Author(s): Kazuaki Ohtsuki; Yasuo Nihei
Keywords: No Keywords
Abstract: Dam construction and sand harvesting have had an impact on the sedimentary system of downstream rivers, not only for the short term, but also for several decades. It is necessary to monitor the sediment conditions on a long-term basis, although these kinds of investigations are limited, especially in tidal flat areas. A lack of data on sediment aggregation, sedimentation and resuspension prevents a welldefined assessment of the estuarine areas of rivers. A numerical simulation would be a first step toward a long-term analysis of the problems, but without the data, we still could not apply the simulation. Therefore, the evaluation of sedimentary conditions in the estuarine tidal flats should be based on long-term observation data, even if it is implicit. Water-level data is one of the options, and has been monitored for a long time at numerous river mouths. Water levels at the river mouth decrease but separates from the sea level at low tide, depending on the land formation of the tidal flat. These characteristics provide the potential for continuously tracing the long-term fluctuations of tidal flat morphology. In this study, we made a long-term estimate of the morphological changes in tidal flats by analyzing water level data in the Kumagawa River in Japan, where the first dam removal project in Japan is being conducted. First, we show the characteristics of water-level fluctuations at the mouth of the river and established two important parameters explaining these characteristics. Next, we present two types of numerical simulations to extract a relationship between the two parameters and morphological changes. Finally, we estimate morphological changes using in-situ observed data and the simulated relationship, then discuss the impact-response for sediment impact on the river.