Morphology, structure and flow phases in soil pipes developing in forested hillslopes underlain by a Quaternary sand-gravel formation, Hokkaido, northern main island in Japan

Preferential flow pathways, such as soil pipes, are usually present in the soil of slopes. Subsurface flow through the soil pipes affects the subsurface drainage system and is responsible for sediment removal from slopes. However, a record of the inner structure of soil pipes has rarely been reported for slopes. A fibrescope examination of the morphology and flow phases in soil pipes in hillslopes underlain by a Quaternary sand–gravel formation provided the following information: the main pores of the soil pipes ran mostly parallel with the slope gradient; the cross-sections of the soil pipes were approximately circular; and occurred on a few occasions; with some triple junctions being present. In addition, both full flow and partly full-depth conditions occurred simultaneously in the soil pipe. The full flow condition has long been used in hydrological studies to model the pipe flow mechanism. Both the full flow condition and the partly full-depth condition, however, must be examined closely in order to evaluate the subsurface hydrology in heterogeneous soil and the hydrogeomorphological processes of subsurface hydraulic erosion.