CLARIFICATION OF SILTATION MECHANISM OF INTERTIDAL FLAT DUE TO RIVER FLOOD BASED ON QUASI THREE-DIMENSIONAL NUMERICAL CALCULATIONS OF REGIONAL SEDMENT TRANSPORT

Abstract

 Sediment transports on intertidal flats have not been cleared enough even in recent years because of a lack of regonal view points and the poor accuracy of amounts of sediment discharges from rivers. Regional sediment transport from mountains to coastal zones through rivers was an important research subject today. In this study, field observations were conducted at the Nakatsu Intertidal Flat and the Yamakuni River which have serious problems of fisheries and sediments, and observation results revealed long-term tendency of erosion/deposition and remarkable siltation due to a flood from the Yamakuni River in 2017, which was caused by the North Kyushu Heavy Rain Disaster. This siltation was recovered until the next spring. However, the long-term siltation from summer to next spring was suggested serious damages on Ruditapes Philippinarums especially in winter. Numerical simulations by using WDM-POM which was a resional sediment transport and morphodynamic change model due to mud and sand considering tides and wind waves and gRSM which was a calculation model of river sediment discharge by using a convenient model and field data, were also conducted in order to verify the siltation mechanism in 2017. Numerical results were well agree with observation results qualitatively and revealed that the muddy water from rivermouth was transported mainly by tidal residual currents toward the Nakatsu Port in the east direction almost during a day. Numerical silmulation considering wind waves in winter and tides was also conducted and numerical results showed that the mud transport toward the Nakatsu Port was exaggerated by coastal currents due to NW wind waves accompanied with tidal residual currents. These strong currents were suggested to recover the siltation in 2017 through the winter. In the future work, quantitaively clarification will be done and future forecast of sediement movements and countermeasures for the siltation will be investigated.