Estuary Processes and Managment - River Dynamics and Integrated River Management - page 485

A scenario with the following characteristics is selected: ocean salinity ( S 0 ) = 29.2 ppt, freshwater
discharge (per unit width q f ) = 0.929 × 10 -3 m 2 /s; tidal amplitude = 0.01524 m; tidal period T = 600 sec. The
instantaneous depth-averaged salinity distribution is shown in Fig. 20(b). Figure 8.20(c) shows the tidally
averaged vertical salinity profiles at selected locations. The observations are made at 5, 40, 80, 120, and
160 ft from the ocean boundary. The distance is used as the reference for the station as indicated in the figure.
To reproduce the salinity transport in the estuary resulting from the salinity difference existing between
the open sea and the freshwater inflow along the intrusion length, the results of two 3D hydrodynamic
models (EFDC and Delft3D) are also presented.
In the dry season, Hong Kong waters are vertically well-mixed due to very low Pearl River discharge,
strong tidal mixing and winds. The salinity (and temperature) is approximately vertically homogenous;
salinities may vary from a low of around 15 ppt at the mouth of the Pearl Estuary to 34 ppt in the eastern
waters (Fig. 8.21(a)). Salinity intrusions may reach to beyond Deep Bay (Fig. 8.21a), and Mirs Bay in
the eastern waters is clearly unaffected by the Pearl River estuarine plume. In the wet season, however,
the interaction of the Pearl River discharge and tidal currents creates significant vertical and horizontal
salinity gradients (Fig. 8.21(b)). The western waters are strongly influenced by the Pearl River. For
example, the surface salinity in the northwestern waters can be as low as 15 ppt, while the bottom salinity
exceeds 32 ppt. By comparison, Victoria Harbor and East Lamma Channel are relatively more affected
by tidal mixing, and the salinity differential is much less pronounced (< 7 ppt). The eastern waters are
relatively sheltered from the Pearl River, and hence they are more oceanic with a typical salinity of 32-34
ppt and much weaker vertical salinity differentials. In the southern waters, the salinity differential is
about 1 to 4 ppt, except for the fresh water main path in the deeper channels such as East Lamma Channel
(with differentials in the range of 6-10 ppt).
Fig. 8.21 General surface salinity field during flood at HHW-3 hr in (a) dry season and (b) wet season
In the wet season, the fresh river water spreads over the sea surface; the turbulent entrainment (mixing)
of the more saline lower layer into the surface layer sets up a saline wedge intrusion which moves shoreward.