Ports and coastal areas rely on coastal protection schemes both for normal operation, as well as for protection against extreme events, such as storm surges and Tsunamis. The failure of many protection structures, particularly massive caisson-type breakwaters during the 2011 Tohoku earthquake and Tsunami provoked intensive efforts to understand how such believed to be robust systems could have failed, and how their performance could be improved. Although sliding and uplifting were often proposed as key failure mechanisms, observations from more recent physical model studies raised the possibility of scour–induced geotechnical failure within the soil, before sliding could take place. The effect of such scouring on geotechnical bearing capacity was captured though a scour-dependent combined failure surface, derived using simple finite element models based on scour geometry, directly captured from the experiments. Expanding on this concept, the same “hybrid” modelling technique also reveals the role of second order effects, due to significant pre-failure rotation due to scour, which increases the horizontal force acting on the foundation. Combined with a sliding soil-foundation interface and appropriate combined (Tsunami) loading, a complex combined sliding/geotechnical failure mechanism can be induced, mimicking the ones experimentally observed. Such a combined mechanism reproduces the reduction of sliding resistance, which has been observed in large scale hydraulic experiments (attributed to a reduction of an “apparent” interface friction coefficient).