Estuary Processes and Managment - River Dynamics and Integrated River Management - page 483

entrance; the peak flood velocity in the harbor is much smaller at about 30%-40% of ebb velocities. The
peak flood and ebb surface velocities in neap tide are about 68%-75% of the spring tide values.
In Lema Channel at the entrance of the Pearl River Estuary, north-easterly residual currents of about
20 cm/s are observed during the wet season and south-westerly residual currents of similar magnitude are
observed during the dry season. These support that besides the tidal induced residual flow, there are two
seasonal Monsoons whose winds drive a strong additional residual current along the South China coast
alternating in direction with the wet and dry season. In summary, the interaction of the freshwater runoff
and the dominating oceanic currents determines the flow regime of the Hong Kong waters.
Fig. 8.19 General surface flow in Hong Kong waters during: (a) flood (HHW - 3 hr); and (b) ebb flow (HHW + 4 hr)
in the dry season; and (c) flood (HHW - 3 hr); and (d) ebb flow (HHW + 4 hr) in the wet season (Lee et al., 2006) (Note
HHW is higher high water)
8.3.2 Density Stratification
The hydrography is also intimately related to the salinity structure of the coastal waters. The coastal water is
a mixture of fresh water derived from the river discharge (with low salinity) and oceanic shelf-water with
relatively high salinity.
To illustrate the salinity transport in estuaries under the action of tide and freshwater runoff, the salinity
intrusion in a rectangular estuary can be used as a simple test case. Ippen and Harleman (1961) have
described the results of a series of salinity intrusion experiments conducted in a tidal flume. The channel
is 327 ft. (99.67 m) long, 0.75 ft. (0.2886 m) wide and 1.5 ft. (0.4572 m) deep. The mean depth is kept at
0.5 ft. (0.1524 m) for all test runs discussed (Fig. 8.20(a). Tides are generated by means of a large