SEDIMENTATION OF THE 2011 GIANT TSUNAMI WAVES ON A SANDY BEACH ALONG THE PACIFIC COAST OF EAST JAPAN

Introduction

The Pacific coast of the Japanese Islands has often been attacked by giant tsunamis. I observed the beach features changed by the 2011 Giant Tsunami on the sandy beach in Ibaraki Prefecture, East Japan, and describe sedimentation of deposits by the giant tsunami on the sandy beach in detail.The 2011 off the Pacific coast of Tohoku Earthquake occurred at 14:46JST on March 11. After about 30 minutes, the giant tsunami has attacked the Pacific coast including Oarai Sandy Beach (OSB). OSB has been attacked three times by major tsunami waves as high as 4.0 m. OSB is about 270 km far from the seismic center around Japan Trench.

Current system on the sandy beach

Run-up flows came in from S-SE direction and return ones went out to SE. These evidences of the directional features are as follows: [Run-up flow] 1, Two steel heavy bridge girders were transported to N direction inland, about 100 m long. 2, Wooden poles in volleyball courts were bent landwards and/or broken. 3, Depressions (plunge pool) in the pavement of parking area were formed at the landward side of low steel fences for dividing parking area. 4, About ten steel poles (1 m in height) have bent inland. [Return flow] 1, An erosional channel, more than 20 m wide at the beach, was formed by return flows. 2, Many floor tiles on a tiled sidewalk had been torn by surge of run-up flows along N side of the sidewalk, and rearranged to make imbrication structures by return flow for seaward.

Discussion

The run-up flow formed by surge at the beach has flowed over the backshore to make artificial works bend and the backshore operate with only very short time. The total amount of erosional sand might be smaller than it of return flow, because the grass on the backshore was sporadically left. Sands eroded by run-up flow were deposited at the roots of hedges more inland. The strength of surge was able to make evidences as mentioned above and move fishery boats landwards. The start point of return flows was the upper limit of surges inland. The flow has operated bottom surface for longer time than surge, and has eroded much backshore sands such as erosional channels The backshore had been flat before coming tsunami, but after that there were 4 or 5 channels with widening seawards.