Extreme Wave Conditions

In the western Baltic Sea, continuous measurements of the sea state started in the 1990s (see Section 7.1.2). Therefore sufficient data are available for statistical analysis of the sea state regime at the measuring position (see Section 7.1.5) and more over for the vahdation of numerical sea state models (see Section 7.1.4). 

The cyclone, whose minimum core pressure was approximately 960 hPa, moved between 7 and 9 January 2005 across the North Sea and the Baltic Sea from Scotland via Southern Norway and Central Sweden to Finland. 

FIGURE 7.23 Wind and Wave Conditions over the Baltic Sea on February 14, 1956 (from Paszkiewicz, 1989). (a) Wind speed, (b) Sea state (solid line, average wave height in meters dashed line, average wave period in seconds). 

In the western Baltic Sea, the cyclone reached its maximum wind force in the evening of January 8, 2005, when the center of the depression was located over Sweden. The wind direction (WSW) was parallel to a line between the Lubeck Bay and the Arkona MARNET Station, that is, in an optimum direction for a fetch of maximum length in this sea area. Wind and sea state measurements are available from the Darss SiU and ArkonaBasin stations (see also Fig. 7.25). Wind speed measured at a height of 10 m was approximately identical at both stations and was about 20 m/s (39 kn) for almost 24 h. The maximum value was approx. 24 m/s (47 kn) and remained at this level at Arkona for 6 h with only slight changes in wind direction. 

The wind caused the significant wave height to increase from 1.5 m to almost 5 m within 3 h foUowing the increase in wind speed with a delay of approximately 2-3 h. There was a systematic difference of 20° to 30° degrees between mean wind direction (WSW) and sea direction (nearly west). A possible cause for this is that due to topography, the most effective direction for wave growth differs from the prevailing wind direction. The peak period increased to 8-9 s. 

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The probability of exceedance p = 0.004 corresponds with extreme wave heights between 7 and 10 m depending on the location. The distribution of the extreme wave conditions is shown in Fig. 7.22 for selected locations. 

Boulder beaches consist primarily of materials that are more than 256 mm in diameter. These beaches are not altered by any conditions other than ice, human activity, or extreme wave conditions. Boulder beaches often give way to mud or sand tidal flats in the lower intertidal zone. Because of the large spaces between individual boulders, oil can be carried down to the sediments and remain there for years. Since animals and plants live in these spaces, oil often has a severe effect on boulder beaches. Boulder beaches are considered to be moderately sensitive to oil and do not recover rapidly from oiling. 

In contrast, aromatic sulphoxides do not need extreme experimental conditions to give a well-defined step in polarography and voltammetry. Thus methyl phenyl sulphoxide (80) exhibits69 a well-defined wave in strongly acidic media at very moderate potential values. The reduction scheme assumes the transient formation of a protonated form prior to the electron transfer ... 

Cirkva and Hajek have proposed a simple application of a domestic microwave oven for microwave-assisted photochemistry experiments [105]. In this arrangement the EDL (the MW-powered lamp for this application was specified as a micro-wave lamp or MWL) was placed in a reaction vessel located in the cavity of an oven. The MW field generated a UV discharge inside the lamp that resulted in simultaneous UV and MW irradiation of the liquid sample. This arrangement provided the unique possibility of studying photochemical reactions under extreme thermal conditions [106]. 

J. W. van der Meer, Extreme shallow water wave conditions. Report H198. Delft Hydraulics Laboratory, The Netherlands (1990). 

The heights, periods, and energies of storm-generated waves are the dominant environmental factors that affect coasts, with their most extreme measured occurrences having resulted in the erosion of beaches and backshore properties, and damaged infrastructme such as harbor jetties and breakwaters. Important is the development of a wave climate, a documentation of the wave conditions experienced dming past decades, with projections of the most extreme waves that could occur in the futme, potentially posing a threat to the coast. 

Ferreira and Soares describe the joint probability distribution function of longterm hydraulic conditions. Especially when the main interest is in the design of flood defence structures, the extreme conditions are important, which implies that the dependence between hydraulic conditions needs to be accounted for. The joint probability analysis of extreme waves and water levels thus is significant in order to estimate more accurately the extreme environmental loading on a coastal structure. Because wind setup (storm surge) and wave conditions depend on the same driving force, a strong dependence between them is observed under extreme conditions. 

A. Repko, P. H. A. J. M. van Gelder, H. G. Voortman and J. K. Vrijling, Bivariate description of offshore wave conditions with physics-based extreme value statistics,... 

H. Hanson and M. Larson, Extreme waves and water levels in the southern Baltic Sea Implications for flooding at present and future conditions, J. Hyd. Res. 46, Extra... 

Natural external events which shall be considered include those which have been identified in site characterization, such as earthquakes, floods, high winds, tornadoes, tsunami (tidal waves) and extreme meteorological conditions. Human induced external events that shall be considered include those that have been identified in site characterization and for which design bases have been derived. The list of these events shall be reassessed for completeness at an early stage of the design process. 

Human shelter destruction due to extreme weather conditions, tidal waves, hurricanes, and tornados washing away housing or triggering floods and landshdes... 

Numerical modelling of extreme storm events is usually applied in areas where extreme storms determine the extreme wave and water level conditions. Input wind fields (and air pressure fields) can be derived from scientific or commercial hind cast databases of which some well-known are ECMWF (www.ecmwf int), Ocean weather (www.oceanweather.com), KNMI (www.knmi.nl), BMO (www.metoffice.gov.uk). 

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