River networks are vital ecosystems that face increasing threats from societal pressures and global environmental changes. Effective biomonitoring of these systems is crucial for maintaining water quality and biodiversity by guiding policy and management actions. Over the past two decades, environmental DNA (eDNA) has emerged as a promising tool, offering advantages over traditional biomonitoring methods. However, the downstream transport and decay of genetic material in river networks poses a challenge for the spatial interpretation of eDNA samples. This study follows from work of Carraro et al. (2021) and aimed to find the optimal sampling density to determine taxon distributions at the catchment scale based on eDNA sampling. This was done by means of a virtual experiment, where a large number of sampling campaigns were simulated over multiple synthetic taxon distributions and river networks. This enabled the development of generalized sampling principles irrespective of system-specific confounding factors. Results showed that taking at least one sample per 10 km2 of catchment area provides a reliable estimate for taxon abundance across the catchment. Taking more samples only marginally improved the accuracy of the predictions when it concerns taxa that are commonly found in different parts of the catchment. In case of finding rare taxa, a minimum of two samples per 10 km2 is necessary to ensure that the taxon is correctly located. This study also underscores the limited capability of eDNA in capturing fine- scale spatial patterns, suggesting its complementary role alongside traditional surveys. In summary, this study provides a generalized sampling guideline for eDNA biomonitoring as a tool for broad-scale environmental assessments and biodiversity conservation.

Rivers play a crucial role in shaping landscapes and supporting ecosystems. This is demonstrated by the tiger habitats in and around Bardia National Park in West-Nepal, which rely on the Karnali River. This study contributes to a larger effort aimed at sustainably managing these tiger habitats. Monitoring the rivers in this remote area is challenging, suggesting a role for remote sensing. An exploration is presented regarding the potential of satellite synthetic aperture radar altimetry (sat-SARA) for monitoring rivers situated in diverse topographic landscapes. Focusing on the Bheri, Karnali, and Geruwa Rivers, the applicability of sat-SARA techniques for water level monitoring, multiple channel identification, and channel activation detection was evaluated. For deriving water surface heights from sat-SARA data, an empirical Gaussian retracker was used. The findings are promising. While resulting water level variations align with field observations, complementary in-situ measurements are imperative for a comprehensive evaluation. Additionally, the study reveals the potential for identifying multiple channels from sat-SARA return signals, extending to channel classification and detecting channel activation. Leveraging the labour-intensive nature of sat-SARA data processing, the technique holds great promise for monitoring rivers in remote and difficult-to-access landscapes. Therewith, this study contributes to advancing the understanding of the hydrodynamics of the Lower Karnali River and opens doors for sat-SARA applications for river monitoring in challenging terrains.

To understand the functioning of a river system it is crucial to have information on the sediment transport. Estimates of long-term sediment fluxes in the Rhine at the German-Dutch border are required to understand the morphological evolution of the Dutch Rhine Delta. Especially information about the long-term flux of bed material load is of interest because this flux is morphologically relevant. Bed material load interacts continuously with the river bed and it influences the bed surface texture and river slope and width.

The objective of this research is to gain more insight in the amount of bed material load at Lobith (at the German-Dutch border) and in the free-flowing Rhine, the region upstream from the border. To accomplish this, a literature study on the definitions of wash load and bed material load is carried out and different methods to make a distinction between the two are discussed.

This study shows that the wash load cut-off size – the sediment grain size that marks the boundary between wash load and bed material load – is a dynamic variable that depends on the local flow and/or bed characteristics. Therefore, the wash load cut-off size is different for every river and can also change within a river system. It is suggested to set the cut-off size for a specific location equal to the diameter for which 1% of the river bed grains at that location are smaller (D1).

D1 as a cut-off size and sediment transport data based on direct measurements (Frings et al., 2014a,b) give an average annual flux of bed material load of 0.5 Mt/a or 200,000 m³/a at Lobith for the period 1991-2010. It should be noted that there are large uncertainties in these numbers. First of all there are large uncertainties in the used sediment data. Second, multiple assumptions have been made in the process of distinguishing between wash load and bed material load.

The last part of this research contains a comparison between the results of this study and the sediment transport determined by three existing computer models. The model results did not differ that much from the results of this study. The results of this study were especially close to the results of the model for which the input was almost identical to the conditions under which the measurements – on which the results of this study are based - took place. It can be concluded that at least the order of magnitude of the bed material load estimated at the German-Dutch border is probably correct.

The hydropower development in the Mekong Basin affects the silt and clay concentrations in the river which has ecological as well as socio-economic effects on the Mekong Delta and its inhabitants. Therefore, the first objective of this research was to obtain a better understanding of the effect of hydropower development on the silt and clay load in the Mekong Delta. Subsequently, the objective was to determine possible sediment management strategies to mitigate the impacts on the silt and clay load in the Mekong Delta.

A conceptual model was developed and applied to analyse these impacts. By means of a sediment balance model with distributed sediment yield and individual trapping efficiencies of dams the annual sediment load has been determined.

This research has shown that the annual silt and clay load downstream has decreased from 140 Megaton ± 40 [Mt] before hydropower development to 26 ± 9 Mt in 2020. Including future hydropower dam projects, this could further decrease to 4-8 ± 3-5 Mt in 2040. Approximately 4% of this sediment load will deposit in the Mekong Delta and therefore it is predicted that the clay and silt deposition will be close to zero in the future.

By 2040, approximately 75% of the total trapped sediment load is trapped in mainstream dams even though they account for 16% of the total dams. The majority (50%) will get trapped in Chinese dams that account for 9% of the total number of dams. This means that almost all sediment coming from China is trapped and will not reach the Mekong Delta. Around 80-85% of the future sediment load that will reach the delta has its source in the 3S region.

This reduction has implications for the sustainability of ecosystems and the productivity of the Mekong Delta. Clay and silt are nutrient rich alluvial sediments that support the ecosystems in the floodplains and delta. These nutrients are crucial for the soil fertility vital for rice cultivation and for various biological processes for fish. Today, nearly 80% of the 20 million people living in the Mekong Delta depend on the river system for their livelihood. However, the rice cultivation is expected to decrease significantly and by 2040, the loss of fisheries could cost close to $23 billion.

Thus, the computed reduction displayed that sediment management strategies are inevitable to maintain the productivity of the Mekong Delta in the future. Therefore, two sediment management strategies were reviewed in this research: cancelling planned dams and reservoir sluicing. Reservoir sluicing is passing incoming sediment-laden water through the reservoir by discharging high flows to enable sediment to move past the dam without depositing.

The analysis of these two strategies showed that the maximum sediment load increase is 1-3 Mt as a result of cancelling planned dams and 2-3 Mt due to reservoir sluicing. This means a very small increase for the deposition in the Mekong Delta (4% deposits). Therefore, the sediment load increase was considered unsatisfactory, suggesting that these strategies are not providing the solution for the Mekong Basin.

The results indicate that effective sediment management strategies are urgently needed and that more research is needed into the effects of other strategies on the silt and clay load in the Mekong Delta.

Humans have a long history of intervening in natural river systems. These human interventions affect how water and sediment interact with the bed and banks of the river. The interaction between water and sediment is called morphodynamics and is an important component of the natural ecosystem of the river and its surroundings. This research aims to investigate if human interventions have disturbed the natural morphodynamics in the Karnali fluvial fan and if this can be the cause of grassland degradation in the adjacent floodplains of the Bardiya national park. To do so, the following research question is answered: What is the relation between human interventions and the morphodynamics of the Karnali fluvial fan?

To do so, a method is developed where a time-series analysis of the morphodynamics is performed, based on discharge and optical remote sensing data. The flow regime is investigated by computing the temporal evolution of discharge, the flow duration curve, the base flow index, and the water distribution at the bifurcation in the upstream part of the fan. Morphodynamic parameters that can be described using remote sensing data concern the planform and the land cover of the river channel-belt. The planform dynamics are described by looking at the evolution of the sinuosity and braiding, and the land cover dynamics are represented by the evolution of the river's channel, sediment bar, and grassland areas. The most important human interventions are identified, after which the relation between the morphodynamics and the human interventions is assessed.

No evidence was found that human interventions have significantly influenced the evolution of the morphodynamics of the Karnali fluvial fan. Only on a small scale, changes or peculiarities in morphodynamic parameters can be attributed to human interventions. In this case study, the construction of embankments reduced lateral erosion, and the Kailashpuri dam influenced upstream sedimentation and erosion processes. A shift of the main stream from the eastern to western river branch in 2009 most likely caused a change in river-floodplain interaction in the branch closest to the Bardiya national park. The quantities of sediment and water extraction were found to be negligible in comparison to the natural water and sediment fluxes.

This research is the first step in quantifying the morphodynamic behaviour of the Karnali fluvial fan and indicates which are the most important drivers of morphodynamic changes. Besides this contribution, the study also includes the development of a methodology that could improve remote sensing-based morphodynamics studies.

Regional climate model (RCMs) simulations are used in hydrological (climate-change) impact assessment studies. However, RCMs exhibit noticeable deviation from observation, and can show large variation in ensemble projections (biases). The objective of this study is twofold, first to assess the robustness of two high-skill bias-correction methods; empirical quantile mapping (QM), and scaled distribution mapping (SDM) in improving the hydrological modelling of the Rhine River. The second is to assess the potential impacts of climate change on low flows at Lobith based on RCP8.5 scenario. The two correction methods are applied to correct the systematic bias from five climate datasets from the Coordinated Downscaling Experiment in Europe (EURO-CORDEX) covering the Rhine domain, using high resolution gridded datasets (1 km2) spanning from 1979 to 2019.
The bias-corrected simulations from the hydrological model provided more accurate discharge estimates than the wet biased simulations, with an average error of less than 100 m³/s at Lobith. The correction methods are also capable of correcting unprecedented temperature and precipitation values, making them useful in climate assessment studies in the Rhine river. However, it appears that the accuracy of the bias correction depends on the parent GCM, performance of the raw RCM and the skill of the hydrological model in estimating discharges at the point of interest. In addition to that, the drizzling effect could not be reduced using these methods.
Noticeable climate change impacts at Lobith are found using the bias-corrected projections. These projections suggest that low flows are going to be more frequent and longer in the coming 38 years. Unprecedented discharges (< 700 m³/s) are projected to occur at least 50 times between 2020 - 2060. This is coupled by a decrease in the long-term mean annual flow by 100 m³/s and a slight shift in the seasonality of low flows (2 weeks shift).
The general hydrograph at Lobith is set to change due to climate change for the period (2020 – 2060), with relatively higher discharges from early June to end of August followed by relatively lower discharges in the last four months of the year. Water levels are projected to decline in average by 20 cm (early June to the end of August) and increase in average by 30 cm (end of August till to end of December). The study recommends the need of combining bias correction, the feedbacks in the climate system (land use changes) and climate adaptation strategies to study these effects further.

Climate change is causing the global sea level to rise. Research and discussion of the effects of sea level rise are often focused along coastlines. However, the effects of higher water level and changing morphodynamics can reach far inland via rivers. This study uses a one-dimensional numerical model to analyze bed level response to sea level rise. The model simulates 100 years of steady sea level rise on a 1000 km long, fixed width channel. Sea level rise creates a backwater curve which grows in the upstream and vertical directions. The transient response of the channel bed is an aggradation wave the grows in the upstream and vertical directions. We studied different cases which vary in sediment flux, flow discharge, grain size, and rate of sea level rise and found changes in the rate of growth of the aggradation in both directions. All runs start in an equilibrium state and run for 100 years.

This study finds a close relationship between the equilibrium slope and depth of the channel and the shape of the backwater curve. This relationship drives the aggradation patterns. For example, for the same amount of sea level rise, a flat, deep channel has a longer backwater curve with a smaller relative increase in depth than that of a steeper, shallower channel. The backwater curve drives the aggradation patters, such that the flat, deep channel then has aggradation over a longer reach, and a smaller increase in bed level than the steeper shallower channel.

With the cases modeled in this study, three general trends in aggradation rates emerge: (1) an aggradation mound that grows quickly upstream, with slower increase in bed level as found in flatter, deeper channels; (2) a faster increase in bed level with slower upstream growth, found in cases with steeper slope and shallower depths; and (3) faster growth in both bed level and upstream direction caused by an increased rate of sea level rise.

Since natural channels are often complex with multiple sources of discharge inputs, a tributary case is also
included. Starting from equilibrium state and applying sea level rise to the downstream boundary, the model
shows aggradation waves in the regions downstream and upstream of the confluence, starting from the downstream boundary and the confluence. There is also degradation just downstream of the confluence. The scour hole grows at first, in depth and the downstream direction then reduces. The aggradation moving upstream intersects with the degradation moving downstream and fills in the scour hole. In some cases, we see the scour fills in within the 100 year time frame, resulting in a net increase in bed level. In a tributary system, the risk of scour is greatest for tributaries with high flow discharge or low sediment flux.

The transient response of the bed level to sea level rise is aggradation. As the water level continues to rise, the bed level is expected to do the same, but at a lower rate. In this study, the nominal rate of sea level rise is 10 mm/yr. The fastest rate of bed level rise in the model results is less than 6mm/yr, creating an ever increasing water depth. This is beneficial for shipping and navigation in the channel, which would not require dredging of the aggradated material. However, the reduction in bankfull volume with rising water levels is dangerous for flood control.

In mid-July 2021, heavy precipitation caused flooding for multiple tributaries of the Meuse and Rhine rivers which resulted in many deaths and exceptional damage. The Ahr in Germany, the Vesdre in Belgium and the Geul in the Netherlands suffered the greatest damage in each country. The first step in reducing such high damage from international, minor catchments is to build an understanding of the flood event.

The goal of this study was to analyse how extreme several hydrological aspects of the flood event of July 13-16 were for the Ahr, Vesdre and Geul. Analysing the extremity of an event requires a description of that event which helps with building knowledge. Its extremity places that knowledge into context. A flood with a 10-year return period will require a different approach than one with a 1000-year return period to balance the risk reduction and required effort. Three international catchments have been selected as case studies to allow an in-depth analysis of catchments and compliment current research on the flood event with a transboundary perspective.

The hydrological aspects considered for this study concern the forcing, flow mechanisms and response. For the forcing and response, a data analysis has been performed to study the extremity of the origin, magnitude, spatial and temporal variation of the precipitation, as well as the magnitude of the flow. Characteristics of both were used to determine the flood type. The data analysis on flow magnitude and seasonality has been supplemented by historical information to give more insight. The flow mechanisms have been derived from two models with varying structure and spatial variation by analysing parameter values and internal states.

The origin of the precipitation is considered as extreme since the cold core low, a low pressure area with a cold centre, was accompanied by several factors in a rare combination that likely contributed to the high precipitation amounts. The precipitation amount of July 14, the wettest day of the event, was of extreme magnitude according to the return periods of over 1000 years for several gauges. Although there was a large spatial variation in precipitation and thus their return periods, the spatial pattern showed a positive correlation of medium strength with the climatic pattern and is thus not considered as extreme. The temporal variation of the event is also not extreme. So during the event, only the amount of precipitation was extreme, but the precipitation amount in the preceding month was also extreme.

The peak discharges have been estimated to be around 990, 600 and 90 $[m^3/s]$ at the downstream gauges of the Ahr, Vesdre and Geul. The higher peak discharges coincide with sharper hydrographs and a stronger relief and impermeable soils. The return periods of the peak discharges were over 1000 years. Such extreme return periods are not surprising as such discharges had never been measured before. However, historical reconstructions show that similar flood events have occurred at least once for the Ahr. The assumed rare occurrence of summer floods in the regions was also debunked by historical information as 21 to 44\% of the historical floods occurred during the summer half-year. Another assumption by reports on the flood event was the flood type identification of a flash flood. The large spatial and temporal scale, as well as lag times for downstream gauges of 13 to 24 hours, do not resemble a flash flood, but rather a long precipitation event with a fast response.

Two models, a lumped, simple model based on the HBV model and a distributed more complex model based on the SBM model, have been calibrated for several years preceding the flood event, resulting in a good performance. Their performance for the flood event was low due to a strong underestimation of the peak discharge. This underestimation was resolved by a re-calibration. The re-calibrated parameters and internal states of the models show an increase in fast responding mechanisms, up to unprecedented magnitudes. The SBM model includes more processes and shows a rare amount of overland flow due to saturation excess. The fast response mechanisms, both in magnitude and relative runoff contribution, were unprecedented and can be considered as extreme.

The magnitudes of the precipitation, flow mechanisms and discharge were extreme for the 2021 flood event. The extreme flows were possible due to fast flow mechanisms, most likely overland flow due to saturation excess, which can be linked to the extreme pre-conditions. The strong relief and impermeable soils also seem to play a role in this extreme response. All catchments are capable of a fast and strong runoff response and although the forcing of the event was extreme, a new flood event in the future can not be excluded and so a higher level of preparedness and protection is required.

This study should be considered as an exploratory first step for action on the flood event. In order to move forward and increase the protection of the region, more detailed research is required. Such research can focus on several areas. Varying the study regions could be helpful for specific purpose. Several examples are an analysis of the entire region for a precipitation analysis, one catchment study for flood measures, or a catchment comparison for detecting hydrological influences. The anthropogenic influences can also provide more insight and opportunities and could be included in these future researches. The use of models was limited in this research but their further development is essential for flood risk management. Uncertainty is another topic that deserves more attention. An important factor in decreasing the uncertainty is to construct robust and extensive measurement infrastructure. Nevertheless, without more international cooperation and data accessibility the additional data will not reach its full potential. International cooperation also provides more value to the previous recommendations. This research shows that the transboundary catchments of the case study can be compared and give more insights to the overall behaviour.

The natural development of the morphology of rivers in time is important due to the functions rivers fulfil, such as navigability and drinking water supply. Rivers adjust their morphology when there is a change in the river controls, which are mainly the water discharge, sediment discharge and downstream water level. The morphology adjustments continue until a river has reached its equilibrium state (i.e. equilibrium bed slope and equilibrium bed surface sand fraction), which is when all sediment that is supplied upstream of the river can be transported downstream. Even though an assessment of how the river changes towards its equilibrium state is important for the performance of the functions of a river, it is often overlooked. This can for example lead to increased maintenance costs and dangerous situations. A quick method to calculate the equilibrium state of a river can help prevent these negative impacts. The analytical relations by Blom et al. (2017) are a promising method to predict how a river will change, since they give useful and rapid insight in the equilibrium river state. However, the degree of uncertainty of the equilibrium river state as a result of the uncertainty in the input parameters is not yet known. It is recommended to examine this uncertainty before interpreting how a river will change. The aim of this research is therefore to investigate what the range of uncertainty of calculated equilibrium river states is, and to execute a sensitivity analysis to examine which input parameters contribute the most to this uncertainty. The river Waal is chosen as a case study for these analyses. The calculated equilibrium state of the river Waal, including the range of uncertainty, is compared to river state measurements to assess how well an equilibrium state can be identified. A model is created based on the analytical relations, and is used to perform three different analyses; a deterministic analysis that results in a spatially varying equilibrium state without uncertainties, a sensitivity analysis that assesses the influence of each input parameter on the equilibrium state, and an uncertainty analysis that results in a bandwidth of possible equilibrium states. The result of the sensitivity analysis is that the mean gravel content of the total sediment supply and the total sediment supply itself have the largest impact on the uncertainty of the equilibrium state. The result of the uncertainty analysis is that the equilibrium bed slope can be approximately 1.8E-5 smaller or larger than the mean bed slope, with an uncertainty bandwidth of approximately 3.7E-5, and that the equilibrium bed surface sand fraction can be approximately 10% to 20% smaller and 10% to 35% larger than the deterministic bed surface sand fraction, with an uncertainty bandwidth between 20% to 50%, depending on which sediment transport relation has been used. From the comparison between the calculated equilibrium state, including uncertainties, and the measured river state, it follows that the current bed slope is steeper than the equilibrium bed slope, and that the current bed surface sand fraction is either larger than or within the uncertainty bandwidth of the equilibrium bed surface sand fraction. The found milder equilibrium bed slope is as predicted. However, the found equilibrium sand fraction does not match the prediction that the bed surface sand fraction is smaller in the equilibrium state. From the comparison and the results of the uncertainty analysis follows that an equilibrium state cannot yet be identified, because 1) the uncertainty range of the calculated bed surface sand fractions is considered to be too large to predict the equilibrium state of the river Waal, and 2) the equilibrium bed surface sand fraction predictions do not match the expectation. To improve the equilibrium state identification of the river Waal, it is recommended to 1) assess why there is a mismatch between the expected and calculated equilibrium bed surface sand fractions, 2) perform research to reduce the input uncertainty, and 3) validate the equilibrium state predictions.

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.

The objective of this thesis is to assess the effects of climate change on the initial, transient and equilibrium response of mixed-sediment river. Climate change will cause changes to hydrographs, which in turn affects sediment transport capacity and thereby the river bed profile. In order to create a better understanding of the processes in play we analyse a theoretical situation using the Lower Rhine as reference. The river Rhine is a heavily engineered river. The exact effects of climate change are therefore difficult to predict, as human intervention is significant. A simplified version of the Rhine can however give an insight into river response to climate change.

First an analyses of historical discharge and future discharge of the Lower Rhine was made, which gave an insight in the changes to the hydrograph that can be expected in the future. Next a model was made representing a theoretical river reach. For this river reach the upstream hydrodynamic boundary is varied and the results of these variations are analysed. Using the results of this model and the hydrograph expectations it was discussed what can be expected to happen to a river bed under these circumstances.

The Meuse river facilitates important socio-economic functions. In the light of Integrated River Management (IRM), the question rises whether these functions can be facilitated in future decades (until 2050) as well. The riverbed is expected to degrade over the coming decades, just like in the past century. On top of that, the discharge regime will alter because of climate change. This research studies the impact of bed degradation and altering discharge regimes on the river functioning regarding navigation and flood safety. This is done by linking hydraulic river conditions to functional performance indicators.

Sediment nourishments have become an increasingly attractive alternative to deal with continuous bed degradation problems in the river system. When supplied to the river, sediment nourishments induce a sediment wave that propagates through the system causing changes in bed level and bed surface texture. In the present study we use a one-dimensional numerical model to analyse how the propagation of these sediment waves varies under different conditions. We find that the propagation of the sediment wave tends to be predominantly dispersive for large spatial variations of the flow, which is the dominant case when the height of the wave is large in comparison to the flow depth, or when the sediment is dispersed over a large enough reach to lead to a backwater effect. In artificial sediment nourishments the height of the sediment wave is usually limited to prevent creating an additional obstacle to navigation, and it is then that the temporal variations in the flow, due to high discharge events for example, determine the propagation of the wave. We study the morphodynamic response in a mixed-size sediment river system to a sediment nourishment, and identify the physical mechanisms that impact the propagation of mixed-sized sediment waves. In order to prevent it from being easily flushed downstream, the grain size distribution of the nourished sediment is typically selected to be coarser than the river bed. This reduces sediment mobility and enhances sediment deposition over the nourished reach, which in turn causes a deficit in sediment supply to the downstream reach and leads to the formation of a degradational wave. The propagation celerity of the degradational wave is found to be significantly faster than that of the sediment wave, which means that the effects of the nourishments in a river system can be observed significantly further downstream than from where the front of the sediment wave is found. We find that the celerities of both the sediment wave and the degradational wave are predominantly affected by the adjustments in surface grain size in the system. These adjustments migrate in the river system as a fining wave, which both accelerates the sediment wave, and decelerates the degradational wave. We learn that higher content of the finer fractions in the nourished sediment accelerate the evolution of the wave and lead to a predominantly translational behaviour. It also affects the magnitude of the incision depth, the closer the composition of the nourished sediment is to that of the bed surface, the less severe the scour. And even though it does not affect the propagation celerity of the degradational wave, it does impact how far downstream the wave travels, affecting a larger river reach the coarser the grain size composition of the sediment wave is. Finally, we analyse the results from a nourishment pilot project carried out in the Bovenrijn from 2016-2019. We find that the propagation of the sediment wave associated with the nourishment is only observed during high discharge events. The wave is predominantly dispersive, and only during a prolonged extreme discharge event did the wave show slight translational behaviour. We observe that lateral sorting mechanisms caused the tracer sediment to have a different trajectory from the sediment wave, which highlights the limitations of using a one-dimensional model. We also observe that the changes in bed level are related to the changes in surface grain size, with sediment deposition over the nourished reach and bed degradation just downstream.

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.

River discharge is an essential parameter in morphodynamic modelling. It proves to be highly variable in both time and space. In this thesis the impact of the schematisation of time-dependent discharge series on morphodynamic change is studied. The impact is governed by several factors such as the long-term discharge statistics, and both short- and long-term sequences of discharge stages. A variable river discharge itself does not necessarily result in morphodynamic change. Changes in conveyance area and roughness are dominant sources of bed fluctuations and bed waves. Moreover, backwater dominated segments are subject to mild fluctuations in the river bed. The combination of natural variation of hydrological processes in the upstream river catchment and the absence of a significant correlation among statistical characteristics of subsequent years makes it hard for river engineers to construct discharge time series for river models that simulate morphodynamic development. The limited predictability of future discharge time series is an important source of uncertainty in model predictions. A way to estimate this uncertainty is by means of a time-consuming Monte Carlo approach, resulting in a mean long-term morphodynamic trend and the associated uncertainty. An alternative method is the application of a deterministic series for which the long-term discharge statistics are translated into a cycled annual hydrograph (CAH). It is expected to yield similar morphodynamic changes with respect to the mean trend from the Monte Carlo approach without the need for a large number of computations. However, as is demonstrated in this study, the CAH-method lacks in performance in simulating average long-term development and the amplitude of fluctuations in the river bed, especially at locations where strong sediment transport gradients are experienced. Using a simplified two-dimensional model that represents a locally widened floodplain, the impact of the included bandwidth, short-term and long-term sequences on morphodynamic change is investigated. The aim is to improve the deterministic hydrograph schematisation and to find a more convenient way to get insight into uncertainty in simulated morphodynamics. These findings result in a set of recommendations for future schematisations of discharge time series in morphodynamic river models. An improved deterministic approach is proposed. Using historical measurements of daily discharge data, years with a similar statistical maximum, mean and standard deviation can be classified. The classified years are translated into multiple cycled annual hydrographs (MCAH). By using these hydrographs, synthetic time series are constructed. Compared to the CAH-approach, the proposed MCAH-method yields a significant reduction of the root mean square error with respect to the long-term average morphodynamic trend from the Monte Carlo simulations. Moreover, the MCAH-series result in amplitudes of bed fluctuations that are closer to the response to natural discharge time series. Finally, the MCAH-series can be used to give a rough indication of the uncertainty in future morphodynamics.