RESEARCH ON STATE OF LAND SUBSIDENCE OF PEATLAND AND ITS MECHANISM

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The purpose of this research project is to clarify the state and the mechanism of large area subsidence in peatland after its opening for agriculture, and to estimate its future sink. The flat lowland of lower reaches of Ishikari River Basin, Shinotsu Mire, has been chosen as research site, where peat covers in wide range and in the average depth of 3 to 5 meters. We collected old and new maps and plans that involve information of ground height. The characteristics of peat were examined from peat boring. GIS was used for analysis of large area investigation and the functions that influences to the peat subsidence. We also established on-site continuous observation point for ground and peat layer fluctuations. Results are as follows ;1) From 1956 to 1996, average of 0.4 meters and maximum of 2.9 meters of land subsidence is detected from this area. In past 40 years, large amount of subsidence was found from newly opened area, while old agricultural area also still shows continuing subsidence of peat surface. No clear relations have been found between peat layer thickness, amount of mineral soil dressing, history of landuse and the amount of subsidence.2) Continuous on-site observation site was established for investigating the ground surface and peat layer movement. The precise fluctuations of peat ground were recorded during the project period. It became clear that these peat layer fluctuations were closely related to changes in shallow-groundwater table, and depth of snow cover.3) From this research, we concluded that the land subsidence in this area, though its speed may reduce, will prolong for long period.