Flow Dynamics/Morphological Impacts of March 11 Tohoku Tsunami, Japan Victor R Baker1, 4,Kazuhisa Goto2, Goro Komatsu2, Takafumi Matsui2,Luke Mcguire3, Takashi Oguchi5, Jon D Pelletier4,Yuichi S Hayakawa5,Hitoshi Saito5

1. Dept. Hydrology & Water Resources, University of Arizona, Tucson, AZ 2. Planetary Exploration Research Center, Chiba Institute of Technology, Chiba, Japan 3. Applied Mathematics, University of Arizona, Tucson, AZ 4. Geosciences, University of Arizona, Tucson, AZ 5. Center for Spatial Information Science, Tokyo University, Tokyo, Japan

The Sanriku coastline of northeastern Honshu, Japan, consists of multiple embayments, or inlets into the Kitakami Mountains. This produces a “ria” (drowned river valley) configuration that is particularly susceptible to the enhancement of erosion effects from tsunami generated at the very active Pacific-Asiatic plate-boundary subduction zone lying approximately 200 km to the east. A preliminary post-Tohoku tsunami survey of areas with minimal artificial construction of sea walls and other coastal defense works revealed that the highest intensity erosion of channels and hill slopes occurred in the narrowest coastal embayments, particularly those with funnel-shaped inlets that open most directly to incident tsunami wave crests. At Aneyoshi (Fig. 1), Miyako-Shi, where wave run-up heights achieved 40.5 meters (among the highest recorded for the March 11 event), there was spectacular erosion of valley sides (Fig. 2), transport and deposition of coarse gravel, and scour-hole generation around large boulders (Fig. 3). As at Tokura, just south of Shizugawa (Minamisanriku), regolith and vegetation, including trees, were stripped off hillsides, exposing and scouring bare rock (Fig. 4). Inundation levels at these sites show an approximate correspondence to levels reached by the 1896 Meiji-Sanriku tsunami event. Combining this observation with local evidence of older tsunami events, e.g., the 1611 Keicho-Sanriku tsunami and the 869 Jogan tsunami, leads to the suggestion that century-scale repetition of very high-energy tsunami events constitutes a critical factor in long-term coastal erosion for northeastern Honshu, Japan. These preliminary observations are consistent with a threshold phenomenon, whereby the energy expenditures by the incidence tsunami waves must be high enough to exceed resistance factors imposed by vegetation-stabilized regolith on hillsides. When the threshold is exceeded, on the approximate order of once per century, catastrophic erosion and deposition are generated in the local zone of tsunami impact. These effects are all remarkably similar to what is observed for catastrophic fluvial erosion in bedrock channels. To quantify the flow magnitudes associated with these phenomena, we are employing 2-D hydraulic flow modeling to estimate peak flow velocities (Fig. 5) and other flow parameters associated with the observed extreme runup values (Fig. 6).

Fig. 2. (Left) Aerial view of scour and erosion in Aneyoshi area.

Fig. 4. Tsunami scour of hillsides at Tokura.

Fig. 1. (Above) Perspective view of Aneyoshi, Sanriku Coastline , northeastern Honshu.

Fig. 3. (Left) Tsunami gravel deposits and scour around 3 m boulder at Aneyoshi.

Fig. 5. (Left) Maximum flow velocities determined by 2-D Hydraulic Flow Model.

Fig. 6. (Left) Maximum runup heights determined by 2-D Hydraulic Flow Model.