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Glossary of Coastal Engineering Terms

The following definitions are based on entries from the USACE's comprehensive Glossary of Coastal Terminology.

Index:   A   B   C   D   E   F   G   H   I   J   K   L   M   N   O   P   R   S   T   U   V   W  


Aback to top

ACCELEROMETER
A device used in wave buoys for measuring acceleration and buoy movement.

ACOUSTIC DOPPLER CURRENT PROFILER (ADCP)
A current measuring instrument employing the transmission of high frequency acoustic signals in the water. The current is determined by a Doppler shift in the backscatter echo from plankton, suspended sediment, and bubbles, all assumed to be moving with the mean speed of the water. Time gating circuitry is employed which uses differences in acoustic travel time to divide the water column into range intervals, called bins. The bin determinations allow development of a profile of current speed and direction over the entire water column. The ADCP can be deployed from a moving vessel, tow, buoy, or bottom platform. In the latter configuration, it is nonobtrusive in the water column and thus can be deployed in shipping channels.

AMPLITUDE, WAVE
The magnitude of the displacement of a wave from a mean value. An ocean wave has an amplitude equal to the vertical distance from still-water level to wave crest. For a sinusoidal wave, the amplitude is one-half the wave height.

ASTRONOMICAL TIDE
The tidal levels and character which would result from gravitational effects, e.g. of the Earth, Sun and Moon, without any atmospheric influences.

ATTENUATION
(1) A lessening of the amplitude of a wave with distance from the origin. (2) The decrease of water-particle motion with increasing depth. Particle motion resulting from surface oscillatory waves attenuates rapidly with depth, and practically disappears at a depth equal to a surface wavelength.


Bback to top

BAR
A submerged or emerged embankment of sand, gravel, or other unconsolidated material built on the sea floor in shallow water by waves and currents.

BASIN, BOAT
A naturally or artificially enclosed or nearly enclosed harbor area for small craft.

BASIN SURGE, ENERGY BASIN
SURGE that occurs within a partially enclosed area such as a man-made harbour or marina.

BATHYMETRY
The measurement of depths of water in oceans, seas, and lakes; also information derived from such measurements.

BIGHT
A bend in a coastline forming an open bay. A bay formed by such a bend.

BOUNDARY CURRENT
A deep ocean current, especially along the western part of the oceans, characterized by sudden changes in temperature and salinity.

BUOY
A float; especially a floating object moored to the bottom. CDIP's buoys are equipped with sensors so that, in addition to floating, they can measure climatological variables such as wave height, swell direction and water temperature.


Cback to top

CAPILLARY WAVE
A wave whose velocity of propagation is controlled primarily by the surface tension of the liquid in which the wave is traveling. Water waves of length less than about 2.5 cm are considered capillary waves. Waves longer than 2.5 cm and shorter than 5 cm are in an indeterminate zone between capillary and gravity waves.

CHOP
The short-crested waves that may spring up quickly in a moderate breeze, and which break easily at the crest. Also known as WIND CHOP.

CONDUCTIVITY
A measure of the amount of salts dissolved in water.

CREST OF WAVE
(1) the highest part of a wave. (2) That part of the wave above still-water level.

CURRENT, LONGSHORE
The littoral current in the breaker zone moving essentially parallel to the shore, usually generated by waves breaking at an angle to the shoreline.

CURRENT METER
Instrument for measuring the velocity of a current.


Dback to top

DECAY OF WAVES
The change waves undergo after they leave a generating area (FETCH) and pass through a calm, or region of lighter winds. In the process of decay, the significant wave height decreases and the significant wavelength increases.

DEEP WATER
Water so deep that surface waves are little affected by the ocean bottom. Generally, water deeper than one-half the surface wavelength is considered deep water. Compare SHALLOW WATER.

DEEP WATER WAVES
A wave in water the depth of which is greater than one-half the WAVELENGTH.

DEPTH
The vertical distance from a specified datum to the sea floor.

DIFFRACTION OF WAVES
The phenomenon by which energy is transmitted laterally along a wave crest. When a part of a train of waves is interrupted by a barrier, such as a breakwater, the effect of diffraction is manifested by propagation of waves into the sheltered region within the barrier's geometric shadow.

DIURNAL TIDE
When there is only one high water (flood) and one low water (ebb) in each tidal day.

DOWNWELLING
Downward movement of surface water caused by onshore Ekman transport, converging currents, or when a water mass becomes more dense than the surrounding water.

DP
See PEAK DIRECTION.

DREDGING
Practice of excavating or displacing bottom sediment from one location to another in order to maintain water depth, or aid in beach restoration, construction, flood management and erosion control.


Eback to top

ECHO SOUNDER
Electronic instrument used to determine the water depth by measuring the time interval between the emission of a sonic or ultrasonic signal and the return of its echo from the bottom.

EKMAN TRANSPORT
Resultant flow at right angles to and to the right of the wind direction (in the northern hemisphere) referred to as UPWELLING and DOWNWELLING.

EL NINO
Warm equatorial water which flows southward along the coast of Peru and Ecuador during February and March of certain years. It is caused by poleward motions of air and unusual water temperature patterns in the Pacific Ocean, which cause coastal downwelling, leading to the reversal in the normal north-flowing cold coastal currents. During many El Nino years, storms, rainfall, and other meteorological phenomena in the Western Hemisphere are measurably different than during non-El Nino years.

ESTUARY
Region near a river mouth in which the fresh water of the river mixes with the salt water of the sea, and which receives both fluvial and littoral sediment influx.

EULARIAN MEASUREMENT
Observation of a current with a device fixed relative to the flow.


Fback to top

FATHOMETER
Copyrighted trademark for a type of ECHO SOUNDER.

FETCH
The area in which SEAS are generated by a wind having a fairly constant direction and speed.

FOAM LINE
(1) The front of a wave as it advances shoreward, after it has broken. (2) Lines of foam such as those which move around the head of a rip current.

FORECASTING OF WAVES
The theoretical determination of future wave characteristics, usually from observed or predicted meteorological phenomena.

FULLY-DEVELOPED SEA
The waves that form when wind blows for a sufficient period of time across the open ocean. The waves of a fully developed sea have the maximum height possible for a given wind speed, FETCH and duration of wind.


Gback to top

GAUGE (GAGE)
Instrument for measuring the water level relative to a datum.

GEOGRAPHIC INFORMATION SYSTEM (GIS)
Database of information which is geographically referenced, usually with an associated visualization system.

GLOBAL POSITIONING SYSTEM (GPS)
A navigational and positioning system developed by the U.S. Department of Defense, by which the location of a position on or above the Earth can be determined by a special receiver at that point interpreting signals received simultaneously from several of a constellation of special satellites.

GRAVITY WAVE
A wave whose velocity of propagation is controlled primarily by gravity. Water waves more than 5 cm long are considered gravity waves. Waves longer than 2.5 cm and shorter than 5 cm are in an indeterminate zone between CAPILLARY and GRAVITY WAVES.


Hback to top

HARBOR OSCILLATION (HARBOR SURGING)
The nontidal vertical water movement in a harbor or bay. Usually the vertical motions are low; but when oscillations are excited by a tsunami or storm surge, they may be quite large. Variable winds, air oscillations, or SURF BEAT also may cause oscillations. See SEICHE.

HEAVE
(1) The vertical rise or fall of the waves or the sea. (2) The translational movement of a craft parallel to its vertical axis. (3) The net transport of a floating body resulting from wave action.

HIGH TIDE, HIGH WATER (HW)
Maximum height reached by a rising tide. The height may be solely due to the periodic tidal forces or it may have superimposed upon it the effects of prevailing meteorological conditions. Nontechnically, also called the HIGH TIDE.

HINDCASTING
In wave prediction, the retrospective forecasting of waves using measured wind information.

HS
See SIGNIFICANT WAVE HEIGHT.

HYDROGRAPHY
The description and study of seas, lakes, rivers and other waters and the description of the physical properties of those waters; also the science of locating aids and dangers to navigation.


Iback to top

INCIDENT WAVE
Wave moving landward.

INCLINATION, MAGNETIC
The angle that the geomagnetic field is tilted with respect to the surface of the earth. Magnetic inclination varies from 90 degrees (perpendicular to the surface) at the magnetic poles to 0 (parallel to the surface) at the magnetic equator.

INFRAGRAVITY WAVE, LONG WAVES
Long waves with periods of 30 seconds to several minutes.

IRREGULAR WAVES
Waves with random wave periods (and in practice, also heights), which are typical for natural wind-induced waves.


Jback to top

JETTY
On open seacoasts, a structure extending into a body of water, which is designed to prevent shoaling of a chanel by littoral materials and to direct and confine the stream or tidal flow. Jetties are built at the mouths of rivers or tidal inlets to help deepen and stabilize a chanel.


Kback to top

KURTOSIS
The quality, state or condition of peakedness or flatness of the graphic representation of a statistical distribution; or the measure of the peakedness of a frequency distribution.


Lback to top

LAGRANGIAN MEASUREMENT
Observation of a current with a device flowing with the current.

LONGSHORE
Parallel to and near the shoreline.

LONGSHORE BAR
A sand ridge or ridges, runing roughly parallel to the shoreline and extending along the shore outside the trough, that may be exposed at low tide or may occur below the water level in the offshore.

LOW TIDE (LOW WATER, LW)
The minimum elevation reached by each falling tide. See TIDE.


Mback to top

MEAN LOWER LOW WATER (MLLW)
The average height of the lower low waters over a 19-year period. For shorter periods of observations, corrections are applied to eliminate known variations and reduce the results to the equivalent of a mean 19-year value. Frequently abbreviated to LOWER LOW WATER.

MEAN SEA LEVEL
The average height of the surface of the sea for all stages of the tide over a 19-year period, usually determined from hourly height readings.

MEAN WAVE HEIGHT
The mean of all individual waves in an observation interval of approximately half an hour. In case of a Rayleigh-distribution 63% of the SIGNIFICANT WAVE HEIGHT.

MONOCHROMATIC WAVES
A series of waves generated in a laboratory, each of which has the same length and period.


Nback to top

NAUTICAL MILE
The length of a minute of arc, 1/21,600 of an average great circle of the Earth. Generally one minute of latitude is considered equal to one nautical mile. The accepted United States value as of 1 July 1959 is 1,852 meters (6,076.115 feet), approximately 1.15 times as long as the U.S. statute mile of 5,280 feet.

NEARSHORE
(1) In beach terminology an indefinite zone extending seaward from the shoreline well beyond the breaker or surf zone. (2) The zone which extends from the SWASH zone to the position marking the start of the offshore zone, typically at water depths of the order of 20 m.


Oback to top

OCEANOGRAPHY
The study of the sea, embracing and indicating all knowledge pertaining to the sea's physical boundaries, the chemistry and physics of seawater, marine biology, and marine geology.


Pback to top

PEAK DIRECTION
The WAVE DIRECTION at the frequency at which a wave energy spectrum (WAVE SPECTRUM) reaches its maximum.

PEAK PERIOD
The WAVE PERIOD determined by the inverse of the frequency at which a wave energy spectrum (WAVE SPECTRUM) reaches its maximum.

PIER
A structure, usually of open construction, extending out into the water from the shore, to serve as a landing place, recreational facility, etc., rather than to afford coastal protection. In the Great Lakes, a term sometimes improperly applied to jetties.

PROPAGATION OF WAVES
The transmission of waves through water.


Rback to top

RANDOM WAVES
The laboratory simulation of irregular sea states that occur in nature.

REFLECTED WAVE
That part of an incident wave that is returned seaward when a wave impinges on a steep beach, barrier, or other reflecting surface.

REFLECTION
The process by which the energy of the wave is returned seaward.

REFRACTION OF WAVES
(1) The process by which the direction of a wave moving in shallow water at an angle to the contours is changed: the part of the wave advancing in shallower water moves more slowly than that part still advancing in deeper water, causing the wave crest to bend toward alinement with the underwater contours. (2) The bending of wave crests by currents.

REGULAR WAVES
Waves with a single height, period, and direction.


Sback to top

SALINITY
Number of grams of salt per thousand grams of sea water, usually expressed in parts per thousand. Ocean water averages 35 parts per thousand.

SEA, SEAS
Waves caused by wind at the place and time of observation. Seas refer to waves that are actively growing.

SEA STATE
Description of the sea surface with regard to wave action.

SEA SURFACE TEMPERATURE
The temperature of water at or near the surface of the sea.

SECCHI DISK
Visibilty disk used to measure the transparency of the water.

SEICHE
(1) A standing wave oscillation of an enclosed waterbody that continues, pendulum fashion, after the cessation of the originating force, which may have been either seismic or atmospheric. (2) An oscillation of a fluid body in response to a disturbing force having the same frequency as the natural frequency of the fluid system. Tides are now considered to be seiches induced primarily by the periodic forces caused by the Sun and Moon. (3) In the Great Lakes area, any sudden rise in the water of a harbor or a lake whether or not it is oscillatory (although inaccurate in a strict sense, this usage is well established in the Great Lakes area).

SEMIDIURNAL TIDE
Tidal current is said to be semidiurnal when there are two flood (high water) and two ebb (low water) periods in each tidal day, with each high/low cycle lasting approximately 12.4 hours. This is the predominant type of tide found throughout the world.

SETUP, WAVE
Superelevation of the water surface over normal surge elevation due to onshore mass transport of the water by wave action alone.

SHALLOW WATER
(1) Commonly, water of such a depth that surface waves are noticeably affected by bottom topography. It is customary to consider water of depths less than one-half the surface wavelength as shallow water. (2) More strictly, in hydrodynamics with regard to progressive gravity waves, water in which the depth is less than 1/25 the wavelength.

SHORE NORMAL
A line at right-angles to the bottom contours in the surf zone.

SIGNIFICANT WAVE
A statistical term relating to the one-third highest waves of a given wave group and defined by the average of their heights and periods. Experience indicates that a careful observer who attempts to establish the character of the waves will record values which approximately fit the definition of the significant wave.

SIGNIFICANT WAVE HEIGHT
The average height of the one-third highest waves of a given wave group or sample. In spectral analyses (like those applied by CDIP), significant wave height is often estimated as Hmo, 4 times the square root of the total energy ( Hm0 = 4(m0)^.5 ).

SKEWNESS
The quality, state, or condition of being distorted or lacking symmetry; the quality or state of asymmetry shown by a frequency distribution that is bunched on one side of the average and tails out on the other side. It results from lack of coincidence of the mode, median, and arithmetic mean of the distribution.

Also a measure of asymmetry of a frequency distribution; specifically the quotient of the difference between the arithmetic mean and the mode divided by the standard deviation. Positive skewness is defined for the longer slope of the plotted distribution in the direction of increasing variate values (mean greater than mode, or coarser particles exceed finer particles in a particle-size distribution); negative skewness is defined for the longer slope of the plotted distribution in the direction of decreasing variate values (mode greater than mean, or finer particles exceed coarser particles in a particle-size distribution).

SONAR
Acronym for sound navigation and ranging, a method used in oceanography to study the ocean floor.

SST
See SEA SURFACE TEMPERATURE.

STORM SURGE
A rise above normal water level on the open coast due to the action of wind stress on the water surface. Storm surge resulting from a hurricane also includes that rise in level due to atmospheric pressure reduction as well as that due to wind stress.

SURF
(1) Breaking waves near the shore. (2) The wave activity in the area between the shoreline and the outermost limit of breakers.

SURF BEAT
Irregular oscillations of the nearshore water level with periods on the order of several minutes.

SURF ZONE
The zone of wave action extending from the water line (which varies with tide, surge, etc.) out to the most seaward point of the zone (breaker zone) at which waves approaching the coastline commence breaking, typically in water depths of between 5 to 10 meters.

SURFACE GRAVITY WAVE (PROGRESSIVE)
(1) The term which applies to the WIND WAVES and SWELL of lakes and oceans, also called a SURFACE WATER WAVE, SURFACE WAVE or DEEP WATER WAVE. (2) A progressive GRAVITY WAVE in which the disturbance is confined to the upper limits of a body of water. Strictly speaking this term applies to those progressive GRAVITY WAVES whose velocity depends only upon the wavelength.

SURGE
(1) The name applied to wave motion with a period intermediate between that of the ordinary wind wave and that of the tide, say from 2 to 60 min. It is low height, usually less than 0.9 m (3 ft). See also SEICHE. (2) see STORM SURGE.

SWASH
The rush of water up onto the beach face following the breaking of a wave.

SWELL
Wind-generated waves that are propagating away or are outside their generating area. Swell characteristically exhibits a more regular and longer period and has flatter crests than waves within their fetch (SEAS).


Tback to top

THERMOCLINE
A layer in which the temperature decreases significantly (relative to the layers above and below) with depth. The principal ones are designated diurnal, seasonal, and main thermocline.

TIDE
The periodic rising and falling of the water that results from gravitational attraction of the Moon and Sun and other astronomical bodies acting upon the rotating Earth. Although the accompanying horizontal movement of the water resulting from the same cause is also sometimes called the tide, it is preferable to designate the latter as TIDAL CURRENT, reserving the name TIDE for the vertical movement.

TP
See PEAK PERIOD.

TRANSECT
A sample area, cross section, or line chosen as the basis for studying one or more characteristics of a particular assemblage.

TROUGH OF WAVE
The lowest part of a waveform between successive crests. Also, that part of a wave below still-water level.

TSUNAMI
A long-period wave caused by an underwater disturbance such as a volcanic eruption or earthquake. Also SEISMIC SEA WAVE. Commonly miscalled "tidal wave."

TURBIDITY
A condition of a liquid due to fine visible material in suspension which may not be of sufficient size to be seen as individual particles by the naked eye but which prevents the passage of light through the liquid; also the measure of fine suspended matter in liquids.

TURBULENCE
The irregular, random velocity fluctuations within a flowing liquid.


Uback to top

UPWELLING
The process by which water rises from a deeper to a shallower depth, usually as a result of offshore surface water flow. It is most prominent where persistent wind blows parallel to a coastline so that the resultant Ekman transport moves surface water away from the coast.


Vback to top

VELOCITY OF WAVES
The speed at which an individual wave advances.


Wback to top

WATER DEPTH
Distance between the seabed and the still water level.

WAVE
A ridge, deformation, or undulation of the surface of a liquid.

WAVE CLIMATE
The seasonal and anual distribution of wave height, period and direction.

WAVE DIRECTION
The direction from which a wave approaches.

WAVE FREQUENCY
The inverse of wave period.

WAVE HEIGHT
The vertical distance between a crest and the preceding trough. See also SIGNIFICANT WAVE HEIGHT.

WAVE PERIOD
The time for a wave crest to traverse a distance equal to one wavelength. The time for two successive wave crests to pass a fixed point.

WAVE ROSE
Diagram showing the long-term distribution of wave height and direction.

WAVE SPECTRUM
In ocean wave studies, a graph, table, or mathematical equation showing the distribution of wave energy as a function of wave frequency. The spectrum may be based on observations or theoretical considerations. Several forms of graphical display are widely used.

WAVE STEEPNESS
The ratio or wave height to wavelength also known as sea steepness.

WAVE TRAIN
A series of waves from the same direction.

WAVELENGTH
The horizontal distance between similar points on two successive waves measured perpendicular to the crest.

WIND ROSE
Diagram showing the long-term distribution of wind speed and direction.

WIND SEA
Wave conditions directly attributable to recent winds, as opposed to swell.

WIND WAVES
(1) Waves being locally formed and built up by the wind; SEAS. (2) Loosely, any wave generated by wind.

Frequently Asked Questions: General Queries



What do Hs values represent?

The wave height (Hs) represents a 30-minute average of the 1/3 highest waves at a sensor. The height is the distance between the trough and the crest of the wave. Statistically, the highest wave during the measurement period is likely to be approximately twice the reported wave height (1.8*Hs).

What are Tp and Dp? Why are there different values given in different products?

The period of waves is the time it takes two consecutive crests to pass a single spot, and the direction is the compass angle (0-360 degrees clockwise from true North) that the waves are coming from. In the ocean, however, no two waves are perfectly identical - they are constantly coming from different directions at different frequencies.

Since there is never just a single direction or period for ocean waves, we can only measure the peak period (Tp) and peak direction (Dp). The peak period is the most common period between consecutive waves, while the peak direction is the most common direction. To come up with these values, all of the wave energy for a station over a specified period of time - approximately 30 minutes, in most cases - is grouped into different bands. For instance, in CDIP's 9-band products, all waves with periods from 6 to 8 seconds go into the 7-second band, those from 8 to 10 secs go into the 9-second band, etc. After all of the wave energy has been divided into these bands, the band with the most energy is selected as the peak band.

The cutoffs between bands, however, are arbitrary, and changing the cutoffs and/or number of bands affects the resulting Tp and Dp values. At CDIP we use two formats for analysis, 9-band and parameter. In the the 9-band products, all of the wave energy is divided into 9 broad bands. In the parameter products, the energy is split into 64 (or 128) narrow bands. As a result, the 9-band Tp and Dp give more general, broad values, while the parameter Tp and Dp identify finer, subtler peaks.

So which Tp/Dp values are better? It depends what you're looking for. For instance, the broad bands of the 9-band values are better for addressing general questions about the sea state (e.g. which is currently predominant - local seas or ground swell?). To pick up more subtle features - like the arrival of long-period swell from a distant storm - the parameters values may be more helpful.

Although the wave height is staying pretty stable, the peak period and peak direction are jumping all around. Which of those Tp and Dp values are really correct?

Over any given period, the waves at a point in the ocean are actually coming from a number of different sources. There may be short-period waves from local winds, long-period waves from one or more distant storms, and a range of other wave fields originating from different weather systems. The peak period and peak direction describe the strongest of all the sources of wave energy. When one source becomes stronger than another, the Tp and Dp values can suddenly change dramatically. And when two sources of wave energy have near-equal strength, the Tp and Dp values may bounce back and forth between them.

For instance, in summer on the US west coast we often have long-period swell from large storms in the southern hemisphere and short-period seas from local winds off the coast. Whenever the south swell is the stronger the peak period could be 18 seconds and the peak direction 180 degrees. Whenever the seas are a bit stronger, the Tp could be 5 seconds and the Dp 290 degrees. And if these two sources have near equal energy, we may find that from hour to hour the Tp and Dp values jump up and down repeatedly, showing whichever of the two sources is briefly the more energetic.

Are your sea temperatures correct? At the beach the lifeguards are reporting 63F, but the buoy says 70F!

There are often big differences between inshore and offshore temperatures due to various phenomena. In the surfzone, for instance, mixing may result in temperatures much colder than in calm surface waters offshore. Our buoys are all located offshore and measure sea surface temperature using a sensor located about 18 inches directly below the buoy.

What causes wave sets?

Very often waves from different parts of a distant storm arrive here at the same time. Their period (or lengths) can be nearly the same size. For a while the crests of the two wave trains begin to coincide. The two trains adding together result in waves that grow bigger. Later, the troughs of one begins to coincide with the crests of the other. The combination wave grows smaller. We see this as a "set" of large waves, followed by an interval of smaller waves. See figure below.

Why are wave heights sometimes underestimated on windy days?

Local winds generate very short-period, high frequency waves, and not all of CDIP's instruments can measure these waves effectively. This is especially true of pressure sensors postioned deep underwater. At Kings Bay, for example, the sensors are mounted at a depth of 55 feet. Due to attenuation, these sensors do not feel high frequency waves, and so on windy days the high frequency energy will not be reflected in the reported Hs.

Why are the times on your buoy data often an hour or more old? Can't the data be more up-to-date?

To determine the wave conditions, you can't simply look at the ocean for a few seconds. Instead, you need to sample data over a long period. For most of CDIP's wave calculations, a data sample of approximately 30 minutes is used. And unlike the NDBC and other data providers who use sample end times on their spectra, CDIP uses sample start times.

This means that when we update our site based on a data sample that ended just a few minutes ago, the time assigned to the data - the start time - will already be more than 30 minutes old. For Datawell buoys, at the end of a 30-minute sampling period, the sensor calculates a wave spectrum and starts to transmit it; it's repeatedly sent in 4-minute blocks over the next half-hour.

This means that the when a CDIP station updates, the new spectrum will have a time that is at least 35 or 40 minutes old (30 min sample + 4 min transmission + a few minutes to process). But sometimes newly-updated data may have a start time that is up to 1 hour and 10 minutes old; it simply depends on which point in the half-hour cycle the buoy is contacted. (I.e. CDIP only grabs the data from stations once each half-hour, and it may happen that we're grabbing the spectrum near the end of its transmission cycle.)

Is the wave direction from the SF/IB Nearshore Buoy correct? All the buoys show a northwest swell, but that buoy is reporting the same swell from the west-southwest!

When looking at buoy data, it's important to distinguish between deep-water measurements and shallow-water measurements. In deep water, swell direction is primarily determined by the location of the fetch that produced the swell. So deep-water buoys on the West Coast register swells from the Gulf of Alaska as NW swells, and swells from the South Pacific as S or SW swells.

In shallow water, on the other hand, the swell direction is determined primarily by the local bathymetery; swell is refracted such that wave crests approach the coast parallel to shore. I.e. whether it's a NW swell or S swell, at your local beach the waves always come in and line up at nearly the same angle, with only slight shifts to the N or S.

For example, the San Francisco Bar buoy is a shallow-water buoy at a location where the shore normal points to the WSW. Thus all long-period swell, regardless of source, has been refracted to the WSW at that spot. So the readings you see are correct given the water depth (15m) at the buoy's location.

Sometimes the swell and wind wave heights reported on the NDBC site for CDIP buoys aren't consistent with neighboring NDBC buoys. The CDIP buoys report larger wind waves, while the NDBC buoys report more swell. Why is this the case?

The relative amounts of swell and wind waves (or seas) reported for a buoy depend upon the cutoff period used to distinguish swell from seas. NDBC buoys include anemometers and use wind measurements to determine the sea/swell cutoff period based on current conditions. CDIP buoys, on the other hand, do not measure winds, and instead of a dynamic sea/swell cutoff they use a fixed 10-second cutoff when reporting swell and wind wave measurements to NDBC.

Sometimes there are significant amounts of wave energy with periods in the 8 to 10 second range. Depending on wind conditions, NDBC may report this energy as swell, whereas CDIP buoys will always report it as wind waves. Hence the NDBC reports will show more swell, and the CDIP reports will show more wind waves. By looking at the swell and wind wave periods, however, it should be clear when this sort of discrepancy occurs; the NDBC buoys will be reporting a low swell period, one that falls below CDIP's 10-second cutoff.

For a more in depth discussion of wave measurements and standards, we recommend the Coastal Engineering Manual (CEM), published by the United States Army Corps of Engineers' Coastal and Hydraulics Laboratory.

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