The terms on the other pieces of the chart.
- ADCP
- Acoustic Doppler current profiler: measures currents through the water column from the Doppler shift of sound reflected by particles in the water.
- Air gap
- The clearance between the highest expected crest, water level included, and the underside of a platform deck.
- Alpha factor
- A reduction applied to an operational limit to allow for forecast uncertainty in a weather-restricted marine operation. It depends on the operation’s duration and the forecast.
- Altimetry
- Satellite radar altimeters measure significant wave height from the shape of the returned pulse, and wind speed from its strength, along the ground track.
- Bathymetry
- The depth of the seabed over the area of interest, from surveys and charts. A nearshore wave or flow model is only as good as its bathymetry.
- Beaufort scale
- An empirical scale from 0 (calm) to 12 (hurricane force) relating wind speed to the observed state of the sea.
- Bias
- The mean difference between model and measurement. Easy to correct; worth understanding before correcting.
- Bootstrapping
- Resampling the data with replacement many times and refitting, to see how much the result moves. A practical way to put an uncertainty band on return values.
- Buoy validation
- Comparing model output with buoy measurements at the same time and place, and reporting bias, RMS error, scatter index and correlation.
- Calibration
- Tuning model parameters, such as bottom friction or wind drag, until the model matches measurements. Validation then tests it on data not used for the tuning.
- Chart datum
- The level to which charted depths and predicted tide heights are referred. In much of Europe it is LAT.
- Crest elevation
- The height of a wave crest above still water level. In steep or shallow-water waves it is more than half the wave height.
- Current shear
- The change of current speed or direction with height above the seabed. It matters for loads on piles, risers and cables.
- Deep water
- Depth greater than half the wavelength, where waves do not feel the seabed. The deep-water wavelength is gT²/2π, about 1.56 T² in metres.
- Delft3D
- Deltares’ modelling suite for flow, waves, sediment transport and water quality in coastal and estuarine waters. Its wave module is SWAN.
- Depth-limited breaking
- In shallow water, depth caps the wave height. The breaker index H/h is about 0.78 for a solitary wave; for Hs in a surf zone it is lower.
- Design basis
- The document that fixes the environmental conditions a structure is designed for, with their sources and methods. Designers and certifiers both work from it.
- Diffraction
- The spreading of wave energy sideways along the crest into the sheltered zone behind an obstacle, such as a breakwater.
- Directional spreading
- How wave energy at one frequency is spread over direction, often modelled as cos2s. Wide spreading means a short-crested sea; narrow spreading, long-crested swell.
- Dispersion relation
- ω² = gk tanh(kh). It ties wave period to wavelength and depth; longer waves travel faster.
- Downtime
- The time an operation waits for weather, estimated from persistence statistics of hindcast or measured data.
- Ensemble forecast
- A set of forecasts from slightly different starting conditions or model settings. Their spread is a measure of the forecast’s uncertainty.
- Environmental contour
- A line of combinations, such as Hs and Tp, with a given return period, used to pick design sea states for structures that respond to both.
- Estuarine circulation
- In an estuary, fresher river water flows seaward at the surface while denser seawater creeps landward along the bed. It traps sediment near the tip of the salt wedge.
- Exceedance
- The share of time a value is exceeded. An exceedance curve of Hs answers most first questions about a site.
- Extreme value analysis
- Fitting a distribution to the largest values in a record to estimate values for return periods longer than the record itself. The extrapolation is where the uncertainty lives.
- Fetch
- The distance over open water across which the wind blows from a roughly constant direction. With wind speed and duration, it limits how high waves can grow.
- Floating LiDAR
- A buoy carrying a laser wind profiler, measuring wind speed and direction up to rotor height without a fixed mast.
- Generalised Pareto
- The limiting distribution of exceedances above a high threshold: the natural partner of the POT method.
- GEV
- Generalised extreme value distribution: the limiting distribution of block maxima, such as annual maxima. Gumbel, Fréchet and reversed Weibull are its three types.
- Group velocity
- The speed at which wave energy travels. In deep water it is half the phase speed; in shallow water the two become equal.
- Gumbel
- The GEV with shape parameter zero. A straight line on Gumbel paper, and a common first choice for annual maxima.
- Harmonic analysis
- Fitting tidal constituents of known frequency to a water-level or current record, to separate the predictable tide from the rest.
- Hindcast
- A model run of the past, driven by analysed winds and pressures, that reconstructs long time series where no measurements exist.
- Hm0
- Significant wave height from the spectrum: 4√m0, where m0 is the variance of the sea-surface elevation, the area under the wave spectrum.
- Hmax
- The highest individual wave in a record. For a three-hour storm of about 1000 waves, the most probable Hmax is roughly 1.86 Hs (Rayleigh distribution).
- Hs
- Significant wave height. Classically H1/3, the mean of the highest third of the waves in a record. In deep water it is a few per cent below Hm0, and the two are often used interchangeably.
- Infragravity waves
- Long waves with periods of roughly 25 to 250 s, bound to wave groups or released at the shore. They drive harbour resonance and the motion of moored ships.
- Inverse barometer
- In equilibrium, sea level rises about 1 cm for each hPa drop in atmospheric pressure.
- Joint probability
- The probability of two variables occurring together, such as Hs with Tp, or waves with high water. Design often turns on the combination, not on either alone.
- JONSWAP
- Spectral shape from the Joint North Sea Wave Project (1973) for fetch-limited seas: a Pierson–Moskowitz spectrum with a sharper peak, set by the peak-enhancement factor γ, typically 3.3.
- K1
- Lunisolar diurnal constituent, period 23.93 hours. With O1 it dominates where the tide is diurnal or mixed.
- Kd
- Disturbance (or diffraction) coefficient: local wave height divided by incident wave height. The basic number of a harbour wave study.
- LAT
- Lowest astronomical tide: the lowest level that can be predicted under average meteorological conditions and any combination of astronomical conditions. A surge can still take the water below it.
- M2
- Principal lunar semidiurnal tidal constituent, period 12.42 hours. The largest constituent along most coasts, the North Sea included.
- Magnetic variation
- The angle between true north and magnetic north at a place. It changes slowly, so a chart gives it with a year and an annual change.
- Metocean
- Meteorological and oceanographic: wind, waves, currents, water level and the rest of the environment a structure or an operation meets at sea.
- MIKE 21
- DHI’s two-dimensional modelling suite, with modules for spectral waves (SW), hydrodynamics (HD) and more.
- Morphodynamics
- The coupled change of seabed and flow: currents and waves move sediment, the seabed changes, and the changed seabed alters the flow.
- MSL
- Mean sea level: the average water level over a long period, ideally a full 18.6-year nodal cycle.
- Nesting
- Running a fine model inside a coarser one that supplies its boundary conditions. It lets a port model feel the ocean outside it.
- Non-tidal residual
- Measured water level minus predicted tide. Mostly surge, plus seiches and errors in the prediction.
- Persistence
- Statistics of how long conditions stay above or below a threshold. Duration, not just frequency, sets the waiting time.
- Pierson–Moskowitz
- Spectrum of a fully developed sea in deep water: the wind has blown long enough, over a long enough fetch, that the waves no longer grow.
- POT
- Peaks over threshold: take independent storm peaks above a chosen threshold and fit them, usually with a generalised Pareto distribution. It uses more of the data than annual maxima do.
- Radiation stress
- The excess flux of momentum carried by waves. Where it changes, as in breaking, it pushes on the water: set-up and longshore currents.
- Rayleigh distribution
- The distribution of individual wave heights in a narrow-banded sea in deep water. It links Hs, Hmax and the other height statistics to the spectrum.
- Reanalysis
- A consistent reconstruction of past weather made with one fixed model and data-assimilation system, such as ECMWF’s ERA5. A common source of hindcast winds.
- Refraction
- The turning of wave crests as they travel over varying depth, or through currents. Crests tend to line up with the depth contours.
- Return period
- The average time between exceedances of a value. The 100-year wave has a 1 % chance of being exceeded in any one year, and about a 63 % chance in 100 years.
- S2
- Principal solar semidiurnal constituent, period exactly 12 hours. It beats with M2 to give spring and neap tides.
- Sand waves
- Large seabed bedforms built by tidal currents, metres high and a few hundred metres apart. They migrate slowly and can bury or expose a cable.
- Scatter index
- RMS error divided by the mean of the observations. A dimensionless measure of how much a model scatters about the measurements.
- Scour
- Erosion of the seabed where a structure speeds up the flow and adds turbulence. Around a monopile it can reach a depth of the order of the pile diameter.
- Seiche
- A standing oscillation of a basin, a harbour or a lake at its natural periods, from minutes to hours.
- Shoaling
- The change in wave height as waves move into shallower water and their group velocity changes. Heights first dip slightly, then grow until the waves break.
- Spring–neap cycle
- The fortnightly variation in tidal range, about 14.8 days, as M2 and S2 move in and out of phase.
- Storm surge
- The rise of water level above the predicted tide, caused by wind stress on the sea surface and by low atmospheric pressure.
- SWAN
- Simulating WAves Nearshore: a third-generation spectral wave model from TU Delft. It solves the wave action balance on a grid, shallow-water effects included.
- Swell
- Waves that have left the area where they were generated. Longer period, more regular, and able to cross ocean basins.
- Third-generation model
- A spectral wave model that computes wind input, nonlinear wave–wave interactions and whitecapping explicitly, without prescribing the spectral shape.
- Tidal diamond
- A lettered diamond marking a position for which the chart’s tidal stream table gives the direction and rate of the stream at each hour before and after high water.
- Tm−1,0
- Energy period, m−1/m0. Used for wave power and, in coastal engineering, for wave run-up and overtopping.
- Tm02
- Mean zero-crossing period from spectral moments, √(m0/m2). It approximates the mean period Tz found by counting zero-crossings in the record.
- Tp
- Peak period: the inverse of the frequency at which the wave spectrum has its maximum. It tells you which waves carry the most energy.
- Turbidity
- Cloudiness of water from suspended particles. It is measured optically and converted to suspended-sediment concentration by calibration against water samples.
- Turbulence intensity
- The standard deviation of wind speed over ten minutes divided by its mean. A key input for the fatigue loads on a turbine.
- U10
- Wind speed 10 m above the surface, the standard reference height. Usually a 10-minute mean; other averaging periods give other numbers.
- Uncertainty band
- The spread around a forecast or an estimate, at a stated probability. A number without one invites more confidence than the data support.
- Under-keel clearance
- The water left between a ship’s keel and the seabed. Squat, wave-induced motion and low water all reduce it, and it decides when a deep ship can use a channel.
- Wave buoy
- A floating instrument that records its own motion, by accelerometers or GPS, from which the wave spectrum and wave direction are derived.
- Wave rose
- A polar diagram of how often waves of each height class come from each direction sector. Waves are named after the direction they come from.
- Wave set-up
- The rise of mean water level in the surf zone, caused by the gradient in radiation stress as waves break.
- Wave spectrum
- The distribution of wave variance over frequency, and in its directional form over direction too. Most wave statistics are its moments.
- Wave steepness
- Wave height divided by wavelength. Individual waves break near 1/7; young wind seas are steep, old swell is gentle.
- WAVEWATCH III
- NOAA’s third-generation spectral wave model, widely used for global and regional wave forecasts and hindcasts.
- Weather window
- A continuous period in which conditions stay within the limits of an operation for long enough to finish it.
- Weibull
- A two- or three-parameter distribution often fitted to wind speeds and wave heights, both to the whole record and to storm peaks.
- Wind sea
- Waves still being driven by the local wind. Short, steep, and running with the wind.
- Wind shear
- The increase of wind speed with height, often described by a power law or a logarithmic profile. Both depend on atmospheric stability and sea-surface roughness.
- Workability
- The share of time an operation can go ahead within its limits. It is lower than a simple exceedance suggests, because an operation also needs time to finish.
