TNAG-0273-FCO40-309-Scheme-for-development-of-container-terminal-at-Hong-Kong-1970 — Page 24

5.29

5.30

5.31

5.32

5.33

If we consider a significant wave of 8-ft. height leaving the fetch in the direction of Kwai Chung, dispersion, shoaling effect, refraction and diffraction would reduce it so that by the time it reaches Kwai Chung its calculated residual height would be about 34 ft. with a period of 7 secs, and it would be travelling towards the north east.

It is also calculated that a lesser wind of 20 m.p.h. would produce a 6-ft. significant wave leaving the fetch and that this would be reduced to less than 3 ft. by the time it reached Kwai Chung and would have a period of 6 secs. The same weather reports indicate that this wind is likely to occur on about 4 days per year on average.

An analysis by the Hydraulics Research Station of ship observations of waves in the South China Sea shows that on average, for about 10% of a south-west monsoon season, waves there exceed both 3 ft. in height and 7 sec. in period. This would probably result in swell of at least 1 ft. in height and of at least 7 sec. period reaching Kwai Chung on 7 to 14 days per year. Such waves would be produced by winds in the fetch of 17 m.p.h.

or more.

Model studies by the Station for other ports have shown that such waves at a 45° angle to a berth would be liable to produce sufficient ship movement to interfere with smooth loading and unloading of containers from a 45,000-ton-displacement ship; they would also be liable to produce very large fender and mooring-line forces. Waves normal to a berth would produce about the same degree of ship movement, though it may be different in character. Much smaller movements with surging and pitching predominant, would occur if the ships were head or stern to the waves.

As it is not known whether waves of less than 1 ft. would also affect operations, it is realised that 7 to 14 days may underestimate the period during which operations would be affected. Wave recordings for the full south-west monsoon season are therefore being taken to enable model tests to be made should these appear necessary as a result of these recordings. From these tests an assessment could be made of ship movements under different swell conditions and of whether or not these movements would result in operating and mooring difficulties.

5.34 Winds from the south-west quadrant are also caused by tropical cyclones and in the 5 years of weather reports 1964 to 1968 there were only 11 occasions recorded when these storms produced winds greater than 23 m.p.h. of which 6 were greater than 29 m.p.h. In cases of this sort however, Typhoon Signals would normally be up and work at the Terminal suspended.

Currents

5.35

Consideration of the currents in the vicinity of Kwai Chung is required for three reasons:- (a) To assess the likelihood of silting or scouring of the seabed.

(b) To ensure that the currents will not interfere unduly with the berthing of container

ships at the Terminal.

Silting or Scouring

5.36

When Tsing Chau was an island and the refuse dump and the reclamation of Gin Drinker's Bay had not commenced, there existed a deep-water channel to the north and east of the Island in addition to Rambler Channel on the west and south. The flow through this channel ensured that silt washed down from the hills surrounding Gin Drinker's Bay was not deposited in the vicinity. When however the rubble mound extending from Tsing Chau to the Mainland was constructed in 1955 to retain the refuse dump, flow through this channel was cut off and it gradually filled with material. This material emanated both from the stream courses flowing into Gin Drinker's Bay and from the silt load carried by the currents from the Pearl River estuary at certain times of the year. How much material was deposited from each of these sources is not known but the result was a reduction in depth of water of up to 15 feet between 1955 and 1966. The breakwater extending from Mong Chau across the mouth of Gin Drinker's Bay in the direction of Tsing Chau was completed in 1966 and this had the effect of speeding up the current in the gap between this breakwater and Tsing Chau. This in turn resulted in scouring of the

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