Temporal and Spatial Variation of Mixing and Movement of Suspended Sediment in the Macrotidal Chikugo River Estuary

The effect of mixing on the movement of the estuarine turbidity maximum (ETM) was examined in the highly turbid Chikugo River estuary, Japan, in consideration of four significant issues: semidiurnal and semilunar tidal variations, spatial distribution, and synchronism of data acquisition. Vertical profiles of salinity and turbidity were measured for 2 weeks at 1 km intervals from the river mouth (0 km) to 17 km upstream, with a short duration for every cruise. Spring-neap transition and the spatial variation of mixing, and suspended sediment concentration (SSC) was found during the semilunar tidal cycle. The estuary gradually changed from a vertically well-mixed condition during spring tide to a stratified condition during the neap tide via a partially mixed condition during the intermediate half tides. The relationship between the flow ratio and the SSC showed that ETM growth was dominated by both tidal forcing and freshwater flow. SSC was high when the salinity was 0.5 practical salinity units (psu) in the upper estuary and 10 psu in the lower estuary. For the same mixing strength (MS), SSC in the lower estuary was greater than that in the upper estuary. Based on flow ratio, salinity, and MS, the occurrence of the ETM and its development at the macrotidal estuary was a consequence of three processes: bed-sediment erosion by seawater in the lower estuary, sediment transport in the estuarine channel by the tidal current and freshwater flow, and sediment accumulation at the salinity front in the upper estuary.