Sea level rise and land subsidence increasingly put more pressure on Dutch flood protections. Additional to regular dike reinforcements, new, more nature-based, solutions are considered, such as a double dike system (DDS). Such system, consisting of two parallel dikes in combination with an interdike area in the form of a salt marsh, offers the opportunity to adapt via hydro-morphological processes to sea level changes. An increased bed level of the interdike area of a double dike system potentially positively affects wave attenuation. Furthermore, if the second (landward) dike breaches, the elevated salt marsh of the interdike limits breach growth and discharge through the breach. However, previous studies show that it is hard to prove the feasibility and effectiveness regarding flood safety of a DDS relative to a single dike and an in-depth hydraulic engineering substantiation for flood risk reduction is lacking. The objective of this research was to investigate which factors determine the effectiveness of a DDS, with the focus on the dynamic development of the system. This leads to the following research question: Under which circumstances can a double dike system be an effective alternative to a regular dike reinforcement? It is possible to distinguish three distinct systems based on the function of the first (seaward) dike: dynamic open DDS, dynamic closable DDS, and static closed DDS. This distinction is primarily based on the degree to which water is permitted to pass through the first dike under both normal and extreme conditions via an opening in the dike. This interaction between the outerdike area and interdike is unobstructed for a dynamic open DDS and almost nonexistent for a static closed DDS. To achieve a more hybrid approach, dynamic closable DDS utilizes adjustable structures like tidal culverts to regulate the inflow. Under normal tidal conditions, the permitted inflow governs the sediment supply and the potential for accretion of the bed level in the interdike area. Depending on the design of the system, during extreme storm conditions, the first dike can act as primary flood defense or as breakwater that provides shelter for the second (landward) dike. This research focused on an open dynamic DDS system, allowing for maximum potential bed level accretion of the interdike area. In doing so, the first dike functions as a breakwater and failure of the system occurs when the second dike is breached. Due to the unrestricted in- and outflow, the assumption is made that water levels are uniform and equal to the hydraulic conditions outside the system. The double dike systems are analysed with the help of two approaches: failure probability approach and local individual risk (LIR) approach. The first approach compares systems based on equal probability of failure. The second approach compares systems based on an equal LIR of 10−5 per year and includes the expected mortality in case of flooding. Systems have been compared to each other with the help of calculated dike heights and total dike volumes. Dikes were tested for failure due to overtopping or overflow. Therefore, a transmission model was applied to describe the transmission of waves through the system: wave transmission over the first dike, wave propagation in the interdike area and wave overtopping over the second dike. The probabilistic calculation method FORM was used to find the failure probability and design point. For the LIR approach, this model was extended with a 1D flood model to determine the flood characteristics and subsequent mortality fraction if failure of the dike occurs. Firstly, a simplified case study was performed based on conditions in the Western Scheldt. Opening the existing dike in the current hydraulic conditions necessitates the construction of a second dike that is only 1.7 meter lower than the single dike system. A sea level rise (SLR) of 0.4 meter in 2070 and 1.0 meter in 2120 was implemented. In addition, morphology scenarios were applied, ranging from no accretion to maximum accretion. In the scenario with maximum accretion, rapid accretion to Mean High water takes place, after which the bed level accretion follows SLR. Comparing systems based on equal failure probability, a single dike should be raised identically to the SLR. In the case of a double dike system and 1.0 meter SLR, this increase in height of the second dike should be approximately 1.15 meter to maintain equal failure probability. Sedimentation in the interdike area has a negligible impact on failure probabilities due to the dominant influence of water depth, meaning the water depth is too high for waves to be affected by the bottom of the interdike area. However, this comparison changes when based on equal LIR. With 1.0 meter SLR, the additional required height of a single dike increases to 1.7 meters. Applying this approach to a double dike system, the effect of the height of bed level of the interdike area on the breach flow does have significant influence. Analysing the effectiveness of a double dike system in comparison to a single dike system based on volume increase, these differences are magnified. However, generally, it can be noted that the extreme storm surge in the Western Scheldt is dominant and a second dike with substantial dimensions is needed to comply to its retaining function. Secondly, a sensitivity analysis was performed on four different parameters and system configurations: 1) height of the first dike in combination with the length of the interdike area, 2) hydraulic conditions with lower water levels, 3) varying area size of the hinterland and 4) implementation of vegetation. The effect of the first dike on the failure probability is not dominant, since the required increase of the height of the second dike is not proportional to the applied decrease in height of the first dike, increasing its effectiveness in terms of dike volume. This effectiveness increases also for hydraulic conditions with lower extreme water levels. In addition, there is a positive relation between the effectiveness and the area size of the flooded hinterland. Vegetation does not have any influence on the dike height due to the high water depths under design conditions. In conclusion, the following three location specific circumstances are important for an open DDS to be an effective alternative to a regular single dike reinforcement: • Sufficient sediments present to allow for a higher rate of sediment accretion compared to sea level rise. • Hydraulic conditions in which waves are more important than storm surge. • Large hinterland area. It is more advantageous to employ the local individual risk approach in comparative studies assessing the effectiveness of double dike systems. The significant positive impact of bed level accretion on flood impact is disregarded when only considering failure probabilities. It is recommended to improve the flood model, implementing a 2D model and considering the effect of the foreshore on breach growth, to gain a more comprehensive understanding of the potential of an elevated bed level of the interdike area on the flood impact, so it can be effectively incorporated into the Dutch flood risk approach. In addition, it is recommended to consider the implementation of an open dynamic DDS in coastal areas with high sediment concentrations and suitable hydraulic conditions (sufficient tidal range and mild storm surge). Furthermore, the wave damping function of the first dike is not critical for interdike areas with sufficient length and reduced depth. It is more efficient to prioritize the flood safety of the second dike. This allows for more effective design and management decisions concerning the height and safety requirements of the first dike. Finally, it is advisable for areas with high water levels during extreme conditions to consider a closable system. This entails the ability to close the gaps in the first dike under specific conditions, such as utilizing tidal culverts, limiting the hydraulic loads on the second dike. However, implementing a closable system necessitates larger investments in modifications to the first dike. Additionally, the maintenance of the first dike becomes more crucial due to its retaining function first. Therefore, it is recommended to conduct a comprehensive cost-benefit analysis to compare the open dynamic double dike system with the closable double dike system.

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.

The risk of flood risk is increasing with global trends such as rising sea levels, land subsidence, increasing populations in coastal areas and economic growth. Traditional solutions to reduce flood risk are a levee or a dam; however, the research on hydraulic engineering is increasingly promoting nature-based solutions for flood protection. Salt marshes and mangroves can attenuate waves and could thus help to reduce failure probability. This research focuses on the attractiveness of mangroves in flood risk reduction. The essence of the reasoning followed in this research is that mangroves in front of a levee enable a lower levee and therefore reduce costs while maintaining the same failure probability. There are three aspects related to this assessment: the biological behavior of mangroves; the physical behavior of waves and storm surge in a mangrove forest; and the economics of flood risk, levees, and mangroves. Mangroves grow under specific conditions, such as high temperatures, saline water with fresh water input, between mean sea level and high tide, and with low wave impact. Mangroves attenuate waves, with the attenuation rates depending on mangrove height, stem diameter and density, in addition to the water depth, wave height and wave period. Lastly, the costs of restoring mangroves and building levees have been considered. In this thesis a method has been developed to define the optimal configuration of a levee-mangrove system. The goal is to reach a desired safety level while keeping costs as low as possible. Costs consist of building a levee, restoring mangroves, and the expected annual damage and maintenance. This depends on the ratio of mangrove and levee costs, the ability of mangroves to attenuate waves, and levee characteristics. Although all variables have equal significance in defining the mangrove forest width, the variety in levee and mangrove costs and significant wave height are the variables most likely to affect the optimal mangrove forest width. Mangroves are only useful to decrease the height of waves, thereby reducing wave run-up. This wave-attenuating effect decreases exponentially with the mangrove forest width. all Based on the literature, when considering common values for levee costs and mangrove restoration costs, levee characteristics and wave attenuation, wave attenuation is strong enough such that the optimal mangrove forest width is larger than zero. The costs of restoring mangroves depend on the required measures to enable mangroves to grow back. Hydrological restoration involves returning the mangrove area to the natural condition where mangroves are able to grow. This could, for example, be restoring a fresh water source that has been blocked by human intervention. If hydrological restoration is required and can be done cheaply, then this is the most effective option. Planting mangroves can also be a cost-effective option. Sheltering the mangrove area by using a permeable structure requires more financial resources. If large-scale filling or excavation is required, costs may increase significantly, and it is likely to be too expensive to use mangroves in flood-risk reduction. However, this cost assessment may change if the ecosystem services of mangroves are taken into account. As a rule of thumb, for mangroves to be economically effective, restoration costs [USD/m/m] in case of a 1 m wave height should remain below 0.003 times the variable levee costs [USD/m/m]. This value increases linearly with the wave height. For levee costs of 1 million USD/km/m, of which 0.6 million USD/km/m variable costs, the mangroves restoration costs should not exceed 0.18 USD/m/m, or 18,000 USD/ha. For Kaback, Guinea, the optimal mangrove forest width and height of the levee are assessed. These are 900 meter and 1.1 meter respectively, with the levee located just behind the mangroves. 900 meter is the maximum width possible, considering the physical conditions where mangroves grow. Since mangroves are growing back naturally already, costs for mangroves restoration is limited to fixed costs. The model developed in this study can be used to create a ``mangrove opportunity map”. This map can indicate—based on physical attributes—the costs, hydraulic conditions such as storm surge and wave height, and whether mangrove restoration for flood risk reduction is effective. Governments or organizations such as the World Bank can use this method to explore whether mangrove-based strategies are an option. Further research can focus on improving the cost estimates on levees and mangroves. In this study, costs are assumed to be linear with the levee height or mangrove area. In practice, levee costs might especially increase exponentially with the levee height, which would make mangroves more effective.

In recent years, nature-based solutions have gained recognition for their capacity to reduce flood hazard on coastal communities. These coastal threats range from waves and tides, to future storm variability and predicted sea level rise. Vegetated foreshores are one of the measures that fall under this palette of solutions, and are the focus of this thesis. While vegetated foreshores can be colonized by several plant species (mainly mangroves or marshes), this thesis presents the study of phenomena occurring at salt marshes in an attempt to determine which are relevant indicators that can predict the presence or absence of this ecosystem at a given location. Moreover, the wave height attenuation capacity of such vegetated foreshores under different conditions is also assessed for different effectiveness indicators, in a way that provides insight into whether this habitat is not just feasible, but also effective as a coastal protection measure, with respect to wave height attenuation.

Results suggest that within a water system with similar sediment composition, inundation-free period and daily-occurring hydrodynamics can explain the presence/absence of vegetation at most sites, while storm conditions can the determining condition for vegetation absence in other locations. For comparison between systems, the dimensionless wave-induced excess bed-shear stress factor may serve as a relevnat proxy. These conclusions indicate the occurrence of sediment re-suspension during approximately 20% of the assessed time, at the most.

From the wave attenuation effectiveness assessment, considerations for tidal regime, surge height, and type of wave forcing were used to determine the degree of effectiveness of salt marshes in the studied water systems, while defining effectiveness in terms relative to a mudflat located at MSL. Results suggest that salt marshes located in macro tidal sites, like in the Western Scheldt, can dissipate more wave energy than habitats located in micro or meso tidal areas, such as the Wadden Sea. On the other hand, results treating the different wave loads further indicates that greater attenuation may be achieved in sheltered coastal locations such as the Western Scheldt estuary, indicating that this system may be more effective in sheltered than in exposed coastal areas.

Finally, the set of conclusions provided by both sections may serve to determine whether an arbitrary location is capable of hosting salt marsh vegetation, as well as the effectiveness of such nature-based solution with respect to wave height attenuation. The elaboration of an assessment tool was not achieved in this thesis; however, in order to perform a quick check for salt marsh introduction as an effective flood defense measure at a given site, results indicate the gathering of necessary data such as bed level elevations - for inundation-free period analysis - as well as occurring hydrodynamic forcing and sediment properties at such arbitrary site (excess bed shear stress determination, evaluated with the previously proposed limits). With respect to the degree of wave height reduction effectiveness, the determination of the occurring tidal regime, design surge heights, and considerations of sheltered/exposed coastal conditions may serve as a first estimate for this purpose.

Flotsam lines may arise on coastal dikes after a storm surge. These lines are generated when material due to debris accumulation on the foreshore has been transported onto the dike surface by wave run-up. Hydraulic engineers who used flotsam line measurements, interpreted a flotsam line as a line which is marking the highest wave run-up. In the past, flotsam lines were used to assess the safety of coastal dikes along the Wadden Sea. Currently, models (SWAN and WAQUA) fulfil this function and this may result in losing the value of flotsam lines. This study aims to rediscover the value of flotsam lines.
The objective of this study was to shed light on the value of flotsam lines. Using the spatial information obtained from flotsam levels it is investigated whether flotsam levels can be used to assess the quality of models (SWAN and WAQUA). This was done by calculating flotsam lines using a sequence of models and wave run-up formulas and then compare to measured flotsam lines. This study showed that the upper side of the flotsam line marked the maximum wave run-up in case of a small layer of flotsam. Besides, a relative thick layer of flotsam was able to absorb the force of the wave run-up and remained in position. This means that the amount of flotsam can disturb the wave run-up and eventually flotsam levels. It also was determined that the analysis of (calculated) flotsam lines gives an indication of the performance of the applied set of models (SWAN and WAQUA) and wave run-up formulas, provided that a systematic comparison between modelled and measured data is performed. A large database of detailed flotsam lines with corresponding storm conditions (water level, wave conditions, wind, etc) can provide valuable information to low costs. This database can be used to detect spatial variations which might give more insight in certain processes during storm conditions

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.