IDENTIFYING A TSUNAMI GEOMORPHOLOGY AND THE IMPLICATIONS FOR IMPROVING COASTAL HAZARDS MITIGATION

There is something of a gentle sea change taking place at the coast. In recent years there has been growing recognition of geomorphological changes at the coast associated with tsunami inundation and their seismic drivers. This talk mainly focuses on New Zealand’s tectonically active coastline, where a recent process geomorphology model proposed a link between coastal transverse dune building phases and seismic activity. In other words, for another coastal transverse dune ridge to form there needs to be a significant supply of new sand to the coast. This is not all however, the start of some parabolic dune phases and dune advances inland coincide with past tsunami inundation.

Other geomorphological evidence for tsunami inundation can also be preserved in the landscape. Distinct features are apparent along post Indian Ocean Tsunami-inundated shorelines. Pedestals of vegetated sand separated by scour depressions are backed by sand sheets extending inland. These sand sheets thin inland and come with their own specific sedimentary characteristics, although in some cases they appear similar to storm deposits. Similar geomorphology features, albeit considerably older, are also present on some of New Zealand’s coastline. In a case study from SE South Island, New Zealand, LiDAR data were used to identify a coastal geomorphology (ground-truthing for the geology) created by tsunami inundation around the late 14^th to early 15^th century. In the examples discussed, landward sand sheets were washed into wetlands and subsequently weathered to form low profile dunes termed “hummocky topography”. To follow this through, a desktop scan for geomorphological features identified evidence of tsunami inundation for which later geological evidence was found. A subsequent search for the most likely source yielded surprising results.

In areas with little perceived threat from tsunamis, an initial geomorphological survey can add value to our understanding of the hazard and improve coastal hazard mitigation.