| 研究実績の概要 |
On a long-term basis, the whole study period was divided into three based on the time of human activities and disasters occurrence; (1) the period of human activities (1953-1998), (2) the period of no human activities and no disasters (1998-2003), and (3) the period of disasters (2003-2022). During period-1, the riverbed was lowered due to extensive human activities such as dredging (flood control), land reclamation, sand mining, etc. During period-2, the bed elevation increased by 0.5-1 m, which was due to the tide-induced landward sediment transport and deposition process. During period-3, the bed elevation decreased as extreme flood disasters eroded the sediment deposited upstream of the estuary. Although the extreme floods and landslide disasters supplied massive sediments into the Chikugo River, it seemed that sediments supplied by disasters were trapped upstream of the river and were not enough to reach the estuary. The erosion of existing deposits and lack of sediment supply from the upstream caused the decrease in estuarine bed elevation during period-3.
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