Hiroto Yamaji

Visualization of large-scale internal seiche in Lake Inawashiro

Lake Inawashiro, located at the center of Fukushima Prefecture in Japan, has the fourth largest area. Investigation on water quality change in this lake has been done by one of the authors ( Fujita and Sasaki, 2001 ), whereas fundamental study on hydraulic characteristics has already been initiated in 1911 ( Honda et al.,1911 ). They reported that surface oscillations with the period of 9 min and 19 min are predominant there. Furthermore, they observed distinct reduction of water temperature in the lake, implying the occurrence of large-scale internal seiche in this lake. Thermocline can be commonly observed in a deep lake during summer due to the rise of temperature, and the flow pattern becomes remarkably different from one observed in a shallow lake because of the stratification in the vertical direction. Large scale internal wave likely to occur more frequently in autumn because of decay of stratification and the effect of strong seasonal wind in Japan. Compared with internal wave, internal seiche causes large oscillation all over a lake and affect water quality largely. Therefore, it is important to investigate the characteristics of internal seiche and its effect to lake water movement and water quality.

A GENERATION METHOD OF ASYMMETRIC OSCILLATORY MOTION SIMULATING CNOIDAL WAVES

A mechanical motion induced by a rotating disc is shown to be almost identical to the oscillatory motion due to rst-order cnoidal waves at small to intermediate Ursell numbers. A dimensionless parameter, b, in the theory is given in terms of Ursell number. The theory can be applied to a generation of an asymmetric oscillatory flow in a closed conduit, simulating near-bottom boundary layer under cnoidal waves.

Depth-Limited Oscillatory Boundary Layers on a Rough Bottom

An experimental study is carried out to investigate the transformation of oscillatory boundary layer from ordinary to depth-limited one by using an oscillating tunnel with triangular roughness pasted on the top and bottom surfaces. The velocity is measured by one-component LDV. It is proved that only longer period of oscillation is not sufficient to induce depth-limited properties. Instead, the ratio of boundary layer thickness to the distance from the bottom to the free surface must be used in order to distinguish a depth-limited oscillatory boundary layer from an ordinary one. The experimental data for mean velocity, phase difference, fluctuating velocity, friction coefficient and boundary layer thickness are presented in order to elaborate the structure of depth-limited oscillatory boundary layers on a rough bottom. On the basis of the present experiments, a modification has been made in an existing criterion to distinguish between ordinary and depth-limited wave boundary layers. A typical field situation is discussed to show the usefulness of the proposed criterion.

Infiltration Flow Through a Sand Spit at the Nanakita River Mouth

Development of a sand spit can frequently be observed at river mouths where sediment transport by wave action is more predominant as compared with flushing ability by tidal and river discharge. Progress of river mouth closure may cause reduction in flow through a river opening, resulting in increase in infiltration flow through a sand spit at a river entrance due to distinct water level difference between inside and outside of a river mouth. However, there have been quite few investigation on this type of flow, although there have been many on coastal aquifer in connection with salinity intrusion and beach stabilization by dewatering of coastal aquifer. In the present study, results of field observation of infiltration flow in a sand spit at the mouth of Nanakita River are described. Comprehensive numerical simulation is also performed to investigate the effect of water level variation in the mouth on the strength of infiltration flow.

Numerical Study on Water Exchange Mechanism in Nagatsura-ura Lagoon

Nagatsura-ura Lagoon is a semi enclosed brackish water body actively used for aquaculture of oyster.In summer of recent years, however, deaths of oysters occur and are in deep concern among stakeholders of the aquaculture.Therefore, field observation was carried out to identify important physical characteristics in the lagoon.Some conclusionso n water exchange characteristich ave been drawn based on analysiso f the data obtained.Int his study, a simple Lagrangian tracer technique was used to further investigate the water exchange mechanism.Tracers velocities were based on the flow field resulted from a calibrated three-dimensional hydrodynamic numerical model.Analyses were carried out through examination of behavior of single-tracer and multiple-tracers release

FIELD OBSERVATION OF WATER QUALITY CHANGE IN RESPONSI TO TIDAL VARIATION IN NAGATSURA-URA LAGOON

In semi-enclosed water areas, aquaculture has been performed actively in many places in Japan by utilizing their calm environment. In case of overproduction of culturing, however, aggravation of water quality environment, such as eutrophication and appearance of anoxic water, is inherent problem in semi-enclosed areas. Nagatura-ura Lagoon, the study area of the present study, is also one of the typical semi-enclosed brackish-water areas. Nagatsura-ura Lagoon has been used mostly for cultivating oyster for these several decades. In the recent years, however, the appearance of anoxic water causes severe damage to the fishery industry. In this study, filed observation has been carried out during summer of 2003. The salinity, water temperature, dissoloved oxygen and water level were measured in the study area. Through these measurements, the characteristics of water quality change in response to tidal variation were investigated to clarify the mechanism of sea water exchange in the lagoon area.