Estuary Processes and Managment - River Dynamics and Integrated River Management - page 486

Figure 8.22(a) shows the salinity contours in a vertical section along a transect down the axis of the Pearl
River, from A to B (Fig. 8.22a). Under an average wet season river discharge ( Q = 19,422 m 3 /s), the
bottom salinity of 18 ppt may intrude to the west of Deep Bay (point C, in Fig. 8.22(a) at higher high
water (HHW), where the surface salinity is about 7 ppt. The bottom salinity of 18 ppt moves southward
with the ebb flow, and reaches point D at lower low water (LLW), after a horizontal tidal excursion of
about 12 km. At LLW, the vertical salinity distribution in the shallow estuary area is vertically well-mixed.
Figure 8.22(b) shows the temporal changes in water level, salinity, and velocity at point C, where the
salinity can change significantly within a tidal cycle. Pronounced vertical gradients can be observed
around high tide, when the velocity is the weakest (high Richardson number and limited vertical mixing).
In general, the vertical salinity gradients are much weaker in Victoria Harbor and adjacent waters.
Fig. 8.22 (a) Computed salinity contours in a vertical section along a transect down the axis of the Pearl River
estuary from A to B (see Fig. 8.17(a) at Higher High Water (HHW) and Lower Low Water (LLW) of a spring tide in
the wet season for a Pearl River discharge of 19,422 m 3 /s; (b) Computed depth-time variation of salinity and velocity
magnitude during a spring tide at location C in the wet season
8.3.4 Tidal Flushing
Wastes and pollutants released into the coastal environment are mixed and “flushed” by the highly variable
estuarine hydrodynamic circulation as previously discussed. If the tide is the main forcing for the
circulation, the flushing mechanism is then produced through repeated exchange of the intertidal water
volume between the embayment and the open water body. Figure 8.23 presents a schematic illustration of
the flushing mechanism for a semi-enclosed system: clean water (from the ocean or estuaries) entering the
embayment during flood tide fills the intertidal volume, mixing with the existing water in the embayment
and, as the tide falls, the intertidal volume of water discharges out of the embayment, removing the
dissolved substance.