There are indications that the mean significant wave height at Seven Stones Light Vessel has increased in the period 1960-85. This is of considerable interest for the design of offshore structures and for coastal defense. In this note, the authors present new results based on the analysis of a collection of more than 20 000 hand-drawn wave charts. These charts were produced routinely between 1960 and 1988 in a manner that remained essentially unchanged throughout this period. The results are in remarkable agreement with the trend observed at Seven Stones Light Vessel.

The European project WASA (Waves and Storms in the North Atlantic) has been set up to verify or disprove hypotheses of a worsening storm and wave climate in the northeast Atlantic and its adjacent seas in the present century. Its main conclusion is that the storm and wave climate in most of the northeast Atlantic and in the North Sea has undergone significant variations on timescales of decades; it has indeed roughened in recent decades, but the present intensity of the storm and wave climate seems to be comparable with that at the beginning of this century. Part of this variability is found to be related to the North Atlantic oscillation.An analysis of a high-resolution climate change experiment, mimicking global warming due to increased greenhouse gas concentrations, results in a weak increase of storm activity and (extreme) wave heights in the Bay of Biscay and in the North Sea, while storm action and waves slightly decrease along the Norwegian coast and in most of the remaining North Atlantic area. A weak increase in storm surges in the southern and eastern part of the North Sea is expected. These projected anthropogenic changes at the time of CO2 doubling fall well within the limits of variability observed in the past.A major methodical obstacle for the assessment of changes in the intensity of storm and wave events are inhomogeneities in the observational record, both in terms of local observations and of analyzed products (such as weather maps), which usually produce an artificial increase of extreme winds. This occurs because older analyses were based on fewer observations and with more limited conceptual and numerical models of the dynamical processes than more recent analyses. Therefore the assessment of changes in storminess is based on local observations of air pressure and high-frequency variance at tide gauges. Data of this sort is available for 100 yr and sometimes more. The assessment of changes in the wave climate is achieved using a two-step procedure; first a state-of-the-art wave model is integrated with 40 yr of wind analysis; the results are assumed to be reasonably homogeneous in the area south of 70 degrees N and east of 20 degrees W; then a regression is built that relates monthly mean air pressure distributions to intramonthly percentiles of wave heights at selected locations with the help of the 40-yr simulated data; finally, observed monthly mean air pressure fields from the beginning of this century are fed into the regression model to derive best guesses of wave statistics throughout the century.

AbstractThree operational shallow water wave models are intercompared for two artificial experiments and verified for a severe storm hindcast, with the objectives of further understanding the effects of the parametrization of shallow water wave processes in numerical models.The models used are the HYPAS (Max‐Planck Institute) and GONO (KNMI) coupled‐hybrid models, and the BMO (Meteorological Office) coupled‐discrete model which are all briefly described. In the first case, depth‐dependent fetch‐limited wave growth in a steady wind is examined. In the second case a steady onshore wind is specified over an idealized constant slope coastal shelf, and the stationary wave spectra at various depths are intercompared. For the third case the wind fields for the North Sea storms of 18‐26 November 1981 were accurately reconstructed and used by each model in its operational configuration to produce a wave hindcast for this period.In case 1 the GONO and BMO models exhibit similar behaviour in the evolution of energy and peak frequency, whereas HYPAS displays less depth attenuation and little variation in peak frequency. In case 2 the energy values at different shelf depths are approximately as predicted in case 1 for HYPAS though rather higher for BMO and GONO. However, GONO and HYPAS show little change in peak frequency with depth here whereas BMO wave spectra become double‐peaked with a wind‐sea peak migrating to higher frequencies in shallower waters. In case 3, the hindcasts, all models produce qualitatively similar results. the time series of wave height and period agree well with measurements, BMO and HYPAS predicting correct energy levels except at storm peaks and GONO generally overpredicting both at lower energy levels and in a duration‐limited strong wind case. the r.m.s. error in wave height at the southern shallow water verification site is 0.5 m for all models, and varies between 0.9 m (GONO) and 1.5m (HYPAS) at the northern deep water site. Some wave spectra are presented and the directional relaxation of wind‐sea in each model is illustrated.The results of cases 1 and 2 are readily explained by the formulation of shallow water processes adopted in each model, but it is difficult to isolate and identify these mechanisms in the measured or modelied spectra from the hindcast. It is suggested that future studies involving detailed verification and intercomparison of wave models should be confined to more carefully designed wave‐measuring experiments so that less ambiguous results are obtained.

A self‐similar spectral shape (the TMA spectrum) to describe wind waves in water of finite depth is presented. The parametric spectral form is depth dependent and an extension of the deep water JONSWAP spectrum. The behavior of the spectrum in frequency and wave number space is discussed. About 2800 spectra selected from three data sets (TEXEL storm, MARSEN, ARSLOE) are investigated to show the general validity of the proposal self‐similar spectral shape.

A rather large number of conventional observations is available for the North Sea. Yet a comparison with the coverage of ERS-1 shows that its data will be a valuable addition to the existing data, especially in the north. The wave forecasting methods in use for swell forecasting in the southern North Sea suffer from a lack of data from the Norwegian Sea. This is illustrated with an example of model mis-prediction. One can conclude that the amount of data from ERS-1 in the Norwegian Sea will certainly improve swell forecasting in the North Sea.

Some climatological data are presented in terms of an average recurrence period of 50 years, cited from a report of Working Group I — Environment of the Conference on Safety and Pollution Safeguards in the Development of North West European Offshore Mineral Resources. The hourly mean wind speed in the northern part of the North Sea may reach 38–40 m/s, which is about 20% greater than the value found in the southern North Sea. On the other hand, wave heights are likely to exceed the 30 m level in the northern North Sea, which is about twice the value found in the south. This is due to the combined effect of differences in wind climate, water depths and fetch lengths.

Bouwa has given a discussion of a severe storm in the southern North Sea, on 3 January 1976 near Texel, one of the Friesian islands. This storm was characterized by a remarkable steadiness of wind and wave parameters. The steadiness of the wave parameters was apparently the result of depth limitations, which prevented further wave evolution. Here the various terms in the energy balance equation for wave growth in shallow water are estimated. The relative importance of wind input, surface dissipation, bottom dissipation, advection and nonlinear transfer is discussed. For a certain choice of dissipation parameters, a good balance can be obtained. This is in agreement with the steadiness of the observed wave conditions.

The equations describing refraction of water waves are based on the geometrical optics approximation, in which wave rays are calculated independently of each other. For this reason the model reacts sensitively to small variations in depth as well as in incident wave frequency and direction. This leads to an uncertainty in the interpretation of the location of individual rays, particularly in cases of large travel distances through regions with weakly irregular bottom topography, as in shelf seas. In this paper, this uncertainty is taken into account from the outset by treating the results of the ray calculations as a case of statistical sampling, in which a tradeoff is made between resolution and confidence. To this end, the area being studied is divided into a number of subregions (bins), in each of which the total wave energy is estimated from the rays passing through it, each ray being considered as a carrier of a certain amount of wave power. The size of the bins determines the spatial resolution. It must be chosen in conjunction with the discretization intervals for ray separation, frequency, and direction, considering also the propagation distance, the length scale of the target area, and the bottom topography. This is illustrated with an example from JONSWAP.