The average overtopping discharge is an important parameter for the design and reinforcement of dikes. Rock armour on the waterside slopes and berms of dikes is widely used to reduce the wave overtopping discharge by introducing slope roughness and dissipation of energy in the permeable armour layer. However, methods for estimating the influence of a rock berm and roughness of rock armour at dikes on the average overtopping discharge still need to be developed and/or validated. Therefore, this study aims to develop empirical equations to quantify the reductive influence of rock armour on wave overtopping at dikes. Empirical equations for estimating the effects of rock berms and roughness are derived based on the analysis of experimental data from new physical model tests. The influence of roughness of the rock armour applied on parts of waterside slopes is estimated by introducing the location weighting coefficients. Results show that the newly derived equations to predict the average overtopping discharge at dikes lead to a significantly better performance within the tested ranges compared to existing empirical equations.

Wave overtopping can cause erosion on the landward slope due to high flow velocities and turbulence that cause high stresses on the cover. Innovative block revetments such as Grassblocks protect the subsoil of the dike against erosion. The blocks are permeable, which reduces the flow velocity and the pressures along the landward slope. The performance of these blocks is assessed in physical tests, which provides insights into the stability of the blocks. However, such experiments are expensive and accurate measurements are difficult due to highly turbulent conditions. Therefore, the goal of this study is to determine the hydrodynamic conditions at the dike cover caused by the wave run-up on the seaward slope and by the overtopping flow over the crest and landward slope. The geometry and wave conditions from the physical test at the Deltares Delta flume are implemented in an OpenFOAM^® numerical model. Using the porousWaveFoam solver, a porous layer on the crest and landward slope is implemented, where the flow resistance of this porous layer largely depends on the resistance coefficients $\alpha$ [-] and $\beta$ [-]. The numerical model is calibrated based on resistance coefficients as introduced earlier in the literature, which showed that the resistance coefficients of $\alpha =500$ and $\beta =2.0$ performed best for the peak flow velocities and the peak pressures. The numerical model is evaluated by using these resistance coefficients in other time series of the physical tests. The evaluated model is then used to determine the hydrodynamic conditions on the landward slope, which showed that the pressure was the most influential hydrodynamic condition at the time of failure. Finally, the model showed that a porosity of $n=0.6$ and the porous layer thickness $\eta =36\mathrm{mm}$ reduced the peak pressure the most.

Climate change and anthropogenic activities are enhancing salt intrusion in many estuaries, which reduces the supply of drinking water as well as freshwater for agricultural and industrial uses and can further cause severe economic losses. Nature‐based solutions emerge as sustainable and ecologically sound strategies to mitigate salt intrusion in estuaries. This review paper explores the effectiveness of nature‐based solutions in salt intrusion mitigation. Advances in the understanding of estuarine salt dynamics are summarized, thus identifying the dominant parameters that determine the salt intrusion in different types of estuaries. Six potential nature‐based solutions (NBSs) for salt intrusion mitigation are conceptualized, including improvement of river discharge management, reduction of channel cross‐section, modification of bottom topography, utilization of friction‐convergence balance, enlargement of intertidal area and introduction of meanders. These NBSs are grouped into two types: (a) advection‐based and (b) mixing‐based. Numerical model assessments indicate that all these NBSs have potential to reduce salt intrusion lengths, depending on the salinity structure of the estuary. Nevertheless, applications of NBSs to real estuaries are rare. Directions toward improving the implementation of nature‐based solutions for mitigating salt intrusion in estuaries are provided.