Boulder transport by the 2011 Great East Japan tsunami: Comprehensive field observations and whither model predictions?

Predicting the local size of a historic high-energy event from its boulders using numerical models is a challenging research topic. Modern high-energy events and their deposits are useful to validate these models; however, validating the accuracy of the results is difficult due to the scarcity of good datasets or the ambiguity of existing field data. Data on boulders transported by the 2011 Great East Japan tsunami at coastal sites (Settai, Taro, and Karakuwa) on the northeast coast of Japan were compiled. Pre-tsunami locations and settings and transport distances were found from evidence such as photographs, aerial images, and the testimony of survivors. The estimated weight of the boulders analyzed ranged from 11 to as much as 167. t, while the transport distance varied from a few to up to 600. m. Modeling results predicted that the minimum limit of maximum flow velocity of the tsunami at the pre-tsunami locations of the boulders varied from 4.2 to 6.8. m/s. The measured maximum flow depths at Settai (17-18. m) and Taro (14. m) were within the predicted range of flow depth when the Froude number. =. 1.0-1.5. Numerical model estimates for an older boulder (285. t) in Settai indicate that it was probably transported by a historical tsunami (1611 Keicho Sanriku event?) which may have been similar to or bigger than that of the 2011 event in the area. The maximum flow velocity could not have been less than 6.1. m/s, and if the boulder was transported to the present location by rolling, the flow velocity must have been within 7.5-23.7. m/s. Following systematic validation, the numerical modeling of boulder transport is proving promising for reconstructing the local magnitude of historical, high-energy events. Further improvements can be made with additional high quality field data from modern high-energy events.