MORPHOLOGICAL ANALYSIS OF AN UPLIFTED COASTAL TERRACE USING HIGH DEFINITION TOPOGRAPHIC DATA: POSSIBLE EROSION BY HIGH WAVES OF TSUNAMIS

A coastal rocky terrace with an elevation of ca. 20 m a.s.l. is located around Cape Todogasaki at the middle section of the ria-type rocky coast in Sanriku, northeastern Japan. In spite of its 20-m height, the tsunami waves induced by the 2011 Tohoku Earthquake were high enough to spread over the terrace. To clarify the effects of tsunamis on the terrace surface micromorphology, we performed field measurements of structure-from-motion multi-view stereo (SfM-MVS) photogrammetry using a digital camera mounted on an unmanned aerial vehicle (UAV). Terrestrial laser scanning was also carried out on the site for data validation. Geographical coordinates of benchmarks for georeferencing the SfM data were measured in the field using a GNSS rover, capable of postprocessing carrier-phase correction with centimeter accuracies. A 20-cm resolution digital elevation model (DEM) was then generated in SfM software, and micromorphology of the terrace surface was analyzed using the DEM. Despite limited basin area draining into the terrace, erosional characteristics likely formed by streamflow were identified. Sedimentological characteristics of thin clasts accumulation likely derived from terrace edge in-situ bedrock were also examined in the field. Based on clast shape and degree of weathering, three layers of deposit were identified: large fresh and angular clasts recently emplaced laying on the top. The spatial pattern of their imbrication for the three layers evidenced a landward flow direction. Furthermore, some angular boulders (2–3 m diameter) showing different degrees of weathering on their faces suggest that they were recently dislodged and displaced landward. These observations indicate that the uplifted terrace surface underwent erosion by not only the 2011 tsunami but also repeated significant tsunamis in the past.