11:00 AM - 1:00 PM
[HGM03-P04] Relationships between debris supply process to the sea and longitudinal profiles of the rivers in the Yaku Island, southern Japan
Keywords:debris transport process, riverbed deposits , longotudinal channel profile, submarine landform, Yaku Island
There are huge boulders sometimes more than 5 meters in b-axis on the riverbeds of most rivers in the Yaku Island, southern Japan. This study aims to discuss the processes of debris transport and debris supply to the sea from the point of view of longitudinal profiles of the riverbed and its change from the LGM period to the present.
Along the rivers with concave longitudinal profiles, which are longer rivers, diameter of riverbed boulders decreases downstream. Near the river mouths riverbed sediments are mainly sand with sometimes cobbles and/or pebbles. Along the steep slope rivers with linear longitudinal profiles, which is relatively short rivers, there are huge boulders at the river mouths.
The Miyanoura River is one of the largest rivers originated in the central part of the island. Maximum diameter of the riverbed boulders decreases downstream to several tens centimeters as the channel slope decreases. There is a continuous terrace along the middle reaches of the Miyanoura River. The terrace deposits consist of huge boulders of several meters in diameter. Although these terraces are covered with riparian forests, boulder deposits are exposed on the terrace surface with thin soil. This terrace is not covered with the Koya pyroclastic flow deposits, which was erupted at 7,300 y.B.P. and which covered the valley side slopes around there.
Existence of terraces with huge boulders along the river and downstream decrease of diameter of the huge riverbed boulders as the channel slope decrease show that these huge boulders are transported along the river. Such boulders were transported by the extraordinary flood caused by the burst of a landslide dam.
Profiles of sea floor from each river mouth to the continental shelf are investigated using submarine structural charts and bathymetric charts. Each longitudinal profile of the riverbed is connected to the profile of the sea floor. In the case of the longer river the extension of the profile of the middle reaches of the river is connected to continental shelf directly. In the case of the steep slope short river the river profile is directly connected to the profile of the submarine canyon. These show that along the longer river huge boulders were transported to the river mouth in the LGM period. In the present environment huge boulders are deposited in the middle reaches and are not transported into the lower reaches with gentle channel slope. The steep slope rivers continue to supply huge boulders to the sea from the LGM period.
Along the rivers with concave longitudinal profiles, which are longer rivers, diameter of riverbed boulders decreases downstream. Near the river mouths riverbed sediments are mainly sand with sometimes cobbles and/or pebbles. Along the steep slope rivers with linear longitudinal profiles, which is relatively short rivers, there are huge boulders at the river mouths.
The Miyanoura River is one of the largest rivers originated in the central part of the island. Maximum diameter of the riverbed boulders decreases downstream to several tens centimeters as the channel slope decreases. There is a continuous terrace along the middle reaches of the Miyanoura River. The terrace deposits consist of huge boulders of several meters in diameter. Although these terraces are covered with riparian forests, boulder deposits are exposed on the terrace surface with thin soil. This terrace is not covered with the Koya pyroclastic flow deposits, which was erupted at 7,300 y.B.P. and which covered the valley side slopes around there.
Existence of terraces with huge boulders along the river and downstream decrease of diameter of the huge riverbed boulders as the channel slope decrease show that these huge boulders are transported along the river. Such boulders were transported by the extraordinary flood caused by the burst of a landslide dam.
Profiles of sea floor from each river mouth to the continental shelf are investigated using submarine structural charts and bathymetric charts. Each longitudinal profile of the riverbed is connected to the profile of the sea floor. In the case of the longer river the extension of the profile of the middle reaches of the river is connected to continental shelf directly. In the case of the steep slope short river the river profile is directly connected to the profile of the submarine canyon. These show that along the longer river huge boulders were transported to the river mouth in the LGM period. In the present environment huge boulders are deposited in the middle reaches and are not transported into the lower reaches with gentle channel slope. The steep slope rivers continue to supply huge boulders to the sea from the LGM period.