PHYSICAL ENVIRONMENTAL CHARACTERISTICS OF THE OTOGO TIDAL FLAT IN THE YOSHII RIVER AND PROPOSAL OF NEW TIDAL FLAT CREATION SITES

Although tidal flats with good natural environment spread on both banksides in the lower reach of the Yoshii River, the tidal flat area considerably decreased owing to the river bank and dam construction works. The Otogo tidal flat is the remaining tidal area in the Yoshii river estuary. This study proposes a candidate site for tidal flat creation in an estuary depending on numerical analysis results. First, physical environment of the Otogo tidal flat was clarified by examining the observed salinity data and numerically obtained salinity and hydraulic data. Second, a promising site for creating a new tidal flat, which has a physical environment similar to that of the Otogo tidal flat was investigated and proposed on the left bank side of 4-4.6 km from the river mouth, which is approximately 2 km upstream of the Otogo tidal flat. Finally, some numerical experiments were conducted assuming the sediment replenishment and construction of new groynes. The results show that the extracted site has a similar sustainability with the Otogo tidal flat against the flood flow with several years return period.