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

Abstract

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 of 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.