Over the past decades, the majority of research into coastal development has been carried out with a focus on open-sea beaches. However, sheltered beaches make up a larger part of the coastlines in the world. Recent studies have shown that the system of sheltered beaches cannot be approached as an open-sea beach with a lower wave energy climate. Doing so results in an inaccurate coastline development prediction. Understanding sheltered beaches and their coastline development is vital because the stresses on these beaches worldwide will increase due to climate change.
Due to the lower energetic wave climate, bed ripples are expected more often on sheltered beaches than on open-sea beaches. The possible presence of these is not yet incorporated in current models. Bed ripples can affect the sediment transport along the coast and can therefore have a substantial role in the necessary understanding of the different coastline development of sheltered beaches with respect to other beaches. Fieldwork data from the Prins Hendrikzanddijk was used to research the behavior of ripples on sheltered beaches. The research was divided into three categories: the formation and existence of ripples under varying hydrodynamic conditions, the performance of current ripple prediction formulas, and ripple migration-induced sediment transport.
Ripples were present with varying heights of 0.023 to 0.078 meters and wavelengths of 0.091 to 0.275 meters and the ripple dimensions responded to the changes in forcing on the sheltered beach. The parameter that related the best to the ripple dimensions was a mobility number that used the 98th-percentile wave-orbital velocity and not a current-related velocity part.
Three ripple predictors were used, of which the predictor by O’Donoghue et al. (2006) performed the best. This predictor relates hydrodynamic conditions to a representing ripple height and wavelength, allowing for changing dimensions and varying hydrodynamic forcing. However, this suggests a strong relation between the hydrodynamic conditions and the dimensions of the ripples. Overall, no strong correlation was visible between measured ripple dimensions and the mobility parameter. This supports the usage of the ripple predictor by Van Rijn (2007), which uses a constant ripple height until conditions become too energetic. This approach performs similarly to the best-performing predictor and could be more stable during varying conditions.
Lastly, ripple migration was measured and was shown to be driven by wave skewness and asymmetry. Both on- and offshore migrating ripples were measured. The source of either on- or offshore migration was not identified clearly. However, the offshore migrating ripples were accompanied by lower skewness and smaller asymmetry values than the onshore migrating ripples. The order of magnitude of the induced bedload transport by migrating ripples was of the same order as bedload transport predicted by the model XBeach without the inclusion of the existence of ripples.