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.

Hybrid dune-dike structures are innovative developments creating coastal defense systems which are more conveniently integrated with the natural environment. In this study, a numerical study was conducted to investigate the temporal evolution of wave overtopping, with the changing profile of the dune under extreme storm conditions with a constant water level, of two types of hybrid dune-dike structures in Katwijk (dike-in-dune type) and Raversijde (dune-in-front-of-dike type). XBeach 1DH was used to firstly calculate bed profiles for different time steps during a 10-h storm duration using the Surfbeat mode and then, in a second step, mean wave overtopping rates were modelled for each calculated bed profile using the Non-hydrostatic mode. According to the simulation results, most of the dune erosion occurs during the first two hours of the storm, and then continues at a slower rate as the sand deposits in front of the dune. Once the hybrid structure is eroding (so for t > 0), the significant wave height at the dike toe and the mean overtopping discharge increase in time for both Katwijk and Raversijde, although it quickly reaches a plateau for Raversijde. The first simulations with the original non-eroded profiles deviate from this trend. The reason for this deviation needs to be further investigated.

The passage of ships in confined waterways creates a stern wave that can overflow bank protection structures such as groins. This overflow, due to the long-period primary ship-induced waves, can be high in velocity, especially at the lee-side slope of groins, potentially causing significant damage to the structure. This study derives an equation to express overflow velocities, intended as a design tool for groins exposed to these types of waves. A detailed experimental investigation was performed on four physical models of groins with different slopes and stone sizes in the armor layer under the influence of different hydraulic heads. Particle image velocimetry (PIV) was used to measure the flow velocities at the crest and lee sides of the structure. All PIV measurements were performed thrice under free-flow conditions with no initial water level at the lee side of the structure. The depth- and time-averaged flow velocities (Uavg) were extracted from four positions along the lee-side slope and accelerated from 0.7 to 2.2 m/s. A dimensionless equation of the overflow velocities was obtained as a function of the hydraulic head (h), slope (θ), freeboard (Rc), and nominal rock diameter (dn50).

Saltmarshes are a promising nature-based alternative for conventional flood protection. However, saltmarshes can erode under storm conditions, whereby the seaward edge of the saltmarsh often forms a vertical cliff. Despite its importance, the effect of storm conditions on erosion at the saltmarsh cliff remains understudied, especially when waves traverse over a cliff. This research investigates the complex flow patterns around a saltmarsh cliff non-intrusively using Particle Image Velocimetry (PIV) conducted through a series of scaled monochromatic wave flume experiments. We adopted realistic foreshore configurations (e.g. cliff heights) and hydraulic loading conditions from the Dutch Wadden Sea. Results show two local near-bed velocity maxima on top of the saltmarsh, created during different wave phases by water depth contraction, wave transmission and interaction between flow and vortices that are shed from the cliff. Under the wave crest, high onshore-directed near-bed velocities were measured at approximately 2.5–4 times the cliff height onshore from the cliff. Under the wave trough, high offshore-directed velocities were found at the marsh edge. Both onshore- and offshore-directed velocities increase with increasing cliff height, larger wave height or lower water depth. Vegetation on top of the marsh reduces both the incoming and outgoing velocities in front of the cliff. Increasing the cliff height resulted in a greater reduction in velocities by the vegetation. These results demonstrate how local near-bed velocity maxima and location are influenced by the presence of a cliff and the interaction with vegetation on top of the saltmarsh. This research highlights the vulnerability of the cliff even during inundation of the cliff and will help to implement saltmarshes as nature-based solutions for flood defence.

This research investigates how salt marshes contribute to both wave energy dissipation and spectral period transformation, advancing their role as a nature-based solution for coastal protection. Using laboratory simulations with a scaled barren foreshore, salt marsh and dike model, we examine the interactions between vegetation, water depth, and wave properties under varied conditions, including storm scenarios with irregular waves. Results indicate a case specific threshold at which the salt marsh model attenuates energy optimally, as for very shallow water depths wave energy is predominantly dissipated by the barren foreshore. The spectral wave period T m − 1 , 0 increases when waves propagate from deep to shallow water depths, as a result of wave breaking and generation of infragravity waves. The presence of salt marsh vegetation further enhances this effect by preferentially damping high frequency components. This highlights that an increase in T m − 1 , 0 in vegetated environments may not always correspond to an increased hydrodynamic load on the dike.

Rock groins in the Elbe Estuary are constructed to maintain proper water levels for navigation and for embankment erosion protection. At certain localities, significant damages to rock groins have been observed due to the primary ship-generated waves. Primary waves are generated along the ship's hull and then propagate toward the river banks and groin fields, appearing in the interaction with the structures as a turbulent overflow phenomenon. Eventually, this overflowing may cause damages mainly to the crest and leeward side of the groins. Since this overflowing is the most pronounced with large primary waves at certain water levels, the estimation of the probabilities of extreme primary waves is a key element for a safe and reliable design of groins. For this goal, nonparametric Bayesian networks (NPBNs) are used here to infer the probability distribution function of the extreme primary wave heights at the tip of a groin in the Elbe Estuary. Results demonstrate the suitability of the NPBN in their prediction. The model framework allows the designer to predict the probabilities of primary ship-generated waves at groins when the information of ship dimensions, nautical parameters, and waterway geometry is available. These probabilities can later be used for design purposes for current and future conditions.

During extreme high-water events, the phreatic water level in levees will rise over time due to infiltration of water. This can promote slope instability or internal erosion, and eventually lead to structural failure. A potential solution is the application of an impermeable seal, such as a geotextile, to the levee’s outer slope to locally reduce the inflow of water. In this study, the spatiotemporal effect of a seal on the phreatic surface level is investigated experimentally, both at laboratory scale for a homogeneous sand levee, and at full-scale for a more realistic levee design. On the two-dimensional laboratory scale, it was found that application of a seal does not significantly change the steady-state phreatic level, as expected from a theoretical perspective. However, the time for the phreatic surface level to reach steady state after a sudden external water rise was found to increase 25% to 50% in the cases with a seal. Similar results were found for the full-scale three-dimensional experiments, which showed that details of the soil-structure interface significantly influenced the effectiveness of the impermeable seal, increasing the time to steady state between 12% and 25%. A simple numerical transient groundwater flow model confirms that the quality of the seal governs the response of the phreatic level. This model required the inclusion of an interface layer to properly model the imperfect soil-seal conditions. It is concluded that application of an impermeable seal to a levee before sudden water rise does not influence the new steady-state phreatic level. However, the seal slows down the infiltration process, especially for a case where the outer slope is damaged.

This study treats a detached homogenous low-crested structure (HLCS) made of Cubipod concrete elements placed seaward of a vertical wall (forming a basin in between) to reduce overtopping. Assessing the complex hydrodynamics and effects of changing the geometry of such a system in relation to overtopping reduction is challenging. The numerical model OpenFOAM was applied to this end. Forchheimer coefficients for wave transmission and the flow through the HLCS were calibrated and validated using existing physical modeling data (α = 500 and β = 1.0, with varying porosity based on the Cubipod shape), while the effect of the basin and vertical seawall was determined fully numerically. The crest freeboard (R_c), crest width (B), and basin length (L_B) were the main geometrical parameters that influenced the performance of the HLCS in reducing overtopping. An exponential decay was observed in the overtopping discharge when the values of these geometrical parameters increased. As L_B increased, this decay was primarily due to the dissipation of the broken-wave bores. The largest gradient in the predicted overtopping discharge was noted at R_c/H_s,i ≈ 0, B/H_s,i ≈ 4.5, and L_B/L_p ≈ 1.2, where H_s,i is the incident significant wave height and L_p is the peak wavelength in the basin.