Significant wave height

As mentioned earlier, the process facilities have been designed to operate in environmental conditions based on a 1-year storm condition. During the 2 years that have elapsed, several bad weather periods have been experienced during which significant wave heights of 21 ft [6.S ml have been observed. It has been established that the worst environmental conditions encountered so far do cause liquid movement in the separators but have not adversely affected (he functioning of the process and utility systems and do not cause level alarms or shutdowns. 

Figure 10. Relationship between diversity index and significant wave height at Stations 1-26 and A-R. The mouth, intermediate, and inner area of the bay are indicated in green, blue, and red, respectively (after [18]).

A procedure for empirical computation of significant wave height, mean wave period and wavelength, and an empirical energy spectrum of wind waves is introduced. 

Very often buoys are used that move with the surface. They have a special elastic mooring, so that the buoy is able to follow the orbital motion of the wave as freely as possible. These measuring buoys contain at least one acceleration sensor for the vertical component. In direction-measuring buoys, the position of the case relative to the horizontal line and to the north has to be determined by additional sensors. The significant wave height and the characteristic periods are then calculated from the variance spectrum of the vertical displacement, and the directional information results from a cross spectral analysis of all components of the buoy movement. A comprehensive description of the measuring instruments and the statistical analysis methods was given by Tucker (1991). 

Ts, significant period (wave period corresponding to the significant wave height). 

The significant wave height corresponds approximately to that wave height that a trained observer determines to be characteristic for the sea state. Comparisons with observed and measured wave heights showed that the match was satisfactory. The standard deviations that were determined were 0.3 m for 1.5 m resp. 1.5 m for 6 m high waves (WMO. 1981). In a further comparison of buoy measurements and observations on board ships deviations of between 0.7 and 0.9 m are reported (Gulev et al., 2003). 

In the relevant literature as well as in sea state analyses and forecasts, in addition to the significant wave height also other statistical characteristics for the wave height (see above) are used, such as the average height of the waves, which comprise the top 10% (Hj/io) or... 

In this atlas, a wave height is depicted that was established as a standard in Soviet specialized literature and that was defined as the wave height at the lower limit of the waves comprising the top 3%. These wave heights must be multiplied with the factor 0.76 to establish a relationship with the significant wave height. 

In accordance with Fig. 7.12, waves with a height of 55 dm are to be expected at the MARNET position north of Arkona. This corresponds to a significant wave height of 4.2 m. 

A parameterization of the JONSWAP spectrum has been worked out by Houmb and Overvik (1976). This work gives recommended values of a, y, and f, when the significant wave height 77 and average zero crossing wave period are given (Table 7.6). 

FIGURE 7.13 Parameterized energy spectrum of the Baltic Sea for significant wave heights of 2, 3, 4, 5 and 6 m. (See color plate)... 

Figure 7.13 shows spectra for significant wave heights 2-6 m. 

Frequency distributions of significant wave height for four intervals (0-1, 1.5-2.5,... 

Two-dimensional frequency distributions of wave period and significant wave height... 

Figure 7.18 shows the monthly distribution of the maximum significant wave height on 11 positions with water depth more than 20 m. 

FIGURE 7.17 Monthly means of significant wave heights. 

FIGURE 7.18 Monthly maxima of the significant wave heights. 

Augustin (2005) found the absolute maximum value of the significant wave heights (10 m) in the eastern part of the Southern Baltic Sea, which corresponds to the latest hindcast studies of Weisse (2007), see Table 7.9. This is also the order of magnitude of the extremes that were determined by various weather services for the hurricanes Anatol (1999) and Erwin/Gudrun (2005—see Section 1.1.6). 

Table 7.11 contains a comparison of significant wave heights for the northern Baltic Sea. The data of Weisse (2007) and Augustin (2005) are based on model hindcast simulations for different time periods. 

TABLE 7.8 Monthly Means of Significant Wave Heights Derived from Ship Ohservations (DWD,) and Hindcast (Weisse, 2007)... 

TABLE 7.10 Frequency Distributions and Extreme Valnes of the Significant Wave Height... 

FIGURE 7.20 Seasonal means of the significant wave height (after Augustin, 2005). 

Significant wave height and zero crossing period... 

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