Urban areas in low-lying coastal zones are typically protected from flooding due to storm surge and waves by a hybrid soft-hard coastal defence structure. Such structures combine a mild-sloped beach (soft structure) and a steep-sloped dike (hard structure). For smaller water depths on the beach (shallow water case), a surf and swash zone starts to develop in front of the dike toe. In this zone, short waves (ss) reduce in wave height by wave breaking, while longer waves, called “infragravity (ig) waves”, increase in wave height due to shoaling and energy transfer from the ss waves. The ig wave-related processes have a highly significant, but insufficiently researched effect on both the beach erosion and wave overtopping, which are leading to flood risks. The proposed research aims at gaining more (quantitative) insight into the influence of ig waves on these processes and in how they can be taken into account in the design of our coastal protection. Computer simulations will be the main tool to study the hydrodynamic and morphodynamic processes. In addition, field observations of ig waves will be performed within new unique measurement site of a hybrid soft-hard coastal defence structure in Ostend (Belgium). This will provide crucial insight into the different types of ig wave sources that exist in the field (currently only one type is taken into account in design methods), and a unique and innovative dataset to validate the computer simulations.