GEOMORPHOLOGICAL EFFECTS OF TSUNAMI RUN-UP AND BACKWASH

Geomorphological processes associated with tsunami run-up and backwash are highly complex. To date, most studies have considered the sedimentology of palaeo-tsunamis while almost no detailed studies have been undertaken in areas known to have been affected by tsunamis in recent times. Research on contemporary tsunamis show that coastal landscapes may be greatly altered not only by direct tsunami run-up orthogonal to the shoreline, but also by episodes of vigorous backwash and by water flow sub-parallel to the coastline. The combined effect of these processes produces hitherto unrecognised coastal landforms that are dominated by the effects of high-magnitude erosion and deposition. Deposition of large boulders is also associated with severe tsunamis, while in adjacent areas run-up processes often result in the deposition of continuous and discontinuous sediment sheets. In many instances the upper limit of sediment deposition lies well below the upper limit of wave run-up which is frequently marked by a well-defined zone of stripped vegetation and soil. The processes described have considerable implications for models of coastal evolution. It may even be the case that the evolution of particular stretches of coastline are dominated by such high-magnitude low-frequency events.