Estuary Processes and Managment - River Dynamics and Integrated River Management - page 482

Fig. 8.18 Comparison of predicted tides (HKO and Delft3D model) with observations at Shek Pik for 19-26 November
2002 (dry season)
The general character of the tide can be expressed by the F -factor (ratio of tidal amplitudes for
(K1+O1)/(M2+S2)). In general, tides would be considered semi-diurnal if F < 0.25 and diurnal if F > 3.0
(Bowden, 1983). In Hong Kong, F varies from 0.89 in Deep Bay (Tsim Bei Tsui) on the west to 1.27 in
Mirs Bay (Kau Lau Wan) in eastern waters (Lee, unpublished; http://www.hko.gov.hk/tide/etide). Since
0.5 < F < 1.5, tides in Hong Kong can be characterized as mixed and mainly semi-diurnal. The mean tidal
range is 1.7 m; corresponding values for spring and neap tides are typically 2.0 and 1.0 m respectively. A
typical spring-neap tidal variation in East Lamma Channel is shown in Fig. 8.17 (inset); it can be seen
that the tide can vary from semi-diurnal tide (late March) to practically diurnal tide (early April) within a
spring-neap cycle.
The hydrography of Hong Kong waters is mainly influenced by three factors: tidal currents, the Pearl
River discharge, and monsoon-induced coastal currents. Tidal currents are determined by the interaction
of ocean tides with the local topography and bathymetry; in general, the flow is from E/SE to W/NW
through Victoria Harbor and East Lamma Channel which are deeper than the surrounding area, up
towards the Pearl River estuary during flood, and from W/NW to E/SE during ebb. Figure 8.19(a) and (b)
show the typical surface flow field during flood and ebb tides in the dry season. In the main tidal stream,
significant peak surface velocities can be found in the northwestern waters north of Lantau Island (up to
2 m/s in narrow channels). During spring tide, the peak ebb velocity in Victoria Harbor is typically about
0.35 m/s near Stonecutters Island, 0.85 m/s in central Victoria Harbor, and increases to about 1.05 m/s at the
eastern harbor entrance; the peak flood velocity is generally smaller at about 70%-90% of ebb velocities.
During neap tide, the peak flood and ebb surface velocities are about 77%-86% of the spring tide values.
The tidal velocity decreases towards the eastern waters; peak ebb velocity in Mirs Bay is about 0.2 m/s-
0.3 m/s near the entrance, and <0.1 m/s inside the bay. In some landlocked tidal inlets (e.g., inner Tolo
Harbor), very weak currents of only a few cm/s are observed. Surface currents in the southern waters
(south of Hong Kong Island, Lamma Island and Lantau Island) flow in accordance with the wind
generated by the monsoon system. Fig. 8.19(c) and 8.19(d) show the surface flow field for the wet season.
The predominant wind direction is E/NE in the dry season and W/SW in the wet season. In the wet
season, during spring tide, the peak surface ebb velocity in Victoria Harbor is about 0.49 m/s near
Stonecutters Island, 1.2 m/s in the central harbor, and increases to about 1.26 m/s at the eastern harbor