A simple analysis method for dose rate of gamma ray in the farmlands contaminated by radioactive cesium is proposed. Vertical profiles of radioactive cesium averaged in each farm plot were laid out on GIS. The dose rate was evaluated based on the point-kernel source method, which is a widely applied as a simplified calculation method for radiation field. The proposed method made accurate estimates of dose rate in farm block scale. However the dose rate estimated in plot scale suffered from large variation that is not observed on the site. This discrepancy is probably attributed to the statistical error included in the Cs concentration evaluated from the soil samples having a diameter of no more than 5cm. The estimated dose rate after performing the decontamination by scraping the ground surface tended to be less than the observed value. The underestimation is supposed to be caused by the overestimates of the thickness of the scraped soil and the remains of contaminated soil on the very surface of the ground after the work.
In pipe flows where the acoustic wave speed of fluid is much faster than the flow velocity, calculation efficiency becomes higher and the limitation of the time step is relaxed by implicitly treating the pressure term. In this study, the fundamental equations are split into a convection phase and a non-convection phase. Then the convection term and the non-convection term are treated by a CIP algorithm and a SMAC algorithm respectively. In the steady state analyses, this method precisely and stably calculated the steady state discharge. In the water hammer analyses, the results do not show high frequency waves and the longitudinal distribution of maximum pressure becomes lower than the existing method. This high frequency wave is probably a numerical error in the existing method. But detailed experimental and analytical investigations is necessary to confirm that that is the numerical error.
This paper discusses the applicability of a two-dimensional unsteady flow analysis method to flood due to a failure of a small earth dam in 2011 off the Pacific coast of Tohoku Earthquake. The method uses the equations for shallow-water flow. The analysis has required the following modifications to input data even though it had adopted the digital elevation model (DEM) of the highest resolution (2 m grid) for topographical data: 1) topographical elevations are partly modified to express the shape of drainage channel and spillways, 2) the bottom elevations of the reservoir located downstream are modified and water is impounded there before the upstream dam breaks, 3) the peak discharge of inflow hydrograph, predicted from the equation by Costa, J.E., is decreased. The analyses with the modified input data are capable to predict the inundation areas. The lower resolution DEM (5 m grid) can also be available for the flood analysis with the modifications mentioned above.
To elucidate the relationship between rainfall-runoff discharge and urbanization, the runoff models were established for each land use and the discharges from unit lots were analyzed in the Tedori River alluvial Fan Area where paddy field are dominant in land use. The area of lots, the shape and height of outlet and the percolation rate before and after mid-summer drainage in the paddy area, and infiltration capacity in no irrigation period of paddy, upland and residential area were investigated to establish the runoff models. The models correspond to five types of land use area, which included irrigated and non-irrigated paddy lots, upland field lots of barley and soy bean and a residential lot. The rainfall-runoff discharges from those lots were calculated for 54 storm events which ware corresponding to planning rainfall amount. The results indicated the followings; the maximum runoff ratio to total rainfall was calculated in the residential lot whereas minimum was in the irrigated paddy plot. The ratio in the irrigated paddy was remarkably increased with accumulated rainfall while those were not remarkable in other agricultural land use types. The peak runoff discharge was calculated by two types data, namely for one hour interval and ten minutes interval. In actual use of flood control planning the former peak discharge estimation method is convenient whereas the latter is theoretically reasonable except in the residential land.
From a previous test result using flexible pipes having a different thickness and an equivalent bending ring stiffness, it is found that the influence of the thickness of the pipe upon the deflection and the distribution of bending strain was extremely small and the larger axial stress acts on the pipe with the thinner wall, and that the change of axial stress in infinitesimal section of pipe was larger with the thinner wall. In this study, shaking table tests using these pipes were conducted in order to verify the influence of the thickness of pipe, a condition of test pit and bedding material on the dynamic behavior of pipes due to a shear deformation of a ground. As the test results, although the bending strain increased in proportion to the thickness of the pipe, the influence of the thickness of the pipe upon the shape of the bending strain distribution was extremely small. In addition, judging from a relationship between shear strains of ground and a maximum absolute axial stress, it is revealed that the thinner the thickness of pipe is, the larger the rate of the maximum absolute axial stress to shear strain is.
To protect against large-scale earthquake disasters, there is an urgent need to strengthen the disaster-response capabilities of main irrigation channels. In this study, the authors identified the causes of disaster response incapability for the typical irrigation facility consisting of a head work and open channels. This identification was done by applying fault tree analysis (FTA) to the disaster response process from the initial reaction immediately after a large-scale earthquake to the closing of intake gates for safety. The effectiveness of measures to prevent disaster response incapability was quantitatively evaluated. It was found that for earthquakes with seismic intensities of 6 or lower on the Japanese scale, some effectiveness in preventing disaster response incapability could be expected through measures that are workable by facility managers. In other words, disaster response can be generally carried out as planned. However, it was shown that such effectiveness can hardly be expected for earthquakes with maximum seismic intensities of 7 on the Japanese scale and that such earthquakes make it difficult for facility managers to close the intake gates. This suggests the necessity of a business continuity plan in preparation for a maximum-scale earthquake.
On islands, such as the Okinawa South-West Islands, freshwater lenses that can be found in the upper layers of underground aquifers are important water sources. However, their availability is expected to decrease significantly due to the sea-level increase caused by global warming. As freshwater is pumped from lenses, the phenomenon of saltwater up-coning may occur and may potentially lead to the entry of saltwater into the freshwater wells and the contamination of the abstracted water. In this study, hydraulic coefficients in the catchment bowling were estimated by numerical experiments and the amount of water up-coning was evaluated by numerical simulations solving the three-dimensional density-dependent groundwater flow coupled with salinity transport. Several well configurations were investigated and relevant experimental data from an actual water-pumping site were used. Results show that it is possible to design well structures to efficiently avoid saltwater intrusion, taking into account the pumping rate and the distance between horizontal strainers among other parameters.
Radioactive substances that were widely dispersed by the accident at TEPCO's Fukushima Daiichi Nuclear Power Plant were adsorbed by soil and can be discharged with suspended solids during rainfall. It is therefore important to quantify the movement of the radioactive substances adsorbed by the suspended solids. We have developed a model to simulate the production and transport of suspended solids depending on their particle sizes, because finer soil particles tend to adsorb more radioactive substances than coarser particles. We then integrated the model into a catchment-scale distributed hydrological model to evaluate the spatiotemporal variation in radioactive transport associated with suspended solids in streamflow. With this model, a continuous hourly simulation was carried out in a dam catchment in Fukushima Prefecture. The results show that the model captures the discharge of radioactive substances both in large and small flood events, indicating that the model can be used for event-based prediction as well as for evaluating long-term influences of radioactive substances on water resources.
Water and solute movements in the freshwater lens of Tarama Island, Okinawa Prefecture, Japan, were investigated with application of environmental tracers. Nitrate in the groundwater originated from multiple nitrogen sources such as chemical fertilizer, livestock manure and domestic wastewater. Composition of major ions in the groundwater showed that a part of the freshwater lens was affected by seawater. Values of δ15N and vertical profiles of dissolved oxygen showed that lateral flow along the transition zone of freshwater and seawater occurred, bringing oxygen-rich and low-δ15N groundwater. Averaged residence time of the groundwater was estimated to be approximately less than 14 years by the SF6 analysis with application of a piston-flow model. Shallow groundwater at the periphery of the freshwater lens appeared to be recharged in relatively recent ages.
Effective soil layer is one of the important factors to determine the amount of irrigation water and to evaluate evapotranspiration from the soil for efficiently scheduling irrigation in Japan. Here we propose a new definition of the effective soil layer based on soil water flux, which can be used to determine the thickness of the effective soil layer of the field under the non-equilibrated soil water conditions. In addition, we developed a new method to determine the depth of the effective soil layer from the field monitoring data of soil moisture content. This method is based on the new definition and uses a computer application of numerical simulation of soil water movement, HYDRUS-1D. To show the detailed calculation procedure of this method, the depth of the effective soil layer in lily field located in Tsunan, Niigata, Japan, was determined using the field observation data. The depth was estimated to be 35 cm, although lily roots mostly reached up to a depth of 20 cm. Downward soil water movement was considered to be not dominant a day after heavy rainfall, suggesting that upward soil water movement was the main factor that dried the soil at depths of 0-35 cm. These results support the availability to determine the depth of the effective soil layer using our new method.
There is a growing number of researches of 17O-excess, which is less sensitive to temperature, in order to develop 17O-excess as a new water tracer in hydrological and climate science. However, there is no data in paddy areas in Japan. In this study, 17O-excess values, its temporal variation and factors affecting that, and the relation with other isotopes in rainwater, surface water including paddy water and irrigation water, and groundwater are presented from the observation in paddy areas in Ibaraki, Japan. The results showed that the 17O-excess of rainwater is lower in summer, and relatively higher during autumn and winter. This temporal variation seems to arise from the following two; the water vapors, i.e., rainwater, formed on sea surface under different evaporative conditions, and re-evaporation of rain drops during precipitation particularly in summer season. The lower 17O-excess values resulting from kinetic fractionation during evaporation were observed in the terrestrial waters, especially paddy waters. The accumulation of further observation data is necessary to verify the validity of 17O-excess as a water tracer in hydrological cycle.
The authors intend to clarify the swimming ability of the Japanese minami medaka fish (Oryzias latipes) in order to consider the habitation of the medaka fish in canal design for ecosystem. The critical swimming speed (CSS) of the medaka fish was measured in a small channel with a rectangular cross-section and a waterway system for circulation under an average cross-sectional water flow velocity of 2-36 cm•s-1. The body length of the Japanese minami medaka fish ranged from 1.3-2.8 cm (the mean body length was 2.1 cm, and the number of fishes was 104). The following results were obtained: (1) the CSS, measured for a duration of 60 min, ranged from 5-19 cm•s-1, and a positive correlation was observed between the CSS and the body length, and a regression formula between 60-min CSS and the body length was obtained (p<0.01); (2) the ratio of the CSS of the body length was 3.0 to 9.0 times (that is, the distance travelled per second based on the body length), and the mean CSS was 5.5 times (with a standard deviation of 1.2).
In the excavation of soil within a cofferdam in a high groundwater area, seepage failure of soil is often a problem. In recent years, a deep and/or large excavation will cause a new type of seepage failure, e.g., due to a concentrated three-dimensional flow. In this paper, three case studies on seepage failure of soil due to a 3D flow were assessed, and the following results were obtained: (1) As for stability analyses of the seepage failure of soil, the method for considering plane shapes of excavation should be recommended. (2) Analyses of FEM seepage flow and stability against the seepage failure of soil, using the prismatic failure concept, can produce the best results for the three cases. The reasons why the soils collapsed were also considered. (3) Three flow conditions: two-dimensional (2D), two-dimensionally concentrated (2DC), and three-dimensional (3D) flows, have a marked influence on stability against seepage failure of soil within a cofferdam. The magnitudes of stability against seepage failure are, in decreasing order, 2D, 2DC, and 3D flows. (4) With respect to the seepage failure problem, an axisymmetric seepage flow through soil within a cylindrical wall can be used to model such a three-dimensional flow.
The purpose of this study is to reveal the change in consciousness of local residents caused by farmers' restaurants enterprises, which have been attracting attention as part of the entrepreneurial activity among farmers. First, we selected two farmers' restaurants for analysis. One was located in rural area and the other was located in residential area. Next, we carried out a questionnaire survey concerning recognition, visit frequency, and change in consciousness about the farmers' restaurants among local residents. As a result, many local residents in the rural area recognized the farmers' restaurants and visited frequently. In the residential area, some residents did not recognize the farmers' restaurants. However, it was found that farmers' restaurants had affected their consciousness.
The aims of this study are evaluating the effect of gate opening of a series of diversion weirs on the temporal and spatial fluctuations of river water level and optimizing the gate opening operation for disaster prevention under the flood condition. A simulation model describing one dimensional unsteady flow was introduced to evaluate the effect of the gate opening of multiple diversion weirs on the downstream current in the Onga River. Using this model, the spatial and temporal variations of the river water level were clarified under the different patterns of gate opening operation. To quantify the impact of the gate opening on the temporal and spatial changes of the river water level, an objective function which represents the temporal changes of the river water level at all computational grids was introduced and the weirs which have the most significant impact on the water level fluctuation was clarified. The method introduced here is effective for quantifying the impact of the gate control on the fluctuation of the river water level and optimizing of gate opening operation to reduce the temporal and spatial changes of the river water level under the flood condition.