Taiwan is located in the subequatorial region of the Western Pacific Ocean. From June until October typhoons originate in the vicinity of Guam. These either pass by or they invade the island and cause catastrophes. If the typhoon directly attacks Taiwan, of course, it generates high waves. However, even if the typhoon does not come to the island but passes nearby, there are always sells of long wave period. The typhoon swell of long wave period. The typhoon swell has a bad effect on the coastal and offshore engineering operations, such as the installation of breakwater caissons, such as the installation of breakwater caissons, armour blocks and underwater pipelines, as well as the dredging works. The typhoon swell may stop these maritime activities or even damage the unfinished maritime structures, especially for those works which are very weak during their construction stage, such as the underwater pipeline. There are cases when the maritime works in construction are damaged by the typhoon swell.

The usually typhoon wave prediction methods are the numerical method of Tang (1970) and the parametric method of Bretschneider (1976). The former can also predict typhoon swell. However, the calculation is quite complicated and not suitable for the climate forecasting operation. Hence, Tang's method has not been used to predict typhoon swell. Also Bretschneoder's method can only predict wind waves. The author has studied the typhoon swell prediction (Liang et al., 1977).

The typhoon swell height is H 1/3 = 0.11H*R R/vDD (metres), in which H*R is the wave height (in feet) in the typhoon centre calculated by Bretschneider's method, R is the radius (in nautical miles) of the maximum wind speed and DD (in nautical miles) is the distance between the typhoon centre and the station. The swell period is in which T1/3 is the wave period in the typhoon centre estimated by Bretschneider's method and U*RS (in knots) is the sea surface wing speed at radius R.

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Wave Height Accumulation And Expansion Correction Factor

When the distance between the typhoon centre and the station changes, the propagating duration of the swell also changes. If the duration becomes shorter, the wave height will increase, i.e. the accumulation effect. If the duration increases, the wave height decreases, i.e. the expansion effect. If more than two typhoons appear or if the propagating duration changes abruptly, for example, the typhoon suddenly becomes strong and its speed of travel increases violently, the swells may overlap. The latter can be solved simply by taking the square sum. For the former, it is considered by the principle of wave energy transmission. The wave power passing through the cross section of a unit crest width is equal to 1/8pgH2Cg' in which Cgis the group velocity and H is the wave height. If the typhoon is stationary, the duration of the swell is D. owing to the moving typhoon, this duration becomes D'. However, the wave power passing that cross section is conserved. They should be equal:

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