Kisaburo Nakata

A model to simulate the transport and fate of gas and hydrates released in deepwater

Methane and natural gas, if released in deepwater, undergo physico-chemical processes as they rise through the water column. In deepwater, these gases are likely to be converted into hydrates. These are dissociated into gas upon reaching shallower regions. The present model accounts for plume thermodynamics and hydrodynamics, and is integrated with the associated physico-chemical processes such as hydrate formation/dissociation, gas dissolution, hydrate dissolution, hydrate shell crumbling and reformation, heat and mass transfer inside gas bubbles, multiple-sized bubbles and their size change, and possible gas separation from the main plume. The model simulations compare well with field data from Deepspill in Norway, and the gas bubble releases off the California coast. The scenario simulations show that the inclusion of multiple bubble sizes impacts the model results. Inclusion of hydrate dissolution is also important for deepwater releases. Without hydrate dissolution, the results tend to overestimate the hydrocarbon mass that will reach the water near the surface.

Analysis of water quality in Lake Hamana using a coupled physical and biochemical model

Abstract The mechanism of water pollution in Lake Hamana, one of the typical highly eutrophicated semi-enclosed estuaries in Japan, has been studied using a numerical 3D physical–biochemical coupling model. In this study, the chemical oxygen demand (COD) was adopted as an index of the lake water quality, and the spatial pattern of the average COD concentration during the summer from 1988 to 1991 was simulated by the model. It was suggested from the numerical result that the water quality in Lake Hamana was essentially influenced by the phytoplankton production, although the dissolved organic matter accounted for the major fraction of COD constituent in the lake. The evaluation of the biological processes over the whole lake region showed that the autochthonous COD flux associated with the primary production may be almost 10 times larger than the external loading flux. Because the phosphate turned out the most limiting nutrient to the phytoplankton production and most of the phosphorus pool in the water column appeared to be supplied from the sediment, it was considered that the benthic phosphorus flux would be the most critical factor to the autochthonous COD in the lake.

Model study of Lakes Shinji and Nakaumi : a coupled coastal lagoon system

Abstract To understand the physical and biological processes that occur in Lakes Shinji and Nakaumi, a coastal lagoon system, a hydrodynamic–ecological coupled model was applied. The model was run to hindcast these processes from March 1, 1995 to the end of September 1995. The simulation of physical processes suggests that the meteorologically induced sea level variation and a large episodic amount of freshwater discharge effectively caused the water exchange between the open sea and this lagoon system. The simulated results of the biological processes using the ecological model show good agreement with field data except for the phosphate concentration in the bottom water. It was also found that the microbiological processes in the water column, as well as in the sediment, contribute largely to the formation of oxygen-depleted water in the bottom layer. It was also clarified from the simulation that the two species of benthic bivalve, which have dense populations in this lagoon system, play important roles on removing organic particulate matter and nutrients from the system.

A New Coastal Marine Ecosystem Model Study Coupled with Hydrodynamics and Tidal Flat Ecosystem Effect

A new coastal marine ecosystem model was developed, which was composed of pelagic and benthic ecosystems, and was applied to Mikawa Bay, Japan. This model deals with variations of biochemical and physical interactions among dissolved oxygen and C-N-P species (composition formed out of carbon, nitrogen and phosphorus elements) so that it resolves the flux dynamics of carbon, nitrogen, phosphorus and oxygen elements. The physical and biochemical mechanism figured in this model is constructed for the purpose of simulating the estuarine lower trophic ecosystem, in areas where the sea was too deep for light to reach the sea-bottom. As a result of coupling the benthic with pelagic system, the effect of process of sedimentation and nutrient diffusion back to the pelagic system could be indicated. In addition, by implementing the tidal flat ecosystem models calculation result, the integrated model can include the effect of water purification in tidal flats where the light can reach the sea-bottom, and where seaweed, sea grass and benthic algae exist. In this study, the model indicates that oxygen-depleted water exists at the sea-bottom especially in summer mainly caused by an increase of oxygen consumption in the benthic system and a decrease of the vertical mixing water process. Furthermore, by comparing the case--with the tidal flat ecosystem model and the case without it, the effect of water purification of tidal flat estuaries was indicated. From the viewpoint of a short time scale, the tidal flat has the potential to restrict red tide (rapid increase of phytoplankton), and from the viewpoint of a long time scale, it restricts the sedimentation of detritus. Restricting the sedimentation prevents oxygen-depleted water occurring in the coastal marine system of Mikawa Bay.

Sensitivity analysis of a coastal marine ecosystem

An ecosystem with four compartments, i.e. PO4-P, phytoplankton, zooplankton and detritus, in a coastal region was considered. Sensitivity analysis of a phosphorus flow model in well-mixed water without currents concludes that (1) the maximal photosynthetic rate,Vm, in Michaelis-Menten relation plays an important role in the distribution of biomass among compartments but the half saturation constant,Ks, is not so important, (2) the natural death rate of phytoplankton is important for the ecosystem, (3) the natural death rate of zooplankton is also effective on the ecosystem.A numerical experiment was also performed on the ecosystem with four compartments are also studied using a dynamical barotropic model of tidal currents of Mikawa Bay (Japan). Diffusion coefficient in diffusion equation plays the role of a linear smoothing parameter in the horizontal distribution of compartment. On the other hand, perturbations of biological parameters cause nonlinear variations in the horizontal distribution of compartment.

A Numerical Simulation of an Oil Spill in Tokyo Bay

Abstract An oil spill accident happened in Tokyo Bay on 2 July 1997. About 1500 m 3 of crude oil was released on the sea surface from the Japanese tanker Diamond Grace . An oil spill model is applied to simulate the fate of spilled oil. The Lagrangian discrete-parcel method is used in the model. The model considers current advection, horizontal diffusion, mechanical spreading, evaporation, dissolution and entrainment in simulating the oil slick transformation. It can calculate the time evolution of the partition of spilled oil on the water surface, in the water column and the sedimentation on the bottom. A continuous source at constant rate is set up as a tanker off the coast of Yokohama. The grid size is 1 km in the calculation domain. The residual flow simulated by a 3-D hydraulic model and observed wind data are used for advection. The simulated distribution of oil spreading agrees well with observations from satellite remote-sensing.

A Numerical Simulation of the Seasonal Cycle of Temperature, Salinity and Velocity Fields in Tokyo Bay

To understand when oxygen-depleted waters occur, how they develop and when they dissipate in inner Tokyo Bay, realistic simulations were attempted with fine spatial and temporal resolution by applying realistic time dependent external forcing. A 3D hydrodynamic model was driven by time-dependent external forcing factors/parameters such as solar radiation, air temperature, relative humidity, wind velocity, and fluvial discharge, under the open boundary conditions of 1995. A simulated time series of salinity and temperature agreed fairly well with observed data, except in summer. The model failed to reproduce the development of the surface mixed layer in summer. Several sensitivity analyses on the external forcing parameters such as wind velocity and vertical diffusivity were conducted to reproduce the mixed layer. However, changing these parameter values did not improve the model results.

Continental shelf waves off the fukushima coast Part I: Observations

Current records obtained in the inshore region along the Fukushima coast are analyzed. The existence of periodical current fluctuations whose period is about 100 hours and whose amplitude is as large as 15–25cm s−1 is recognized. Auto-spectral analyses are made also for sea level, atmospheric pressure and wind records. Each spectrum has significant peaks at the similar period to the current spectrum. The wind spectrum has a broad peak compared with the current. The periodical current fluctuations propagate southward with speed of 3–5 km h−1. These propagation speeds seem to correspond to those of the second-and third-mode shelf waves.

Estimation of primary production in the ocean using a physical-biological coupled ocean carbon cycle model

A lower-trophic marine ecosystem model that takes into account both the grazing food web and the microbial food web has been developed to investigate the ocean carbon cycle. The ecosystem model was coupled to an oceanic general circulation model and a simulation was performed to examine the temporal and spatial distribution of primary production in the world ocean. Numerical results revealed that the total amount of annual net primary production reaches nearly 61.2GtC, showing fair agreement with the recent estimates based on the satellite image analysis. The annual flux of particulate organic carbon toward the subsurface layer, viz., the export flux, was evaluated as 5.5GtC. The model results reproduced the general tendency that the regions of low latitude are characterized by high primary productivity as well as low export flux, and suggested a dominant role for the microbial food web in the oceanic carbon cycle.

Ecological risk assessment of TBT in Ise Bay.

An ecological risk assessment of tributyltin (TBT) in Ise Bay was conducted using the margin of exposure (MOE) method. The assessment endpoint was defined to protect the survival, growth and reproduction of marine organisms. Sources of TBT in this study were assumed to be commercial vessels in harbors and navigation routes. Concentrations of TBT in Ise Bay were estimated using a three-dimensional hydrodynamic model, an ecosystem model and a chemical fate model. Estimated MOEs for marine organisms for 1990 and 2008 were approximately 0.1-2.0 and over 100 respectively, indicating a declining temporal trend in the probability of adverse effects. The chemical fate model predicts a much longer persistence of TBT in sediments than in the water column. Therefore, it is necessary to monitor the harmful effects of TBT on benthic organisms.