The main objective and secondary objective, which corresponds to the in-depth research are as follows: (1) To develop guidelines for engineers to include salt marshes in hydraulic infrastructure projects, subject to the Building with Nature philosophy. (2) To investigate the effect of grazing at salt marshes on the coastal protection property. This research concerns the development of guidelines for engineers to include salt marshes in coastal protection projects, subject to the Building with Nature philosophy. The results include important design aspects for engineers and an analysis of trade-offs between biodiversity and the coastal protection property of salt marshes. This research is based on a literature study about the natural and artificial development of salt marsh. Thereafter, the effect of grazing on wave attenuation is modelled by the numerical model SWAN (Simulating WAves Nearshore). The effect of vegetation and the bathymetry is analysed. The vegetation data results from a grazing experiment at the salt marsh at Noord Friesland Buitendijks, in the north of the Netherlands. The results from the literature study and the in-depth research are translated to important design aspects within the engineering design guidelines for salt marsh development. The main conclusion of the in-depth research is that the effect of vegetation on wave attenuation is small compared to the effect of wave breaking and bottom friction. Moreover, the variations in wave attenuation between the different grazing strategies is also small. However, grazing could be used as an ecosystem management strategy, as from literature it follows that grazing positively affects biodiversity. Although the effect of biotic factors on wave energy dissipation is small, the presence of a salt marsh indirectly affects wave attenuation. The salt marsh vegetation enhances sediment accumulation, which leads to a higher bottom profile elevation compared to the bare foreshore. This should be substantiated by future research.

Climate change puts under pressure existing and future coastal interventions. Growing threats like sea-level rise and intensity of storms require solutions to be adaptable and resilient. Nature-based solutions have shown to tackle these challenges while providing social, environmental, and economic benefits. The role of vegetation in coastal protection is increasingly recognized. Aquatic vegetation reduces erosion, storm surge, and incoming wave height. Large-scale modeling of waves with spectral wave models such as SWAN is indispensable for the design of coastal structures and the assessment of flood risk. Wave dissipation due to vegetation can be modeled in SWAN as increased bottom friction (implicit modeling) or as an additional dissipation function (explicit modeling). The second assumes that vegetation can be represented as rigid cylinders or plates (canopies) with different properties. While some studies concluded that implicit modeling reproduces the spectral evolution of field measurements more closely, others concluded the opposite. Within the BE-SAFE project, field campaigns measured the spectral energy distribution over salt marshes in the Dutch Wadden Sea during several winter storms. The vegetated foreshore in front of the coastal dike got submerged over 2 m of water during high tide and storm surge. The measurements deployed wave gauges over the study transect, which was defined between the pioneer zone marsh edge and the near-dike location (300 m behind the salt marsh). Calibrating the implicit and explicit models in SWAN brought the modeled total wave energy decay closer to the measurement. Nevertheless, the spectral shape, which describes the energy distribution over frequencies, still showed significant and not yet understood differences near the dike. A methodology was executed to investigate the mechanisms that could reduce the spectral mismatch between the SWAN wave model and measurements over vegetation. First, the literature highlighted possible mechanisms that could be incorporated for this purpose. Next, a new frequency-distributed explicit dissipation model of Jacobsen et al. (2019) was implemented in SWAN and compared to implicit and explicit models using lab and field measurements. The results showed that the newly implemented model accurately captures the physics and the change of spectral shapes for all experimentally tested wave conditions and submergences. In contrast, the existing implicit and explicit dissipation models in SWAN reproduce the spectral evolution only under certain circumstances. In the validation and comparison to the field measurements with a much larger water depth than the vegetation height, the model of Jacobsen et al. (2019) correctly captured the vegetation's physical representation and the dissipation on the wind-sea frequencies. Nevertheless, the amount of energy on low frequencies was largely underpredicted by all frequency-distributed models. Therefore, the model of Jacobsen et al. (2019) was modified to include flexibility in a frequency-dependent reduction factor that reproduced the energy decay of the measurements in all frequency regions. Other mechanisms that could be responsible for the mismatch before and over the marsh are the redistribution of energy by non-linear triad interactions, generation of infra-gravity waves, and near-shore currents caused by horizontal variations on the vegetation properties. The present research provides the range of conditions in which the tested explicit and implicit energy dissipation functions in SWAN are able to simulate the spectral evolution over rigid canopies and flexible salt-marsh vegetation. A new version of SWAN includes a new frequency-distributed explicit model that performed more accurately than existing models for rigid canopies. The physical insights from the research contributed to developing additional versions of SWAN, which performed closely to the energy distribution of the measurements over deeply submerged and flexible salt marsh vegetation species. References: Jacobsen, McFall, Van der A (2019). A frequency distributed dissipation model for canopies. Coastal Engineering, 150, 135-146.

For the design of coastal structures, the hydraulic loads that act on the coast should be known. These are often based on extrapolation of measurements. However, if the physical relation is unknown, large errors could be made. Therefore, a numerical model can be set up to take the most important processes into account.Yet, uncertainties are present in these numerical models as well. Taking for example the spectral wave model SWAN, it is hypothesised that the wave bottom refraction is not always properly modelled. Previous studies concluded that SWAN overestimates this refraction process near tidal channels, leading to an underestimation of waves entering the channel.This study uses the model SWAN in which different sensitivities are tested to assess the effect on refraction. The focus will be on the spatial and directional resolution as well as the wave propagation scheme that SWAN uses to discretise the propagation terms, which are the default SORDUP scheme and the optional BSBT scheme. From previous studies it is known that also physical processes could impact the bottom refraction, however these are only addressed shortly in this study. The aim of this study is to analyse the current performance of SWAN with respect to refraction and to assess how this performance can be improved.First, an analysis of a simple schematic channel case is made in SWAN, where waves propagate from shallow to deeper water. Physical source terms and wind wave-growth were deactivated in this part. It is found that a coarser spatial resolution can lead to a weaker wave refraction. This is caused by the way SWAN determines the turning rate, which is underestimated by SWAN compared to the theory of Snellius. On the other hand, an overestimation of wave refraction was observed for cases were waves should enter the channel. This is partly caused by the increased diffusion. These findings were taken into practice in a study of the Eastern Wadden Sea, specifically along the Ems channel, where waves propagated from deeper to shallower water. A coarser spatial resolution and the BSBT scheme showed less wave penetration from offshore into the Ems channel. The storm of January 2017 was simulated in SWAN after which a comparison was made to measurements. It was found that SWAN underestimated the wave energy for frequencies<0.15Hz. However, this did not improve by refining the resolution in both spatial and directional domain or by applying a different propagation scheme. SWAN thus overestimates the wave refraction due to bottom gradients if waves propagate to shallower water. In case waves travel to deeper water, refraction can be underestimated. If all physical source terms are deactivated, the effects of resolution on refraction are clearly visible. Including the source terms diminishes this effect. Along the channel edges, effects of increased spatial resolution are notable giving a significant wave height of 12-14cm (6-9%) closer to the measured condition. However, the effects at the coast near Uithuizerwad are negligible.

Including vegetation in flood defense systems has the potential to be a more cost-effective solution than conventional dike reinforcement measures, as vegetation contains wave damping properties. However, more insight is required on the complex physical processes due to wave-vegetation interaction in order to predict the amount of wave damping. Past research found that these processes are dependent on both flow conditions and vegetation characteristics. Therefore, reliable quantification of these vegetation characteristics is of importance to understand these processes and thus to improve predictions for wave dissipation due to vegetation. This study aims to gain insight on practical methods for quantifying relevant vegetation characteristics for wave damping. Data is used from full scale physical experiments conducted in the Delta Flume at Deltares, with 40 meters of willow forest. The vegetation characteristics are quantified in this study both by manual measurements on the willow trees and by executing Terrestrial 3D laser scans (TLS) of the whole forest. The frontal area of vegetation is found in literature to be a relevant parameter for determining the wave attenuation, therefore the focus in this study lies on schematizing this parameter over the vertical, which is seen as representations of the average willow tree. This schematization is referred to in this report as “tree model”. In this study, four tree models are obtained. The four tree models include: tree model 1a (manual measurements on the primary branches, excluding side branches), tree model 1b (branching method based on Strahlers ordering scheme, including side branches), tree model 1c (adjusted branching method) and tree model 2 (from terrestrial laser scanner). The reconstructed area from the TLS point cloud is determined by using Matlab built-in alpha shape function. The tree models are compared in terms of frontal area distribution over the vertical and of their effect on the corresponding wave attenuation. The latter comparison is achieved by using the numerical wave model SWAN and confronting the results with the measured wave attenuation from the physical experiments. With regards to the manual measuring methods, the frontal area of the average tree from tree model 1a serves as a lower limit, while tree model 1b gives the upper limit. The TLS outcome (tree model 2) underestimates the frontal area as computed by the other tree models. In particular, the underestimation is 70% when compared with tree model 1b, and 30 % when compared to tree model 1a. Adjustments on tree model 1b leads to tree model 1c. This tree model accounts for the tapering form of the branches and results in a total frontal area in between tree model 1a and tree model 1b. In this study, the representative area for the willow trees can be best captured with tree model 1c. This tree model results in a drag coefficient (CD) of 1.15 averaged over the tests with leafless willows and showed a negative correlation with the Keulegan-Carpenter number (KC). However, the capabilities of the TLS should be analyzed further and this study encourages to use a larger data set of trees in order to find a relation between the laser penetration and corresponding mismatch. Gathering of vegetation parameters by hand is in fact not economically attractive forwoody riparian vegetation, as these trees are characterized by complex canopy structures and high elevations. The TLS can serve as a practical tool for obtaining these relevant vegetation parameters for applying willows in hybrid solutions.

Flood defenses are important to keep large parts of the Netherlands dry. These primary flood defences are every twelve years assessed on safety during the Dutch national safety assessment. In 2017, the new safety standards (WBI2017) were introduced. These stricter standards caused that in total 251 km of the primary flood defences was rejected on height, which includes the failure mechanisms wave overtopping and overflow. Traditionally, dike strengthening is required. However, the new Delta Plan gives also spe- cial attention to the implementation of innovative nature-based solutions, for example the integration of shallow foreshores without or with vegetation in the safety assessment. These foreshores can reduce the incoming wave load by dissipating energy through wave breaking, bottom friction and vegetation attenu- ation. (Vegetated) foreshores are also present in front of some river dikes, called floodplains. Probably, 5 to 15 procent of the foreshores in the current reinforcement program (HWBP) is not or conservatively consid- ered in the safety assessment, which entails unnecessary costs (approximately 0.5 - 1 billion euros). In a quickscan of the riverine area several locations are indicated which meet the requirements as promising locations for wave reduction by vegetation. However, despite a renewed focus on building with nature, implementation of nature-based flood protections is hindered by te dike managers because of a knowl- edge gap in the effects of the variable vegetation on wave energy dissipation. The state and functioning of vegetation during design conditions is uncertain. This uncertainty is the most important reason for dike managers to hamper the implementation of a nature-based flood protection. The main objective of this thesis is to determine the reliability of willow vegetation as wave load reduction for river dikes. In order to assess the variability of vegetation on the wave damping, a field test and simulation models are used. The measurement set-up of Deltares consists of a plot of 7 x 7 meters with young willow branches and pressure transducers perpendicular to the river, which measure the (by ships produced) waves in the river Noord. Based on the field measurements, a model is developed and applied to determine the drag coefficient of willow branches. This model is an one-dimensional SWASH model which contains the bathymetry and the willow vegetation, characterised by a density, branch diameter and drag coefficient for three height ranges. These parameters provide the obtained wave attenuation by vegetation. The diameter and density were measured in the field. Therefore, the drag coefficient can be determined by calibrating the outcomes of the numerical model with the field measurements. From the field measurements by Deltares, it can be concluded that willow branches reduce incoming waves with a height higher than 0.20 meter with approximately 4 % per meter willow vegetation width. The wave attenuation depends mainly on the water depth and shape of the willows. The stiff stem of old willows damps waves less than high dense, more flexible branches above the stem, and the wave damping decreases with increasing water depth mainly when branches submerge. The field measurements are used to calibrate the drag coefficient. The 25th and 75th-percent value of the calibrated drag coefficient were calculated on -1 and 5, a large range. A negative value may not be possible, because this means an increase of the wave height by the willow vegetation. This large range in calibrated values and the existence of negative values are caused by the measurement set-up. The plot with willows was too small for accurate measurements and the crates, in which the willow branches were planted, caused distortion of the waves due to the sudden bottom elevation. Because of this, the first sensor at the edge of the crate (at the river side) has probably encouter measurement errors. Third, the angle of the incoming ship waves caused an underestimation of the calculated wave damping capacity of willows. The set-up should be improved to give proper results and conclusions about the drag coefficient of willows. The drag coefficient remains therefore uncertain and gets a large standard deviation in the second part of the research. In this second part, a study is carried out to the uncertainties of implementing wave damping wil- low vegetation. A one dimensional wave model based on the wave energy balance, is created for a dike section at the Heesseltsche Uiterwaarden, alongside the river Waal. Waterboard Rivierenland suggest this floodplain as opportunity for planting wave damping willow vegetation to compensate the required heightening of the dike. In the wave model, various input parameters are required to calculate the wave damping: boundary con- ditions, dike-foreshore parameters and the vegetation parameters. These parameters have all certain stan- dard deviations or variations. First, the effect of these standard deviations on the wave damping is deter- mined, and so the overtopping discharge and required crest level. Second, a thorough analysis is made to exogenous threats which affect the vegetation parameters. For willows this could be diseases, insects, ice, animals, drought, flood, fire and storm. The effects of these threats on the vegetation parameters are estimated by the knowledge and expertise of willow experts and a literature study. It is concluded that from all input parameters with standard deviations, the variations of the vegetation pa- rameters has the largest effect on the obtained wave overtopping. The wave reduction depends mostly on the vegetation height and water depth, which was also concluded by the field measurements. Second im- portant vegetation parameter is the density of the branches or the drag coefficient, if the drag for willows can be lower than 0.4. The most relevant influential threats with a high risk and significant consequence are storms, floodings and droughts. These effects of the standard deviations of input parameters, exogenous threats and annual maintenance on the overtopping discharge (and required crest level) are used for a recommendation about the design and maintenance strategy of willow vegetation as solution. The willow vegetation should ensure enough wave damping during the total lifespan of the reinforcement to comply the safety assessment. Therefore it is recommended to extend the width of the willow vegetation with a buffer zone in which all the vegeta- tion uncertainties can be accommodated. The width of the vegetation plot is therefore the main parameter that should be assessed during inspection. Note that the uncertainties and probabilities of the exogenous threats are assumed. Further research is required to validate these assumed values. To conclude, willows can be used as wave damping vegetation at floodplains along rivers. The branches of willows will reduce the incoming waves significantly if they are not submerged. The height of the willows is therefore the most important vegetation factor and should be assessed during inspections just like the required vegetation width to ensure the wave damping capacity during the total lifespan of the dike reinforcement. The reliability of willows for wave load reduction on river dikes is sufficient if the maintainer apply the design and maintenance principles as recommended.

Sandy foreshores, beaches and dunes play an eminent role in flood risk reduction in coastal areas, reducing the impact of wind waves and storm surges on the hinterland. In some areas, sandy protection is naturally present. In other coastal areas, engineering solutions are needed to provide safety. “Soft” sediment-based solutions often serve multiple objectives, including flood safety, but also provide other ecosystem services. Knowledge of morphodynamics of these “soft” solutions (i.e. beaches) is crucial for protecting and managing coastal areas prone to flood risk. The aim of this thesis is to understand and quantify how hydrodynamic processes drive morphological development of low-energy, non-tidal, sandy beaches.@en

Sea level rise will increase the risk of flooding in coastal areas. This poses a risk to the coastal protection as well as rivers and lakes close to the coast. Solutions are needed to cope with this threat. The past decade, nature based solutions have gained significant interest. One of these solutions could be sandy foreshores. Due to the use of natural materials, sandy foreshores are a ‘nature-based solution’ opposed to a more traditional approach of dike reinforcement. For sandy foreshores to be a viable alternative to regular dike reinforcements, the order of magnitude of construction and maintenance costs need to be known. For this reason, it is necessary to be able to calculate a failure probability of a dike with sandy foreshore, to predict the required maintenance and to optimize the design based on life-cycle costs. To improve the calculation method, a step-wise calculation of the failure probability for wave overtopping of a hybrid structure was developed. The calculation included an iterative process. The calculation methods consists out of calculating the failure probability for wave overtopping in Riskeer and a dune erosion calculation in Xbeach. In this research, overtopping was considered as the dominant failuremechanism. For the assessment, the dike was seen as an impermeable hard layer and the foreshore as a beach. Therefore, dune erosion could not erode the main structure and is considered a sub-mechanism of overtopping. Next, a calculation was performed to predict the longshore transport along the beach in Almere Duin. The occurrence of different wind directions, together with a Delft3D model of the Markermeer, was combined to find the longshore transport. The LST was calculated with transport formula calibrated for coastal areas. The life cycle costs (LCC), including design and maintenance costs, were calculated for different strategies. The calculated alongshore transport together with cost estimates were used to calculate the LCC. Subsequently, the net present value of the maintenance costs was calculated to determine the LCC for each maintenance strategy. At the end the uncertainty and sensitivity of each strategy were analysed. The time-varying protection level can be optimized by calculating the failure probability due to wave overtopping, with Riskeer and calculating the erosion of the foreshore, with Xbeach. To use this method, Riskeer and Xbeach should have the same design point and if the design point shifts, an extra iteration is necessary. For a 1/10 profile, only a 0.25 m lowering of the foreshore height was found and one extra iterative step was required to carry out the calculation. A maintenance strategy with a 100 year maintenance interval period was found to be optimal in this thesis and a Monte Carlo simulation, which included uncertainties, led to similar results. However, the total LCC were 21% to 34% higher, when uncertainties were included in the calculation. These findings suggest that it is not necessary, to carry out a probabilistic calculation, to find the optimal maintenance strategy. However, the models used, aswell as the local boundary conditions, such as the longshore transport and the cost of sand, were found to influence the life cycle costs significantly. For this reason, it is concluded that a location specific analysis is important to optimize the maintenance strategy.