@misc{oai:niigata-u.repo.nii.ac.jp:00005484,
 author = {LIU, HAISHENG},
 month = {Mar},
 note = {The seawater intrusion renders the quality of river water unsuitable for agricultural use. It is likely to occur near the estuaries of rivers draining into the sea. The agricultural area irrigated using the water from Shinkawa River has been suffering from the presence of saltwater. To avoid saltwater mixture to the irrigation water, closing the sluice gates at the river mouth and stopping intake pumping stations have been implemented when the salinity of water is expected to the standard of irrigation water. However, operating drainage pumps incurs a cost and stopping pumps causes shortage of irrigation water. In fact, the first class rice harvested in the year 2010 remained only 10 % of the total rice production of this area (Average rate 75 % for the whole prefecture). This study, therefore, aims at finding inexpensive but effective countermeasures to control the seawater intrusion for the irrigation water quality conservation. To start with the study, the characteristics of seawater intrusion in Shinkawa River were investigated by visualizing the behavior of seawater intrusion through field surveys and numerical calculation. In the field survey, an acoustic reflection profiling system (SC-3) and electronic conductivity (EC) meters were employed to obtain the longitudinal and vertical profiles of the seawater intrusion. Then, the behaviors of the seawater intrusion, in response to the tidal force and the discharge change from upstream during the whole irrigation season without operation of sluice gates, were reproduced by a numerical calculation with a one-dimensional and two-layer unsteady flow model. As a result, it was clarified that when the interface between the saltwater and freshwater rises as high as 1.2 m below the inlet of intake pump, electrical conductivity of irrigation water exceeds the standard of irrigation water, 1500 μs/cm, which was evaluated as about 23 % of the irrigation period if the countermeasures were not practiced. Based on this analysis, three countermeasures, discharge increase, the operation of sluice gates and selective intake, were conducted to control the seawater intrusion in Shinkawa River. The discharge increase was achieved by the operation of all drainage and irrigation pumping stations. The method of the operation of the sluice gates is to II create a submerged orifice by controlling the aperture of sluice gates to keep the constant water level difference of 25 cm between inside and outside the gates, pushing-out the saltwater layer outside the river. The basic idea of selective intake is constructing one kind of structure which can only let freshwater freely enter the gate of pumping station, and block saltwater outside of the gate. As a result of these three countermeasures, the operation of the sluice gates was determined as the most effective method. The elevation of the density interface between saltwater and freshwater decreased by 1.04 m, and the extent of seawater intrusion decreased by 1.85 km within 3 hours. Therefore, the most inexpensive and effective countermeasure for controlling seawater intrusion is the operation of sluice gates, and this method can be applied to other estuaries suffering from the presence of seawater intrusion., 学位の種類: 博士（農学）. 報告番号: 甲第3939号. 学位記番号: 新大院博（農）甲第142号. 学位授与年月日: 平成26年3月24日, 新大院博（農）甲第142号},
 title = {Study on the controlling seawater intrusion for irrigation water quality conservation},
 year = {2014}
}