Known for its low-lying and flood-prone geography, the Netherlands has a long history of flooding. Flood events in the 1990s, which led to large-scale evacuations, shifted the country's focus from simply strengthening dikes to creating more space for rivers to flow and storing water to prevent similar disasters. With changing climate conditions, rising water levels, land subsidence and sediment supply issues, the safety of primary flood defences needs to be regularly reassessed. The latest safety assessment, using a new methodology that assesses dikes based on the probability of flooding, found that many dikes do not meet the required safety standards. These dikes will need to be redesigned and reinforced, and there are opportunities for innovative and environmentally friendly solutions.
Previous research has extensively investigated the potential of different ecosystems, such as salt marshes, mangroves and willow forests, to reduce the impact of waves on coastal and riverine structures. Studies have shown that vegetation can significantly dissipate wave energy, thereby reducing the load on flood defences. However, the dynamic and unpredictable nature of vegetation has made it difficult to incorporate it reliably into defences. While there is general agreement on the benefits of vegetation for wave attenuation, its reliability in reducing failure probabilities requires further investigation.
This thesis tests the potential of riparian forests for wave attenuation and dike design optimization. The main objective is to investigate how wave attenuation by riparian forests can improve dike safety and to identify the critical variables that most significantly affect the probability of flooding. Key research questions include how wave attenuation can contribute to dike design and safety assessment, and which variables most influence the probability of flooding.
Vegetated foreshores can reduce the height of incoming waves, with positive implications for dike design and safety assessment. A model was developed to test this by combining current wave impact and run-up models with a wave energy balance that includes energy dissipation by vegetation. The model was subjected to a Monte Carlo simulation to determine the probability of dike failure due to erosion of the outer slope by wave impact and run-up. The model analyzed a dike section near Kampen and showed that riparian forests can significantly reduce the impact of waves on dikes. The results showed that the forest length, frontal-surface area and stiffness of the forest must be large enough to be effective. Strategically placing trees with the largest frontal-surface area at critical points can significantly reduce the probability of dike failure.
In conclusion, this thesis highlights the potential of riparian forests to improve dike safety and contribute to a more resilient and environmentally friendly flood protection strategy in the Netherlands.

The wave attenuation of vegetation plays an increasingly important role in flood control in coastal areas. Past studies have found that the interaction of waves with vegetation mainly depends on hydraulic conditions and vegetation characteristics. Therefore, it is necessary to build vegetation models and quantify vegetation characteristics related to wave attenuation. Mangroves are one of the typical tropical intertidal vegetation. This study aimed to test the potential of using a low-cost, convenient smartphone-based structure-from-motion with multi-view stereo-photogrammetry (SfM-MVS) to accurately measure mangrove parameters related to wave attenuation.

SfM-MVS is a computer vision technique in which the point cloud coordinates of an object can be calculated from a series of 2D photos to generate a 3D point cloud model. This study first tested the optimal photography distance （about 25cm） and optimal weather conditions （cloudy and no sunlight） using the smartphone-based SfM-MVS method. Then, based on the optiaml usage conditions, 3Dpoint cloud models of 10 individual mangrove samples were reconstructed using this method, then the mangrove parameters related to wave attenuation were estimated according to the model: linear parameters (tree height, crown diameter, stem diameter, branches diameter) and the frontal area at a certain height. The true values of these parameters were measured using traditional hand measurements to evaluate smartphone-based SfM-MVS-derived parameter estimates.

The results show that the estimation of the linear parameters of the mangroves generally achieves high accuracy (RMSE of tree height, canopy diameter, stem diameter and thick branch diameter are all lower than 15%). There were significant negative biases in the estimates of tree height and crown diameter, and no significant biases in the estimates of stems and branches. The results of linear regression show that there is a strong positive correlation between the estimated values of all parameters and the true values. There are large errors and negative biases in the estimates of the frontal area at different heights. Overall, RMSE of frontal area at a certain height is 45.58% with a bias of -38.83%. This result showed that the SfM-MVS model resulted in a large underestimation of the frontal area at each height. A series of analyses showed that the main reason for the large error in the frontal area at a certain height was that the terminal branches with the lowest branch order (smallest diameter) were difficult to visualize in the SfM-MVS model. And this part of the branches actually occupies a large proportion of the frontal area.

This study demonstrates that smartphone-based SfM-MVS is capable of estimating mangrove tree height, canopy diameter, stem diameter, and thick branch diameter. Its accuracy is comparable to other existing methods such as TLS, ALS, camera-based SfM-MVS. However, the resolution is insufficient for very thin branches (<=5mm), which makes the method inaccurate in estimating the frontal area at a certain height. Factors such as photography distance and ambient lighting can affect the accuracy of the model. Compared with the currently used remote sensing technologies such as TLS, the smartphone-based SfM-MVS has the advantages of low cost, high flexibility, and low difficulty for those who lack professional knowledge or training, which makes it an alternative with great potential.

Conventional dike reinforcement measures do not take vegetation into account, while vegetation contains wave damping properties. Reasoning for this is that vegetation in general is seen as temporary and hence may be removed in the future. In this case study for the Dutch waterboard WDOD, the wave damping properties of vegetation in a floodplain along the river IJssel (Duursche Waarden) are studied. Duursche Waarden consists of mostly willow species and is adjacent to a dike. It is part of Natura2000 and hence it is a protected vegetation area and not considered to be temporal. A general approach is developed for similar sized (riparian) forests as Duursche Waarden (1.1 km2) to map the vegetation and obtain the vegetation parameters. This approach is also applied to Duursche Waarden. First of all the different vegetation areas within the riparian forest are identified with the help of aerial photos and field observations after which they are prioritised with the help of a Multi Criteria Analysis. The prioritised vegetation areas are divided into uniform and non-uniform areas and studied in detail. Uniform and non-uniform areas are distinguished based on tree species, structure, height and density with the help of field observations, aerial photos and airborne Light Detection and Ranging (LiDAR) data. A representative distribution of the vegetation parameters over the different vegetation areas is obtained with the help of one method. The vegetation parameters interesting for wave damping are the frontal area per volume and the bulk drag coefficient of leafless vegetation. Leafless vegetation is of interest since most extreme storms occur in winter. On top of that leaves seem to have insignificant contribution to wave damping in the full scale physical experiments of white willow trees conducted in the Delta Flume at Deltares. These experiments were performed in 2018. A new definition is introduced for the frontal area per volume. The frontal area density distribution over the height is estimated with the help of point clouds retrieved from TLS measurements and validated with hand measurements and studies of similar vegetation. Two methods are used in estimating the frontal area with the help of literature: the alpha shape and grid method. These methods are used for merged point clouds from Multiple Scanning Stations (MSS) and a point cloud from a Single Scanning Station (SSS). The SSS method includes a shadowing correction factor. Finally, the values of the frontal area per volume are chosen based on assessing the outcomes of the MSS method, SSS method and hand measurements. The bulk drag coefficient is estimated using selected values from literature. Dense vegetation contains relatively higher values for the vegetation parameters than sparse vegetation. This will result in more wave energy dissipation and hence more wave damping for dense vegetation. The frontal area per volume of sparse to moderate dense woody vegetation (values on average of about 0.01-0.20 m-1) in uniform areas is believed to estimated best and most efficiently with the MSS method. In non-uniform areas hand measurements are believed to be estimated best and most efficiently by hand measurements. The frontal area per volume of dense woody vegetation (values on average greater than 0.20 m-1) is most uncertain. Therefore, both hand and TLS measurements (with the SSS method) should be used to estimate the frontal area per volume. The wave attenuation by vegetation is estimated using the numerical wave model SWAN 1D and 2D computations including and excluding vegetation using the obtained vegetation parameters. No currents are included. Hydraulic conditions of Hydra-NL calculations using Bretschneider are used for both SWAN 1D and 2D computations. A comparison between Hydra-NL and SWAN 1D computations including model uncertainty for exactly the same conditions show similar outcomes for SWAN. SWAN 1D computations showed wave attenuation due to vegetation with a magnitude of 0 to about 44 cm for all dike locations adjacent to Duursche Waarden. SWAN 2D computations without vegetation show significantly lower outcomes than 1D computations without vegetation: on average about 12 cm. This difference is explained by refraction and directional spreading resulting from the spatial variability in water depth and fetch of the studied area. Therefore the studied area is believed to be better described by 2D computations. One combination of the wind direction and water level is proven to be governing for all dike locations. Following 2D computations, the failure mechanism ‘erosion of the outer slope’ with a return period of 66666 year results in an estimated maximum wave height of 77 cm for dike locations adjacent to Duursche Waarden. This is a reduction of about 23 cm compared to the results of Hydra-NL. Indicative calculations suggest that dike reinforcements are still needed at Duursche Waarden. The wave damping is estimated to be 16 to 24 cm at dike locations adjacent to relatively large vegetation areas of sparse to dense vegetation. Dike locations at which the significant wave height is the greatest are called critical dike locations. The wave damping for these critical dike points is estimated to vary between 4 to 7 cm. In a report by Deltares an overview is made of promising wave damping foreshores in the Netherlands. Rough estimations show possible wave damping at Duursche Waarden of about the same order of magnitude as found in this study. Also studies on similar vegetation show significant greater values for the wave damping, suggesting rather conservative than optimistic outcomes in this study.