To numerically analyze the hydraulic and hydrological aspects of water environment in an alluvial area that has three-dimensional (3-D) complexity in its geological structure, a composite dimensional physically-based numerical model is developed by use of the finite volume method (FVM). The model consists of the surface and subsurface flow submodels which are based on the 2-D shallow water equations of diffusion type, and the 3-D mixed-form Richards equation, respectively, and fully coupled. The FVM-based numerical formulation and the use of the pentahedral cell with a triangular-shaped top face for piecewise discretization are of great advantage in economizing the computational tasks while finely resolving the land surface, say, on a field-plot-scale resolution. The example application of the model developed is made to an actual area embracing the hillside as well as the alluvial fan where paddy rice farming is implemented, aiming at holistically and heuristically surmising whether the multiple clayey layers of the subsurface stratum are perfectly or imperfectly compacted with clayey soils. The result shows that the model has capabilities as a helpful tool for simulating the interlinked surface and subsurface flows in an alluvial farmland area expanding over the subsurface stratum of 3-D complexity.

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Understanding water flows in open channels is of great importance in agricultural engineering because water is necessary for farming. Although water flow is fundamentally a three-dimensional (3D) phenomenon, one-dimensional (ID) models are frequently used in numerical simulations of open channel flows because more detailed data and a greater computational cost are required for higher-dimensional models. Open channels include various structures. A bend is one such structure; however, the effects of a bend are usually neglected in ID models. Therefore, a mathematical model of the effect of a bend on open channel flows is derived, and a numerical model of ID flows in open channels with bends is shown by combining the mathematical model with the earlier model. The numerical model is applied to flows in two sets of experiments. The computed results are compared with the experimental data. The first set of experiments for steady-state flows in an open channel with two bends is conducted in this study; the second set of experiments was carried out as part of the Concerned Action of Dam Break Modelling project by Frazao et al. (1999). Dam break flows in bend channels were thus investigated. The comparison between the computed results and the experimental data indicates that the ID numerical model can suitably reconstruct flows in open channels with bends.

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Abundant and heavy rainfall in addition to large amounts of rock, rough terrain, and unstable surface deposits in Japanese mountainous forests demands very high cost in maintenance of road segments both during construction and after completion. This paper presents a simple system of quantitative descriptions of irregular surfaces of forest road segments based on computer simulation techniques. The system used to generate two-dimensional surface roughness is formulated by using a first-order Markov process model (AR model). In order to verify the applicability of this model for practical purposes, several types of road surface consolidated by different combinations of operations and machinery were analyzed.

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This paper presents a simple system of quantitative descriptions of drainage processes on irregular surfaces of road segments based on computer simulation techniques, and it proposes an alternative flow model which more closely takes into account linked geomorphic and hydrologic properties. In earlier papers, the author formulated such a model in the one-dimensional manner. The approach employed here is extension of this model into two dimensions. The model derived here represents the mode of accumulation of water in depressions on the irregular surface and the development of oversurface flow. The surface flow system is simulated by a simple storage model. The assumptions behind a model postulated as describing the model of accumulation of water on irregular surfaces are that the infiltration capacity of soil is exceeded and the rainfall continues. Accordingly, the residual water ration (the depression-storage of irregular surfaces) can be derived in a simple way. In order to verify the applicability of these models for practical purpose, several types of road surface consolidated by different combinations of operations and machinery were analyzed.

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In recent years, the drainage condition of rice fields has been improved for upland crops planting by the farm land consolidation in order to increase farm productivity. On the other hand, the improvement of drainage conditions in paddy field area greatly influenced the habitat of freshwater fishes. However, there are few researches on how freshwater fishes have been influenced by the drainage improvement. In this research, we investigated the difference between the environment for freshwater fishes in drainage canals in the year when the whole area was planted to rice and in the year when a part of the rice field area was planted to wheat. When an upland crop such as wheat was planted in paddy field area, the water depth in the drainage canal among all the environmental components greatly changed. The vegetation on the levees was not available for freshwater fishes due to the fact that the water level in the canal did not increase up to the level of levees, and this drastically decreased the number of fishes living in the canal. Furthermore, from the analysis of the relationship between the number of fishes and the distance from vegetation zone, it could be suggested that at least one vegetation zone of length 100m must be created along the canal with the aim of maintaining the habitat for freshwater fishes.

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The investigation was carried out for two years, to clarify which types of environment-conscious canals are suitable for freshwater fish habitation as an alternative environment for disappearing wetlands. It was found that freshwater fishes could inhabit the environment-conscious canals if vegetation zones are created at the water edges. Moreover, it was found that demersal fishes needed to be moved artificially because they could not move by themselves from the disappearing wetlands to the environment-conscious canals. However, the environment in the agricultural drainage canal changed drastically due to the significant changes in water depth; therefore the canal could not provide a stable habitat for freshwater fishes.

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An infiltration trench with wood chips is more efficient method to prevent reddish soil runoff in an upland field. To put the measure to practical use, this study unvails two factors influencing the sediment reduction efficiency and estimates the USLE support practice factor P value. For the purpose of this study, field experiment was conducted under natural rainfall condition using six experimental slopes. The slope dimensions are 30 m long by 1 m wide. It was found that the efficiency increased as the trench volume was gained, and was higher in the slope with multiple trenches than with single one. It seems likely that the amount of unsaturated gap volume in the trench during rainfall event would be directly related to sediment removal. USLE P value of the infiltration trench was in the range of 0.1 to 0.3, and was smaller than that of conventional measures. In conclusion, the volume and the arrangement of the infiltration trench are factors affecting the sediment reduction efficiency, and USLE P value is estimated as the efficiency.

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In the case of a sedimentation tank constructed in an upland field, its design conditions are affected differently by on-site environment such as rainfall and inlet structure, so that the changes in the calculated suspended soil trap efficiency can be of significant concern. The purpose of this study was to compare the calculated suspended soil trap efficiency with the observed in a sedimentation tank. For the purpose of this study, field observation was conducted to get the observed trap efficiency, and calculation was made using grain size distribution of suspended soil to get more precise planned trap efficiency. As a result, the observed trap efficiency ranged from 59% to 98%. The calculated trap efficiency changed according to suspended soil particle density ρ_p to be 29%-64%, 40%-73%, and 48%-77% to ρ_p = 1.5, 2.0, and 2.5Mg・m^<-3>, respectively. The results show that the observed trap efficiency and the calculated trap efficiency are in good agreement, and suggest that the flow similar to the one in an ideal tank was formed in the investigated sedimentation tank.

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Deposition in a sedimentation tank may become important resource because it contains soil and nutrients. The purpose of this study was to examine the form characteristics of paniculate nitrogen as related to its settling and particulate nitrogen trap efficiency in an ideal settling tank. For the purpose of this study, observing suspended soil particles with a digital microscope, measuring particulate nitrogen concentrations classified by the suspended soil particle size, and calculating trap efficiency based on the clarification theory of an ideal settling basin were conducted. As a result, it was found that the particulate nitrogen concentration of the suspended soil classified as fine sand was higher than that of silt and clay. It was also found that particulate nitrogen was nitrogen as a constituent of the organic matter in a micro aggregate, so that it was relatively easy to settle. The results show that particulate nitrogen is removed easier than suspended soil, so that its trap efficiency in sedimentation tanks is higher than that of suspended soil.

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