BSH

Sea state

Definition

Sea state is the state of oscillation of the sea surface (waves) generated by wind energy. The significant wave height (Hs), which is defined as the mean wave height of the upper third of the wave height distribution, is used as a measure of the sea state strength. The wave height is the vertical distance between the wave crest and the wave trough.

The sea state is composed of the wind sea and the swell:
The wave heights of the wind sea depend on the wind strength, the duration of the wind effectiveness and the fetch, which is the distance over which the wind acts on the sea surface.
The swell is an "old" sea state from more distant storm areas that spreads over great distances independently of the local wind. In contrast to waves of the wind sea, swell waves are rounded. This means that they have a lower slope, which is defined as the ratio of wave height to wavelength, since they have in particular longer wavelengths (a greater distance between two neighbouring wave crests). Thus swell waves in the Atlantic can reach wavelengths of more than 200 m.

Aim and purpose of sea state measurements

The purpose of the measurements is to obtain data for the

  • general information on sea state conditions in the North Sea and Baltic Sea,
  • safety in shipping, offshore operations (e.g. in offshore wind farms), ferry services and ocean surveying,
  • verification of the numerical sea state prediction model of the German Weather Service, which is used for route planning and the planning of offshore operations,
  • verification, validation and improvement of Hindcast data sets which are used as an assessment basis for the construction of offshore structures,
  • general investigation of sea state conditions and their change,
  • verification of model results for future climate development.

The measurement of sea state

At BSH operational sea state measurements are carried out in different ways. Radar gauges are installed on fixed structures (e.g. offshore wind turbines, research platforms or oil rigs). The time course of the vertical deflection of the sea surface is measured here. If three or more gauges are used, the wave direction can also be determined. Where there are no fixed structures in the sea, the sea state is traditionally measured with sea state measuring buoys anchored to the seabed. The buoys are equipped with GPS, a compass, inclination and acceleration sensors. The buoy data is then transmitted to land and further processed there.
Satellite-based sea state measurements are gaining in importance. For example, radar backscatter signals or radar-supported imaging methods are used to determine sea state parameters.

Measuring stations

The BSH operates its own measuring stations and also uses the buoys WESTERLAND /SYLT and BUNKER HILL. These buoys are measuring stations of the Landesbetrieb für Küstenschutz, Nationalpark und Meeresschutz Schleswig-Holstein (LKN). The buoy BUNKER HILL is operated in cooperation with the Helmholtz Zentrum Geesthacht (HZG). The stations FINO1, FINO2 and FINO3 are operated within the FINO project. The BSH operates the stations alpha ventus, Butendiek and Nordsee One in the area of the corresponding wind parks and in cooperation with the operators of the wind parks.

StationPosition (Latitude)Position (Longitude)Water depthAvailable from
Nordseeboje-255° 00.000' N06° 20.000' E42 m1993
Nordseeboje-354° 41.000' N06° 47.000' E40 m2010 (until 2012)
Helgoland-Süd54° 09.600' N 07° 52.089' E20 m1989
Helgoland-Nord54° 13.164' N07° 49.109' E30 m2008
Elbe53° 59.825' N08° 06.630' E25 m1990
Fino 154° 00.860' N 06° 35.031' E30 m 2003
Fino 255° 00.420' N13° 09.250' E24 m2014
Fino 355° 12.000' N07° 09.000' E25 m2011
Arkona54° 53.116' N 13° 51.644' E46 m2002
Darss/Zingst54° 51.900' N12° 42.000' E21 m1991
Westerland54° 55.030' N 08° 13.300' E 13 m 1989
Butendiek55° 01.480' N07° 47.216' E19 m2018
Nordsee One53° 59.110' N06° 50.316' E40 m2017
Bunker Hill54° 47.083' N08° 16.050' E10 m2011
Alpha Ventus54° 00.256' N06° 36.866' E30 m2012