Purpose of this study was to understand the properties and processes of natural fluvial geomorphology in fans and floodplains. In my researches in the past, characteristics of channel geomorphology, factors influencing channel course change were investigated. In the process of the research, ground water was remarked as an important factor for the alluvial processes. In this study, dynamics of the ground water and detailed geomorphology was investigated, function of the ground water was studied.
A floodplain in the northern part of Hokkaido was chosen as a study site. Detailed field survey and intensive observation of the ground water were carried out to a trench which was dug in the flood basin. Pezometers were installed in and around the trench. Chemical quality of waters taken from the piezometers, seeps, puddles and stream were analyzed. Responses of water tables in some pezometers to the water table of stream and precipitation were recorded. And tracer test was carried out around th
e trench with saturated sodium chloride solution to examine the existence of some specific underground flow path.
The analysis of water quality showed that there were two kinds of ground water that the sources differed. Acceding to the research of the fluctuation of water tables of the piezometers, the water tables were influenced by precipitation and it was strongly suggested that piezometric surface was mainly influenced by not percolation but other kind of ground water movement. Tracer test supported the existence of specific ground water vine. The following conclusions were led by these results that 1) there were some kinds of ground water of which origin and history differ, 2) a kind of ground water flowed quickly through a specific route like pipe flow in a slope does, and 3) path of the quick flowing ground water was likely to be a relic channel of buried channel because its sediment structure was further porous than bank sediment structure is.
Process of formation of the trench and other eroded microgeomorphology in the floodplain was considered. Surface flow over the flood basin could contribute to the formation, however, the most important processes was the piping. In other words, the ground water, the cause of piping, was supplied from the stream water, flowed through some specific path or relic channel of porous sediments. In fluvial process of the floodplain, the ground water must contribute to the formation of alluvial channels as other functions such as bank erosion and bar formation, debris deposition, etc.do. Less