抄録
It is well known that volcanic ash often generates surface flow on hillslopes as it decreases the infiltration rate of surface soil. Volcanic ash itself has a grain size of sand and has non-cohesive properties so it commonly has a high rate of water infiltration in laboratory tests. In spite of this, hillslopes covered by volcanic ash often generate surface flow, which results from the low infiltration rate of surface soil. Ordinarily, one of the reasons given for this low infiltration rate is the soil crust which develops on the surface of soil as a result of compression by drops of rainfall. This paper describes the vegetation and soil properties of Sakurajima, a volcanic island in southern Japan, where surface water and debris flow frequently occur. Laboratory tests on samples of the volcanic ash soil and in situ testing of surface flow were carried out. The test results demonstrated that slope surfaces covered by leaf litter or moss had very low infiltration rates, generating surface flow. To compound this, surface water was found to transport the ash and its deposition may have led to the formation of soil crust layers through a grading effect in the deposition process, giving the surface soil a low infiltration rate. However, if the slope has a large amount of living vegetation, such as Japanese pampas grass, surface flow rarely occurs. The presence of vegetation greatly improves the infiltration rate, limiting surface flow.
