Sea state

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NOAA ship Delaware II in foul weather on Georges Bank. Wea00810.jpg
NOAA ship Delaware II in foul weather on Georges Bank.

In oceanography, sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell—at a certain location and moment. A sea state is characterized by statistics, including the wave height, period, and power spectrum. The sea state varies with time, as the wind conditions or swell conditions change. The sea state can either be assessed by an experienced observer, like a trained mariner, or through instruments like weather buoys, wave radar or remote sensing satellites.

Oceanography The study of the physical and biological aspects of the ocean

Oceanography, also known as oceanology, is the study of the physical and biological aspects of the ocean. It is an important Earth science, which covers a wide range of topics, including ecosystem dynamics; ocean currents, waves, and geophysical fluid dynamics; plate tectonics and the geology of the sea floor; and fluxes of various chemical substances and physical properties within the ocean and across its boundaries. These diverse topics reflect multiple disciplines that oceanographers blend to further knowledge of the world ocean and understanding of processes within: astronomy, biology, chemistry, climatology, geography, geology, hydrology, meteorology and physics. Paleoceanography studies the history of the oceans in the geologic past.

Free surface surface of a fluid that is subject to constant perpendicular normal stress and zero parallel shear stress; surface not created by the container

In physics, a free surface is the surface of a fluid that is subject to zero parallel shear stress, such as the interface between two homogeneous fluids, for example liquid water and the air in the Earth's atmosphere. Unlike liquids, gases cannot form a free surface on their own. Fluidized/liquified solids, including slurries, granular materials, and powders may form a free surface.

Wind wave Surface waves generated by wind that occur on the free surface of bodies of water

In fluid dynamics, wind waves, or wind-generated waves, are surface waves that occur on the free surface of bodies of water. They result from the wind blowing over an area of fluid surface. Waves in the oceans can travel thousands of miles before reaching land. Wind waves on Earth range in size from small ripples, to waves over 100 ft (30 m) high.

Contents

In case of buoy measurements, the statistics are determined for a time interval in which the sea state can be considered to be constant. This duration has to be much longer than the individual wave period, but smaller than the period in which the wind and swell conditions vary significantly. Typically, records of one hundred to one thousand wave-periods are used to determine the wave statistics.

The large number of variables involved in creating the sea state cannot be quickly and easily summarized, so simpler scales are used to give an approximate but concise description of conditions for reporting in a ship's log or similar record.

WMO sea state code

Winter, North Atlantic - Water over deck and hatches, storm with huge waves (1958) Wasser uber Deck und Luken WNA.png
Winter, North Atlantic – Water over deck and hatches, storm with huge waves (1958)

The World Meteorological Organization (WMO) sea state code largely adopts the 'wind sea' definition of the Douglas Sea Scale.

WMO Sea State CodeWave heightCharacteristics
00 metres (0 ft)Calm (glassy)
10 to 0.1 metres (0.00 to 0.33 ft)Calm (rippled)
20.1 to 0.5 metres (3.9 in to 1 ft 7.7 in)Smooth (wavelets)
30.5 to 1.25 metres (1 ft 8 in to 4 ft 1 in)Slight
41.25 to 2.5 metres (4 ft 1 in to 8 ft 2 in)Moderate
52.5 to 4 metres (8 ft 2 in to 13 ft 1 in)Rough
64 to 6 metres (13 to 20 ft)Very rough
76 to 9 metres (20 to 30 ft)High
89 to 14 metres (30 to 46 ft)Very high
9Over 14 metres (46 ft)Phenomenal
Character of the sea swell
 0. None
Low1. Short or average
2. Long
Moderate3. Short
4. Average
5. Long
High6. Short
7. Average
8. Long
 9. Confused
The direction from which the swell is coming should be recorded.

Sea states in marine engineering

In engineering applications, sea states are often characterized by the following two parameters:

In physical oceanography, the significant wave height is defined traditionally as the mean wave height of the highest third of the waves (H1/3). Nowadays it is usually defined as four times the standard deviation of the surface elevation – or equivalently as four times the square root of the zeroth-order moment (area) of the wave spectrum. The symbol Hm0 is usually used for that latter definition. The significant wave height may thus refer to Hm0 or H1/3; the difference in magnitude between the two definitions is only a few percent.

Wave height The difference between the elevations of a crest and a neighbouring trough

In fluid dynamics, the wave height of a surface wave is the difference between the elevations of a crest and a neighbouring trough. Wave height is a term used by mariners, as well as in coastal, ocean and naval engineering.

The sea state is in addition to these two parameters (or variation of the two) also described by the wave spectrum which is a function of a wave height spectrum and a wave direction spectrum . Some wave height spectra are listed below. The dimension of the wave spectrum is , and many interesting properties about the sea state can be found from the spectrum.

The relationship between the spectrum and the wave amplitude for a wave component is:

Fetch (geography) The length of water over which a given wind has blown

The fetch, also called the fetch length, is the length of water over which a given wind has blown. Fetch is used in geography and meteorology and its effects are usually associated with sea state and when it reaches shore it is the main factor that creates storm surge which leads to coastal erosion and flooding. It also plays a large part in longshore drift as well.

where

and

(The latter model has since its creation improved based on the work of Phillips and Kitaigorodskii to better model the wave height spectrum for high wavenumbers. [4] )

An example function might be:

Thus the sea state is fully determined and can be recreated by the following function where is the wave elevation, is uniformly distributed between 0 and , and is randomly drawn from the directional distribution function [5]

In addition to the short term wave statistics presented above, long term sea state statistics are often given as a joint frequency table of the significant wave height and the mean wave period. From the long and short term statistical distributions it is possible to find the extreme values expected in the operating life of a ship. A ship designer can find the most extreme sea states (extreme values of H1/3 and T1) from the joint frequency table, and from the wave spectrum the designer can find the most likely highest wave elevation in the most extreme sea states and predict the most likely highest loads on individual parts of the ship from the response amplitude operators of the ship. Surviving the once in 100 years or once in 1000 years sea state is a normal demand for design of ships and offshore structures.

See also

Footnotes

  1. International Towing Tank Conference (ITTC) , retrieved 11 November 2010
  2. International Ship and Offshore Structures Congress
  3. Pierson, W. J.; Moscowitz, L. (1964), "A proposed spectral form for fully developed wind seas based on the similarity theory of S A Kitaigorodskii", Journal of Geophysical Research, 69 (24): 5181–5190, Bibcode:1964JGR....69.5181P, doi:10.1029/JZ069i024p05181
  4. Elfouhaily, T.; Chapron, B.; Katsaros, K.; Vandemark, D. (July 15, 1997). "A unified directional spectrum for long and short wind-driven waves" (PDF). Journal of Geophysical Research . 102 (C7): 15781–15796. Bibcode:1997JGR...10215781E. doi:10.1029/97jc00467.
  5. Jefferys, E. R. (1987), "Directional seas should be ergodic", Applied Ocean Research, 9 (4): 186–191, doi:10.1016/0141-1187(87)90001-0

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