Tsunami Erosion: Geomorphology Before and After the 15 Nov 2006 Tsunami in the Middle Kuril Islands, Russia (abstract)

Because the 15 Nov 2006 middle Kuril Island earthquake (Mw 8.3) and tsunami occurred between two field expeditions of the Kuril Biocomplexity Project, we have detailed topographic profiles and photographs from 3 months before and 9 months after the event. Thus, we are in the position to define tsunami-induced changes in coastal geomorphology both quantitatively and qualitatively. The locations where we conducted our surveys varied in the type of coastline (defined by the width, geomorphology and sediment type of the coastal plain), and the local size of the tsunami (defined by runup and inundation). The change from 2006 to 2007 was most extreme in the sandy beach ridge plain of central Ainu Bay, Matua Island, which experienced a tsunami with 15-20 m runup and 300-400 m inundation. Topographic profiles measured in 2006 and re-measured in 2007 show that the beach was eroded landward 25-50 m, with entire beach ridges removed or reduced in size, and troughs between ridge deepened. Farther landward, there are many zones of soil stripping, both large and small-- the tsunami exploited rodent networks and cinder layers to flay the surface. There are also scours associated with concentrated outflow. Erosion was about as extensive as deposition-- within 10s of m of inundation distance and meters of runup elevation). However, the volume of sediment eroded is an order of magnitude greater than the volume of sediment deposited onland. In areas with high runup (10-20 m) and narrow rocky beaches (eastern Matua Island and parts of Dushnaya Bay, Simushir Island), patchy to extensive erosion occurred over almost all of the inundation distance up to a few meters below maximum runup. Erosion was evinced by trim lines, where soil and vegetation were stripped to an approximately uniform elevation, and by other forms of soil removal. Soil scouring also commonly appeared as scalloped "bite marks" along the beach edge or edges of stream channels. Of studied sites, erosion was minimal where runup was less than 10 m (central Dushnaya Bay, Simushir Island). In general, changes were subtle and present mainly near the shoreline. Examples of erosion in this location are back-beach cliff retreat, surficial sediment removal on sand dunes (which lack coherent soils), and small-scale scouring associated with focused water withdrawal.This abstract is available in Eos.