Abstract
A newly developed topographical methodology was applied to the prediction of subsurface-layer permeability of the Rokugo alluvial fan in the Yokote basin, Akita prefecture, Japan. Rokugo was selected as a typical example of the gently sloped alluvial fans which occupy more than half of lowland acreage in the Tohoku region of Japan.
As the first step of the analysis, the latest depositional surface of about 20, 000 yr B. P. was identi-fied by three-dimensional analysis of aerial photographs and chronological information. The subsequent erosional surfaces were marked with assemblages of abandoned fluvial channels (detailedscale analysis).
As the next step, a large-scale flooded trace on the pre-determined erosional surfaces was examined in a 1 m contoured topographic map. The trace was detected as a characteristic topography with the combination of concaved-contour in the proximal fan and convexed-contour in the distal fan on the map (micro-scale analysis).
Clay and silt contained in the deposits have been washed and leached out of the subsurface-layer of the fan by repeated scour and fill in the process of degradation over the last 20, 000 years. Water supplied by either precipitation or irrigation can percolate, seep and run downward more easily in the erosional surfaces than in the original depositional surface. In the large-scale flooded trace, in particular, permeable materials which are deposited in succession, forming a thick subsurface-layer with high per-meability from the fanhead over the distal fan, can work as a trunk pass for shallow groundwater.
Geological and hydological data were combined to support this prediction. Specification of the depositional surface at about 20, 000 yr B. P., therefore, is considered to provide useful information not only for the prediction of subsurface-layer permeability but also for land evaluation.
