The infiltration rate of slopes covered by volcanic ash is crucial for correctly estimating the volume of surface runoff. However, measuring the infiltration rate in situ during an eruption is not always possible; therefore, developing a method for calculating the infiltration rate of slopes covered by volcanic ash using the available data immediately after an eruption is essential. The fine fraction content of volcanic ash, which is easy to analyze via sampling, influences the infiltration rate of a slope covered by volcanic ash. Furthermore, the infiltration and final infiltration rates of ashfall‐deposited slopes have been reported to decrease through repeated rainfall. This study examined the mechanism of the final infiltration rate through repeated rainfall and the effects of fine fraction content on the final infiltration rate of the slope covered by volcanic ash.
We performed a laboratory experiment by sprinkling water on experimental plots that imitated the slopes of volcanic mountains. The experimental plots were covered by volcanic ash, with varying fine fraction contents of 19wt%, 28wt%, 36wt%, and 43wt%. The experimental plots received 1st rainfall, followed by a dry period, and subsequently received 2nd rainfall of the same intensity as the 1st. Results indicated that the experimental plots with fine fraction contents of 28wt%, 36wt%, and 43wt% during the 2nd rainfall exhibited a lower final infiltration rate than those during the 1st. The dry density of the experimental plots increased following the 1st and 2nd rainfall. This suggests that the decrease in the final infiltration rate with repeated rainfall may result from changes in soil structure due to rainfall. The experimental plot with a fine fraction content of 36wt% showed the minimal ratio between the final infiltration rates during the 2nd and 1st rainfall. Despite claims that the final infiltration rate recovers over time, this study did not observe such recovery in the experimental plots.