Hiroyuki Yamanishi

Chapter 22 Numerical simulation of hydrodynamics, water quality and sediment transport in the Ariake Sea, Kyushu Island, Japan

Abstract The objective of this chapter is to develop a water quality model coupled with a hydrodynamic model to simulate flow, current velocity, water quality and sediment transport in the Ariake Sea, west coast of Japan. The computed spatial distribution and temporal variations of flow velocity, salinity, chlorophyll-a, nutrients, dissolved oxygen and suspended solid showed good agreement with field observations. The simulated results demonstrated the seasonal changes of water quality constituents in response to the weather and the flow boundary conditions. Higher concentrations of nutrients and chlorophyll-a are observed in the inner bay during the summer periods. High suspended-solid concentrations are found in the western part of the inner bay.

Chapter 23 Effect of the shape of submerged dike/mound on mud transport

Abstract The effect of the shape of submerged dike/mound on seabed mud transport has been investigated by three-dimensional mud transport analysis. The numerical results show that the narrow hexagonal shape (plan view) has an advantage over the rectangular shape. This is because the outline of a narrow hexagonal shape is close to a streamline and it will cause less eddy flow and can divert suspended solid particles (SS) moving outside the cultivated area. Also, the effects of the height of the dike/mound and the average flow velocity have been studied numerically. Limited analyses show that increase in the height of the dike from 0.3 to 0.5 m can reduce SS concentration on the cultivated area by about 30% and increase the height of water column mixed with SS by about 0.5 m. Under the condition of a constant SS concentration at upstream boundary, increasing the average flow velocity will bring more SS into the area considered and there will be higher SS concentration on the cultivated area.

Field study and modelling on the characteristics of bed mud formation processes at the Rokkaku River

Field observations of bed mud formation prosesses were continuously carried out at a site 11km upstream from the river mouth of the Rokkaku River in Japan from November 28, 1994 to February 4, 1995. A large experimental reservoir was constructed along a mudbank in the midstream of the Rokkaku River. From the observations, the maximum sedimentation rate on the flats per one tide cycle reached about 0.01m at spring tide. Based on the observations, modelling the bed mud formation process in the reservoir was performed. The process of the bed mud formation consists of three stages: deposition-consolidation, sweep-out and evaporation processes. The simulation results explain the formation process of the bed mud layers in the reservoir very well, so that, the process in tidal rivers can be explained in terms of this method by composing shear stress in the bed itself and that generated by the flow.