In-situ measurement on development process of salinity stratification due to flood events in the Northern Ariake Sea, Japan.

Yohei Kitagawa, Shinichiro Yano, Akira Tai, Hiroaki Taniguchi, Sangyeob Kim, Akihide Tada

Thursday 2 july 2015

17:45 - 17:48h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Morphodynamics of estuaries and coastal areas, Poster pitches

Parallel session: Poster pitches: 13A. Sediment - Coast

A concept of the “regions of freshwater influence (ROFIs)” was proposed by Simpson (1997). Generally, ROFIs are considered as the one of key factors to determine conditions of marine environment and ecosystem in coastal area. Thus, we focus on freshwater dynamics in the gulf type ROFI of the Ariake Sea, Japan. To understand salinity stratification due to freshwater input from multiple rivers through flood events, in-situ measurement for 25hours on tidal current, CTD, turbulence micro-structure, and suspended solid (SS) grain size property was conducted at a single point in the northern Ariake Sea, Japan, in July, 2014. The first measurement was conducted in neap tide (from 18:00 on 3 July, 2014 to 18:00 on 4 July, 2014), and the other in spring tide (from 14:00 on 11 July, 2014 to 15:00 on 12 July, 2014). On 3 July, large flood with total peak discharge of 4,500m3/s from all A-class rivers flow into the bay occurred. On the other hand, big flood with peak discharge of 4,000m3/s occurred from 7 to 10 July. From the results of the measurement, the followings were clarified: 1) Sub-tidal development and decay process of saline-freshwater stratification during flood season was measured. The time scale of stratification was able to change with a dimension of flood; 2) Relationship between particle size distribution of SS and development of stratification was exhibited; and 3) Dependence of tempo-spatial distribution of chlorophyll-a (phytoplankton) on the different mechanism from stratification due to freshwater transport and mixing in the bay was seen.