Characteristics and depositional pattern of fluvial-flood deposits on the river mouth: case study of a fluvial-flooding event at 2009 in the Kushida River Delta, Ise Bay, central Japan
河口沿岸域における洪水起源堆積物の特徴と堆積様式―2009年10月 伊勢湾櫛田川河口干潟の例―

Shota Yamashita, Takeshi Nakajo, Naohisa Nishida, Hajime Naruse
2011 Journal of the Sedimentological Society of Japan  
On October 2009, the typhoon No. 18 with a heavy rain passed the Ise Bay, central Japan, and caused a fluvial flooding at the Kushida River mouth. Here, drastic geomorphological changes on the coastal area by this fluvial flooding event were reported, and characteristics and depositional patterns of fluvial-flood sediments deposited on the tidal sand flat were described in detail. The geomorphological changes observed by this fluvial flooding event are following; 1) the spit on the right bank
more » ... the Kushida River was cut off by the flooding of the branched channels, 2) a large amount of terrestrial organic matters such as plant debris deposited along the shoreline, and 3) a fluvial flooding deposit which is mainly composed of muddy particles covered the sandy tidal flat. As a result of sedimentary facies analysis and SEM observation, two different sedimentary facies were distinguished from the fluvial-flood sediments. Facies 1 is composed of medium-to very coarse-grained sand layer with sharp erosional base, and interpreted as an "overbank" deposit. Facies 2 is composed of massive muddy layer showing microscopic granular structure in clay fabric, and interpreted as a fluid-mud deposit. The fluid-mud deposits (Facies 2) were thickly (>10 cm) developed in the local depressions such as troughs between sand bars, and a part of them was still observed 6 months later. Furthermore, fluid-mud deposits interpreted as deposited from historical flooding events were recognized from the subsurface sediments of the sandy tidal flat. These facts suggest that fluid-mud deposits by fluvial flooding events are potentially preserved in the subsurface sediments of the tidal sand flat without winnowing by wave or tidal activities.
doi:10.4096/jssj.70.81 fatcat:7dx3k2ebvjahdlmnrri7pijyaq