Debris flows promoted by mechanical deterioration of the ground due to eutrophication of hillside ecosystems

摘要

There are many hazards caused by debris flows and landslides all over the world, and especially in Japan, every year. In June 1999 and August 2005 in Hiroshima prefecture and in July 2009 in Yamaguchi prefecture there were many large hazards of debris flows and landslides owing to torrential downpours. Through the survey of these hazards it seems that there is biodeterioration of the mechanical structures of the ground in the hillsides caused by eutrophication (nutrient enrichment) originating in the transition of ground ecosystems in hillside slopes. The eutrophication promotes microbial activities that deteriorate the mechanical structures of the ground in the hillside slopes. Microorganisms promote the weathering of rocks to soils and cause aggregation of soils by their enzymes. Aggregated structures of soils wet the ground to decrease resistance to landslides. It seems that microbial activities accelerate the occurrence of debris flows. This paper involves the investigation of the above-mentioned relationships between debris flows and mechanical deterioration of the ground due to eutrophication of ground ecosystems in hillside slopes. It was found that the values of the eutrophication indexes are low in the stable slopes and high in the unstable slopes. The values of the stability of ground formation are high in the stable slopes and low in the unstable slopes. Many microorganisms are found in the unstable slopes.