Estuary Processes and Managment - River Dynamics and Integrated River Management - page 495

Using this model, it was found that in the main tidal stream, peak velocities of the west-east ebb flow
are found to be about 0.15 m/s near Stonecutters Island, 0.60 m/s in the central harbor, and to increase to
about 1.05 m/s at Lei Yue Mun. The east-west flood flow is similar, with a little smaller peak flood
velocities. The flow is qualitatively similar to that before reclamation, but there is a notable decrease of
about 8%-17% with a mean value of 11.5% in peak discharge along the harbor channel. This leads to a
decrease in tidal velocity in the east of section 2, but an increase in tidal velocity in the west of section 2
due to significant reduction in width after reclamation of a large area (Fig. 8.31). For example, the depth-
average peak velocity decreases around 9.6% at point 1 and increases 19% at point 3; both will slightly
increase by less than 1% after completion of the proposed reclamation (maximum possible Central and
Wan Chai reclamation).

Fig. 8.31 Comparison of time variation of depth-averaged velocity magnitude at points 1 and 3 in the wet season
with different coastlines ( T = 0 at 0:00 on 5 August, 2003)

The computed flushing times in the wet and dry seasons for different coastlines are listed in Table 8.3.
After reclamation of a large area, the self-cleansing capacity of Victoria Harbor is decreased by 30%-50%.
Further Central and Wan Chai reclamation will slightly improve the flushing capacity in Victoria Harbor
by 1%-1.5%. In present coastline, the flushing time in Victoria Harbor ranges from 1.5 to 2.5 days in the
wet season and from 5 to 7 days in the dry season.

T able 8.3 Flushing time (days) in Victoria Harbor for different coastlines
Coastline Wet season Dry season

1975 1.10 4.85

2003 1.67 6.25

Proposed 1.65 6.16

8.4 Eutrophication and Algal Bloom

Eutrophication refers to the enrichment of a water body by nutrients—the chemical substances essential
to the growth of plants—to a level that results in high biological productivity, deterioration in water
quality, and undesirable disruption to the balance of aquatic ecosystem. Eutrophication can occur in rivers