Effects of Past Human Activities and Recent Disasters on Riverbed Morphology of the Chikugo River Estuary

Abstract

Effects of past human activities and recent climate change disasters and their interaction on the riverbed morphology of the Chikugo River estuary, Japan were investigated using the long-term measurement of annual and seasonal topographic surveys. The results showed that human activities caused the extensive riverbed incision of the Chikugo River estuary with a maximum value of about 4 m from 1953 to 1998. After stopping the sediment extraction in 1999, the elevation of the estuary increased by 1.2 m due to the sediments transported by the tide, especially during 1998-2003. Starting from 2009, the occurrence of floods and landslide disasters increased in the upstream of the Chikugo River. Although disasters carried huge sediments, a decrease in elevation was observed between 10-17 km with a value of 1.5 m, which means that sediments supplied by disasters had been deposited in the upstream where sediments were extracted in the past, thus, they couldn't be able to reach the estuary yet. So increased river flow with insufficient sediments eroded not only the surface silt and clay layer but also the bottom sand layer, resulting in a drastic decrease in the elevation of the estuary. Therefore, this study concludes that the riverbed morphology of the Chikugo River estuary is affected by both tidally induced sediment transport from downstream and an increase in river flow by recent disasters and human disturbances that were implemented in the past.