providing guidance for similar projects and contributing to the mainstreaming and transferability of the NBS practice.
The framework was guided by a review of existing NBS assessment frameworks and methodologies, already realized NBS projects, and the UN 2030 Agenda. The assessment frameworks and methodologies worked as a starting point both for the structure and the components of the deliverable framework. Already realized NBS projects were treated as case studies calibrating the literature findings with what was applied in practice. The UN 2030 Agenda was used for developing indicators that address contribution to the SDGs. Finally, the framework was tested by applying it to an already realized NBS project, the Eddleston Water Project (different from the calibration ones), by giving the project metadata as input to the framework indicators.
Overall, the framework performed well as most of its indicators are having indicative data that were found in publicly available references. Expert consultation was needed for acquiring some of the data but was kept to a limited extent as much as possible. Additionally, some overlaps between the needed data per framework indicator were observed that can probably be attributed to the scope and current state of the Eddleston project. Regarding the Eddleston project it was found that meets its initial objectives, contributes to SDGs 1, 6, 8, 9, 11, 12, 13, 15, and 17 while enhances several provisioning, regulating, cultural, and supporting ecosystem services of the Eddleston Water.
To the downsides of the framework, its testing with only one case study due to the time frame of the project. Furthermore, it was developed and tested focusing on the developed world. Additionally, the link between NBS and SDGs is based on the author’s self-explanation and gained insights throughout the project. Hence, a more experienced person might create different links.
In line with the aforementioned limitations, future recommendations suggest further testing with ranging, both in scale and in location, case studies. Expert judgment could also be used as another testing way by examining experts’ suggestions and willingness to use the framework. Finally, the inclusion of the SDGs in the indicators offer several opportunities: (i) re-evaluating potential NBS-SDG links and (ii) examining framework changes in case of considering the developing world.