The crest level of seawalls is often based on estimates of the amount of wave overtopping. Methods to estimate the mean overtopping discharge have been provided in several guidelines. One of the important parameters affecting wave overtopping is the wind. However, the effects of wind have not been accounted for in detail in present design guidelines although some guidance for coastal structures with crest elements is provided in literature. For onshore wind the expected wave overtopping discharge at coastal structures with a crest element can be up to a factor 5 larger than for situations without wind. In the present study the maximum influence of wind on wave overtopping at impermeable seawalls with crest elements has been studied based on physical model tests. The result of the study is a guideline to estimate the maximum influence of wind on wave overtopping at seawalls with crest elements.

Physical model tests have been performed to study wave overtopping at rubble mound breakwaters, including breakwaters with a crest wall, breakwaters with a berm, and breakwaters with a crest wall and a berm. For rubble mound structures with a protruding crest wall or with a stable berm, limited information is available in literature even though protruding crest walls and berms clearly affect wave overtopping discharges. Adding a crest wall to an existing structure, increasing the height of a crest wall, adding a berm, or increasing the width or height of a berm, can be effective measures to account for effects of sea level rise if the sea level rise appears to be more severe than the amount of sea level rise for which the structure was designed for. The present wave flume tests were used to develop guidelines for rubble mound breakwaters, including breakwaters with a crest wall or with a berm. The relative height of the protruding part of a crest wall dominates the effect of a crest wall. The berm width, berm level and wave steepness all affect the influence of a berm on the wave overtopping discharge. Moreover, it was confirmed that the wave steepness also affects wave overtopping discharges for rubble mound breakwaters without a berm or without a crest wall. The developed set of expressions for rubble mound structures has also been validated based on existing data for oblique wave attack on rubble mound breakwaters with a crest wall.

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.

Underneath rock slopes of rubble mound structures often one or more granular filter layers are present. These filter layers prevent base material washout. In contrast to traditional filters, geometrically open filters allow for some movement of base material. In order to design such open filters the amount of erosion and accretion of base material needs to be predicted. Based on 2D physical model tests Van Gent and Wolters (2015) presented a method to estimate the erosion and accretion of sand underneath rock slopes. Since the developed method was only valid for perpendicular wave attack, a new study has been performed to assess effects of oblique wave attack on the amount of erosion and accretion of sand underneath a rock slope. For that purpose 3D physical model tests were performed in a wave basin with 5 different wave angle between 0° (perpendicular wave attack) and 75°. Based on the test results it was shown how the effects of wave obliquity can be taken into account in the design of open filters. In addition, the effect of the storm duration on open filters has been studied. This led to an extended design method to predict the amount of erosion and accretion of sand underneath rock slopes under wave loading.

Moving water exerts drag forces on vegetation. The susceptibility of vegetation to bending and breakage determines its flow resistance, and chances of survival, under hydrodynamic loading. To evaluate the role of individual vegetation parameters in this water-vegetation interaction, we conducted drag force measurements under a wide range of wave loadings in a large wave flume. Artificial vegetation elements were used to manipulate stiffness, frontal area in still water and material volume as a proxy for biomass. The aim was to compare: (i) identical volume but different still frontal area, (ii) identical stiffness but different still frontal area, and (iii) identical still frontal area but different volume. Comparison of mimic arrangements showed that stiffness and the dynamic frontal area (i.e., frontal area resulting from bending which depends on stiffness and hydrodynamic forcing) determine drag forces. Only at low orbital-flow velocities did the still frontal area dominate the force-velocity relationship and it is hypothesised that no mimic bending took place under these conditions. Mimic arrangements with identical stiffness but different overall material volume and still frontal area showed that forces do not increase linearly with increasing material volume and it is proposed that short distances between mimics cause their interaction and result in additional drag forces. A model, based on effective leaf length and characteristic plant width developed for unidirectional flow, performed well for the force time series under both regular and irregular waves. However, its uncertainty increased with increasing interaction of neighbouring mimics.

Rubble mound breakwaters and revetments typically contain granular filters in one or more layers. The transition from the armour layer to the filter layer, and transitions between other layers within the structure, are normally geometrically tight to prevent material washout. This requires a limited ratio of the material size of the upper layer and neighbouring layer. An alternative is a geometrically open filter where in principle underlayer material can be transported into the upper layer, but if the hydraulic load at this transition between two layers remains low, the transition can be designed such that no or limited transport occurs, see for instance Van Gent and Wolters (2015), Van Gent et al (2015) and Jacobsen et al, (2017). This allows for larger ratios of material sizes, which can reduce the number of filter layers, and relax the material requirements with respect to the width of gradings. This can lead to considerable cost savings. In Van Gent and Wolters (2015) physical model tests for the transition between a layer of rock and an underlayer that consists of sand have been performed and design guidelines have been derived. Here, additional physical model tests are presented to study the influence of the storm duration and water level variations on the response of sand underneath a layer of rock.

This paper will address the validation and application of a volume of fluid method for coastal structures under the influence of normal incident irregular wave fields. Several physical processes will be addressed as part of the validation process, namely: (i)wave reflection from permeable and impermeable structures, (ii) wave transformation over a small shoal, (iii) wave damping inside of a permeable structure, (iv) the resulting wave induced internal setup and (v)wave induced forces. The numerical model will be validated against a multiple of experimental data sets for two dimensional coastal problems. The impact of air-relief gaps on the modelled wave induced pressures on a crest wall is analysed for the two dimensional layout of the structure. This is analysed to study the importance of the cushion effect from the incompressible air phase. Subsequent to the validation of the numerical model the internal setup in permeable coastal structures is given separate attention. A combination of an analytical prediction of the magnitude of the internal setup and numerical results is used to derive numerically based empirical formulae for the magnitude and the time scale of the internal setup. The formulae include the effects of wave height, wave period, material properties of the coastal structure and the dimensions of the structure.

Permeable hydraulic structures that consist of rock material typically contain granular filters in one or more layers. These filters are normally geometrically tight to prevent material washout. Geometrically tight filters are often difficult to realize in the field and expensive. An alternative is a geometrically open filter. A geometrically open filter has a large ratio of the size of toplayer material (rock) and underlayer material (e.g., sand) and is designed in such a way that only minimal base material loss or settlement occurs. Potential applications of open filters include bed protections, toe structures, and slope protections. Proper guidelines on the design of open filters under wave loading could lead to significant cost and material savings, and to more practical applications of filters in the field. Physical model tests were conducted in a wave flume of Deltares. The analysis of the tests with 1:4 and 1:7 slopes and fixed test durations of 3. h (<. 10,000 waves) has led to a method of predicting the amount of erosion of the sand underneath granular filters and of the sand accretion within a granular filter. Ranges of applicability have been provided for the underlying formulae. The tested material ranges included filter rock sizes close to prototype scale. In addition, a criterion has been developed to define the amount of acceptable transport in open filters.

Coastal communities around the world face an increasing risk from flooding as a result of rising sea level, increasing storminess and land subsidence¹². Salt marshes can act as natural buffer zones, providing protection from waves during storms³⁷. However, the effectiveness of marshes in protecting the coastline during extreme events when water levels are at a maximum and waves are highest is poorly understood^8,9. Here we experimentally assess wave dissipation under storm surge conditions in a 300-metre-long wave flume tank that contains a transplanted section of natural salt marsh. We find that the presence of marsh vegetation causes considerable wave attenuation, even when water levels and waves are highest. From a comparison with experiments without vegetation, we estimate that up to 60% of observed wave reduction is attributed to vegetation. We also find that although waves progressively flatten and break vegetation stems and thereby reduce dissipation, the marsh substrate remained stable and resistant to surface erosion under all conditions. The effectiveness of storm wave dissipation and the resilience of tidal marshes even at extreme conditions suggest that salt marsh ecosystems can be a valuable component of coastal protection schemes.

Rubble mound coastal structures typically contain granular filters in one or more layers. These filters are normally geometrically tight (to prevent material washout), often difficult to realize in the field, and expensive. An alternative is a geometrically open filter. A geometrically open filter has a large ratio of the size of toplayer material and underlayer material and is designed in such a way that it fulfills its filter functions with only minimal base material loss or settlement. Potential applications of open filters include bed protections and toe & slope configurations of coastal structures. Proper guidelines on the design of open filters under wave loading could lead to significant cost and material savings, and to a more practical application of filters in the field. Physical model tests were conducted in a wave flume. These tests focussed on granular open filters on a 1:7 sand slope under wave loading. The analysis of the tests has led to guidelines on the amount of erosion of the sand underneath a granular filter and of the sand accretion within a granular filter.

Rubble mound coastal structures typically contain granular filters in one or more layers. These filters are normally geometrically tight (to prevent material washout), often difficult to realize in the field, and expensive. An alternative is a geometrically open filter (i.e. a large ratio of the size of toplayer material and underlayer material), designed in such a way that it fulfills its filter functions with only minimal base material loss or settlement. Potential applications of open filters include bed protections and toe & slope configurations of coastal structures. Proper guidelines on the design of open filters under wave and current loading could lead to significant cost and material savings, and to a more practical application of filters in the field. The physical model tests conducted in this study focus on granular open filters on a horizontal sand bed under wave and combined wave & current loading.

Stability formulae for armour layers of rubble mound breakwaters are usually being applied assuming perpendicular wave attack. Often it is assumed that for oblique wave attack the reduction in damage compared to perpendicular wave attack is small. This seems however a very conservative assumption. Wave basin tests at Deltares provide information to assess the effects of oblique waves on the stability of rock slopes and cube armoured rubble mound breakwaters. This includes cubes in a single layer and cubes in a double layer. The results show that the few available formulae that include wave obliquity underestimate the effects of oblique wave attack; the observed damage to breakwaters with armour layers of rock and cubes is lower and therefore new stability increase factors and mass reduction factors have been developed. The tests were performed for wave directions between perpendicular (0°) and 70°. The results show that large potential savings in diameter and mass can be obtained for large angles of wave obliquity.

Most established laboratories that perform physical modelling have particular methods for hydraulic model studies. Physical modelling procedures in different laboratories vary in e.g. wave generation techniques; typical storm sequences; wave calibration techniques; scaling of short duration (impulsive) loadings; scaling of permeable materials; monitoring of damage and quantifying small armour movements; overtopping analysis; analysis and verification procedures; factors of safety etc. The potential differences in modelling results are difficult to judge, but may be significant in some cases. Reliable comparison of model test results will only be possible if the test set-up, measurement techniques and modelling approach are verifiably (i.e. geometrically, dynamically and kinematically) similar. This is often not the case, so comparisons between model results from different laboratories and data transfer between laboratories are complicated. This paper presents new guidelines (available for download at http://www.hydralab.eu/) to form a basis for a more unified physical modelling approach, which simplifies data transfer and the interpretation of modelling results gathered by varying laboratories and modelling approaches. To achieve this more unified modelling approach, guidelines for good laboratory practice in physical (experimental) model testing of coastal structures exposed to wave action have been developed.

Crown walls or superstructures on top of permeable breakwaters are often used as a measure in existing structures to counteract insufficient design protection against overtopping. These crest elements, generally located well above the design water level, have been found to effectively decrease the mean overtopping discharge over a dike or rubble mound structure. The current study focusses at investigating the reduction in overtopping which can be expected from crest elements on a permeable breakwater.