Nature-based solutions are increasingly recognized as effective and multifunctional components of climate-resilient flood protection. While tropical mangroves have received substantial attention, temperate riparian forests, particularly willow systems, offer comparable wave attenuation and biodiversity benefits, yet remain understudied. This study assesses the ecological and protective value of three types of willow floodplain forests: a so-called wild-grown willow forest, a pollard willow forest, and a willow plantation. Using field data from the Biesbosch National Park (the Netherlands), we quantified forest structure, ground-dwelling invertebrate diversity, and modelled wave attenuation under storm scenarios. Structural complexity and biodiversity were highest in the wild-grown forest, with significantly greater invertebrate order richness, larger body sizes, and more heterogeneous canopy architecture. The pollard forest showed the highest wave attenuation efficiency due to their dense, low-lying crown structures. The plantation forest showed lower values across both axes. We integrated these findings into a trade-off model evaluating ecological value, flood protection efficiency, and a 50-year simple cost analysis of each forest type as a hybrid solution alongside traditional dikes. While the pollard forest is the most spatially efficient for flood attenuation, the wild-grown system provides greater ecological value at lower lifecycle cost. Our results underscore the importance of tailoring hybrid flood defense strategies to local priorities - balancing biodiversity, spatial constraints, and economic feasibility. The framework developed here can inform ecosystem-based design in delta regions worldwide, supporting integrated climate adaptation that aligns safety with ecological resilience.

Riparian forests in front of levees can dampen incoming waves and reduce erosion, thereby decreasing the risk of flooding. This makes them a sustainable addition to stand-alone dikes, potentially decreasing the overall costs for future dike reinforcement measures. However, there are currently no guidelines for the design, monitoring, and maintenance of dike-forest combinations as significant uncertainties remain. For instance, most studies on the effectiveness of forests in attenuating waves focus on mild wave climates or rely on scaled tests using simplified tree mimics. As a result, prototype-scale studies on wave damping by forests under extreme conditions—essential for design and assessment purposes—are lacking.

This thesis investigated wave damping by riparian forests, with a specific focus on pollard willow trees, which are commonly found along the riverbanks in the Netherlands and other parts of Europe. The primary aim was to reduce the uncertainties associated with the application of forest-dike combinations. The foundation and source of innovation for this thesis is the data from real-scale flume tests, conducted on a 40-m-long live pollard willow forest, subjected to significant wave heights up to 1.5 metres. The analysis of these tests revealed several important areas for further investigation. First, the vertical frontal-surface area (Av) distribution of leafless trees should be measured in detail, as leaves were found to minimally affect wave damping. Second, flume studies at various scaled often form the basis of calibration and validation of analytical, numerical and empirical wave-vegetation models, however, the extent to which small-scale tests accurately represent wave-vegetation interaction at real scale remains unknown. The data from the real-scale tests made it possible to design scaled tests (with 3D-printed tree mimics) and compare the results between both scales. Lastly, during real-scale tests, the live tree branches were observed to sway by nearly 180 degrees under the highest water levels and wave conditions, highlighting the importance of and need for further research into branch flexibility.

Numerical models of vegetation largely underestimate the vegetation surface by assuming that vegetation consists of only stems and a single branch order and by neglecting tapering of branches. In Chapter 2, we investigate methods to obtain accurate Av distributions over the height of live willow trees. One method used a combination of manual measurements and tree allometry relations to create tree models (resulting in a detailed representation of Av). This method was compared to the results of a relatively more practical method: Terrestrial Laser Scanning. The findings showed a large variation of (calibrated) bulk drag coefficients between measuring methods and highlight the importance of reliable frontal-surface area estimations and consequently for reliable wave attenuation predictions.

Until now, no prior studies have compared real-scale and scaled tests with woody vegetation. We therefore conducted scaled tests with complex 3D-printed willow tree mimics to explore scale effects in scaled tests with vegetation (Chapter 3). The maximum measured wave damping (30%) was shown to be roughly 1.5 times higher than the real-scale tests (20%) for water levels just above the knot of the trees. The amount of wave height damping decreased for larger water levels, following the same trend as that of the real-scale tests. The largest effects were attributed to increased viscous damping (due to smaller branch Reynolds numbers), and non-exact flexibility scaling. These notable deviations illustrate that real-scale tests, though expensive, may still be needed to validate the results of scaled tests for woody vegetation. Alternatively, accounting for these discrepancies can increase the reliability of scaled tests for wave damping studies on woody vegetation and reduce the need for more expensive real-scale tests.

Additionally, scaled tests with flexible conical shapes were conducted to study the effects of flexibility on wave damping in greater detail (Chapter 4). The first-mode cone deflection was determined at ~0.7 times the length of the cone to avoid higher-order modes in the measurements. The findings showed that cone deflections greater than 5 degrees had a large spread in force reduction and resulted in a significant decrease in measured forces of up to 50% compared to their rigid counterparts. This work demonstrated that the effective length principle, which has already been successfully applied to grassy vegetation such as salt marshes and seagrass, is a promising dimensionless parameter for predicting force reduction in conical shapes--and could potentially be extended to tree canopies.

Lastly, the experimental data was used as input for analytical wave damping models, which allowed us to discuss the opportunities for riparian forest-dike solutions in the Netherlands (Chapter 5). The outcome of our probabilistic study suggested that pollard forests in front of existing dikes offered the greatest benefit in mitigating failure caused by the erosion of grass on the outer-slope of the dike due to wave impact. We also discussed that the height of the trunk, which determines the location of the knot—where the frontal surface area, and consequently wave damping, are greatest—can serve as a key design parameter for forest-dike systems.

The thesis offers an overview of key parameters and their associated uncertainties, contributing to the ongoing integration of (riparian) forests into dike design and assessment methodologies.

Riparian forests in front of dikes can dampen incoming waves and thereby contribute to flood safety. In real-scale flume experiments with live pollard willow trees (forming a 40-m-long forest), it was observed that during storm conditions, a maximum reduction of 20 % in incoming wave height could be achieved (van Wesenbeeck, et al., 2022). Notably, this amount of wave damping occurred at a water depth of 3 meters, aligning with the section of the trees with the maximum frontal-surface area. For a larger water depth, measured wave damping however declined. This is potentially partly caused by the natural tapering form of the trees. Typically, trees are characterized by smaller branch diameters and more flexible branches higher up in the canopy (McMahon & Kronauer, 1976); and flexible vegetation mimics are known to dampen less than rigid mimics due to motion (Van Veelen, T, Reeve, & Karunarathna, 2020). Hence, both the frontal-surface area and branch rigidity decrease along the height of the willow trees, potentially leading to less wave damping by the forest when subject to large waves at higher water levels.

Vegetation in front of levees, dikes and seawalls can decrease wave energy and therefore contribute to the safety against flooding. However, wave damping predictions by vegetation are still inaccurate due to measurement and modelling uncertainties. Many studies focused on finding reliable predictive tools using scaled flume tests with vegetation mimics. Scaling down vegetation can however lead to discrepancies with realistic scales, known as scale errors. In this work scaled tests were conducted on 3D-printed elastic replicas of willow trees and compared with real-scale experiments. We identified differences in measured wave dissipation between the scaled hydraulic model (1:10) and its real-scale prototype with 5m high live willow trees under storm conditions (1:1). The maximum measured wave damping (30%) was roughly 1.5 times higher than the real-scale experiments (20%). Following the same trend of the real-scale experiments, this amount of wave height damping declined for larger water levels. Largest effects are argued to be due to increased viscous damping (smaller branch Reynolds numbers) and non-exact flexibility scaling. These significant deviations illustrate that full-scale experiments, although expensive, are still needed to validate the results of scaled experiments for woody vegetation. Alternatively, accounting for these discrepancies can make scaled experiments more reliable and expensive real-scale experiments less needed for wave damping studies on woody vegetation.

Worldwide, communities are facing increasing flood risk, due to more frequent and intense hazards and rising exposure through more people living along coastlines and in flood plains. Nature-based Solutions (NbS), such as mangroves, and riparian forests, offer huge potential for adaptation and risk reduction. The capacity of trees and forests to attenuate waves and mitigate storm damages receives massive attention, especially after extreme storm events. However, application of forests in flood mitigation strategies remains limited to date, due to lack of real-scale measurements on the performance under extreme conditions. Experiments executed in a large-scale flume with a willow forest to dissipate waves show that trees are hardly damaged and strongly reduce wave and run-up heights, even when maximum wave heights are up to 2.5 m. It was observed for the first time that the surface area of the tree canopy is most relevant for wave attenuation and that the very flexible leaves limitedly add to effectiveness. Overall, the study shows that forests can play a significant role in reducing wave heights and run-up under extreme conditions. Currently, this potential is hardly used but may offer future benefits in achieving more adaptive levee designs.

The last years, capacity of vegetation to reduce wave impact is receiving considerable attention. To predict wave attenuation processes within vegetation fields reliable estimates of vegetation parameters are needed. This proves to be difficult for woody vegetation as it consists of complex branch structures, characterized by varying branch densities, diameters and angles. State of the art physical and numerical models effectively use a single value for the diameter, b _v and density, N of vegetation, which is unrepresentative for complex vegetation, such as trees. Trees can be better described by the projected frontal-surface area, A _v. Hence, this work compares methods to quantify the A _v in space for a pollard willow forest, and determines suitability of these methods for predicting wave attenuation using a spectral wave model (SWAN). We use data from manual measurements and Terrestrial Laser Scans (TLS), to estimate the vertical distribution of A _v; and data from large-scale flume experiments performed on a willow forest to verify model sensitivity to A _v inferences. As a baseline for comparison, tree models that describe the structure of the trees in various degrees of complexity are compiled. The most realistic tree model is used to quantify potential errors in TLS and basic manual measurements of N and b _v. An initial comparison shows that the TLS data underestimates A _v, which indicates that conducting manual measurements is more suitable to quantify a homogeneous forest. We found that the TLS suffers from shadowing effects (i.e., blockage of laser beams) and we recommend to apply a correction factor to improve its measurements. Furthermore, we identified the impact that the different methods to determine A _v have on the estimation of wave attenuation using SWAN; in addition we verified the model results with data from large-scale flume experiments performed on the willow forest. The modeled sensitivity tests indicate large differences in wave attenuation and, consequently, a wide range (0.94–1.70) of bulk drag coefficients, (Formula presented.), for the various methods applied. This shows the variation of outcome between measuring methods and highlights the importance of stating the selected method for reliable frontal-surface area estimations, and consequently for reliable wave attenuation predictions.

Bomen op voorlanden langs dijken dissiperen golfenergie en bevorderen opslibbing. Welk soort bos levert de meeste biodiversiteit én grootste bijdrage aan de waterveiligheid? En hoe zal dit in tijd en ruimte verschillen? Op zoek naar optima, als aanvulling op het wettelijke dijkenbeoordelingsinstrumenterium.