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New South Wales Wave Climate Glossary

Index


Average Hsig

Average significant wave height recorded during a storm event.

Average TP1

Average spectral peak period recorded during a storm event.

Average Tsig

Average significant wave period recorded during a storm event.

Average Wave Power

Average wave power level recorded during a storm event.

Bore

A broken swell wave travelling shorewards across the surf zone.

Breaker Index

The ratio between the height of a wave and the water depth in which the wave breaks.

Breaker Zone

That area of coastal waters where shoaling effects cause swell waves to break. This typically occurs in the shallow waters over
an offshore bar.

Breaking Waves

As waves increase in height through the shoaling process, the crest of the wave tends to speed up relative to the rest of the wave. Waves break when the speed of the crest exceeds the speed of the advance of the wave as a whole. Waves can break in three modes: spilling, surging and plunging.

Correlation Coefficient

Measurement (between -1 and 1) of the quality of fit of a line through a set of data points. The closer the number to +-1.0 the better the fit.

Data Capture or Data Recovery

Number of records collected divided by total number of possible records. Normally expressed as a percentage.

Date / Time

For start of record.

Deepwater

Water sufficiently deep that surface waves are little affected by the ocean bottom. Generally, water deeper than one-half the surface wave length is considered deepwater.

Design Wave Height

The wave height adopted for the purposes of designing coastal structures such as breakwaters and seawalls. It is chosen to ensure that the structures are not at undue risk of wave damage.

Diffraction

The "spreading" of waves into the lee of obstacles such as breakwaters by the transfer of wave energy along wave crests. Diffracted waves are lower in height than incident waves.

Directional Waverider Buoy

A floating device used to measure ocean wave height, period and direction. It is a registered trademark of the Dutch company Datawell.

E

Normalised Spectral Estimate.

Effective Record Length

Total record length multiplied by the data capture rate.

Electromagnetic Current Meter

A device which measures current and water pressure variations. If deployed in shallow water current and pressure data can be converted to wave height, period and direction. Current meters manufactured by the American companies Marsh McBirney and InterOcean are used by MHL to collect wave data.

ERR

Number of corrections or `patches' in a record.

Fetch

The horizontal distance over which a wind blows in generating waves.

Filtered Data

 Raw data modified to remove wind waves, tide or some other frequency components.

Hindcast

 The prediction of wave characteristics using meteorological information as opposed to the measurements of these features.

H1

Average height of the waves that comprise the top 1%.

H10

Average height of the waves which comprise the top 10%.

Hmax

Maximum wave height in a recorded burst of raw data.

Hmean

Mean wave height.

Hrms

Root mean square wave height.

Hsig

Significant wave height = average height of the waves which comprise the highest 33% of waves in a given sample period (typically 20 to 30 minutes).

LEN

Accepted record duration (normally in seconds).

Logger

Device for recording digitised data.

Long Wave

 Waves with periods greater than 30 seconds. Often associated with storm wave activity along the NSW coast.

M0, M1, M2, M3

 Spectral Moments. These provide parameters describing the shape of the spectrum.

Nearshore Zone

Coastal waters between the offshore bar and the 60m depth contour. Swell waves in the nearshore zone are unbroken, but their behaviour is influenced by the presence of the seabed.

NPTS

Number of sample points in a record.

Offshore Bar

Also known as a longshore bar. Submerged sandbar formed offshore by the processes of beach erosion and accretion. Typically, swell waves break on the offshore bar.

Offshore Zone

 Coastal waters to the seaward of the nearshore zone. Swell waves in the offshore zone are unbroken and their behaviour is not influenced by the presence of the seabed.

P2P1 or SP2SP1

Ratio of second highest spectral peak to the highest.

Peak Hmax

Highest maximum wave height recorded during a storm event.

Peak Hsig

Highest significant wave height recorded during a storm event.

Peak Wave Power

Maximum wave power level recorded during a storm event.

Percentage Exceedance

Percentage of time that a given value is exceeded.

Percentage Occurrence

Percentage of time that given value (or range of values) occurs.

Plunging Waves

The wave crest breaks suddenly and with tremendous force by curling over a near vertical wave face.

Record

Burst of data from the wave measuring device (usually 2048 seconds).

Record Interval

Time between records (usually 1 hour). Prior to June 1984 the standard for wave data collection at MHL was 6 hours.

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.

Refraction

The tendency of wave crests to become parallel to bottom contours as waves move into shallower waters. This effect is caused by the shoaling process which slows down waves in shallower waters.

Return Period

Expected average interval between the occurrences of events at a particular threshold.

Sample Increment

Time between sample points measured by the transducer. Sample points are typically spaced at 0.5 second intervals for ocean wave measurement.

S.E.

Standard Error = rms deviation of data points from a fitted line.

Sea Waves

Waves in coastal waters resulting from the interaction of different wave trains and locally generated waves. Typically, sea waves are of short wave length and of disordered appearance.

Shallow Water

Water of such a depth that surface waves are noticeably affected by bottom topography. Generally, water depth less than
one-half the surface wave length is considered shallow water.

Shelf Wave

Long period waves of low height that travel along the continental shelf and may modify coastal water levels off New South Wales by up to 0.2m. Shelf waves are generated by the pressure gradients associated with atmospheric disturbances in Bass Strait.

Shoaling

The influence of the seabed on wave behaviour. Such effects only become significant in water depths of 60m or less. Manifested as a reduction in wave speed, a shortening in wave length and an increase in wave height.

Storm Event

Period of high wave activity. For the NSW coastline is normally defined as the time when a Hsig greater than 3 metres is recorded at an offshore wave recording station.

Spilling Waves

The wave crest breaks gradually as the wave travels to the shore. Characterised by the appearance of white water at the crest.

Surf Beat

Periodic rise and fall in coastal water levels caused by two or more wave trains arriving at the shoreline.

Surf Zone

Coastal waters between the breaker zone and the swash zone characterised by broken swell waves moving shorewards in the form of bores.

Surging Waves

The wave does not "break" but maintains its basic shape as it moves towards the shore, where it surges up the beach. Very little white water is evident before surging waves reach the shore.

Swash Zone

That area of the shoreline characterised by regularly spaced wave crests.

Swell Waves

Wind waves remote from the area of generation (fetch) having a uniform and orderly appearance characterised by regularly spaced wave crests.

Total Record Length

Elapsed period from the date of commission to the end of data collection at a recording site.

Tc

Crest period = average time between successive crests.

TP1

Period of the peak of the energy spectrum.

TP2

Period corresponding to the second biggest peak of the energy spectrum.

Tsig

Significant period = average period of the waves used to define Hsig.

Tsunami

Long period ocean waves generated by geological and tectonic disturbances below sea level. Incorrectly referred to as "tidal waves", tsunami travel at speeds of up to 800 km/h in the open ocean, where they are of low height. However, tsunami can rise to a height of 10m or more through the shoaling process as they approach land.

Tz

Zero crossing period = mean period.

Wave Direction

The direction from which ocean waves approach a location. Generally, the principal wave direction is represented by the direction which corresponds to the peak period of the energy spectrum (TP1).

Wave Height

The vertical distance between a wave drest and the next trough.

Wave Length

The distance between consecutive wave crests or wave troughs.

Wave Period

The time taken for consecutive wave crests or wave troughs to pass a given point.

Wave Power

The rate at which wave energy is transmitted in the direction of wave propagation. Normally expressed in kilowatts per metre of wave crest length.

Waverider Buoy

A floating device used to measure water level variations caused by ocean waves. It is a registered trademark of the Dutch company Datawell.

Wave Runup

The vertical distance above mean water level reached by the uprush of water from waves across a beach or up a structure.

Wave Setup

The increase in water level within the surf zone above mean still water level caused by the breaking action of waves.

Wave Train

A series of waves originating from the same fetch with similar wave characteristics.

Wind Setup

The increase in mean sea level caused by the "piling up" of water on the coastline by wind.

Wind Waves

The waves initially formed by the action of wind blowing over the sea surface. Wind waves are characterised by a range of heights, periods and wave lengths. As they leave the area of generation (fetch), wind waves develop a more ordered and uniform appearance and are referred to as swell or swell waves.

Yrms

Root mean square amplitude (not to be confused with Hrms).

 

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