Evaluation of the relationships between rainfall infiltration properties and the lateral flow discharge of a slope based on artificial rain experiments

Go YANAGISAKI, Naoki IWATA, Satoshi WATANABE, Yuichiro MARUOKA, Atsushi KAJIYAMA and Katsuo SASAHARA

For the purpose of detecting a premonitory phenomenon involving shallow landslides due to heavy rainfall, we observed the process of rainfall infiltration in soil on a slope of weathered granite during artificial rainfall experiments. Measurements of lateral flow discharge, volumetric water content, and matric potential were taken during conditions where the artificial rainfall intensity was the same but the initial soil moisture contents were variable. The following results were obtained. (1) In the case where the ground was initially dry, a large discharge of lateral flow occurred from the litter and the root mat layer that was shallower than 20 cm in depth. The discharge of lateral flow decreased with increases in the matric potential and matric potential gradient in the lower layer. (2) In the case where the ground was wet prior to rainfall, lateral flow from the litter and root mat layer that was shallower than 20 cm in depth did not occur. This may have been caused by enhanced vertical rainfall infiltration. (3) In the case where the ground was wet prior to rainfall, in comparison to the case where the ground was initially dry, lateral flow was slight in the layer of decomposed granite soil with many roots from 20 to 45 cm in depth. When the moisture conditions approached saturation, and when the difference in the volumetric water content decreased, the lateral flow tended to flow out easily.