Field monitoring of pore-water pressure in fully and partly saturated debris flows at Ohya landslide scar, Japan

Abstract

Abstract. The characteristics of debris flows (e.g., mobility, sediment concentration, erosion, and deposition of sediment) are dependent on the pore-water pressure in the flows. Therefore, understanding the magnitude of pore-water pressure in debris flows is essential for improving debris flow mitigations measures. Notably, the pore-water pressure in a partly saturated flow, which contains an unsaturated layer in its upper part, has not been understood, due to a lack of data. The monitoring performed in Ohya landslide scar, central Japan, allowed us to obtain the data on the pore-water pressure in fully and partly saturated flows during four debris flow events. In some partly and fully saturated debris flows, the pore-water pressure at the channel bed exceeded the hydrostatic pressure of clean water. The depth gradient of the pore-water pressure in the lower part of the flow, monitored using water pressure sensors at multiple depths, was generally higher than the depth-averaged gradient of the pore-water pressure from the channel bed to the surface of the flow. The low gradient of the pore-water pressure in the upper part of partly saturated debris flow may be affected by the low hydrostatic pressure due to unsaturation of the flow. Additionally, excess pore-water pressure was observed in the lower part of partly saturated surges. The excess pore-water pressure may have resulted from the loading of particles and contraction of interstitial water. The pore-water pressure at the channel bed of fully saturated flow was generally similar to the hydrostatic pressure of clean water, while some saturated surges portrayed higher pore-water pressure than the hydrostatic pressure. The travel distance of debris flows, investigated by the structure from motion technique using unmanned aerial vehicle (UAV-SfM) and the monitoring of time lapse cameras, was long during a rainfall event having high intensity, even though the pore-water pressure in the flow was not significantly high. We conclude that the flow type (fully or partly saturated flows) should be considered to estimate the depth gradient of pore-water pressure in debris flows.