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Submerged shore-parallel breakwaters for coastal defence (henceforth SBWs) are a good compromise between the need to mitigate the effects of waves on the coast and the ambition to ensure the preservation of the landscape and water quality. However, if not properly designed, such structures can force circulation patterns that enhance shoreline erosion rather than shoreline accretion. Numerical models can be used to investigate the structure-induced circulation patterns and the resulting shoreline response. However, being computationally demanding these models are most suitable for advanced stages of the design process. The aim of this paper is to present a simple criterion to qualitatively identify whether an accretive or erosive circulation pattern can be expected in the lee of the structures. The criterion is based on analytical considerations and builds on the model presented by Bellotti (2004Bellotti ( , 2007. It is validated against the non-hydrostatic free surface numerical model SWASH (Zijlema et al. 2011) and experiments performed by . The validation indicates that the proposed analytical model is capable of providing a rapid first assessment of the potential shoreline response mode for SBW design.

Abstract. The accuracy of morphological predictions is generally measured by an overall point-wise metric, such as the mean-squared difference between pairs of predicted and observed bed levels. Unfortunately, point-wise accuracy metrics tend to favour featureless predictions over predictions whose features are (slightly) misplaced. From the perspective of a coastal morphologist, this may lead to wrong decisions as to which of two predictions is better. In order to overcome this inherent limitation of point-wise metrics, we propose a new diagnostic tool for 2-D morphological predictions, which explicitly takes (dis)agreement in spatial patterns into account. Our approach is to formulate errors based on a smooth displacement field between predictions and observations that minimizes the point-wise error. We illustrate the advantages of this approach using a variety of morphological fields, generated with Delft3D, for an idealized case of a tidal inlet developing from an initially very schematized geometry. The quantification of model performance by the new diagnostic tool is found to better reflect the qualitative judgement of experts than traditional point-wise metrics do.