The authors conducted a field swimming experiment using cultured masu salmon (Oncorhynchus masou masou) fingerlings in order to study their critical swimming speed during their release into the river in the Iwaki River diversion weir. The experimental equipment was a small, rectangular cross-section channel, which was installed in a local riverbed at the fishway. The experiment was conducted using an average cross-sectional water flow velocity of 17 to 92 cm·s^-1, and using masu salmon fingerlings from 4.8 to 7.1 cm in the length. River water temperature was between 13.7 and 20.6 °C. The critical swimming speed measured over 60 minutes was between 16 and 41 cm·s^-1 and a positive correlation was found between the critical swimming speed and body length. The critical swimming speed measured by body length (BL) was 3.5 to 6.9 times (that is, the distance travelled per second based on body length), and the mean critical swimming speed was 5.5 (with a standard deviation of 1.1). Results showed that water temperature differences in the experiment had no significant effect on the critical swimming speed measured over 60 minutes.

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Electric conductivity of groundwater was measured every 1 m of depth in wells on Tarama Island, Japan to clarify the thickness of the freshwater lens. In addition, electric terrain conductivity was measured by electromagnetic surveys. As a result, it was clarified that the thickness of the freshwater lens (electric conductivity less than 200 mS/m) on Tarama Island was 0 - 7.2 m at 12 wells. In addition the freshwater-saltwater interface depths that were estimated to be 14 - 23 m by electromagnetic surveys corresponded to depths where electric conductivities were 1,840 mS/m. There was a high correlation (R² = 0.9) between depths where electric conductivity of groundwater was 200 mS/m and the depth of freshwater-saltwater interface estimated by electromagnetic surveys. From these results, the thicknesses of freshwater were estimated at 7 wells where the thickness of the freshwater lens could not be determined by measurement of electric conductivity in groundwater. Finally, the amount of the freshwater lens was estimated to be 6.8 million m³.

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The factors affecting the groundwater level and chemistry in abandoned terraced paddy fields on Sado Island, Japan, were investigated. Seasonal changes of the groundwater level in site A, which is on the bottom of the slope, was slight throughout the year compared with that in site B, which is on the middle part of the gentle slope. EC, pH, and some components involved in mineral weathering (Na⁺, Ca²⁺ and alkalinity) in the groundwater from site A were higher than those from site B. These results showed that the infiltrating water from the upper part of the slope was the main source of the groundwater in site A. On the other hand, the impact of sea salt components (Na⁺ and Cl^-) was evident in site B, because the concentration of these components increased during winter. Groundwater chemistry was also affected by other factors such as the dilution of groundwater caused by rainfall, sulfur redox, and nutrient uptake by vegetation. The effects of each factor on groundwater chemistry differed between sites because the topography of the two adjacent sites was different. The characteristics of water environment, such as the groundwater level and chemistry, should be considered in the management of abandoned terraced paddy fields on Sado Island.

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Water purification characteristic evaluation was done for adding aeration and circulation to a canal planted with Carex dispalata, which is a Japanese native species whose height does not obstruct the view. Circulation improved the BOD removal efficiency, irrespective of the use of aeration. Aeration improved the BOD removal efficiency in the condition of circulation ON. The BOD removal rate without any circulation was 15g·m^-2·d^-1, which was the maximum, suggesting that it can be raised higher than 20g·m^-2·d^-1 through the use of aeration and circulation. The D-BOD removal effect is high. Therefore, removal of the organic pollution load by bacteria increased. The nitrogen removal efficiency did not increase because nitrification by aeration did not advance. Instead of energy-intensive aeration, it is necessary to consider the oxidative capacity of nitrification.

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Although the disaster restoration in the agricultural land and rural villages has progressed since the 2004 Chuetsu Earthquake, potential small-scale damages of the agricultural infrastructure have been continuously coming to surface. The notable difference between the earthquake disaster and other disasters is that the former induces extensive ground damages which, in turn, lead to a long-term continuation of small-scale damage emergence. However, there exists no measure devised to deal with those small-scale damages due to the difficulty in technically proving the relationship between the damages and the earthquake even though the damages are actually caused by the earthquake. Although it has been pointed out that longer-term restoration measures are necessary to cope with the peculiar characteristic of a earthquake disaster, these are not realized in practice owing mainly to the unavailability of the secular change data of damages. In this research, the damage characteristics in the Chuetsu area were clarified through the analysis of tracking records of the Small-scale Disaster Restoration Project (SDRP: In Japanese "Tezukuri-tanaoshi-tou-shien" Project), and the results of questionnaires for the farmers. In conclusion, a proposition was made on long-term measures for the disaster restoration after the occurrence of a large-scale earthquake.

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This paper presents a series of erosion tests of sand-clay mixtures, which investigated the relationship between erosion rates of the soils and shear stress exerted onto the soil surface, and laboratory experiments of embankment erosion due to overflow, where the same sand-clay mixture was used as the embankment material. The effects of the dry density and the clay content on the erosion rates of the soils were investigated in the erosion tests. When an embankment is subjected to overflow, the height of the embankment decreases while the erosion occurs at the crest, where the water flow is Froude-critical. The experiments of embankment erosion have revealed that the decrease rates of embankment height corresponded to the erosion rate of the embankment material under the critical flow. This fact has shown that the estimation of the shear stress under the critical flow enables the temporal development of embankment erosion to be predicted with erosion rates of the embankment material.

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From among the methods of computing one-dimensional unsteady open channel flow, space-time conservation method is picked out, which has flux limiter functions to prevent numerical oscillations and whose settings of various boundary conditions are easy to handle, that are decided by water control structures, discharge, water depth, etc. By extension of the scheme we propose a new algorithm of computing the flow in channels with irregular geometries where the channel breadth and the bed slope change variously along the flow. It is showed that the new scheme is effective as a result of computing steady quiescent flow, comparing numerical steady solution with the steady water surface obtained by calculation of water surface profile, and comparing numerical unsteady solution with the time variation of the water depth obtained by the experiment in rectangular channels that have breadth and bed slope changing.

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Three prototype escape countermeasures for frogs that can be easily installed in U-shaped canals with widths of 30-50 cm and depths of 30-50 cm were experimentally produced because frogs cannot escape from agricultural canals with deep concrete walls after falling into the canal. The differences of effectiveness of the 3 prototypes in places for the countermeasures (1 and 2) and flow conditions (dry and water running) were investigated for 2 frog species (Tokyo Daruma Pond Frog and Japanese Brown Frog). The brown frogs escaped from the canals more easily than the pond frogs. The brown frogs escaped regardless of their body size, but the small pond frogs escaped more easily than the large pond frogs. The prototype with slopes beside both canal walls and a net spread across the center line of the canal enabled frogs to escape from the canal more easily than the prototypes with only slopes or nets beside both canal walls. Increasing the number of places for the countermeasures enhanced frog escape. The differences in frog escape between dry canals and canals with water running were not significant. Therefore, the prototypes were confirmed sufficient as escape countermeasures that is inexpensive and can be easily placed in and removed from agricultural canals.

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The pavement rate of the farm road which becomes important in activities of agricultural production, circulation of agricultural products and rural life is low. There are many farm roads to which the function of traveling performance, traveling comfort and prevention of the damage of agricultural products in transportation is not secured. Maintenance including improvement in the pavement rate of a farm road must be economically carried out based on the service environment, the circumference environment and the required function according to the kind of farm road. In this research, the problem of the farm road in a paddy area was extracted from the questionnaire to a land improvement district as an administrator, and the conditions which should be taken into consideration in maintenance of a farm road were clarified. The problem of a farm road is deformation of a road surface and a request is a period which does not need to repair. Moreover, the present performance of ground property and road surface of sediment pavement on-farm road was evaluated. Positive correlation is between the standard deviation of modulus of elasticity of the soil and surface roughness, negative correlation is between the modulus of elasticity of the soil in the rut and rutting depth.

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Sediment of drainage canals plays significant roles for aquatic plant rooting, fish spawning and invertebrate habitats. In steep areas, it is difficult to maintain the sediment during floods because water flows rapidly. Fast flowing water causes sediment loss and habitat loss through intense sediment transport. Therefore, we installed spur dikes in field drainage canals to control sediment transport and form sandbars to provide habitats for aquatic organisms. The results showed that sandbars were formed by controlling the sediment transport and the process of sandbar formation depended on the magnitude of flood discharge. When the flood discharge was small, the sediment was washed out between the spur dikes, and distinct sandbars were not formed except in areas aquatic plants. When the flood discharge was large, the sediment accumulated between the spur dikes, and sandbars were clearly formed. Without spur dikes, most sediment was flushed away by flood discharges. Consequently, spur dikes facilitate an increase in invertebrate populations by establishing sandbars for habitats.

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The authors developed a model for planning the transportation and application of digested slurry to farmlands as a fertilizer. The model requires the following parameters, daily production of the digested slurry, concentrations of fertilizer elements in the digested slurry, distances from the plant to each field where the slurry will be applied and application loads of slurry for each field. According to the data, the model simulates daily workloads for transportation of digested slurry from the plant to the fields by a vacuum truck and application of slurry to each field with a spreader. The model calculates the daily changes in digested slurry storage at the plant. Validity of the model was confirmed by data obtained from a pilot scale plant. The model is a useful tool for planning the transportation and application of digested slurry to farmland as a fertilizer.

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