Shoreface nourishments have become a popular management option to mitigate coastal retreat for sites with abundant sand supplies. With shoreface nourishments, relatively large volumes of off-site sand are placed under water in typical water depths of 4–10 m. This part of the nearshore zone has a high bed level variability and contains a myriad of (rhythmic) morphological features. As a result, understanding and forecasting shoreface nourishment morphodynamics and impacts is challenging. Significant progress on this topic is needed in due time, especially in light of emerging climate-change effects. This review paper presents an overview of field, laboratory and numerical modeling studies on shoreface nourishment morphodynamics. We have identified 4 key knowledge gaps. First, the spreading of nourished sand through the coastal zone is poorly understood, and has not been quantified. Second, it is unclear how design variables such as size, placement location and grain-size affect the lifetime, spreading and impact of shoreface nourishments. Third, the cumulative effect of repeated shoreface nourishments (scale: 1–10 km, 1–10 years) on the coastal system as a whole (100+ km, 50+ years) is largely unknown. Fourth, numerical models cannot reliably predict the complete morphological development and impact of shoreface nourishments. To tackle these knowledge gaps we propose a research agenda to ensure the generation and valuation of scientifically robust and societally relevant knowledge.

Wave nonlinearity plays an important role in cross-shore beach morphodynamics and is often parameterized in engineering-type morphodynamic models through a nonlinear relationship with the Ursell number. It is not evident that the relationship established in previous studies also holds for sheltered sites with fetch-limited seas as they are more prone to effects of local winds and currents, the waves are generally steeper, and the beaches are typically reflective. This study investigates near-bed orbital velocity nonlinearity from wave records collected at two sheltered beaches in The Netherlands and contrasts them to earlier observations made along the exposed, wave-dominated North Sea coast. Our observations at sheltered beaches show that the Ursell number has comparable skill in predicting wave nonlinearity as it has on previously studied exposed coasts. However, the orbital velocities at sheltered coasts are more asymmetric for the same Ursell number than on exposed coasts. When exposed coast data were examined for moments with comparable high-steepness waves, a similar effect on asymmetry was observed. In addition, following and opposing winds were found to have a clear relationship with total nonlinearity, while they did not affect the phase between skewness and asymmetry at the sheltered beaches. Refitting the free parameters of an Ursell-based predictor improved the bias for the asymmetry parameterization. Whether this has implications for modeling of the magnitude of wave-nonlinearity-driven sediment transport using engineering type models is strongly dependent on the sediment transport formulation used, as these formulations depend on additional calibration coefficients too.

Shoreface nourishments are commonly applied for coastal maintenance, but their behaviour is not well understood. Bathymetric data of 19 shoreface nourishments located at alongshore uniform sections of the Dutch coast were therefore analyzed and used to validate an efficient method for predicting the erosion of shoreface nourishments. Data shows that considerable cross-shore profile change takes place at a shoreface nourishment, while an impact at the adjacent coast is hard to distinguish. The considered shoreface nourishments provide a long-term (3 to ~30 years) cross-shore supply of sediment to the beach, but with small impact on the local shoreline shape. An efficient modelling approach is presented using a lookup table filled with computed initial erosion-sedimentation rates for a range of potential environmental conditions at a single post-construction bathymetry. Cross-shore transport contributed the majority of the losses from the initial nourishment region. This transport was driven partly by water-level setup driven currents (e.g., rip currents) and increased velocity asymmetry of the waves due to the geometrical change at the shoreface nourishment. Most erosion of the nourishment takes place during energetic wave conditions (Hm0 ≥ 3 m) as milder waves are propagated over the nourishment without breaking. A data-model comparison shows that this approach can be used to accurately assess the erosion rates of shoreface nourishments in the first years after construction.