Reply on RC3

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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.

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