Distribution of Debris Flow Deposits from Unzen Volcano on the River Mouth Area of Mizunashi River

Volcanic debris flows repeatedly occurred along the Mizunashi River by the eruption of Unzen Volcano since November 1990. Debris flow sediments were dredged on the river mouth area of the Mizunasi River from May 1993 to November 1996. We measured the thickness and analyzed the grain size of these debris flow sediments. The appearance of the debris flow sediments look like the volcanic ash from Unzen Volcano. After the intense debris flow occurred, the debris flow sediments were widely distributed. On March 1994,these sediments have the largest distribution. The river mouth area, sediments were characterized by the mud that median diameter was larger than 4φ. Before the influx of debris flow in this area, most of surface sediments used to be a fine grained sand. On November 1996,the debris flow sediments remained on the river mouth area of the Mizunashi River