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Glossary of Coastal Terms

Definitions of common terms.

  • Alluvium: Loose, unconsolidated (not cemented together into a solid rock) soil or sediments which has been eroded, reshaped by water in some form, and re-deposited in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles silt and clay and larger particles of sand and gravel. When this loose alluvial material is deposited or cemented into a lithological unit, or lithified, it would be called an alluvial deposit.
  • Backshore: The area of shore lying between the average high-tide mark and the toe of the dune and vegetation, affected by waves only during severe storms.
  • Bathymetry: The science of measuring the depths of oceans, seas etc. Topographic maps of the seafloor that result from such measurements.
  • Beach: A deposit of sand that consists of a narrow backshore and foreshore (the part of the beach affected/largely built by waves) in front of a sea cliff or man-made structure such as a seawall. The deposit of sand is often thin and sits on a rocky wave cut shore platform, with the rock out cropping in the low tide and/or shallow intertidal area. Occur in locations of low sediment supply and/ or where there is limited accommodation space for a dune to form because of the backing cliffs and wave reflection off the cliffs at high tides and storms do not permit sediment to accumulate. . 
  • Beach cusps: Beach cusps are shoreline formations made up of various grades of sediment in an arc pattern. The horns are made up of coarser materials and the embayment contains all the finer grain sediment. They can be found all over the world and are most noticeable on shorelines with coarser sediment such as pebble beaches, however they can occur with sediment of any size. They nearly always occur in a regular pattern with cusps of equal size and spacing appearing along stretches of the shoreline. These cusps are most often a few metres long, however they may reach 60 m across. Although the origin of beach cusps has yet to be proven, once cusps have been created they are a self-sustaining formation. This is because once an incoming wave carrying sediment hits the horn of a beach cusp it is split and forced into two directions, with coarser sediment being deposited on the horn and finer sediment is being eroded away from the embayments. This process causes the horns and embayments to at least maintain their size, if not grow larger.
  • Beach hazards and rating: Refers to the scaling of a beach according to the physical hazards relating to water safety associated with its beach type under normal wave conditions, together with any local physical hazards. It ranges from the low least hazardous rating of 1, to a high and most hazardous rating of 10. It does not include biological or water quality hazards, such as sharks or water pollution.
  • Beach ridge barrier: A single low, essentially continuous mound or ridge of beach material predominantly built by the action of waves (swash) on the backshore of a beach, and occurring singly or as one of a series of approximately parallel deposits. The barrier is generally composed of coarse sandy, pebbly, cobble and/or shelly material. 
  • Beach ridge barrier plain: A sequence of (relict) ridges separated from the shoreline by progradation representing a successive positions of an advancing shoreline (also called a strandplain).
  • Beach type: Refers to the prevailing morphology (modal state) of a beach, including the waves and currents, the extent of the nearshore zone, the width and shape of the surf zone, including its bars and troughs, and the dry or subaerial beach.
  • Boulders: A rock fragments with grain size of usually no less than 256 mm (10 inches) diameter, which makes it bigger than a cobble.
  • Breaking waves: Waves break at beaches because wave heights are amplified in the region of shallower water (because the group velocity is lower there). There are four basic types of breaking water waves. They are spilling, plunging, collapsing, and surging. Spilling breakers occur when seabed has a gradual slope. Here the wave will steepen until the crest becomes unstable, resulting in turbulent whitewater spilling down the face of the wave. This continues as the wave approaches the shore, and the wave's energy is slowly dissipated in the whitewater. Because of this, spilling waves break for a longer time than other waves, and create a relatively gentle wave. Onshore wind conditions make spillers more likely. Plunging breakers occur when the ocean floor is steep or has sudden depth changes, such as from a reef or sandbar. The crest of the wave becomes much steeper than a spilling wave, becomes vertical, then curls over and drops onto the trough of the wave, releasing most of its energy at once in a relatively violent impact. A plunging wave breaks with more energy than a significantly larger spilling wave. The wave can trap and compress the air under the lip, which creates the "crashing" sound associated with waves. Offshore wind conditions can make plungers more likely. If a plunging wave is not parallel to the beach, the section of the wave which reaches shallow water will break first, and the breaking section (or curl) will move laterally across the face of the wave as the wave continues. This is the "tube or barrel" that is so highly sought after by surfers. A plunging wave that is parallel to the beach can break along its whole length at once, rendering it unrideable and dangerous. Surfers refer to these waves as "closed out". Collapsing waves are a cross between plunging and surging, in which the crest never fully breaks, yet the bottom face of the wave gets steeper and collapses, resulting in foam. Surging breakers originate from long period, low steepness waves and/or steep beach profiles. The outcome is the rapid movement of the base of the wave up the swash slope and the disappearance of the wave crest. Surging waves are typical of reflective beach states. On steeper beaches, the energy of the wave can be reflected by the bottom back into the ocean, causing standing waves.
  • Chenier plain: An accretionary feature consisting of a long, low lying, narrow strip of gravely sand (typically up to 3 m high and 40 to 400 m wide), often shelly, deposited in the form of wave-built beach ridge on a swampy, deltaic, or alluvial coastal plain of fine sediment.
  • Coastal erosion: The wearing away of land and the removal of beach or dune sediments by wave action, tidal currents, wave currents, or drainage. Coastal accretion is the process of coastal sediment returning to the visible portion of a beach or foreshore to raise the level of the beach and make it wider.
  • Coastal inundation: Coastal flooding from the sea occurs when normally dry, low-lying land is flooded by sea water. The extent of coastal flooding is a function of the elevation inland flood waters penetrate which is controlled by the topography of the coastal land exposed to flooding. The sea water can inundate the land via direct flooding of low lying area (where sea height exceeds the elevation of the land, often where waves have not built up a natural barrier such as a dune system) and over topping and/or breaching of a barrier. Coastal flooding is largely a natural event, however human influence on the coastal environment can exacerbate coastal flooding
  • Coastal sensitivity index: The Coastal Sensitivity Index (CSI) provides a snapshot of the potential sensitivity of the New Zealand soft shore coastline to coastal inundation (coastal flooding) and coastal change (erosion and accretion) due to future climate change. The CSI can be viewed as maps of for coastal inundation and coastal change using the Coastal Explorer web tool. Mapping the CSI for New Zealand is a first step in understanding where the impacts of climate change on coastal margins may be most significant and where potential adaptation activities may most usefully be targeted by management agencies. Coastal vulnerability and sensitivity indices (CVI's and CSI's) have been developed as a rapid and consistent method for characterising the relative vulnerability of coasts to hazards. In respect of hazards and disaster management, vulnerability is generally perceived as the extent to which people can be affected by the physical, social and economic impacts of a hazard. The CSI mapped in Coastal Explorer only includes physical variables (coastal inundation and coastal change) and not any socio-economic aspects; hence 'coastal sensitivity' rather than 'coastal vulnerability' was chosen as the description of the index. 
  • Cobble: A rock fragment between 64 and 256 mm in diameter, especially one that has been naturally rounded, which makes it smaller than a boulder.
  • Cuspate foreland: An accretionary feature consisting of a triangular accumulation of sand or shingle projecting seawards from the shoreline. It can have straight or concave shores and multiple beach ridges marking stages in progradation.
  • Delta: An alluvial deposit, usually triangular in planform, at the mouth of a river of other stream. It is normally built up only where there is no tidal or current action capable of removing the sediment as fast as it is deposited, and hence the delta builds seaward.
  • Dissipative: Highest wave energy (breakers 2 to 3 m high) of the wave-dominated beaches. Wide surf zone (up to 300 to 500 m) with 2 or 3 shore-parallel (straight) bars separated by subdued troughs. Waves dissipate their energy as they break passing over bars in the surf zone. Wide, low gradient intertidal beach composed of firm fine sand.
  • Embayment: An indentation or recess of a shoreline, larger than a cove but smaller than a gulf, that forms a bay.
  • Estuarine coast: A shoreline inside an estuary.
  • Estuary: Day's (1981) variation of Pritchard's (1967) definition and defines an estuary as: "A partially enclosed coastal body of water that is either permanently or periodically open to the sea in which the aquatic ecosystem is affected by the physical and chemical characteristics of both runoff from the land and inflow from the sea". The NZ Estuarine Environment Classification uses a broad definition for estuaries to include the many different types of coastal water bodies that need to be managed. This definition includes estuary types and coastal water bodies described in other New Zealand classifications (e.g., Heath 1976, Healy and Kirk 1982, Hume and Herdendorf 1986, 1993) as drowned river valleys, lagoons, coastal lakes, fjords, and river mouths. It includes features variously named on the NZMS 1:50,000 topographic maps as estuary, creek, firth, inlet, gulf, cove, river, bay, lagoon, harbour, stream, fjord, sound, port, arm, small craft retreat, haven, and basin.
  • Exposed coast: A shoreline that faces the open sea and is subject to ocean swell.
  • Exposure: The degree to which a coast is exposed to wave energy and ocean swell.
  • Foredune barrier plains: Systematic beach progradation over time frames of 10s to 1000s of years may lead to the development of wide foredune plains. Such plains may also develop during sea level regression and during sea level transgression as long as there is a significant sediment supply.
  • Foredune barriers: Shore-parallel dune ridges formed on the top of the backshore by wind and sand deposition within vegetation. Actively forming foredunes occupy a foremost seaward position in a dune system. Foredunes generally fall into two main types, incipient and established foredunes, within which there can be wide morphological and ecological variations. Also known as foredunes.
  • Foreshore sediment type: The type of sediment that makes up the foreshore. Made up of minerals and rock and shell fragments.
  • Foreshore: That part of a beach that is exposed by the low tides and submerged by high tides. This area can include many different types of habitats, including steep rocky cliffs, sandy beaches or vast mudflats. Also called an intertidal beach.
  • Gravel: Unconsolidated sediment of particle sizes that includes granule 2 to 4 mm, pebbles 4 to 64 mm and cobbles 64 to 256 mm, and boulders >264 mm particle sizes.
  • Hapua: Is an elongated lagoon separated from the sea by a narrow barrier and situated at the mouth of large rivers that are usually braided. They form on mixed sand/gravel coasts, where the hinterland is a steep alluvial fan and are indicative of a coast being continually reshaped, by wave- and river-dominated processes (waves most important).
  • Headlands: Land masses of erosion resistant material having a considerable elevation that border beaches and compartmentalise sand transport along the shore, and reduce sand exchange between adjacent beaches.
  • Hinterland: Land or region behind the beach and dune or ridge systems. Can be rising ground, sea cliffs, lagoon or wetlands.
  • Incipient barrier beach:  A deposit of sand that consists of a backshore and foreshore and a low incipient dune (sand) or beach ridge (gravel) (generally less than 1 m tall), in front of rising ground or a sea cliff (often fossil). Compared to a 'beach' they occur in areas of greater sediment supply and therefore the nearshore is mostly sandy with some rock outcrops. 
  • Inland headland bypass: Locations where transgressive sands climb from one coastal compartment over land to an adjacent coastal compartment.
  • Island: An area of land, smaller than a continent that is completely surrounded by the sea.
  • Lagoons: Lagoons are shallow, often elongated bodies of water separated from a larger body of water by a sand or gravel barrier or similar feature, and they generally have some degree of salinity. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as estuaries. 
  • Lagoons/estuary/river: The area behind the beach is a lagoon, estuary or river.
  • Longshore bar and trough: Consists of a shore parallel bar separated from the beach by a deep trough. Breakers 1.5 to 2.0 m high. Moderate rip currents. Straight beach composed of medium sand with moderate to steep beach face and cusps.
  • Low energy coast: A shoreline that is sheltered from large waves and long period waves. Occur in gulfs and behind islands and reefs on the open coast.
  • Low lying plain – Drained: The land backing the beach is very low lying (near or below sea level) but is drained (cut with drains).
  • Low lying plain – Dry: The land backing the beach is very low lying (near or below sea level) but is dry.
  • Low lying plain – Wetlands: Low lying land (near or below sea level) behind the beach where water saturation is the dominant factor determining the nature of soil development and the types of plant and animal communities living in the surrounding environment. Other common names for wetlands are bogs, swamps and marshes.
  • Low tide terrace: Moderately steep beach face joined to an attached bar or terrace exposed at low tide. The bar extends alongshore, is flat and featureless, or cut every several 10's of metres by small rips. Breakers are 0.5 to 1.0 m high. Beach composed of fine to medium sand. Commonly occur in areas sheltered from direct wave attack.
  • Microtidal: Is a characteristic of a coast defined by the tidal range (the vertical distance between low and high water). Estuaries for instance can be defined as macrotidal (tide range >4 m), mesotidal (tide range 2 to 4 m) and microtidal (tide range <2 m).
  • Mixed sand/gravel beach: Consists of: (1) poorly sorted mixtures of sandy and gravelly sediment, or (2) alternatively there is a clear distinction between the textural type between the upper (usually the gravel) and lower foreshore (usually the sand).
  • Morphologic and hydrodynamic controls: Features that control the morphodynamic response of a coastal compartment/beach unit e.g., headlands at either end of a pocket beach, offshore island, streams over the beach.
  • Mud: A sticky fine-grained material comprised of unconsolidated sediment of particle sizes that includes silt 0.063 to 0.004 mm and clay <0.063 mm.
  • Offshore reefs: A ridge of rock with the top just below or just above the surface which is located at some distance from the shore.
  • Platform beach: A small body of sand deposited by wave action on and toward the rear of a wave cut platform. The sand extends out from the base of the backing cliff but rarely to the outer edge of the platform.
  • Progradation: Seaward growth of a beach, delta, fan, etc., by progressive deposition of sediment by rivers or shoreline processes.
  • Reflective + bars and rips: Tide-modified system with relatively straight, moderately steep, narrow, usually coarser and cusped reflective high-tide beach, fronted by a lower gradient, relatively featureless intertidal zone and wave dominated low-tide surf zone usually characterised by bar and rip morphology. Breakers 0.5 to 1.5 m high (height increases with onshore winds). At high tide waves break across a narrow continuous surf zone. At low tide a wider surf zone has rips (spacing 100 to 150 m).
  • Reflective + low tide terrace: Tide-modified beach type. Lowest energy of the tide-modified beaches with the coarsest sand. Steep cusped high-tide beach composed of medium to coarse sand, which changes at an abrupt break in slope into a low-gradient wide (av.120 m but can range from 20 to 1000 m) low tide terrace composed of finer sand. Breakers 0.5 to 1 m high. At high tide waves surge at base of steep beach and there is no surf zone. At low tide there is a flat sand bar exposed, waves of >0.5 m height plunge on outer end of bar, and waves >1 m height may cut rip channels across the terrace.
  • Reflective + rock flats: Tide-dominated system with steep reflective high tide beach fronted by rock extending seaward as an intertidal rock platform and/or rock flat. Bedrock control in the form of reefs and headlands means the beaches are short and waves average only 0.5 m height.
  • Reflective + sand flats: Tide-dominated beach type. Small steep (3° to 10°), low-gradient, very low-energy high-tide beach composed of coarse sand, fronted by flat featureless sand flats up to several hundred meters wide composed of finer sand. No waves unless strong onshore winds.
  • Reflective + sand ridges: Tide-dominated beach type. Steep (3 to 10°), narrow high-tide beach composed of coarse sand, fronted by an abrupt break in slope and a wide (several hundred metres) low gradient, usually finer sand intertidal zone containing shore parallel, numerous low amplitude sand ridges and runnels. At high tide there are no waves unless strong onshore winds, and relatively deep water off high tide beach.
  • Reflective + tidal mud flats: Tide-dominated system with narrow reflective high-tide beach composed of coarse sediments, fronted by wide (100's to several 1000's of metres), low gradient (<1°) mud flats with tidal draining channels. Mangroves or other vegetation may grow in the higher intertidal zone. Usually calm, only low wind chop during strong onshore winds. Occur where there is a source of mud nearby and where waves are insufficient to remove muds.
  • Reflective + tidal sand flats: Tide-dominated beach type. Narrow reflective high-tide beach composed of coarse sediments, fronted by wide (hundreds of metres) low gradient (<1°) sand flats, that become muddy on lower intertidal with tidal draining channels. Mangroves or other vegetation may grow in the higher intertidal zone. Entire tidal flat is covered at spring high tide. Usually calm, only low wind chop during strong onshore winds.
  • Reflective: Lowest wave energy of the wave-dominated beaches (breakers 0 to 1 m high). Steep narrow beach face with cusps on upper beach and narrow and swash zone. Short beaches composed of soft coarse sediments. Occur in locations sheltered by rocks, reefs and headlands.
  • Rhythmic bar and beach: High energy, beach consists of rhythmic (undulating) bar, trough and beach. Distinct rip troughs separated by detached bars. Breakers 1.5 to 2.0 m high. It has beach cusps (in lee of bars) and typically fine-medium sand.
  • Ridges and runnels: A series of asymmetrical ridges running parallel to the coast and separated by shallow troughs (runnels) 100 to 200 m wide. This topography is developed on the foreshore of sandy mesotidal or macrotidal beaches. The development of these forms is favoured by moderate wave-energy conditions acting on a flat beach with an abundant sediment supply.
  • Rising ground: The land backing the beach is land that is rising inland. In terms of its topography it is somewhere between low lying and a cliff.
  • River mouth: Location where a river emerges on the coast.
  • Sand: Unconsolidated sediment of particle sizes 0.062 to 2.0 mm.
  • Sea cliff – Active: The area backing the beach is a sea cliff that is actively being eroded.
  • Sea cliff – Fossil: The area backing the beach is a sea cliff comprising hard rock that is not actively eroding.
  • Shell material: Shell material is made up of the discarded calcium carbonate shells of animals such as gastropods and clams that live in the seabed.
  • Sheltered coast: A shoreline inside an estuary or embayment that is sheltered from the open sea and ocean swell.
  • Spit: An accretionary feature, formed by waves consisting of a long narrow accumulation of sand or shingle, lying generally in line with the coast, with one end attached to the land the other projecting into the sea or across the mouth of an estuary.
  • Storm surge: Storm surge is an abnormal rise of water generated by a storm, over and above the predicted astronomical tides. This rise in water level can cause extreme flooding in coastal areas particularly when storm surge coincides with normal high tide, resulting in storm tides reaching up to 6 m or more in some cases. Storm surge is produced by water being pushed toward the shore by the force of the winds moving cyclonically around the storm. The impact on surge of the low pressure associated with intense storms is generally minimal in comparison to the water being forced toward the shore by the wind. Storm surge should not be confused with storm tide, which is defined as the water level rise due to the combination of storm surge and the astronomical tide.
  • Streams: Location where a small watercourse emerges onto the coast.
  • Submarine canyons: Submarine valleys that run across the continental shelf and down the continental slope. They are conduits for sand transport and are a one way trip for sand which is lost from the coastal system.
  • Surf zone: The region of breaking waves and white water offshore from a beach.
  • Territorial sea limit: Defined by the 1982 United nations Convention on the Law of the Sea as a belt of coastal waters extending at most 12 nautical miles (22 km; 14 miles) from the baseline (usually the mean low-water mark) of a coastal state. The territorial sea is regarded as the sovereign territory of the state, although foreign ships (both military and civilian) are allowed innocent passage through it; this sovereignty also extends to the airspace over and seabed below.
  • Tidal delta: An intertidal or subtidal bar or shoal, typically triangular or lobate, formed on the inner (flood delta) and outer/ocean (ebb delta) sides of a tidal inlet. Usually sand sometimes gravel with diverging channel systems.
  • Tidal entrances: All entrances to estuaries and harbours that are not tidal inlets.
  • Tidal inlets: The entrance to estuaries on sandy shores that have formed where sand barriers or spits enclose bays. They comprise a flood and ebb tidal deltas, and a deep narrow throat through which strong currents flow.
  • Tides: The periodic variation in the surface level of the oceans and of bays, gulfs, inlets, and estuaries, caused by gravitational attraction of the moon and sun. King tide is a popular term used to refer to an especially high tide. They are simply the very highest tides. Conversely, the low tides that occur at this time are the very lowest tides. They are naturally occurring, predictable events.
  • Tide-dominated beaches: Occur in areas of high tide range and usually lower waves. Occur when the tide range is between 10 and 15 times the wave height and the wave height is very low. Consists of 5 types: Reflective + sand ridges, Reflective + sand flats, Reflective + tidal sand flats, Reflective + tidal mud flats, Reflective + rock flats.
  • Tide-modified beaches: Occur in areas of high tide range and usually lower waves. Occur when the tide range is between 3 and 15 times the wave height and the wave height is <0.3m. Consist of 3 types: Reflective + low tide terrace, Reflective + bars and rips, Ultradissipative.
  • Tombolo: An accretionary feature comprising sand or gravel beach sediment developed by wave refraction, diffraction and longshore drift to form a 'neck' of land connecting a coast to an offshore island or breakwater.
  • Transgression: A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused either by the land sinking or the ocean basins filling with water (or decreasing in capacity). Transgressions and regressions may be caused by tectonic events such as orogenies, severe climate change such as ice ages or isostatic adjustments following removal of ice or sediment load. The opposite of transgression is regression, in which the sea level falls relative to the land and exposes former sea bottom.
  • Transgressive dunes: The land backing the beach consists of sand dunes migrating inland in the direction of the prevailing wind and burying the topography that includes anything except sand.
  • Transverse bar and rip: Bars transverse (perpendicular) to and attached to the beach separated by distinct rip troughs at 150 to 300 m spacing. Breakers 1.0 to 1.5 m high. Surf zone 50 to 150 m wide with cellular circulation pattern. Undulating beach, with cusps, composed of fine to medium sand.
  • Ultradissipative: A tide-modified beach type. Relatively straight, steeper, cusped high tide beach, with a low gradient concave, featureless, wide (averages 400 to 500 m) intertidal zone. Occur on the higher energy, tide modified coasts with waves averaging 0.5 m, and favoured by higher tide ranges and fine sand. Spilling breakers of 0.5 to 1.5 m height (height increases with onshore wind velocity). At high tide there is a wide zone of spilling breakers. At low tide there is a very wide zone of spilling breakers.
  • Volcanic ring plain: The area immediately surrounding the volcano, but not including the constructional edifice itself. Lithofacies of the ring plain are dominated by fall tephra and by the tephra that has been rapidly reworked from the cone. Interbedded with fall tephra are the fluvial and laharic deposits, which may form the distal edges of wedge-like fans of debris from the lower cone slopes or may be concentrated along well-established drainage channels through the ring plain. The distal edges of the most extensive of flows (lava and pyroclastic) from the cone may also reach as far as the ring plain.
  • Waituna: Similar to a hapua but has a wider brackish lagoon separated from the sea by a narrow barrier and located at the mouth of smaller rivers. They form on mixed sand/gravel coasts, where the hinterland is a steep alluvial fan and are indicative of a coast being continually being reshaped, by wave- and river-dominated processes (waves being most important).
  • Wave-dominated beaches: Beaches exposed to persistent ocean swell and waves and low tides (range <2m). Consist of 3 types: Reflective, Intermediate (longshore bar and trough, rhythmic bar and beach, transverse bar and rip, low tide terrace) and Dissipative.
  • Wave refraction: The process by which the direction of a wave train moving in shallow water at an angle to the seabed contours is changed, and in doing so the wave train becomes more perpendicular to the shore.
  • Wave diffraction: The bending of waves around obstacles in their path. An effect seen as waves pass through an opening in a breakwater into protected waters. The waves fan out from the opening into the region beyond, but as they do so their height is diminished accompanying a redistribution of energy.