All over the world, coastal protection measures are taken, which can be soft (e.g. sand nourishments on sandy beaches) or hard (e.g. seadikes or seawalls). Special care has to be taken to design the transition between these hard and soft flood defences, as they are often vulnerable components of a coastal defence system. This study therefore aims to give more insight into the morphological processes around hard-soft transitions, in order to make recommendations on design and nourishment plans for hard-soft transitions in the future. To this end, the hard-soft transition of Maasvlakte 2 (MV2) was studied. For the case of MV2 the limited knowledge of hard-soft transitions resulted in a conflict of the assessment method of the flood defence and the nourishment strategy according to the client requirements. The most important factors governing the morphological behaviour were studied using different numerical models (UNIBEST-LT/CL , SWAN and XBeach. From the results of the case study of Maasvlakte 2, it was found that the most severe erosion at the hard-soft transition is found during periods with strong southward longshore transports, which causes positive longshore transport gradients at the transition. Cross-shore processes such as storm erosion also indirectly contribute to this by transporting sediment from higher in the profile to lower in the profile. Regarding the influence of wave reflection, preliminary results were found. Using SWAN and UNIBEST, the effect of short waves was investigated, which showed that reflection of short waves is larger at the deeper located part of the hard flood defence. The preliminary results from XBeach showed that bound long wave reflection is larger closer to the hard soft-transition, and that these reflected waves also reach quite far offshore. As regards to the role of the tide, it was found that the dominant northward directed flood tidal current along the soft flood defence strengthens the supply of sediment from south during northward transport. On the other hand, the dominant southward directed ebb current along the hard flood defence hardly picks up any sediment and therefore it does not contribute to the morphological changes at the hard-soft transition.
Some generic observations in the morphological behaviour of MV2 are likely to be found at other hard-soft transitions: - A strong dynamic variability is usually present at the hard-soft transition, in the form of coastline retreat which gives a rotated coastline shape. - The reflection of short waves will likely be larger at the deeper part of the hard flood defence. Closer to the transition zone, where sediment from the soft flood defence is deposited, short wave reflection will be smaller, but long wave reflection will be larger. - In general the highest erosion will be observed in periods with oblique wave incidence where sediment is lost due to longshore sediment transport gradients. Based on these findings, several recommendations are proposed to design a hard-soft transition. Moreover this study presents some other design examples to minimise the nourishment frequency. Finally, recommendations are presented for further research.