Current Measurement in the Nagara River Estuary with Acoustic Doppler Current Profiler

The Nagara Rivermouth Barrage, constructed at the Nagara River estuary in 1994, has divid- ed the newly created freshwater environment from a brackish one. Therefore, a flow change and an anoxic water mass formation due to the barrage construction were one of the major water environmental issues because the Nagara River estuary was famous as one of fisheries of the Corbicura japonica. Average depth of the Nagara River estuary is 3.0 m in depth, and the hollows of 5 m or more in depth exist in the estuary. It is well known that the highly saline water mass below the halo- cline can easily become anoxic due to both weak vertical mixing between the upper and lower layer in such hollows. In this study, we investigated the current with an acoustic doppler current profiler (ADCP) in detail to clear the current distribution, and the highly saline water exchange process in the hollow which exists in the Nagara River estuary. As a result, the highly saline water run-up according to the tide was confirmed from this field observation. We calculated theoretically using the density distribution and the current velocity data which were obtained by this observation, in order to examine whether complete exchange of the water in a hollow are generated. In this condition, it was found that complex exchange did not occur.