台風来襲時の高波と高潮の相互作用によって沿岸低地に生じた小規模な砂質イベント堆積物の堆積過程

―1959年9月18日午前9時から19日午前1時の間に渡島半島西岸平浜海岸低地において生じた事例―

抄録

 Features and depositional processes are revealed of sandy event deposits (SED) caused by a storm surge and high waves during the 1959 Miyakojima typhoon around the Hirahama coastal lowland, along the western coast of the Oshima Peninsula, southwestern Hokkaido. Three new trenches were excavated in the lowland to study sedimentary features and grain size. Sedimentary features imply that the 1959 SED was deposited from an unidirectional run-up flow. The deposits can be subdivided into three units: T, S, and F in ascending order. Unit T shows 3D dunes. Unit S shows bedform transition from 2D dunes to ripples. Unit D consists of a mud layer including suspended plants and pieces of wood. Grain-size analysis shows that Units T and S peak at around 2.0 Phi (P-2 population), which is the same as beach sand from the Hirahama Coast, and the wide grain-size distribution is over 0–4 Phi due to mixing with the fluvial bed of Yumiyama River (P-1 and P-3 populations). According to Dmax, Unit T shows coarsening upward from －0.25 to 0.25. On the other hand, Unit S shows finning upward to 0.75 from 0.25. Therefore, Unit T recorded the amplification process of the storm surge and high wave energy due to the typhoon after 09:00 on September 18. Unit S recorded the decay process of high waves and storm surge energy associated with the movement of the typhoon from 13:00 to 14:00 or later. After the period 00:00 to 01:00 on September 19, suspended solids and wood fragments in stagnant water covered Unit S, then deposited Unit D because the typhoon had passed.