Watabe, Y. and Sassa, S., 2016. Sedimentation history of sandbars in flood-tidal delta evaluated by seismic method in Lake Tofutsu, Japan.
An extensive pattern of sandbars within a flood-tidal delta complex can be seen at the tidal inlet of Lake Tofutsu, one of the lagoons located along the Okhotsk Sea in Hokkaido, Japan. The primary aim of this study was to investigate the sequential sedimentation history of these sandbars. The shear-wave velocity structure, which is equivalent to the stiffness variation, of these sandbars was examined using a surface-wave method called the multichannel analysis of surface waves (MASW) to identify and describe the stratigraphy of the flood-tidal delta at the tidal inlet of Lake Tofutsu. The MASW results demonstrated that even though the sandbars appeared to comprise the same sandy material, the sediment under the sandbars had a very varied stratigraphy, comprising the river-borne muddy sediment derived from the upstream side of the lagoon and wave-induced marine sandy sediment derived from the coastal sand of the Okhotsk Sea. The second aim of this study was to verify the sedimentation history described on the basis of the MASW results through laboratory testing of sediment samples collected at characteristic points along the MASW array. The sampling method was modified to be applicable to low-cost sediment investigation at tidal flats and was conducted by human power without the aid of an engine. The collected samples were examined in the laboratory to obtain the depth profiles of grain-size distribution and carbon 14 dating. The results obtained from the sampling are consistent with the sequential sedimentation history evaluated by MASW.