Narrow Results

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.

The sediment supply to the delta and adjacent beaches of the Elwha River in Washington State, USA is expected to increase significantly after removal of two dams. This paper describes the initial implementation of a process-based hydrodynamic and sediment transport model to predict sediment transport pathways and delta morphological response to changes in sediment supply in a mixed sediment system. The hydrodynamic model is calibrated and validated against water levels and currents measured in the Strait of Juan de Fuca and on the Elwha delta. Strong instantaneous and residual tidal currents are responsible for the transport and dispersal of fine-grained and sand-sized sediments across the delta. If sediment supply is large enough, some sediment will accumulate on the delta, modifying the delta substrate, which is presently dominated by hardbottom and coarse sediments.

In this paper, we highlight the various properties of wind-blown sand within a coastal dune, which is located on an eroded beach on the Enshu-Nada coast of Japan. Short term topographic changes of the Nakatajima dune have investigated with continuous field surveys over two years period (2007-2009). Measurements of wind and wind-blown sand transport rate were also carried out several times in winter. We found that the windward slope/dune crest in middle part of the dune is an actively eroding region by wind. The windward slope/dune crest of the dune was also migrated eastward as predominant wind direction in years or decades. The source of the sand transported mostly from the dune itself and limited amount of sand come from the shoreline, resulting the decreased in the elevation significantly in the West side of the dune.