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Tanshui River mouth is important for coastal defense purpose in north of Taiwan. The sea condition over there is of high concern to the ROC Coast Guard. The in situ observation in real time at sea is not quite straightforward, encouraging the application of coastal radar remote sens-ing for currents and waves. However, its results ought to be examined by various surveys at sea, providing an opportunity to study the interaction of current field and hydrographic field. The coastal remote sensing is done by the Wavex radar system. The surface currents are tracked by GPS drifters, while the subsurface currents below 2.5 m depth are scanned by the Acoustic Doppler Current Profiler (ADP). The time series of hydro-graphic variation are derived by CTD casts. A temperature and pressure recorder (TD) is used to get tide and waves information. The results show that, due to the coastal southwest flood current, the surface currents at the river mouth rotate counterclockwise and reduce speeds gradually to enter the river channel with a cyclonic moving track. On June 13 of 2006, the temperature of river water was lower than that of sea water outside the river mouth, constructing the vertical hydrographic structure with surface water of low temperature and low salinity. The temperature difference between surface and bottom waters reaches 3℃ and 10 ppt for salinity. At high tide, such stratification disappears with a similar temperature and salinity all over the water column. In ebb tide, sea water entered during flood tide and river water discharge together to strengthen the westward flow up to a speed of 4 knots. The hydrographic stratification appears again. Af-fected by the coastal northeast ebb current, the strengthening outflow of the river rotates counterclockwise continuously, but with an anti-cyclonic moving track instead. The 2.5 m subsurface current scanned crossing the river mouth indicate that the northern half of the mouth is the main drain-age of the river and the current is about the same as of the surface. The results of coastal radar measurement are not quite comparable, requiring further investigation.
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