[A-CG43] Water and sediment dynamics from land to coastal zones

[ACG43-P02] The prolonged impact of Hageyama - a severe anthropogenic disturbance 100 years ago - on sediment discharge from a mountain watershed.

*Yuko Asano¹, Koju Kishimoto¹, Nobuaki Tanaka¹, Yusuke Mizuuchi¹ (1.Graduate School of Agricultural and Life Sciences, The University of Tokyo)

Keywords:forest restoration, hillslope, channel, sediment supply, seiment transport, erosion control works

Since mountainous areas are a source of sediment supplied to downstream areas such as plains and coasts, it is necessary to understand the amount of sediment discharged from mountainous areas and to predict its changes. Sediment discharge from mountain areas is thought to change with changes in land cover, climate, and other factors, but there are few long-term observation and investigation. In particular, it is not clear how the sediment dynamics have changed during the past 100 years, when many of Japan's mountainous areas were devastated in the beginning but now covered with forests. The Ecohydrology Research Institute of the University of Tokyo was established in an area where there were many bald hills "hageyama", and has been monitoring sediment dynamics since around1930. This study demonstrates the changes in sediment discharge from Shirasaka experimental watershed, and clarifies the impact of the intense anthropogenic disturbance on the sediment discharge from the mountainous area. The Shirasaka experimental watershed is an 88.5 ha watershed consisting of granite, with a gauging weir at the exit of the watershed. Erosion control works have been conducted mainly along the river in the watershed. The bare land area was about 10% of the watershed in 1930th, but it decreased to less than a few percent around 1980th. In the beginning, sediment discharge measurement was not intended but record was left when sediment was dredged from weir pond in order to continue the water discharge measurement. During 1930 to 1965, sediment volumes were assumed to be estimated from the human work. From 1966 onward, the volume of sediment removed from the weir pond was recorded each time sediment was dredged based on simple surveying. In 1930, the sediment discharge was already about 20% of that from bare slopes (about 5,000m³/km²/y), but nearly one order of magnitude greater than that from forested slopes. Sediment discharge was high in the year of Torrential rains in the Tokai region in 2000 and for a few years thereafter, but returned to pre-storm levels within a decade. From about 2010 onward, sediment discharge from watersheds has leveled off to about the same level as sediment discharge from forested slopes. "Bald Hill" in 100 years ago impacted sediment transport from mountain watersheds until about 1990^th. Since sediment production on the slopes would have already been significantly reduced by 1930th, the main reason for this time lag in sediment transport could be due to sediment storage in the river channel. Sediment supplied to the channel by severe anthropogenic disturbance up to 100 years ago may have influenced sediment runoff from the mountain watershed until about 1990.