Keiji Nakatsuji

Anti-cyclonic circulation driven by the estuarine circulation in a gulf type ROFI

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Baroclinic residual circulation processes are examined in gulf type Regions Of Freshwater Influence (ROFIs), which have large rivers discharging into a rounded head wider than the Rossby internal deformation radius. Theoretical and observational investigations concentrate on Ise Bay, Japan, with supporting data from Osaka Bay and Tokyo Bay. Simplified analytical solutions are derived to describe the primary features of the circulation. Three dimensional residual current data collected using moored current meters and shipboard acoustic doppler current profilers (ADCPs), satellite imagery and density structure data observed using STDs, are presented for comparison to the theoretical predictions. There are three key points to understanding the resulting circulation in gulf type ROFIs. First, there are likely to be three distinct water masses: the river plume, a brackish upper layer, and a higher salinity lower layer. Second, baroclinic processes in gulf type ROFIs are influenced by the Earths rotation at first order. Residual currents are quasi-geostrophic and potential vorticity is approximately conserved. Third, the combined effects of a classical longitudinal estuarine circulation and the Earths rotation are both necessary to produce the resulting circulation. Anti-cyclonic vorticity is generated in the upper layer by the horizontal divergence associated with upward entrainment, which is part of the estuarine circulation. The interaction between anti-cyclonic vorticity and horizontal divergence results in two regions of qualitatively different circulation, with gyre-like circulation near the bay head and uniformly seaward anti-cyclonicly sheared flow further towards the mouth. The stagnation point separating the two regions is closer to (further away from) the bay head for stronger (weaker) horizontal divergence, respectively. The vorticity and spin-up time of this circulation are −(f−ω l )/2 and h/2w 0 , respectively, where f is the Coriolis parameter, ω l is the vorticity of the lower layer, h is the depth of the upper layer and w 0 is the upward entrainment velocity across the pycnocline. Under high discharge conditions the axis of the river plume proceeds in a right bounded direction, describing an inertial circle clearly seen in satellite images. Under low discharge conditions the river plume is deflected in a left bounded direction by the anti-cyclonic circulation of the upper layer.

Tidal-jet and vortex-pair driving of the residual circulation in a tidal estuary

Abstract Many tidal estuaries are composed of narrow constrictions and wide basins, forming a “strait-basin system”. A typical example of such a system is that of the Akashi Strait and Osaka Bay in the Seto Inland Sea, Japan. In this system, tidal currents in the straits are much stronger than those within the basins. Therefore the mass transport processes are advection rather than diffusion and governed by strongly non-linear effects. In order to reveal the flow and transport mechanism in the strongly non-linear effect, field observations have been conducted. The observations include flow-visualization experiments by the simultaneous use of several shipboard acoustic Doppler current profilers. In the strait-basin system, the tidal-jet from the strait generates a moving vortex-pair, which is smaller than the tidal excursion. This vortex-pair carries vorticity originally generated at the separation point in the strait far into the basin during each tidal cycle. The moving vortex replenishes the vorticity of the basin-wide residual circulation, which is larger than the tidal excursion. This residual circulation further acts on the trajectories of the moving vortices. Generation of moving vortices from a tidal-jet is not unique but common and essential phenomena in many strait-basin systems. These systems are filled with vortices.

Influences of Land Reclamation Near the River Mouth on River Plume Spreading

ABSTRACTThe present study examines the effects of land reclamation in the vicinity of the river mouth on the flow structure of flood river plumes and on the surface water level rise by using a three dimensional baroclinic flow model with a κ-ϵ turbulence model. The models potential to simulate three dimensional behaviors of the river plume near the river mouth is examined by comparing the hydraulic experiments. As a result of the comparison of the surface water level rise, the spreading and the acceleration of river plume and the cross-sectional flow structure, it is confirmed that the reclamation affects significantly on the vertical and horizontal spreading pattern of the flood discharge when the expansion angle is beyond 11°, which is determined as a standard of the design for river works by the Ministry of Construction. The availability of the expansion angle 11°, therefore, can be proved from the physical viewpoint.

STUDY OF UPWELLING PHENOMENA OF ANOXIC WATER 'A-OSHIO'

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

SPATIAL AND TEMPORAL DISTRIBUTIONS OF DISSOLVED OXYGEN AND NUTRIENTS AROUND ARTIFICIALLY RECLAIMED LANDS IN ESTUARIES

Water mass which is deficient in oxygen spreads in the Port of Amagasaki-Nishinomiya -Ashiya in the head of Osaka Bay from April to November. In the past two years field survey of dissolved oxygen has been carried out in the port, but the detailed dynamics of the oxygen-deficient water is not clear totally in the head of the port. In the present paper a new field survey of DO and nutrients is conducted in small water areas around the reclaimed islands in the head of the port. The water areas are divided into two types, that is, the small and enclosed areas in the head and the other area in the port. In the head of the port excess production of phytoplankton increases, and the great deal of organic nutrients makes the concentration of DO very low in spring. This oxygen-deficient water spreads out to the other water area in early summer. The behavior of the oxygen-deficient water in each area is linked together in high summer.

FACTORS THAT INFLUENCE THE DEVELOPMENT AND DECLINE OF ANOXIC WATER AT AN ENCLOSED WATER AREA IN THE HEAD OF A BAY

Water near sea bottom in the head of a bay into which a lot of nutrients flow becomes deficient in oxygen during summer. The characteristics of the anoxic water are different between in the whole bay and at the enclosed area in the head of the bay. The vertical difference of temperature and DO (dissolved oxygen) in a bottom layer are correlative in the whole bay, and not so at the enclosed area. It is because there is excess consumption of oxygen at the enclosed area, which is influenced well on by the water temperature under a thermocline. In the present paper the factors which have influence on the development and decline of the anoxic water are discussed. When the anoxic water mass develops early in summer, the development of that depends on the temperature of the middle layer close under the thermocline. The higher temperature leads more consumption of oxygen in the process of the degradation of detritus. This consumption of oxygen is more important factor of the dynamics of anoxic water at the enclosed area than in the whole bay.

Water Quality variation and habitat environment for Corbicula japonica in Lake Ogawara

In a blackish lake, rich organic matters and nutrients are supplied from ariver, and salinity is provided from a sea. There exists a balanced ecosystem, and its stability greatly depends on the nutrient cycle. On the other hands, the matter cycle depends on the ecosystem. Therefore, it is important to understand the characteristics of the water qualities considering the ecosystem to manage and preserve the water environment in a blackish lake.In this study, we evaluate the habitat environment for Corbicula japonica using its growth model. And, we analyze the seasonal and annual variations of water qualities, using a hydrodynamic model coupled with an ecological model to clarify the relation between thewater qualities and the amount of Corbicula Japonica.

Variability on Flow and Density Structure Near the Mouth of Mutsu Bay Focused by Passage of Migratory Cyclone

The Mutsu Bay is a semi-enclosed bay located in northern Japan. In the present study, we clarify the influence of a passage of migratory cyclone on a flow and density structure around the bay and a water exchange through the bay mouth. A three-dimensional baroclinic flow model and wind calculation models are used. Five cases of simulations under various meteorological conditions are examined. Moreover a Lagrangian particle tracking model is also adopted in order to examine meteorological effects on the water exchange in the bay. The numerical model was able to hindcast the observed sea surface setup and rapid change of density structure. The computed results show that the density structure is mainly affected by wind stress, while tidal anomaly is affected by time change of atmospheric pressure. The water exchange at the bay mouth is enhanced by the passage of migratory cyclone. These results suggest that meteorological disturbances may play an important role on conservation of water quality in Mutsu Bay.

Seasonal Changes of Baroclinic Circulation and Water Exchange in the Bohai Sea

In four principal sea areas in China, accelerated water pollution has recently become more widespread and severe with rapid economic growth. In enclosed areas such as the Bohai Sea, red tides are occurring with greater frequency, degrading ecosystems and causing eutrophication. As the first step in clarifying the mechanisms of ecosystem and coastal eutrophication and for considering concrete enforcement measures to reduce the damage, the present study focuses on physical phenomena such as hydrodynamics and transport processes. First, the flow structure and density field in the Bohai Sea are examined using a three-dimensional baroclinic flow model. Second, the seasonal change of wind and surface heat transfer on the residual current system and the water exchange through the Bohai Strait are examined using a Lagrangian particle tracking method. This computation makes clear that the residual currents in the majority of the Bohai Sea are very weak, less than 0.05m/s. The effect of seasonal winds on hydrodynamics such as residual currents in the Bohai Sea, water exchange between the Bohai Sea and the Yellow Sea, and stratification induced by heat transfer at the sea/water surface are studied.

FIELD SURVEYS OF ANOXIC WATER IN ENCLOSED COASTAL AREA OF OSAKA BAY

Appearances of algae blossom at the sea surface and anoxic water over the sea bed have often been observed in the enclosed coastal area at the head of Osaka Bay. In particular, special attentions have recently been paid to the anoxic water, because it finally relates to lives of fish and shellfish. In this paper, we present the results of our field observation of temperature, salinity, chlorophyll-a, dissolved oxygen, and so on at 15 stations in the Port of Amagasaki-Nishinomiya-Ashiya every week from June to September, 2002. During the period, blue tide, that is, upwelling phenomena of anoxic seabottom water was able to be observed in the enclosed coastal area under the north wind blowing, but the water existed strongly over the seabed after the dramatic change. The effect of rainfall was limited at the sea surface and did not have an impact on anoxic water in the port. In addition, the thickness of the water depended on the depth of thermocline, not the strength of stratification.