At the crest of rubble mound structures a crest wall is often present to reduce the consumption of material compared to breakwaters without a crest wall. To design the crest wall several methods based on estimates of the hydraulic wave loads are available. These methods are focussed on waves that approach breakwaters perpendicular. In practice, the dominant wave conditions are often not perpendicular but approach the breakwater under an angle. Oblique waves can significantly reduce the amount of wave overtopping and reduce the wave loads on coastal structures compared to perpendicular waves. Therefore, physical model tests have been performed in a wave basin where the wave loads on crest walls and the wave overtopping discharges have been measured under oblique wave attack. Design guidelines have been developed to take the reduction due to oblique waves into account. Together with available methods to predict the wave forces and wave overtopping for perpendicular wave attack, the obtained results enable a more advanced design of crest walls.

Most guidelines on wave overtopping over coastal structures are based on conditions with waves from one direction only. Here, wave basin tests with oblique wave attack are presented where waves from one direction are combined with waves from another direction. This is especially important for locations where wind waves approach a coastal structure under a specific direction while swell waves approach the coastal structure under another direction. The tested structure was a dike with a smooth and impermeable 1:4 slope. The test programme consisted of four types of wave loading: (1)Wind waves only: "sea" (approaching the structure with an angle of 45°), (2)Wind waves and swell waves from the same direction (45°), (3) Wind waves and swell waves, simultaneously from two different directions (45° and -45°, thus perpendicular to each other), and (4) Wind waves, simultaneously from two different directions (45° and -45°, thus perpendicular to each other). Existing guidelines on wave overtopping have been extended to predict wave overtopping discharges under the mentioned types of wave loading (oblique sea and swell conditions).

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.

The toe structure of a breakwater provides support to the armour layer and protects the structure from damage due to scour at the toe. Often a toe structure consists of rock material. Several design formulae exist to predict the amount of damage to the toe structure under wave loading. These design formulae for the required rock size include effects of the wave height and the water depth above the toe structure. Here, rock toe stability has been studied by means of physical model tests to provide information on the required rock size in the toe structure. The tests and analysis are focussed not only on the influence of the wave height and the water depth above the toe structure, but also on the influence of the width of the toe structure, the thickness of the toe and the wave steepness. The wave steepness, width of the toe and the thickness of the toe appear to affect the damage to the toe; these parameters need to be taken into account in order to derive accurate predictions of the damage to the toe structure. Based on the test results a prediction formula has been derived including these effects. The formula can be used to determine the required rock size in the toe of rubble mound breakwaters within the ranges of the performed tests.

The toe of rubble mound structures is important since toe structures need to provide support to the armour layer and prevent scour to occur directly beneath the armour layer. In Van Gent and Van der Werf (2014) an expression for the prediction of the required stone size in toe structures has been derived. This prediction formula was derived for toe structures that consist of rock in combination with rock slopes. Additional tests have been performed a) to analyse the influence of the slope of the rock armour layer, b) to analyse the influence of the roughness of the armour layer, and c) to analyse the potential use of interlocking toe blocks, either in combination with rock slopes or slopes that consist of Cubes in a single layer. It appeared that the derived prediction method is accurate for 1:1.5 slopes and 1:2 slopes, that the roughness of the slope plays an important role and can be incorporated in the prediction method as well, and that interlocking toe blocks with a V-shape are an alternative to rock toe structures.

The stability of rock slopes with a horizontal berm has been studied by means of physical model tests. This paper is focussed on the rock slope stability of the slopes above and below the berm. By applying a berm the rock size can be reduced compared to the required rock size for a straight slope without a berm. This reduction can be significant for the slope above the berm. The influence of the slope angle (1:2 and 1:4), the width of the berm, the level of the berm, and the wave steepness have been investigated. Based on the test results prediction formulae have been derived to quantify the required rock size for rubble mound breakwaters with a berm.

The presented study focussed on issues related to the hydraulic stability during the construction of rubble mound breakwaters. During construction the temporary roundheads differ from the roundheads in the final stage. Often a submerged uncompleted part of the breakwater is present in front of the section that reached its final crest elevation. This can occur because often the breakwater is being constructed with maritime-based equipment up to a certain elevation while the upper part of the breakwater is being constructed with land-based equipment. Three-dimensional physical model tests were performed to analyse the influence of the submerged part on the stability of the emerged part. The locations incurring the most damage clearly differ. Furthermore, for some combinations of water level, wave direction and structure geometry more damage was found for structures with a submerged part, although most combinations showed that the amount of displaced stones is generally lower for the structures with a submerged part.

This paper focusses on some fundamental aspects of gravel beaches and revetments with dynamic rock slopes. Obviously, the response of gravel beaches depends on parameters such as the wave height and the stone size. Here, other parameters such as the permeability of the subsoil, the influence of sand in the pores of gravel, and the influence of seawalls have been studied. The results from the physical model tests indicate that a homogeneous mound of gravel with an initial slope of 1:8 leads to a more dynamic response than a gravel beach with an impermeable subsoil. The results also show that if the pores are filled with sand, the response becomes less dynamic. Placing a seawall on a gravel beach may be interesting for gravel beaches that serve as (man-made) sea defence. Furthermore, the results provide insight into the magnitude of scale effects.