Waves
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5.19
Three types of waves are likely to affect the design of the terminal and each of these has been taken into account in assessing the suitability of the site and the design of the berths. They are:-
(a) Swell.
(b) Storm waves.
(c) Standing waves.
Swell comprises trains of waves of comparatively long wavelength that have left the area in which they were generated. At Kwai Chung, swell originating in a fetch situated in the South China Sea beyond the islands to the immediate south of Hong Kong must be considered when determining whether the unloading of containerships would be hampered by surging and if so, in assessing for how many days of the year this state of affairs would persist.
Storm waves are those produced in the fetch extending from the site to the nearest land obstruction in the direction from which the wind is blowing and in Hong Kong are usually associated with tropical cyclones. These will comprise a wide spectrum of waves of various lengths, periods and heights. The size of the largest storm waves and the likely frequency of their occurrence must be considered in designing the berths.
5.20 Standing waves are those produced by the reflection of incident waves from a seawall or other structure. They may cause erosion in front of the berths as their influence on the seabed is greater than that of the incident waves. They may put fine seabed material into suspension which in turn could then be removed elsewhere by the tidal current.
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5.23
Standing waves also have a marked effect on the motion of a ship berthed alongside. Their amplitude at their antinodes is twice that of the incident waves thereby producing increased vertical movement, while at their nodes the motion produced is purely horizontal. It can be seen therefore that not only are the height and wave length of the incident waves important factors in determining the effect on a ship but also the position in which the ship is moored in relation to the node or antinode of the standing wave. The fact that ships berthed alongside would at times have little depth of water between their keels and the seabed would modify the foregoing effects both with regard to scouring and to the ship's movements.
In assessing swell at Kwai Chung it has not yet been possible to observe wave heights and periods at the Terminal site under full south-west monsoon conditions and therefore wave-forecasting techniques employing weather records have been used. As waves of many different wavelengths spread out of a generating area or fetch into shallower water they begin to "feel" the bottom and become modified by an ever-increasing amount as the depth decreases. In calculating the residual waves at the site, dispersion, refraction, shoaling effect and diffraction have been considered.
5.24 "Dispersion" is the spreading out and reduction in height of waves that have left the
area in which they have been generated.
5.25
"Refraction" is the process by which waves change direction and height as they pass over water that is shallower or deeper than the adjacent areas.
5.26 "Shoaling Effect" is the modification of the wave form due to the influence of the sea-
bed as it becomes shallower.
5.27
5.28
"Diffraction" is the phenomenon which occurs when a train of waves is interrupted by a barrier resulting in the spreading of wave energy along the wave crests.
An analysis of reports made over the 5-year period ending in 1968 indicates that the maximum daily-average wind speed of the south-west monsoon is 23 m.p.h. These reports were for the area of the South China Sea taken to be the fetch in which swell would be generated. Winds of 23 m.p.h. blowing for 24 hrs. would produce a "fully arisen sea" and these conditions are likely to occur for only 2 days a year on average. With this wind speed and duration it is calculated that in deep water the waves generated would have an average height of 5 ft., a significant-wave height of 8 ft. and a greatest individual height of 15 ft. The significant-wave period would be 7 secs.
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