Spacial Distribution of Salinity and the Mechanism of Saltwater Intrusion in the Modaomen Water Channel of Pear River Estuary

Modaomen channel is an important fresh water resource in Pearl River Delta. It has been impacted by saltwater intrusion frequently in the last decade. This has drawn more and more attention from scientists and engineers. The hydrodynamic mechanism of saltwater intrusion is still impercipient. In the present paper, hydrographs of velocity and salinity in the channel are analyzed based on field observations of velocity and salinity of upper, middle, and lower water layers at several stations along the Modaomen channel. It is found that the transport of salinity in Modaomen channel is obviously different from other estuaries. As the tidal range increases from neap to spring tide, the salinity in each water layer decreases unexpectedly. This peculiar phenomenon is attributed to the extraordinary flow process in the channel. When salinity value in each layer and vertical salinity gradient are lower during spring tide, no matter on rising or ebbing tide, the flow velocity monotonously decreases from water surface to the bottom, which is suggested by common sense. However, when salinity values and vertical salinity gradient are higher during neap tide, the flow velocity unexpectedly increases from water surface to the bottom during flood period, and flood duration of the bottom current is surprisingly as long as 15-18 hours. In addition, an inflexional velocity profile may remain amazingly for about 9 hours. This could be driven by the baroclinic pressure under the condition of tides, topography and upstream runoff discharge of this channel