Definitions

Acoustic

Having to do with sound.

Acoustic beam

Wave of sound.

Acoustic backscatterance sensor (ABS)

Instrument that measures the suspended sediment concentration by emitting a beam of sound, and them detecting how much of that beam is scattered back to the instrument.

Andesitic

A type of volcanic rock of intermediate composition. (Mt Taranaki is an andesitic volcano.)

Aspect ratio

Ratio between height and width.

Baroclinic

The model allows for currents to be driven by density, temperature and salinity differences in the water.

Barometric pressure

Air pressure at sea level. In the SI system, it is measured in hectoPascals, hPa (and is the same as the old millibar (mb) unit).

Bathymetry

Shape of the ocean floor in terms of a map of its depth. Click here for a picture of the bathymetry in the region around New Zealand.

Beating

Occurs when two signals of similar period interact with each other. The largest response occurs when both signals are exactly in phase and the smallest response when they are out of phase.

Bedform

Shape in the sediment, such as a ripple.

Bedload

Sediment that is transported very close to the bottom by saltation (bouncing along).

Boundary layer

Layer close to the seabed, where friction causes the current velocities to be deformed by the proximity to the bed.

Break-off range

Ripples that occur when the wave orbital excursion and ripple spacing uncouple and ripple steepness decreases through a decrease in ripple height.

Coastal flooding

Product of tide and storm surge or flooding. Storm surge and flood events are stochastic and can therefore occur at any time and be unknown in magnitude. Tides on the other hand are deterministic. As a result we can predict their occurance and magnitude.

Coastal hazard zone

Designated planning zone including the upper beach and coastal dunes, where no new dwellings are permitted because of a defined risk of damage from coastal storm erosion, tsunami and rising sea levels.

Coastal margin

Relatively narrow strip of water adjoining the shoreline (mean high water spring) and out to approximately 10-20 m depth where wave processes usually dominate sediment transport. Includes the surf zone, nearshore bar and the shoreface.

Compound tide

A constituent arising from the interaction between other primary constituents; e.g., MNS₆ is the constituent arising from the interaction between M₂, N₂ and S₂ constituents. Compound tides occur when the tide enters large expanses of shallow water (e.g., Tasman Bay).

Continental shelf

Relatively shallow, often broad, region adjacent to the coastline out to a depth of around 200-500 m. The outer edge of the continental shelf is called the shelf break or continental slope and it is often defined as the position where the depth changes over a short distance from a few hundred metres to a thousand metres or more. Click here for a picture of New Zealand’s continental shelf (light blue area surrounding the dark land mass).

Cyclone

Intense storm generated in tropical regions. Cyclones (also known as hurricanes and typhoons) in the Southern Hemisphere have winds that rotate clockwise around a central low pressure region. As a cyclone propagates south from the tropics into higher latitudes it often weakens into a tropical storm, which is much less severe. Cyclones seldom propagate below Cook Strait.

Darwin symbols

Were assigned by G.H. Darwin (an uncle of Charles Darwin of HMS Beagle) in the mid-1800s. They comprise one or more letters (Roman or Greek) and a subscript. The letters often denote the origin of the constituent and the subscript denotes the approximate period. For example, S₂ is the solar semi-diurnal constituent (S for Sun, 2 for twice daily); M₄ is the ter-diurnal (4 times daily) lunar constituent, called an over-tide; MNS₆ is the constituent (called a compound tide) arising from the interaction between M₂, N₂ and S₂ constituents.

Diurnal tides

Tidal constituents which occur once daily, i.e., they have a tidal period of approximately 24 hours, e.g., the solar-lunar constituent K₁ (23.93 hour period).

EEZ

Exclusive Economic Zone, the area including all marine waters from the outer edge of the Territorial Sea (12 nautical miles) seaward for 188 nautical miles until the 200 nautical mile limit (370 km).

El Niño-Southern Oscillation (ENSO)

A natural global climate phenomenon involving the interaction between the tropical Pacific and the atmosphere, but has far-reaching effects on the global climate, especially for countries in the Pacific rim. ENSO is the strongest climate signal on time scales of one to several years. The quasi-periodic cycle oscillates between El Niño (unusually warm ocean waters along the tropical South American coast) and La Niña (colder-than-normal ocean waters off South America). To learn more about ENSO, click here for a link to the Climate Variability and Change page.

Equilibrium range

Ripples that have maximum steepness and where the ripple length scales to the water movement due to the waves above.

Finite difference

Numerical model which calculates currents by approximating the ocean as a series of cubes. All the cubes are the same size and currents are calculated by assuming mass and momentum are conserved between each of these boxes.

Fluorometry

Measuring the degree of fluorescence of a substance.

Grades

Particle-size distributiuon.

Hydraulic roughness

Level at which the current goes to zero on a rough bottom.

Intermediate

Classification of beaches ranges from reflective (such as the steep and gravelly beaches south of Banks Pennisula) to dissipative (such as the flat and fine-sand beach at Muriwai). Intermediate is somewhere in the middle.

Inverted barometer

Static response of the ocean to changing barometric pressure in which 1 hectoPascal rise in pressure causes a 1 cm drop in sea level and vice versa.

Mathematical model

Mathematical representation (usually in a computer) of a physical situation. All models used in this programme are hydrodynamic models, which means they are based on the laws of Newton (conservation of mass and conservation of momentum).

Model calibration

Altering model tuning parameters, e.g. the degree of sea bed “roughness”, until the best match is obtained at specific sites between model output and observations such as data from sea-level gauges.

Model grid

Mathematical representation of the geometry and bathymetry of the ocean using particular geometrical shapes called elements. In this programme we use triangles. The corners of the triangles are called nodes and at each of these we specify a depth. At each of the nodes the mathematical model calculates sea level and southerly and westerly currents.

Native sand

Locally derived sand.

NIWA Sea-level Network

Network of ten open-coast sea-level gauges around the New Zealand coast co-ordinated by NIWA, with assistance from Canterbury and Auckland Regional Councils, National Tidal Facility (Adelaide) and University of Canterbury.
For more details contact Rob Bell [ r.bell@niwa.co.nz ].

Optical backscatterance sensors (OBS)

Instrument that measures the suspended sediment concentration by emitting a beam of light, and detecting how much of that beam is scattered back to the instrument.

Oscillatory

Movement back and forth.

Over-tide

A constituent arising from the interaction of a primary tide with itself; e.g., M₄ is the ter-diurnal (4 times daily) lunar constituent resulting from M₂ interacting with itself. Over-tides occur when the tide enters large expanses of shallow water (e.g., Tasman Bay).

Perigean/apogean tides

Occur every month (27.555 days to be exact) in conjunction with the position of the Moon in its elliptical orbit around Earth. When the Moon is closest to Earth, it is in its perigee and larger than normal perigean tides occur. When the Moon is farthest from Earth, it is in its apogee and smaller than normal apogean tides occurs. Another equivalent definition is that perigean and apogean tides are the result of M₂ (the lunar semi-diurnal constituent) beating against N₂ (the elliptic semi-diurnal constituent).

Perigean spring tides

Occur about every 7 months (206.6 days to be exact) when New or Full Moon occurs at the same time as the Moon is in its perigee. Usually, these are the tides with the largest tidal range.

Petrological

Study of rocks.

Provenance

Source or origin.

Quartzo-feldspathic

Sand made of grains of quartz and feldspar (both minerals).

Reflective

The classification of beaches ranges from reflective (such as the steep and gravelly beaches south of Banks Pennisula) to dissipative (such as the flat and fine-sand beach at Muriwai).

Renourishment

Adding sand to a beach to repair damage caused by erosion.

Rhyolitic

Type of acidic volcanic rock (Mayor Island is comprised of acidic rocks).

Sea-level gauge

An instrument that automatically registers the rise and fall of the tide and other non-tidal sea-level variations. The registration is accomplished by recording the sea-level heights at regular time intervals (e.g. 5-10 minutes) in digital or analogue (chart) format. A variety of sensors used for measuring the sea surface height, the main ones being a pressure gauge, ultrasonic sender/receiver (or a mechanical float), and counter-weight system on a pulley.

Sea-level rise

Sea-level rise is a long-term trend of increasing sea level caused by global changes in climate and related effects (e.g. ice melt, thermal expansion of seawater).

Sediment fluxes

The mass of sediment moving a particular direction per unit time.

Sediment-transport model

Sediment-transport model a model that simulates the movement of sediments, such as sand, from tides, winds, waves and currents, leading to whether erosion or accretion will occur.

Semi-diurnal tides

Semi-diurnal tides are tidal constituents which occur twice daily, i.e., they have a tidal period of approximately 12 hours, e.g., the solar semi-diurnal constituent S₂ (12 hour period); the lunar semi-diurnal constituent M₂ (12.42 hour period); the elliptic semi-diurnal constituent N₂ (12.66 hour period).

Significant wave height

the average wave height of the highest 33% or one-third of ranked wave heights measured or modelled over a given sampling period, usually 10–20 minutes. H_1/10 is another wave height used in extreme design of coastal structures, being the average wave height of the highest 10% or one-tenth of ranked wave heights.

Southern Oscillation Index (SOI)

Southern Oscillation Index (SOI) is an index of the relative strength of an El Niño (negative SOI) and La Niña (positive SOI). The most common SOI is based on the difference in sea-level atmospheric pressure between Tahiti and Darwin. Click here for a link to the Climate Variability and Change page.

Spring/neap tides

Spring/neap tides occur every fortnight (14.765 days to be exact) in conjunction with Moon’s phase: spring tides occur just after New and Full Moon; neap tides occur just after First and Last Quarter. Spring tides have a much larger tidal range than neap tides because at New and Full Moon, the Moon and Sun are lined up and they pull together upon Earth’s waters; whereas at First and Last Quarter the Moon and Sun are opposed and the pull is less. Another equivalent definition is that spring and neap tides are the result of M₂ (the lunar semi-diurnal constituent) beating against S₂ (the solar semi-diurnal constituent).

Storm surge

Storm surge is the temporary elevation of the ocean above the mean tidal level as a result of a storm or cyclone. Usually, it comprises two components: the response of the ocean to low barometric pressure (called inverted barometer) and wind set-up. Storm surge is also known as hurricane surge.

Swell

Long-period waves (8-15s) that are generated by distant storms.

Surf zone

Area on a beach where the waves are breaking.

Tidal constituents

Individual components which comprise the tides. Each constituent arises either from a specific astronomical feature or from the interaction between two or more constituents. For example, N₂, the elliptic semi-diurnal constituent (N₂ is its Darwin symbol), arises from the elliptic orbit of the Moon around Earth. Each constituent has a unique tidal period.

Tidal model

Model that specifically calculates tides or tidal constituents. See mathematical model.

Tidal period

Interval of time over which a tidal constituent varies. Each constituent has a unique period, governed by a particular astronomical effect. For example, the solar semi-diurnal constituent S₂ has a period of exactly 12 hours and this arises because the Sun passes over the same spot on Earth every 24 hours. Similarly, the lunar semi-diurnal constituent M₂ has a period of 12.42 hours and this arises because the Moon passes over the same spot on Earth every 24.84 hours.

Tidal range

Difference in height between a consecutive high tide and low tide.

Tidal residual currents

Normally, if you average the currents generated by the tide over several tidal cycles, the results will be zero. However, if the spatial pattern of tidal currents is different when the tide is rising from when the tide is falling, then there will be an average current which is called the tidal residual current.

Tides

Caused by the gravitational attraction of astronomical bodies (the Moon and the Sun) upon Earth’s waters. The tide consists of many tidal constituents.

Time of transition

Time that a particle spends in the beam of a laser.

Titanomagnetite

Mineral made up of titanium and magnetite.

Tracer

Grains that are used to minic the movement of natural sediment, and can be tracked because they are different. Native sediment may be coated in a thin layer of fluorescent or radioactive material. Alternatively, fluorescent plastic particles may be matched to the density and size of the natural sediment.

Tsunami

Surface wave or series of waves generated by a seismic event such as an earthquake or a submarine volcano. Often a tsunami can also be triggered by a co-seismic activity such as a submarine landslide following an earthquake. (Probably the 1998 PNG tsunami was one of these.) In deep water (i.e., 5,000 m or greater) tsunami are very small (less than 1 m usually) and travel very fast (800 km per hour or more), but as the water gets shallower near land, they slow down and their height can increase enormously.

Undertow

Seaward flowing current inside the surf zone, which is greatest near the sea bed.

Wave model

Model that simulates how waves are generated by winds and travel across the ocean.

Wave orbital ripples

Ripples caused by waves (rather than currents).

Weather system

Pattern of weather occurring over the entire New Zealand region and lasting a few days.