Acceleration of gravity (g) - the rate of change with time of the velocity of an object caused by the earth's gravitational force.
Amplitude - The magnitude of the displacement of a wave from its mean value. In a pure sine wave, the amplitude is one-half the wave height.
Attenuation - a reduction in wave amplitude.
Average period (Tav, Tz) - Average zero down-crossing wave period. The average period of the waves observed, weighted by wave energy.
Average Wave Height (Hav) - Average zero down-crossing wave height. The average height of the waves observed.
Breaker - A wave that has reached maximum steepness and is breaking.
Capillary Wave - A wave in which the velocity of propagation is a function of the surface tension of the water. Wind waves of wavelength less than about 2.5 cm (1") are considered capillary waves.
Cat's paw - a puff of wind; a light breeze that temporarily ruffles patches of the water surface.
Chop - short-crested waves that spring up following onset of a moderate breeze, and break readily at the crest.
Crest - The highest point on a wave.
Crest Period (Tc) - the average time between successive maxima, or crests. Calculated from moments of wave frequency spectrum as Tc = square root of (m2/m4) .
Data block - A continuous block of wave samples. Due to processing constraints, the number of wave samples in a block must be a power of two.
Density - the ratio of the mass of any substance to the volume it occupies. Typical ocean water has densities in the approximate range of 1.020-1.028 g/cm3.
Deep Water Wave - A wave for which water depth is greater than one half the wave length. Ocean wind waves are negligibly affected by the bottom in deep water.
Diffraction - A wave process in which energy is transmitted along wave crests. When a wave train passes a barrier, diffraction causes energy to propogate into sheltered regions behind the barrier.
DIREC - An reciever manufactured by DATAWELL to receive digital data from a DATAWELL WAVEC Buoy and translate into RS232 digital data out.
DIWAR - An receiver manufactured by DATAWELL to receive analogue tone shifting data from a DATAWELL WAVERIDER, and supply a digital output. Ie. DIgital WAverider Reciever
Dispersion of Waves - The tendency of longer waves to travel faster than shorter waves due to the proportionality between wave phase speed and wave length.
Dominant Wave Period - The period corresponding to the frequency of maximum variance as represented by a wave frequency spectrum.
Drogue - a current measuring device with sufficient subsurface area and depth penetration to be mainly carried by the oceanic current rather than by the effects of winds and waves.
Duration - In terms of wave growth, the time over which the wind blows at a constant velocity.
Fetch - In terms of wave growth, the distance on the ocean over which the wind blows at a constant velocity.
Fourier Transform - a mathematical transform that converts a time series of data that has been collected at a specific sample interval into a series of frequency energy data. Ie data in meters is converted into m2/Hz. A FFT or Fast Fourier Transform is a computationally efficient method of doing a Fourier Transform. For Wave analysis it assumes that a time series of wave height data is the sum of many waves of different frequencies. For example a time series of 512 samples is converted into 256 wave energies of frequencies from 1/(sample interval *512) Hz to 256/(sample interval *512) Hz.
Fully-Developed Sea - A sea state in which waves have reached maximum energy. Additional energy added to the spectrum is dissipated by wave breaking.
Frequency - a measure of the number of oscillations or cycles per unit time; the reciprocal of the time duration (period) of an oscillation. (A wall outlet in North America, for example, has a voltage oscillation frequency of 60 cycles/s.)
Gravity Wave - A wave in which the velocity of propagation is a function of gravity. Water waves over a few inches in length are considered gravity waves.
Group Velocity - The velocity at which wave energy propagates. In deep water, it is equal to half the velocity of the induvidual waves in the group.
Harmonic - a quantity whose frequency is an integral multiple of the frequency of a periodic quantity to which it is related.
Height - The vertical distance between a wave crest and the next wave trough.
H10 - Zero down-crossing wave height, H10, where H10 is the average of the highest tenth of the waves.
Intermediate Water Wave - A term used to describe waves that are neither deep water nor shallow water waves. For both of these cases, equations for waves can be easily approximated. Waves are usually considered in intermediate water when the ratio of the water depth to wave length is between about 1/20 and one half ( 1/2 ) .
JONSWAP Spectrum - was established during a joint research project, the "JOint North Sea WAve Project", and is presented in literature by K.Hasselmann & al., in "Measurements of Wind-Wave Growth and Swell Decay during the Joint North Sea Wave Project (JONSWAP)" Deutsche
Hydrographische Zeitschrift, Reihe , No.12, 1973.
Kinetic energy - the energy of an object or parcel of fluid by virtue of its motion. Kinetic energy is proportional to mass and the square of the speed.
Knot - a unit of speed equal to 1 nautical mile per hour, approximately 51 centimetres per second.
Leeward - The direction toward which the wind and waves are going.
Locked - The receiver is locked when it can detect and lock onto the carrier signal transmitted by the buoy.
Maximum wave height (Hmax) - This is the largest peak to trough height seen during a record. Mean zero down-crossing wave height.
Mean Wave Direction (Dm)- Overall mean wave direction in degrees obtained by averaging the mean wave angle (theta) over all frequencies with a weighting function S(f). Theta is calculated by the KVH method in the TriAxys Directional Wave Buoy.
Moments - Calculated from surface elevation spectrum. i.e. mk = sum of {(f)k * S(f)*df} where f is the band frequency, k is 0 to 4, and S(f) is band energy.
Nyquist Frequency - The highest resolvable frequency in a digital wave record. Frequencies above the Nyquist frequency appear as lower frequencies.
Period - A measure of wave repeatability. The wave period is usually considered as the time between two successive crests or the time between two successive zero crossings in the same direction.
Peak period Tp - The period with the maximum wave energy, determined from the wave spectrum.
Phase Velocity - Propogation velocity of an individual wave. In deep water it is proportional to the wave length, otherwise it depends on water depth.
Program Disk - The disk supplied with the system con-taining the Real Time Wave System program, and other files necessary to run the program.
Record interval - The time between the start of sequential records, which must be greater than the record length.
Record length - The total time required to collect the data for a wave record.
Record offset - The time to offset record start times from the integral multiple of the record interval. For example if the record interval is 30 minutes, and the record offset is -10 minutes, then records will start 10 minutes before each hour and half hour.
Reflection - The process by which wave energy is returned in the opposite direction after a wave strikes an object or a water boundary.
Refraction - The process by which the direction of a moving wave is changed due to its interaction with the bottom topography. Wave heights may be increased or decreased by refraction.
Response function - The correction applied to the spectrum of a record to allow for the frequency dependant errors introduced by the wave buoy and receiver.
Return Period - The average time interval between occurences of wave heights equal to or greater than the height associated with the return period. It is a measure of the infrequentness of higher wave heights.
Sample interval - The time between wave samples. A typical value would be .78125 seconds, or 1.28 Herz for a DATAWELL wave buoy.
Sea - Waves formed due to the direct action of local winds.
Shallow Water Wave - A wave for which the depth divided by the wave length is less than approximately 1/20. Equations for waves can be approximated by special equations for such shallow water where waves are strongly affected by bottom depth.
Shoaling - Changes in wave height as waves move into shallow water. Except for a limited depth region, shoaling increases wave heights. Shoaling occurs even if wave heights and directions do not change as a result of wave refraction.
Significant Wave height (Hs,Hmo, H1/3) - This is the average of the highest 1/3 of all waves in a time series. It can be closely approximated from a time series of wave heights as four times the standard deviation of the time series. The value can also be approximated from four times the square root of the area under the energy spectrum of a FFT analysis. This is typically called Hmo.
Significant Wave Period (Ts) - The average period of the one-third highest waves in a wave record. The significant wave period is somewhat shorter than the dominant wave period. Calculated from moments of wave frequency spectrum as Ts = square root of (m0/m1). In the TriAxys Directional Wave Buoy it is the average period of the significant zero down-crossing waves(s) .
Spectrum - A method of representing the distribution of wave energy as a function of frequency.
Stokes Wave Theory - A nonlinear wave theory in which higher order terms proportional to wave slope are not neglected as in linear wave theory.
Surf - Waves as they reach the area between the shore and the area where breakers start to occur.
Swell - Wind waves that have traveled out of a storm generating area. Swell has longer periods and a smoother appearance than wind waves in the storm area.
Te - Wave Energy Period. Derived from the zeroth and first negative moments of the frequency spectrum. ie. Te=M(-1)/M(0). Typically used by wave energy engineers because it is the period corresponding to the weighted average of the wave energy.
Tp5 - Peak wave period in seconds as computed by the READ method. Tp5 has less statistical variability than Tp because it is based on spectral moments. The Tp5 is determined from calculating Fp5 which is the average frequency computed with the weighting function S(f)**5 over the defined upper and lower frequency range.
The algorithm
is:
Fp5= [Sum of
Fi*(Ei/Epeak)^5]/[Sum of (Ei/Epeak)^5]
where:
For i = low
frequency band(i) to high frequency band
Fi is frequency
of band
Ei is energy
of band i
Epeak is Energy
of Peak Frequency
therefore, Tp5=1/Fp5
T10 - Average period of the H10 zero down-crossing waves.
TRIAXYS Directional Wave Buoy - A directional wave buoy manufactured by Axys Technologies Inc. that is able to determine wave motion with 6 degrees of freedom.
Trough - The lowest part of the wave between successive crests.
Utility disk - The disk supplied with the system con-taining various programs to test and initialize the system. The files on this disk are not required while the system is running.
WAREP - An older receiver that was manufactured by DATAWELL to receive data from a DATAWELL WAVERIDER, and supply an analogue output.
WAREC - An reciver manufacterd by DATAWELL to receive digital data from a DATAWELL Directional Waverider Buoy and output as RS232 digital data. Ie. WAvedirection RECeiver.
Wave Direction - The direction from which a wave approaches.
WAVEC - A Directional Wave Buoy Manufactered by DATAWELL.
Wave record - A wave record is a group of continuous data blocks.
Wave sample - A single wave height measurement.
Wind Direction - The direction from which the wind is blowing.
Zero Crossings - Number of waves detected by zero crossing analysis of the demeaned wave elevation record.
Zero Crossing
Wave Period (Tz, Tav) - The average time interval between similar direction
crossings of mean water level for a wave record. The zero crossing period
can also be calculated from the moments of wave frequency spectra. Tz =
square root of(m0/m2). Also called the Mean
Spectral Period.