LONG-TERM (1953-2020) MORPHOLOGICAL CHANGES OF CHIKUGO RIVER, JAPAN

This study aims to predict the long-term morphological changes of the Chikugo River using river cross-sectional surveyed data from 1953-2020. The study area was divided into the estuary (0-23 km), middle stream (23-50 km) and upstream (50-64 km). The mean bed elevation of every cross-section in each surveyed year was calculated to analyze the long-term longitudinal elevation changes. Spatial (0-64km) and temporal (1953-2020) changes in channel shapes were found. In addition, three different morphological trends were found during the study period of 67 years. From 1953 to 1993, dredging, sand & gravel mining, and dam construction decreased the bed elevation of Chikugo River; 1-4 m for estuary, 2-4 m for middle stream and 1-3 m for upstream. From 1993-2009, elevation changes became less compared to the first period and elevation increased uniformly (about 0.5 m). From 2009-2020, non-uniform morphological changes were observed due to changes in river flow and sediment supply by climate change disasters. The disaster events not only increase the river flow but also carries more sediments from the watershed. Most of these sediments were deposited in the middle stream (50-64 km) increasing the bed elevation by about 1-1.5 m. The upstream (50-64 km) showed a quick and significant response to the type of disasters although elevation change was almost stable due to the net result of deposition by landslide disaster and erosion by the flood. However, the elevation of the estuary (8-23 km) decreased due to net erosion by high river flow and insufficient sediment supply from upstream while the elevation of 0-8 km increased slightly due to the deposition of eroded sediments from 8-23 km. Therefore, climate change disasters affect the morpho dynamic equilibrium of Chikugo River non-uniformly although elevation changes were less compared to that of human activities.