[in Japanese]

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Heat, liquid water, vapor movement theory, not including Kelvin's relation and equation of state, was proposed and was combined with a solute transfer model to assess the secondary salinization, i. e. the re-accumulation of salt near the soil surface following the elution from an initial salt accumulated layer after precipitation. The proposed model was, therefore, extended to analyze the moisture movement from funicular to pendular moisture state. The computation could reproduce vertical profiles of temperature, volumetric water content, vapor density and salt concentration in a soil column including a salt accumulated layer after watering the soil surface. It was confirmed that the secondary salinization is sensitively affected by the watering rate, watering times and the depth of the initial salt accumulated layer.

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Land developments often result in adverse environmental consequences, especially for local surface and subsurface water systems. Thus, it is important to assess potential adverse effects in advance of any development. For this purpose we propose a combined rainwater-recharge model with quasi three-dimensional unconfined groundwater flow analysis in the present paper. The catchment water balance for a planned new campus area of Kyushu University is studied by the suggested method with simultaneous analysis of both surface runoff and groundwater flow. The results show that the direct surface runoff rate estimated by the present method is nearly equal to that of the graphical base flow separation or a four-level tank model and that modelled daily, monthly, and annual surface runoff agree reasonably well with the observed discharge. Several hydrological components were investigated in order to understand the characteristics of the water movement in the study area.

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Significant and monotonous expansion in luxuriant, stable vegetation areas like wood-lands has been observed in gravel-bed reaches of the Tama River. Changes in vegetation, substrate and channel shape in micro- and macro-scale was traced carefully together with hydrologic and hydraulic conditions experienced. A scenario for the expansion was obtained, where the positive effect of floods by forming a “top fine-sediment layer” necessary for the stable vegetation growth was emphasized, as well as their negative effect by disturbing and removing a gravel surface with vegetation. A simple but comprehensive simulation model based on the scenario was developed and used for identifying hydrologic, geomorphic and fine-sediment-supply conditions encouraging or discouraging the growth of stable vegetation.

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Regarding air-entraining vortices in a suction sump, critical submergence S_c were specified concerning five parameters, such as the Froude number Fr, the Reynolds number Re, back clearance X, sump breadth B, and bottom clearance Z. Here, all parameters are non-dimensionalized by suction-pipe outer diameter D and Bell-mouth-entrance velocity. Consequently, the authors confirmed that Re effect is rather small, while Fr effect is large. X/D effect is also small. Most of X/D, B/D and Z/D effects can be explained on the basis of Fr-S_c/D relation, except for the X/D effect by which S_c/D attains the maximum value.

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Numerical analysis has been performed for the developing turbulent flow in curved open-channel flow with roughened wall by using algebraic Reynolds stress model. It is important to clarify the relationship between flow structure and roughened wall, because riverbed and bank of almost all rivers have been roughened by means of transporting sand. In order to examine the validity of the present method, preliminary calculation has been done for turbulent flow in curved open-channel with smooth wall comparing with the experimental data. As a result of this research, it is found that the calculated results are in good agreement with the experiment and downward secondary flow is generated near outer bank, which is not only distinctive feature in curved open-channel but also recognized in both results. As for calculated results of roughened wall, it is summarized that the secondary flow is greatly changed by the location and area of roughened walls and remarkable changing is especially shown for the case of roughened bank, which phenomenon must be caused by anisotropic turbulence.

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A fifth-order numerical scheme (FVM-QSI scheme) has been proposed for linear and non-linear convection equations, which are discretized with a finite volume method. In this scheme, the fluxes on cell boundaries are spatially interpolated with quintic spline functions and they are controlled not to cause numerical oscillations by the proposed DC and FP methods. As a result of the computations, it was shown that the FVM-QSI scheme with the DC method enables us to obtain stable and more accurate results than fifth-order TVD schemes and that the FP method can shorten the computational time. The proposed scheme is also applicable to the incompressible flows without increasing computational time.

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This paper presents a mathematical model of the formation of gullies on slopes composed of material with relatively weak cohesivity such as freshly deposited volcanic ash. A linear stability analysis is performed with the use of the momentum equations of flow, the dispersion equation of suspended sediment, and the Exner equation extended for beds subject to erosion and deposition of sediment. The analysis shows that the dominant gully spacing is 600-1500 times the Froude-critical depth of the sheet flow on the slope. Evaluating the critical depth to be 5cm, the spacing of gullies is 30-75m, which is cosistent with observations.

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To clarify horizontal structures of hydrodynamic environments in a mangrove swamp, we have conducted field measurements in a mangrove swamp with two surrounding creeks, which is located in Fukido River estuarine section of the Ishigaki Island, Okinawa. The results indicate that the tidal currents in the swamp flow towards not only the normal direction to the creek, but also the parallel direction. We have also found the tidal asymmetry of the swamp currents that the differences of the flood and ebb water velocities appear appreciably. These patterns of the swamp currents are governed with the water surface slope in the swamp, which is closely related with the differences of the water elevation along the surrounding creeks.

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Using a rather small number of irregular quadrilateral lattices, the weighted finite difference method (WFDM) may represent the geometrically complicated shape of a closed water area. In WFDM, an unknown value of the point under consideration can be calculated as the weighted sum of the known values of the neighboring points. These weights of the neighboring points are obtained from the several polynomials that satisfy the governing equation. In this paper, we developed the two-dimensional WFDM for calculating the tidal flow of the hydraulic model simulation, which has the geometrically complicated shape of a closed water area. The numerical solutions of WFDM show good agreement with the results of the hydraulic model simulation in our laboratory.

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Principal component analysis (PCA) and complex PCA (CPCA) are applied to examine medium-term geomorphological behavior of an exposed sandy beach before Gamo Lagoon located at the mouth of Nanakita River along Sendai Coast, Japan, using a 12-year series of bathymetry survey data. The results of PCA and CPCA for the bathymetry data set show that erosion and subsequent accretion of the submerged terrace formed in front of the river mouth appear in the first mode of CPCA and have the most significant influence on the medium-term geomorphology of the study area. The first mode is mostly caused by northward alongshore sediment transport driven by wave energy flux, explained from the observed wave data. The second mode of CPCA demonstrates that the sediments previously discharged from the river would return again into the nearshore region, and furthermore, the topography changes due to cross-shore sediment transport emerge in the third mode of CPCA.

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An ensemble ocean wave prediction sysytem is developed to predict medium range wave fields using a weekly weather ensemble prediction system. The characteristics and validity of the ensemble ocean wave prediction system are discussed analyzing the numerical results. The predicted mean wave height by the ensemble ocean wave prediction system is highly correlated with analysis data and is slightly underestimated as forecast length becomes longer. The predicted wave height by the ensemble ocean wave prediction system is correlated with spread, scatter of ensemble member, as well as the wind and atmospheric pressure in the ensemble weather prediction. The value of spread is increased with the forecast length, monotonically. Moreover, the probability density function for the wave heights of ensemble members is proposed using the two-parameter Weibull distribution for practical purpose.

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Field observation on the supply of sediment and nutrients from Nagura river basin, Ishigaki island, was studied in this paper. They were estimated by measuring water velocity, depth, turbidity, concentrations of carbon, nitrogen and phosphrous, in which the former 3 items were measured continuously for 3 months and the concentrations of latter 3 materials were observed only for rainy days. The result has revealed that the sediment yield from the basin in these 3months is 134 tons, and the specific sediment yield in this basin is 8.3gr/m² for the 3months. It was found that heavy rains yield nutrients mainly in particulate type. However, sugar cane field yields dissolved nutrients gradually after the rain. The timing of flood and tide is an important factor to control the discharge of sediment and nutrients to the tidal area such as lagoon and river mouth.

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We have carried out field measurement on water quality in the inner area of Ariake Bay in the summer of 2002 in order to investigate the occurrence of hypoxic water. It was observed that hypoxic water widely appeared in the bottom layer along the coastal area of Saga Prefecture during the observation period. It also appeared two times in Isahaya Bay in early July and August. Formation of the hypoxic water is influenced by stratification coursed by temperature and salinity distribution. It was also found that the hypoxic water disappeared when wind broke the stratification of the sea water.

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The convergence and computational-speed of the C-HSMAC and SOLA methods were discussed and compared in detail. It was shown that only the diagonal elements in the pressure matrix are taken into account in the SOLA method, while the numerical solutions of the matrix equation are obtained in the C-HSMAC method. In addition, the iterative numerical procedures in the SOLA method are based on the classical Jacobi method, while a high-performance iterative method can be available to solve the pressure matrix in the C-HSMAC method. As a result of the computation of 3D cavity flows, it was confirmed that the computational performance of the C-HSMAC method is higher than that of the SOLA method.

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