Definitions of coastal terms

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This article gives an overview of terminology frequently used in the Coastal Wiki. The focus is on terms related to physical coastal processes and engineering. A complementary list of definitions related to the living environment is given in the article Definitions of marine ecological terms.

Many definitions in this glossary are derived from Mangor et al. 2017 [1] and from the USACE Coastal Engineering Manual [2]. Other definitions are based on the related Coastal Wiki articles. Terms related to the coastal profile are illustrated in the figure below.

Fig. 1. Definition of coastal sub-zones, adapted from the Shore Protection Manual, 1984[3]


The erosive action that occurs when rock particles of varying sizes are dragged over or hurled against a surface.

Accretion or Aggradation

Growth (vertical and/or horizontal) of morphological structures (beach, bar, dune, sand bank, tidal flat, salt marsh, tidal channel, etc.) by sedimentation.

Active coastal zone

The active coastal zone (also called active coastal profile) is the cross-shore coastal zone that is highly dynamic, with up and down redistribution of sand by the action of tides, waves and wind. It extends from the closure depth up to a fixed land boundary (rock, cliff, seawall, sea dike). In the case of a dune coast the front dune is part of the active coastal zone. See: Active coastal zone.

Advective transport

Transport along with the main flow.


Pertaining to the wind, wind-blown.

Alluvial fan

Deposit rock material formed when streams run off mountains to plains below.

Angle of incidence

The angle between the wave propagation direction and the normal to the coastline or the angle between the wave front and the coastline (often denoted by the symbol [math]\alpha[/math] or [math]\theta[/math]). When waves enter shallow water, the wave propagation direction tends to become perpendicular to the depth contours, by refraction. See: Shallow-water wave theory, Wave transformation. The deep water angle of incidence is often denoted [math]\alpha_0[/math].


Underground layer of water-bearing permeable rock, rock fractures or unconsolidated materials (gravel, sand, or silt). A confined aquifer is an aquifer that is overlain by a confining poorly permeable layer. An unconfined aquifer is an aquifer extending from the base of the aquifer to the land surface.


Poorly permeable underground layer that limits the flow of groundwater from one aquifer to another. An aquifuge is an absolutely impermeable unit that will not transmit any water. An aquiclude is a formation that has very low hydraulic conductivity and hardly transmits water.


  1. Applying a hard protective layer, a so-called armour layer, on a breakwater or seawall, composed of armour units.
  2. The formation of a top layer of coarse sediment hiding and protecting a sublayer of finer sediment from erosion, see Seabed armoring.

Astronomical tide

The tidal levels and water motion which would result from Earth's rotation and gravitational effects of, in particular, the Earth, Sun and Moon, without any atmospheric influences. See also: Tide


The sudden abandonment of an existing tidal channel in favour of a newly formed channel. Channel avulsion is a common process in tidal deltas.

Back barrier

Pertaining to the lagoon-marsh-tidal creek complex in the lee of a coastal barrier island, barrier spit or baymouth barrier.


The part of the beach lying between the beach face and the front dune, cliff base, vegetation line or coastal protection structure. The backshore is dry under normal conditions; it is often characterised by berms. Vegetation is generally sparse or even absent. The backshore is only exposed to waves under extreme events with high tide and storm surge. It is influenced by occasional storm overwash and windblown sand deposition. Site of embryonic dune and subsequent foredune development.


Elongated sand body created by (tidal) currents or by waves . Bars in estuaries and tidal lagoons develop naturally in flow convergence zones, often in relation with channel meandering. Estuarine bars also occur where ebb- and flood-dominated channels meet. Bars that partially block the openings to minor streams and lagoons are mainly due to littoral drift; these bars are generally referred to as spits. Wave action is responsible for the development of breaker bars, beach berms and swash bars. Bars can raise above the high water level, but they are often intratidal or subtidal.


Mapping of the seafloor depth with respect to the mean water level. The term is also used for the description of seafloor topography or coastal morphology. The depths are generally measured indirectly by an acoustic device (recording the time required for a signal to travel from a transmitter, to the bottom, and back to a receiver).


The beach (or shore) zone of unconsolidated material that extends from the mean low water line to the place where there is a marked change in material or physiographic form (e.g. dunefoot), or to the line of permanent vegetation (the effective limit of storm waves and storm surge), i.e. to the coastline. The beach or shore can be divided in the beach face and the backshore. Often a distinction is made between dissipative and reflective beaches.

Beach berm

A beach berm is a nearly horizontal shore parallel ridge formed on the beach due to the landward transport of the coarsest fraction of the beach material by the wave uprush. There may be several beach berms and in some cases no berms. Under normal conditions a beach berm is formed on the upper part of the beach face, and over the backshore during severe events. Berms can also form on the higher intertidal zone of a tidal flat; these berms are generally called "swash bars". Beach berms are sometimes artificially reinforced as coastal protection measure.

Beach cusps

Regularly spaced shoreline structures (spacing typically between a few meters and a few tens of meters) consisting of small embayments between protruding horns. They are a common feature of reflective beaches. See: Beach Cusps.

Beach face

Beach face (also called foreshore) is the zone between the mean low water (MLW) and the seaward beach berm, which is equivalent to the upper limit of wave run-up at high tide, see Fig. 1. The beach face is the part of the shore/beach which is wetted due to the varying tide and swash under normal conditions. This means that the beach face in morphological terms extends further up on the beach than the intersection between the mean high water (MHW) and the coastal profile (MHW line). However, for practical reasons the upper delineation of the beach face is often defined as the intersection between the MHW line and the coastal profile, which is identical to the usual definition of the shoreline. On the lower part of wide intertidal beaches several intertidal bars (also called beach ridges) can be present, related to onshore moving nearshore sandbars. The depressions between these intertidal bars are called runnels.

Beach fill

The supply of beach sand for the construction of an artificial beach.

Beach nourishment

Beach nourishment is the supply of sand to the beach to increase the recreational value and/or to compensate for the effect of shore erosion by feeding sand on the beach. See Shore nourishment.

Beach ridges

Relict, semiparallel, multiple ridges, either of wave origin (beach berms) or wind origin (multiple backshore foredunes). They usually form strandplains.


Hard coastal sedimentary formations consisting of various beach sediments, lithified through the precipitation of carbonate cements.

Bed forms

The seafloor is seldom flat, but generally undulated by the interaction with currents and waves . Undulated bedforms exist over a large range of spatial scales, from centimeters up to kilometers, see: Wave ripples, Wave ripple formation, Sand ridges in shelf seas, Stability models. The smallest bedforms play an important role in the friction exerted by the seabed on water motion, see Boundary layer, Bedforms and roughness, Bed roughness and friction factors in estuaries, Wave ripples, Wave ripple formation.


Bedload transport refers to sediment transport by rolling and saltating sediment grains over the seabed. Bedload transport is the dominant sediment transport mode when the flow velocities (currents and wave-orbital velocities) are above the critical velocity for setting bed particles in motion (in the order of 0.2-0.4 m/s for sandy sediments), but insufficient for bringing sediment particles in suspension. Bedload transport occurs for non-cohesive medium-coarse sediments and is associated with the formation of bed ripples and dunes. The migration of these bedforms yields an estimate for the bedload transport. See: Sand transport.


The solid rock that underlies gravel, soil and other superficial material. Bedrock may be exposed to the surface (an outcrop) or it may be buried beneath a few centimeters to thousands of meters of unconsolidated material.


Organisms living on or in the seabed, including vegetation.


Wedge-shaped, relatively narrow ridge situated at high tide level between the backshore and the upper foreshore; may be composed of sandy or gravelly sediments.


Wet, spongy, poorly drained vegetated area containing a high percentage of organic remnants and residues, frequently associated with a subsurface water source. A bog sometimes represents the final stage of eutrophication by which the upstream part of an estuary or a lagoon slowly transform into land areas.


Upward flow of water in a sandy formation due to unbalanced hydrostatic pressure.


Stone or rock fragment that exceeds 256 mm in size.

Boulder rampart

Storm-deposited, occasionally very substantial, and high supratidal gravel-boulder ridge, located on the backshore.

Boundary layer (turbulent)

The fluid layer where momentum and energy are dissipated as a result of friction exerted by the seafloor or a nearby hard boundary. In the boundary layer, fluid momentum is dissipated through transfer from the large-scale flow pattern in a cascade process to increasingly smaller turbulent flow structures. The large-scale flow profile in the turbulent boundary layer has a logarithmic profile. The thickness of the turbulent boundary layer corresponds to the size of the largest turbulent eddies generated by friction at the seafloor. Because the development of the turbulent boundary layer takes time, the boundary layer thickness for slowly varying currents is much greater than for rapidly varying currents. The boundary layer thickness for steady flow and tidal flow is typically of the order of 10-50% of the water depth, whereas the boundary layer thickness for wind-driven waves is only a few centimeters. Hence, steady flow and tidal flow experience much stronger friction than propagating wind-driven waves. See: Bed forms, Wave ripples.

Braided river

River type with multiple channels separated by shoals, bars and islands.

Breaker zone

See surf zone.

Breaker bar

Breaker bars, also called nearshore sandbars, are elongated (approximately) shore parallel bodies of sand or gravel built in the surf zone due to the action of breaking waves and cross-currents. There can be several rows of bars. Breaker bars are very mobile formations, which tend to be in unstable equilibrium with the wave climate and tide conditions, which means that they are constantly changing. The overall tendency is that the bars are moving seawards during storm wave conditions and landwards during conditions dominated by smaller waves and swell. At intervals there are gaps in the breaker bars formed by rip currents. See Nearshore sandbars.

Breaker index

The ratio of wave height and still water depth at the shoreface location where incident waves start breaking. See Breaker index.


A structure built for reducing wave activity in the waters at the leeside. It can be linked to the shore or it can be positioned offshore. A common type is the detached breakwater. This is a structure approximately parallel to the coast, built inside or outside the surf zone. The main purpose of detached breakwaters is either to protect a harbor entrance or a ship wharf from wave action or to reduce wave activity at the beach. See: Application of breakwaters, Detached breakwaters, Detached shore parallel breakwaters, Floating breakwaters, Stability of rubble mound breakwaters and shore revetments.


Structure to retain or prevent sliding of land, and to protect upland against damage from wave action.


Upward force experienced by a body of lower density (water body of lower salinity, higher temperature, for example) than the surrounding fluid.


Wave propagation speed, see Wave propagation.


An accretionary feature consisting of a long, low lying, narrow strip of (gravelly) 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.

Closure depth

There are two closure depths: (1) the depth [math]h_{out}[/math] at the seaward limit of the lower shoreface, beyond which no significant wave-induced sand transport takes place; (2) the depth [math]h_{in}[/math] marking the transition between the lower shoreface (wave shoaling zone) and upper shoreface (wave breaking zone). See Closure depth, shoreface and Shoreface profile.


The strip of land that extends from the coastline inland to the first major change in the terrain features, which are not influenced by coastal processes. The main types of coastal features are dunes, cliffs and low-lying areas, possibly protected by dikes or seawalls.

Coastal area

The land and sea areas bordering the shoreline.

Coastal cell

A coastal cell (or littoral cell or sediment cell) is a coastal compartment that contains a closed cycle of sedimentation including sources, transport paths, and sinks. The cell boundaries (often corresponding to headlands or jetties) delineate the geographical area within which the budget of sediment is balanced, providing the framework for the quantitative analysis of coastal erosion and accretion (see Sediment Budget).

Coastal development

Any activity likely to alter the physical nature of the coastal zone in any way, including construction of buildings and works, the deposit of waste or other material from outfalls, vessels or by other means, the removal of sand, sea shells, natural vegetation, sea grass and other substances, dredging and filling, land reclamation and mining or drilling for minerals, but excluding fishing activities.

Coastal erosion

Coast erosion is the process of wearing away material from the coastal profile due to imbalance in the supply and export of material from a certain section. Distinction must be made between incidental coastal erosion and ongoing coastal erosion.

Coastal hinterland

The land that extends landward of the coast and which is not influenced by coastal processes.

Coastal morphodynamics

The mutual interaction of coastal morphology with hydrodynamic agents (tides, currents, waves). This interaction takes place through sedimentation, erosion and sediment transport processes. Tides, currents and waves adapt to constraints imposed by the morphology of a coastal system (e.g., delta, estuary, beach, etc.). The morphology of a coastal system adapts to the tides, currents, waves to which it is exposed. This mutual adaptation, which is always highly nonlinear, generates morphological patterns, such as channel meanders, tidal flats, ebb tidal deltas, nearshore sandbars, beach berms, sand ridges, ripples, etc. As a result, the large-scale coastal morphology develops into a slowly evolving morphodynamic equilibrium state in which smaller morphological patterns evolve in a quasi-cyclical (usually non-deterministic) manner at much smaller timescales.

Coastal morphology

Coastal morphology (or coastal geomorphology or morphology) is the (study of the) shape and structure of coastal systems or subsystems. For example: the morphology of a delta, the morphology of an estuary, the morphology of a beach, the morphology of a bedform. The meaning of the Greek word "morphè" is form or shape. See also: Characteristics of sedimentary shores, Classification of sandy coastlines, Morphology of estuaries.

Coastal profile

The cross-shore profile of the active coastal zone. See also: Shoreface profile.

Coastal protection

Three different protection/defence definitions are used as follows:

Coastal squeeze

Loss of natural buffer zones in front of sea defense structures when the shoreline retreats (term used in particular for shoreline retreat due to sea level rise).

Coastal zone

General, wide planning-oriented characterisation: The interface between land and sea, defined as the part of the land affected by its proximity to the sea (influence of marine processes), and the part of the sea affected by its proximity to the land (influence of terrestrial processes).

Coastal zone management (CZM)

Coastal zone management is the process aiming to balance the different interests in coastal areas, related to safety, environment, economy, social equity and esthetic and cultural values. Coastal area planning and coastal engineering (hard or soft measures) are major components of coastal zone management, together with legislation, administration, communication, education, monitoring, research. Because of this comprehensive scope the term Integrated coastal zone management (ICZM) is more widely used than CZM. The dynamic nature of the coastal zone is a highly challenging aspect of coastal zone management. See also: Integrated Coastal Zone Management (ICZM), Some definitions of Integrated Coastal Zone Management (ICZM), The Integrated approach to Coastal Zone Management (ICZM), Spatial Planning and Integrated Coastal Zone Management, Policy instruments for integrated coastal zone management, Shoreline management, and other articles in the category Integrated coastal zone management


The boundary between land and sea, a proxy of the instantaneous shoreline for the assessment of erosion or accretion trends. See: Coastline.

Coastline retreat

Landward shift of the coastline caused by a long-term erosional trend or by sea-level rise.

Continental shelf

The continental shelf (or shelf sea) is the continental border which is submerged in relatively shallow sea (water depths typically less than 200 m). The continental shelf extends from the coastline of a continent to a drop-off point called the shelf break. From the break, the shelf descends toward the deep ocean floor in what is called the continental slope. Water motion, water quality and ecosystem of shelf seas are strongly influenced by the adjacent ocean, see Shelf sea exchange with the ocean, Continental shelf habitat.

The legal definition of a continental shelf is different from the geographic one. According to the UN Convention on the Law of the Sea, every nation has a continental shelf extending no more than 200 nautical miles from the nation's coastline. See: Legislation for the sea.


The acceleration experienced by a current due to earth rotation, see: Coriolis acceleration.


A small sheltered recess in a coast, often inside a larger embayment.


An arrangement of relatively thin layers of rock inclined at an angle to the more nearly horizontal bedding planes of the larger rock unit. Also called 'cross-stratification'.

Crown wall

Concrete superstructure on a rubble mound.


Any permanent line, plane or surface used as a reference to which elevations are referred.

Deep water

Water too deep for waves to be affected by the seafloor; typically taken as half the wavelength, or greater.


The removal of loose material from a beach, dune or other land surface by wind action.


The fan-shaped mouth of a river or a tidal basin, formed by several distributary channels. River deltas are formed when the supply of sediments to the coast by a river is faster than they are dispersed by waves, tides and the associated currents. They are the result of depositional and erosional processes under the influence of currents, waves and tides. Because of their different morphologies, often a distinction is made between river-dominated deltas, wave-dominated deltas and tide-dominated deltas. See: Morphology of estuaries, Wave-dominated river deltas.

Flood tidal deltas are sedimentary bodies deposited by flood currents. Ebb-tidal deltas are sedimentary bodies deposited by ebb currents. Flood deltas and ebb-tidal deltas are generally present at the inshore and offshore sides (respectively) of tidal inlets of estuaries and tidal lagoons.


Process by which wave energy is transmitted, radiated and dissipated when the wave is bending around an obstacle such as an island or a breakwater in order to propagate into the sheltered region.


  1. The passive dispersal of dissolved substances in the marine or estuarine environment. Dispersion is the overall mixing effect of all hydrodynamic processes including turbulence, flow circulations and gradients in current velocity. See: Seawater intrusion and mixing in estuaries, Transport and dispersion of pollutants, nutrients, tracers in mixed nearshore water, Shelf sea exchange with the ocean.
  2. Wave field transformation due to the dependence of propagation speed on wave frequency. See Dispersion (waves), Shallow-water wave theory.

Dissipative and reflective beaches (also Dissipative and reflective coasts)

Dissipative beaches have typically gentle coastal profiles; they are subjected to energetic short-crested waves, which are strongly damped in the nearshore zone. Dissipative beaches have medium to large intertidal zones which consist of fine (sandy or muddy) sediment. Reflective beaches have typically steep slopes and are subjected to low-energy swell waves. An intertidal zone is almost absent and the beach consists mainly of coarse (sandy or gravelly) sediment. Beach types can be characterised by the so-called ' Dean parameter ' [math]\Omega=H_0/(w_s T_p)[/math], where [math]H_0[/math] is the deep-water wave height, [math]w_s[/math] the mean fall velocity of beach sediment and [math]T_p[/math] the peak spectral wave period. Reflective beaches correspond to [math]\Omega \lt 1[/math] and dissipative beaches to [math]\Omega \gt 6[/math]. See Shoreface profile and Characteristics of sedimentary shores.

Drainage basin

Total area drained by a stream and its tributaries.


  1. The term 'dunes' generally indicates subaerial dunes. These dunes are ridges or moulds of loose, wind-blown sand (fine to medium) forming on the backshore and forming the coastal features at certain locations. Dunes are more or less vegetated. Dunes are active coastal form elements acting as a flexible sand reservoir. At coasts subject to structural coastal erosion they are moving backwards in parallel with shoreline retreat. Dunes act as a kind of flexible natural protection against erosion and flooding, see Dune erosion. If the vegetation is damaged by too much traffic or grazing etc. the integrity of the dunes may be endangered.
  2. The term 'dunes' is also used for subaqueous dunes, which are usually called sandwaves in shelf seas. Subaqueous dunes are bed forms induced by the interaction of the seabed with (mainly tidal) currents. Sandy seabeds are often covered with dune fields in regions where the maximum tidal current velocity is in the range 0.5-1.5 m/s. Dune spacing (more than 10 m and less than 1000 m) depends mainly on water depth. Dunes can reach a height of 10-50% of the water depth; high dunes present a risk for navigation.

Dune foot

Transition between the backshore beach and the much steeper dune.

Ebb shield

High, landward margin of a flood-tidal shoal that diverts ebb-tide currents.

Ebb tidal delta

The self-organized pattern of shoals at the seaward mouth of tidal inlets formed in interaction with tidal currents and waves. Sometimes called: 'inlet-associated bars' or 'entrance shoals'.

Ebb and flood tides

The following two definitions of ebb and flood can be found in the literature:

  1. Ebb is the tidal phase during which the tidal current is flowing seaward (ebb current) and flood is the tidal phase during which the tidal current is flowing inland (flood current);
  2. Ebb is the tidal phase during which the water level is falling and flood the tidal phase during which the water level is rising.

The two definitions do not coincide. The first definition is more usual for tidal inlet systems: estuaries, tidal lagoons and tidal rivers; the second definition is more usual for the open coast.

Edge wave

Obliquely incident waves trapped to the shore by wave refraction and reflection. Non-breaking long-period waves (infragravity waves) that are not strongly dissipated in the nearshore zone cannot escape to deep water after reflection but continue travelling along the shore.

Ekman transport

Depth-integrated water transport at right angles (anticyclonic) to the wind direction as a result of Coriolis acceleration. Often associated with upwelling or downwelling.

Environmental impact assessment (EIA)

A written analysis of the predicted environmental consequences of a proposed development activity, including

  • a description of the avoidable and unavoidable adverse environmental effects (in conjunction with the cumulative effect of other human interventions);
  • a description of alternatives to the activity which might be less harmful to the environment, together with the reasons why such alternatives were rejected;
  • a description of any required irreversible or irretrievable commitments of resources required by the proposed development activity.


The transition zone between the riverine and the marine environment. A usual definition is: a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which sea water is measurably diluted with freshwater derived from land drainage. Strong tides intrude in general much further upstream than seawater. From a morphological and sedimentary point of view it is therefore more logical to consider as upstream estuarine boundary the location where tidal discharges become much smaller than river discharges, instead of the seawater intrusion limit. See: Seawater intrusion and mixing in estuaries, Morphology of estuaries.

Estuarine circulation

Residual flow pattern in an estuary driven by density differences between fluvial water and seawater. See: Estuarine circulation, Salt wedge estuaries.


See Mining


Length in the wind direction of the marine area where water waves are generated by wind.


The process by which fine sediments aggregate into larger flocs that are held together by electrochemical bonds and then settle out of the water column. See: Flocculation cohesive sediments.


Flood (tide): Two definitions of ebb and flood can be found in the literature

  1. Ebb is the tidal phase during which the tidal current is flowing seaward (ebb current) and flood is the tidal phase during which the tidal current is flowing inland (flood current);
  2. Ebb is the tidal phase during which the water level is falling and flood the tidal phase during which the water level is rising.

The two definitions do not coincide. The first definition is more usual for tidal inlet systems: estuaries, tidal lagoons and tidal rivers; the second definition is more usual for the open coast.
Flood (overflow): the rising of a body of water and its overflowing onto normally dry land.

Flood tidal delta

The bulge of sand formed at the landward mouth of tidal inlets as a result of flow expansion.

Fluid mud

Fluid mud is a high-concentration colloidal suspension of fine sediment particles (< 63 µm, with often a high percentage of clay < 2 µm). It is formed by settling of mudflocs into a near-bottom suspended layer or by fluidization and/or liquefaction of an underconsolidated mud bottom.


Front dune formed by eolian processes usually from fine-to-medium sand by accretion of embryonic backshore dunes.


The vertical distance between the water level and the top of a structure (breakwater, seawall, etc.; positive for emerged structures and negative for submerged structures). For a ship: the distance between the waterline and the main deck.


Shear stresses generated by the seabed (sediment grains, bedforms) and by flow obstacles, causing loss of fluid momentum. The shear stresses are transmitted to the flow via the turbulent boundary layer. See: Bed forms, Bed roughness and friction factors in estuaries, Bedforms and roughness


Same as beach face.

Froude number

The ratio of inertial and gravitational forces for a fluid.


Bank or bottom protection structure containing rocks, rubble or masonry (1) held together by a wire mesh, or (2) enclosed within a steel wire-mesh basket.


Instrument for measuring the water level with respect to a datum.


Study of the formation and characteristics of earth surface structures.

Graded sediment

A sediment bed composed of a mixture of fine- and coarse-grained sediment particles.

Gravel beach

Beach built of granular material with a size larger than 1 mm: very coarse sand (>1 mm), gravel (2-4 mm), pebbles (4-64 mm) and cobbles (>64 mm), see Coastal and marine sediments. Beach material often originates from erosion of nearby cliffs or is supplied by gravel rivers draining nearby mountains. Gravel beaches have steep slopes and occur on coasts exposed to strong wave action. Cusp patterns are a common beach feature. See: Gravel Beaches.

Groyne or groin

A straight structure perpendicular to the shoreline. Groynes work by blocking (part of) the littoral drift. They trap/maintain sand on their updrift side and cause erosion on the downdrift side. Groynes can have special shapes and they can be emerged, sloping or submerged, they can be single or in groups, the so-called groyne fields. Groynes are normally built as rubble mound structures, but they can also be constructed in other materials, such as concrete units, timber, etc. See: Groynes, Groynes as shore protection, Deteriorated groynes.


Land mass with a considerable elevation that borders beaches. Headlands form the boundaries to sediment cells, compartmentalising sand transport along the shore, and reducing sand exchange between adjacent beaches.
Artificial headlands are smooth structures built from the coastline over the beach and some distance out on the shoreface. They work by blocking (part of) the littoral transport. A headland combines the effects of groynes and detached breakwaters and at the same time, minimises some of the disadvantages of groynes and breakwaters – see Modified breakwaters and headlands.

Hydraulic radius

Quotient of the wetted cross-sectional area and the wetted parameter. Approximately equal to the depth for wide systems.

Inlet (Tidal inlet)

  1. A short waterway connecting a back-barrier basin to the open sea;
  2. An elongated sea arm extending inland (e.g. estuary).

Infragravity waves

Ocean surface waves with a period of typicallly 30-300 s. They arise in particular through non-linear interactions within wave groups in shallow water. They play an important role in beach dynamics of dissipative coasts because their amplitude increases shoreward, relative to the breaking short waves. See: Infragravity waves.

Internal waves

Waves on the interface of fluid layers with different densities. Internal waves can be very large when the density difference between the fluid layers is small. Breaking of internal waves contributes to mixing of the layers and destruction of the layer structure.

Intertidal zone

Area which is dry at low water (LW) and submerged at high water (HW), where LW and HW refer to mean spring tide. The beach face, tidal flats and (parts of) salt marshes are intertidal zones.


A contour line connecting points of equal depth on a chart.


  1. Breakwater protecting a harbour entrance channel from wave action (also called harbour mole);
  2. Structure (often a bridge) connecting an offshore ship mooring to the coast.


Area of relatively shallow water situated in a coastal environment, separated from the open marine conditions by a natural barrier (a sand spit, a barrier island or a coral reef), but with an access to the sea. One may distinguish between microtidal and macrotidal lagoons. Examples of microtidal lagoons are Great South Bay and Pamlico Sound at the US Atlantic coast. Examples of macrotidal lagoons (also called tidal lagoons) are the Wadden Sea at the Dutch-German-Danish North Sea coast and the Bassin d'Arcachon at the French Atlantic coast. Tidal lagoons are distinct from microtidal lagoons by the existence of deep tidal inlets and large tidal flats. See also: Morphology of estuaries.


A natural or artificial ridge or embankment along a stream.


Cyclic wave loading can liquify a water-saturated soil, which starts behaving as a fluid, losing stiffness and bearing, see Wave-induced soil liquefaction.

Littoral cell

Same as coastal cell.

Littoral drift

Littoral drift or longshore sediment transport is the term used for the longshore transport of non-cohesive sediments, i.e. mainly sand, along the upper shoreface due to the action of breaking waves and longshore currents. Formulas for the longshore sediment transport are given in: Littoral drift and shoreline modelling; see also Coastal Hydrodynamics And Transport Processes.

Littoral zone

In marine ecosystems the shore area or intertidal zone, where periodic exposure and submersion by tides is normal. It may also include the adjacent shallow zone.

Longshore current

The longshore current or nearshore current is the dominating current in the nearshore zone and is running parallel to the shore. The longshore current is generated by the shore-parallel component of the stresses associated with the breaking process for obliquely incoming waves , the so-called radiation stresses. See: Longshore current and Shallow-water wave theory.

Longshore sediment transport

Same as Littoral drift.


Thin transition layer between a low-turbidity upper water layer and a denser high-turbidity lower layer. Also: pycnocline separating layers of low and high suspended sediment concentration.


Tidal range greater than 4 m. (Megatidal: tidal range exceeding 8 m, mesotidal: tidal range between 2 and 4 m, microtidal: tidal range less than 2 m)

Managed retreat (managed realignment)

The deliberate setting back of the existing line of sea defense in order to obtain engineering or environmental advantages by avoiding coastal squeeze.

Management unit (MU)

A management unit is a coastal stretch with coherent characteristics in terms of both natural coastal processes and land use. The MU is used as boundary for Shoreline Master Plans.

Marine regression

Coastal extension due to a falling relative sea level.


Natural propensity of a flow to scour the outer bend of a channel. When a channel meander becomes very large a cutoff channel (a so-called channel chute) is often formed, leading to avulsion of the meander.


Mining (or excavation) is the mechanical removal of consolidated soil or unconsolidated material (aggregates like sand, gravel, shells) from seabed, beach or dunes.


Measures aimed at countering, alleviating or partially obviating the adverse consequences of threatening developments or events that have a human or natural cause.


Physical or mathematical representation of nature, for studying or predicting coastal behaviour; see Modelling coastal hydrodynamics.

  • Physical models. These models copy salient features of nature at a reduced scale in a laboratory setting. Scale effects are an important issue for the translation of observed model results to the natural scale. Major scale effects generally arise for the representation of phenomena influenced by friction, by sediment transport (including sediment erosion and deposition) and by biota. See Scaling Issues in Hydraulic Modelling.
  • Mathematical models. These models solve in one way or another the governing hydrodynamic (and/or morphodynamic) equations . They exist in many different types, see for example Estuarine morphological modelling. Most common types are: Analytical models, Numerical process-based models, Behaviour-based models, Stochastic models, Data-driven models, Particle-based models and Cellular models. Analytical models are based on explicit solutions of the governing equations, which are strongly simplified to retain only those features which are most pertinent to the studied phenomena. Numerical process-based models describe nature on a discretised grid, retaining all physical features relevant for a reliable and accurate representation of the studied phenomena; the governing equations are solved numerically for successive discrete time steps, see Process-based morphological models. Behaviour-based models (also called Aggregate-scale models) consider the coastal system as an assembly of interacting subsystems and solve the governing equations numerically for the interactions at system level, whereas empirical relationships are used for representing the dynamics at subscale levels; see: Behaviour-based models. Stochastic models are process-based models which use probability distributions for certain input data for taking into account natural fluctuations (wave climate, river discharge, for example) or uncertainty (sea-level projections, model parameters, for example). Data-driven models are used to make moving forecasts by integrating observed data of the recent history to calibrate uncertain model parameters or uncertain model input data, see: Reduction of uncertainties through Data Model Integration (DMI). Particle-based models describe numerically the motion of discrete fluid parcels taking into account their mutual interaction according to the laws of physics; in the most popular method (called Smoothed-particle Hydrodynamics (SPH)) the fluid is divided into a set of discrete moving parcels with properties which are smoothed over the parcel size according to a prescribed smoothing function. Computational fluid dynamics (CFD) models solve the Navier-Stokes equations for multiphase flows, including the interaction of the fluid (liquids and gases) with surfaces defined by boundary conditions. Cellular models divide the coastal system in fixed discrete cells which behave in a prescribed way according to the evolution of neighbouring cells, taking into account physical conservation laws.
  • Hybrid models combine physical and numerical models.


The (study of the) shape or structure of natural objects, whether living or not. In the coastal context, morphology or geomorphology is generally used for designating the coastal bathymetry, including its sedimentary composition and structure. See also Coastal morphology.


See Coastal morphodynamics.


Fine cohesive sediment deposit containing a high fraction (≥20%) of clay minerals. Fine sedimentary particles, consisting of clay minerals, but also other particles (silt, fine sand, organic matter), can be glued together by large organic molecules (extracellular polymeric substances, EPS) into large mud flocs. These flocs, with a diameter of 0.1-1 mm, settle much faster than the individual particles and can form a colloidal suspension on the seabed. This so-called fluid mud layer can move along the seabed (driven by pressure gradients at the interface, by flow entrainment or by bed slope effect) and be a major cause of harbor siltation. After consolidation, which is often a lengthy process of months to years, a mud bed can become highly resistant to erosion by currents. See: Mud, Dynamics of mud transport , Sediment deposition and erosion processes, Coastal and marine sediments.

Nearshore zone

Same as upper shoreface.


Nourishment (or sand nourishment) is a method to neutralize the effect of coastal erosion by maintaining the sand volume of the active coastal zone. Sand is extracted (generally by dredging) from nearby sources and applied to the beach, the shoreface or the dunes. The costs highly depend on the location of available sand sources, which should be situated outside (seaward of) the active coastal zone. Dune nourishment is usually meant for safety against flooding, beach nourishment for restoration of the beach and shoreface nourishment for stabilizing the shoreline. See: Shore nourishment, Beach nourishment, Shoreface nourishment.

Offshore zone

Different definitions of the offshore zone can be found in the literature. It can be the zone off the shoreface, off the surf zone or off the littoral zone. In the present context, the offshore zone is defined as the zone off the shoreface.


Surface exposure of bare rock, not covered by soil or vegetation.


Passing of water over the top of a structure as a result of wave run-up or surge action. See Wave overtopping.


Overflowing water that causes erosion of the crest of a dike, barrier or berm. The term overwash is also used for the resulting sand deposit at the leeside of the barrier.


A low wall built along the edge of a structure such as a seawall or a quay.

Perched beach

A beach or fillet of sand retained above the otherwise normal shore profile by a submerged dike.


Erosion of closed flow channels by the passage of water through soils. Flow underneath structures, carrying away sediment particles, may endanger the stability of the structure.

Pore pressure

The interstitital pressure of water within a mass of soil or rock.


Percentage of total volume of a soil sample not occupied by solid particles but by air and water.


Coastal extension into the sea due to natural aggradation. Coastal progradation occurs specifically where rivers supply large amounts of sediment.


Transition layer between water layers of different density (due to strong gradients in temperature, salinity and/or suspended sediment).

Radiation stress

The radiation stress is defined as the excess flow of momentum due to wave orbital motions (with units of force/unit length). Gradients in the radiation stress induce an effective momentum transfer from wave motion to steady motion that takes place when the wave amplitude changes along the direction of propagation.


The transformation of areas which were formerly part of the marine or estuarine domain into non-floodable land.


A ridge of material at or near the surface of the ocean. Natural reefs are made of rocks or the skeletons of small animals (mainly corals). Reefs can also be created artificially for several reasons:

  • Protect coastlines from erosion;
  • Promote sea life for recreation and aquaculture;
  • Create a wave pattern that promotes the sport of surfing.

See: Coral reefs, Nature-based shore protection, Artificial reefs.

Reflective beaches

See Dissipative and reflective beaches

Refraction (Wave refraction)

The propensity of waves to align the wave front in shallow water with the depth contour, according to Snell's law. Also: Change of wave propagation direction due to the interaction with currents.

Rip current

Rip currents are strong offshore directed currents that occur when high waves break over nearshore sandbars. When breaking, high incident waves raise the water level in the area between the beach and nearshore sandbars. This elevated water is discharged by rip currents that flow seaward through narrow gaps between the nearshore bars. The corresponding morphological pattern is called "rip cell". It results from the morphodynamic feedback between the coastal morphology and the incident wave field. A local setback in the shoreline is often seen opposite the rip opening. The rip opening travels slowly downstream. Rip currents are dangerous for swimmers. See Rip current and Rhythmic shoreline features.


A revetment is a facing of stone, concrete units or slabs, etc., built to protect a scarp, the foot of a cliff or a dune, a dike or a seawall against erosion by wave action, storm surge and currents. This definition is very similar to the definition of a seawall, however a revetment does not protect against flooding. Furthermore, a revetment is often a supplement to other types of protection such as seawalls and dikes. See Revetments, Seawalls and revetments, Stability of rubble mound breakwaters and shore revetments.


Rough irregular fragments of broken rock.


Depression between intertidal bars or between the dry beach and the upper intertidal bar (beach ridge). After high tide, the depression is filled with water, which is gradually discharged to the sea during falling tide via a channel (a so-called rip channel) that cuts through the beach ridge.


A bulge in the shoreline projecting towards an offshore island or breakwater, but not connected to it as in the case of a tombolo. Developed by wave refraction and longshore drift. See Detached breakwaters .


Sediment particles travelling by a series of hops and bounds, not retained in suspension.

Salt marsh

A densely vegetated coastal ecosystem situated in the upper coastal intertidal zone between land and intertidal mudflats, or bordering directly open saltwater or brackish water if mudflats are absent.

  • Salt marshes are a key habitat of transitional waters lying at the interface between the land and the sea, depending on, and periodically covered by tidal sea water.
  • Salt marsh vegetation is usually composed of grasses and other low plants, but not trees.
  • Water saturation is the dominant factor controlling plant and animal communities and soils.
  • The soil may be composed of deep mud and peat.
  • Salt marshes are drained by tidal creeks that form spontaneously depending on local soil characteristics and gradients in the hydraulic head of infiltrated water.
  • Salt marshes usually form in sheltered coastal systems, such as lagoons and estuaries where fine sediments can be deposited. Salt marshes can also form behind spits and artificial sea defences where tidal waters can flow gently and deposit fine sediments.
  • Salt marshes are sometimes referred to as schorre or kwelder.

See: Salt marshes.

Salt wedge

Seawater intrusion in an estuary as a wedge-shaped bottom layer which hardly mixes with the overlying fresh water layer. Salt wedges occur in estuaries where tidal motion is very weak or absent.

Sand bank

Popular term for large bars and ridges in tidal waters, especially in relation to navigation hindrance.

Sand nourishment

See Nourishment

Sand spit

A sand spit is an accretionary feature formed by littoral drift, consisting of a long narrow accumulation of sand or gravel, lying generally in line with the updrift coast, with one end attached to the land and the other projecting into the sea or across the mouth of an estuary or lagoon. During dry periods, a spit can develop into a barrier that temporarily blocks the mouth of a lagoon or a small river. See Sand spit.


  1. A subaqueous dune with high length to height ratios and continuous crestlines;
  2. A slowly migrating shoreline undulation with a wavelength in the range of a few hundred meters to a few kilometers, see: Rhythmic shoreline features.

Sea level rise

The so-called greenhouse effect or global warming causes a rise of the mean sea level, which will have a great impact on long-term coastal morphology, see Sea level rise. The long-term gradual sea-level rise will cause a general coastline retreat and an increased flooding risk depending on local conditions. Instead of the absolute rise of the mean sea level it is more relevant to consider the relative sea-level rise: the rise relative to vertical land motions that can be positive (uplift) or negative (subsidence). An estimate of coastline retreat due to relative sea-level rise can be derived from the so-called Bruun rule, which is valid under certain rather restrictive conditions, see: Bruun rule.


Fine-grained loose particles such as gravel, sand, mud. These sediments are produced by chemical or physical weathering of rocks, by seabed erosion and by soil erosion in river basins. They are called clastic sediments and consist of quartz, feldspar, mica and clay minerals. Other particles have a biotic origin, for example shell debris, peat, detritus, fecal pellets and plankton. Sediment particles have widely different grainsizes: clay (< 0.002 mm), silt (0.01-0.02 mm), sand (0.1-2 mm), gravel (2-5 mm). The density is of the order of 2.65 times the density of water. Sand or gravel beds occur in zones with strong (tidal) currents and strong wave activity, whereas fine sand, silt and mud cover the seabed in sheltered zones where currents and wave activity are weak. See: Coastal and marine sediments, Sediment deposition and erosion processes, Gravel Beaches.

Sediment cell

Same as coastal cell.

Sediment transport

The amount of sediment transported by water motion (currents and /or waves). Sediment transport is a crucial link in the interaction between coastal morphological evolution and waves, currents and tides. Sedimentation is related to convergence of sediment transport and erosion to divergence of sediment transport. Sediment transport takes place in several ways: Suspended-load transport, Bedload transport and Fluid mud motion. Suspended-load transport is the transport of sedimentary particles that are suspended in the fluid. Bedload transport is the transport of sedimentary particles that are rolling or leaping along the seabed. Fluid mud transport is the motion of a fluid mud layer along the seabed. Formulas for bedload transport are based on empirical relationships involving characteristics of sediment, currents and waves. See: Sediment transport formulas for the coastal environment, Sand transport.


Harbour seiches are resonant (or near-resonant) standing oscillations in a semi-enclosed water body caused by incoming long-period waves (periods typically in the range 200-2000 s). Incoming waves can be strongly amplified if the period is close to the harbour resonance period, causing damage to ships and moorings. Long-period waves can be generated by nonlinear interaction of random short waves with a peaked frequency distribution (see Infragravity waves), generated mainly in shallow water and reflected from adjacent coasts. Long-period waves can also be generated by meteorological effects, in particular strong wind speed fluctuations during storms, related to the passage of a cold front. Other generation mechanisms include deep-sea internal waves, seismic activity, or tsunamis. Seiches occur also in closed basins, such as lakes, often induced by strong fluctuations in the wind field.

Setback area

A strip along the coastal zone where certain development activities are prohibited or significantly restricted. See: Setback area.

Sheet flow

Sheet-flow sediment transport refers to transport of sandy sediments as a fluidized thin surface layer (thickness of ten to several tens grain diameters). This type of sediment transport occurs under strong wave action (wave orbital velocity greater than 1 m/s), where bed ripples are flattened out. In the sheet-flow layer, continuous contacts between sand grains create an intergranular stress. This stress decreases the velocity in *the sheet-flow layer to about one half the velocity in the top layer. The sediment concentration in the sheet-flow layer is in the order of 100 to 1000 kg/m3. See: Sediment transport formulas for the coastal environment.

Shelf sea

See Continental shelf.


Shoaling is the deformation of incident waves on the lower shoreface that starts when the water depth becomes less than about half of the wavelength, causing the waves to become steeper: increase in amplitude and decrease in wavelength.

  • Wave amplification is due to (approximate) continuity of the wave energy flux [math]F=c_g E[/math] seaward of the surf zone, where [math]E = \frac{1}{8} \rho g H^2[/math] is the wave energy and [math]c_g[/math] the wave group propagation speed. The landward decrease of the wave group propagation speed ([math]c_g \approx \sqrt{gh}[/math] in shallow water of depth [math]h[/math]) results in a landward increase of [math]E[/math], thus in a landward increase of wave height [math]H[/math]. See: Shallow-water wave theory for a more detailed treatment.
  • Wave propagation in the shoaling zone has a strongly non-linear character because the wave height is no longer negligible compared to the water depth. This produces wave asymmetry, with the wave orbital velocity being greater in the onshore than offshore direction and the offshore-to-onshore orbital acceleration being greater than the onshore-to-offshore acceleration.
  • Wave shoaling precedes wave breaking on the upper shoreface when the wave steepness exceeds a critical limit.


Usually same as Beach.

Shore protection

See Coastal protection.


The shoreface is the nearshore zone of the inner continental shelf that is bounded landward by the mean low-water line and that extends seaward to where the influence of wave action on sediment transport is on average minor compared to other influences. A similar definition is: The shoreface is the zone seaward of the shoreline where offshore generated waves interact with the upward sloping seabed. The shoreface can be divided in two zones, the upper shoreface and the lower shoreface (also called shoaling zone). The upper shoreface is the zone where most energy is dissipated by wave overturning and breaking and the lower shoreface the zone where waves shoal. The lower part of the shoreface extends to the so-called outer closure depth [math]h_{out}[/math]; beyond this depth the seabed is hardly influenced by waves and wave-induced sediment transport is (on average) insignificant. The transition between lower and upper shoreface is generally defined by the closure depth [math]h_{in}[/math] related to the significant wave height, which is exceeded 12 hours per year, [math]H_{s,12h/y}[/math]. See Shoreface profile.


The intersection between the water line and the shore. The line delineating the shoreline on Nautical Charts (Sea Maps) approximates the Mean High Water Line. See the article Shoreline for an overview of shoreline detection techniques.

Shoreline management

The act of dealing – in a planned way – with actual and potential coastal erosion and its relation to planned or existing development activities on the coast, see Shoreline management. The objectives of Shoreline Management are:

  • To ensure the development activities in the coastal area follow an overall land use plan and a general environmental policy;
  • To ensure the development activities in the coastal area do not cause to or aggravate erosion;
  • To ensure that development activities do not occur in sensitive areas;
  • To ensure that erosion control techniques are cost-effective and socially and environmentally acceptable.

Shoreline management is typically based on coastal sediment cells.


Accumulation of fine sediments (sand, silt, mud) in channels, harbors and fairways. See: Siltation in harbors and fairways, Dynamics of mud transport , Sediment deposition and erosion processes, Sediment transport formulas for the coastal environment.

Slack water

Tidal phase at which the current turns from flood to ebb (high-water slack tide) or from ebb to flood (low-water slack tide). See also Definition of ebb and flood (tide).


See Sand spit.

Spring tide

A tide that occurs at or near the time of new or full moon (SYZYGY) and which rises highest and falls lowest from the mean sea level.

Still water level

Average water surface elevation at any instant, excluding local variation due to waves and wave set-up, but including the effects of tides, storm surges and long period seiches.

Storm surge

The rise in water-level on an open coast as a result of the combined impact of the wind stress on the water surface, the atmospheric pressure reduction and local topographic features. The storm surge does not include the effect of the astronomical tide. The storm at a location is in first approximation inversely proportional with the water depth in the offshore zone. This implies that shores out to deep oceans will only be exposed to relatively small surges whereas shores out to shallow seas can be exposed to high surges. See: Extreme storms.


Less dense water layer overlying a water layer of higher density (related to higher salinity, lower temperature or higher suspended sediment concentration). Density differences in the vertical inhibit turbulent mixing, which causes the interface of the layer to be sharpened. See: Seawater density, Salt wedge estuaries.


Downward motion of the land surface. It is most often related to soil compaction (underground material movement) caused by the removal of water, oil, natural gas, or mineral resources out of the ground by drainage, pumping, fracking, or mining activities. Subsidence can also be caused by natural events such as earthquakes, glacial isostatic adjustment, erosion, sinkhole formation, sediment loading by river deposites, and adding water to fine soils deposited by wind (a natural process known as loess deposits). See also: Coastal cities and sea level rise.


  1. Collective term for waves breaking on shore and on reefs.
  2. The wave activity in the area between the shoreline and the outermost limit of wave breaking.

Surf beat

Long-periodic oscillation of the water line on the beach. The oscillation is related to Infragravity waves in the surf zone. See: Infragravity waves, Edge waves, Shallow-water wave theory.

Surf similarity parameter

The surf similarity parameter is defined as [math]\; \xi = \Large\frac{\tan \beta}{\sqrt{H_s/L}}\normalsize , \;[/math] where [math]L=\Large\frac{g T^2}{2 \pi}\normalsize[/math] is the wavelength at the seaward boundary of the breaker zone and [math] H_s/L[/math] the wave steepness at this location. It compares the wave surface slope to the bed slope in the surf zone and represents important features of the hydrodynamics of the surf zone.

Surf zone

The surf zone (or breaker zone) is the zone where waves break as a consequence of depth limitation and surf onshore as wave bores. The width of the surf zone varies depending on the wave conditions and water level. The surf zone is narrow and close to the shoreline in a gentle wave climate and can be very wide under storm conditions, extending from the seaward boundary of the upper shoreface to the dunefoot. It is estimated that 80 to 90% of the yearly littoral transport takes place within the breaker or surf zone.


Elongated, relatively narrow low miniature valley forms between two wave-built berm ridges or foredune beach ridges.


Up and down propagation of bores formed after collapse of waves on the beach. Swash is the decelerating uprush phase and backwash is the accelerating downrush phase. On dissipative coasts swash processes are dominated by infragravity waves. See: Swash zone dynamics.

Swash bar

The term "swash bar" usually designates a berm that is formed by wave uprush on the higher intertidal zone of a tidal flat. See also Beach berm

Swash zone

Zone where wave bores run up and down the beach face.


Waves generated far offshore in the deep sea that propagate onshore. Ocean swell waves have longer wavelengths than locally generated wind waves. The longest waves in the ocean travel faster and dissipate less energy than the shorter waves generated by the same strong wind event. They therefore can reach distant shores whereas the shorter waves cannot. See Waves.


Concrete armor unit with four stubby legs, used on breakwaters and seawalls to dissipate wave energy.


The line joining the deepest points of an inlet or stream channel.

Tidal asymmetry

Difference between the ebb and flood phase of the tide: different duration of ebb and flood flow, different strength of ebb and flood currents, different duration of rising tide and falling tide. Tidal asymmetry is mainly caused by the nonlinear dependence of tide propagation on water level variation in shallow coastal systems with complex geometry (e.g., estuaries, lagoons). Tidal asymmetry can also result from the superposition of certain astronomical tidal constituents in estuaries with strong diurnal tides. Tidal asymmetry can strongly influence sediment transport and is an important morphodynamic feedback mechanism for the morphological development of estuaries. It is also plays an important role in the formation of the estuarine turbidity maximum. See Tidal asymmetry and tidal basin morphodynamics, Estuarine turbidity maximum.

Tidal bore

Breaking tidal flood wave – the ultimate stage of tidal asymmetry. Tidal bores occur in shallow funnel-shaped estuaries with a large tidal range (generally more than 6 m). See Tidal bore dynamics.

Tidal channel

Seabed incision concentrating the main tidal flow. Tidal channels form naturally in sedimentary environments where tide-induced water motion is stronger than wave-induced water motion.

Tidal creek

A tidal channel that drains a salt marsh.

Tidal flat

Shallow, sandy or muddy area which is covered and uncovered by the rise and fall of the tide. Muddy tidal flats are also called mud flats or slikke. As a rule of thumb, tidal flats generally occur in sheltered coastal areas where the relative tidal range RTR, defined as the ratio between the mean spring tidal range and the annual average significant wave height [math]H_s[/math], is about 15 or higher.

Tidal inlet

The connection between the sea and a tidal basin (lagoon) or estuary partially shielded by a barrier island, sand spit or headland. A tidal inlet consists of a narrow deep channel through which strong currents flow, with flood and ebb tidal deltas at the landward and seaward sides.

Tidal prism

Volume of water flowing during flood through a tidal inlet, or through a cross-section of an estuary or a tidal lagoon.

Tidal range

Water level difference between high water and low water, i.e. twice the tidal amplitude. The tidal range changes from tide to tide depending on the positions of moon and sun relative to the earth; the most important change is the fortnightly variation from spring tide to neap tide. See Ocean and shelf tides.

Tidal ridge

Submarine sand ridge with a length of several tens of kilometers and height of a few tens of meters, generated by the nonlinear interaction of tidal currents with the seabed on the shelf sea. Sand ridge fields consist of a number of tidal ridges with a spacing of the order of 5 km. See: Sand ridges in shelf seas.

Tidal wave

The wave associated with tidal motion. The term "tidal wave" is also frequently used as a popular expression for an unusually high and destructive water level along a shore, thus including the combined effect of astronomical and meteorological surges.


The tide (more precisely, the astronomical tide) is the large-scale water motion generated by the rotation of the earth in combination with the varying gravitational influence on the ocean of celestial bodies, especially the moon and the sun. These phenomena cause predictable and regular oscillations in the water level, which are referred to as the tide. The astronomical tide at a specific location can be predicted and is published in Tidal Tables. See: Ocean and shelf tides, Tidal motion in shelf seas. The term 'tide' is sometimes used for the combined effect of astronomical tide and wind-driven set-up or set-down of the sea level (including storm surges).


A bar or spit that connects an island to the mainland or a sand accumulation between land and a detached breakwater.

Training wall

A low wall, dike or jetty that directs the current flow with the purpose to improve navigability and reduce dredging needs.


Flooding of land by the sea related to relative sea level rise. Transgression implies coastline retreat.


Long waves caused by a strong local disturbance of the water mass. The most important generation mechanisms are subsea earthquakes and subsea slides of unconsolidated seabed slopes. Tsunamis propagate very fast over the ocean with very few energy loss due to the great ocean depth. Tsunamis generally consist of a few successive waves with wavelengths typically much larger than the wavelength of wind-generated waves and much smaller than the wavelength of tidal waves. In nearshore waters the amplitude increases dramatically due to shoaling, see Tsunami.

Turbidity maximum

Convergence zone of suspended sediment transport in an estuary, where turbidity levels are high due to high suspended sediment concentrations. Upstream (landward) transport of fine suspended sediment in an estuary is possible due to estuarine circulation and tidal asymmetry (maximum flood current stronger than maximum ebb current). This produces a maximum in the suspended sediment concentration just downstream of the zone where the influence of river discharge on sediment transport becomes dominant. Fine sediments in the turbidity maximum zone settle to the bed in periods where currents are small (slack tide, neap tide), which may result in the formation of fluid mud layers. High turbidity often causes oxygen depletion and mortality of estuarine organisms. It also enhances sedimentation of tidal flats and harbours. See: Estuarine turbidity maximum, Dynamics of mud transport.


Irregular flow pattern of individual fluid parcels related to small-scale flow instability, see Turbulent boundary layer. Turbulence plays a major role in mixing processes and in energy dissipation. See: Transport and dispersion of pollutants, nutrients, tracers in mixed nearshore water, Currents and turbulence by acoustic methods.


Undertow is the current flowing offshore near the seabed driven by the cross-shore wave set-up gradient. It compensates for the onshore directed wave-induced mass transport and roller transport in the upper layers of the water column.

Upwelling, downwelling

See Ekman transport.

Wash load

Very small particles in near-permanent suspension that are transported without deposition.


Deposit resulting from overwash.


The term "wave" designates in most cases surface water waves generated by wind (other types of waves are explicitly referred to as tidal wave, tsunami wave, etc.). A wave field is generally a superposition of waves of different height, period and direction that can be described by a wave spectrum. A unidirectional wave field is often characterised by the significant wave height [math]H_s[/math] (representing approximately the mean wave height of the highest third of the waves) and the spectral peak period [math]T_p[/math] (wave period with the highest energy). See: Statistical description of wave parameters. Coasts situated on the open ocean are mainly subjected to long-period swell waves. Coasts situated on inland seas are mainly subject to locally generated short-crested waves, also called "sea". See Waves and Shallow-water wave theory.

Wave breaking

Wave breaking is the ultimate stage of wave deformation due to strong nonlinearity of the wave propagation process when waves enter shallow water (water depth less than half the wavelength). Waves start breaking (by surging, collapsing, plunging or spilling) where the still water depth is smaller than one or two times the wave height. Waves may reform in the surf zone, but remain depth-limited by spilling or by plunging a second time, until final collapse and uprush on the beach. See: Wave transformation, Shallow-water wave theory.

Wave energy

Energy carried by waves when propagating. The wave energy [math]E[/math] is proportional to the square of the wave height [math]H[/math] (formula: [math]E=g \rho H^2 / 8[/math]). For irregular waves, [math]H[/math] is the root mean square wave height [math]H_{rms}[/math], see Statistical description of wave parameters.

Wave group

Incident waves generally arrive in groups, corresponding to the superposition of incident waves with slightly differing wave lengths and frequencies that are present in the wave spectrum. Wave groups generate infragravity waves with the same group wavelength and period, by non-linear interaction. Wave groups also cause surf beat. See: Shallow-water wave theory, Infragravity waves.

Wave height

The water level difference between wave trough and wave crest, or twice the wave amplitude. In an irregular wave field, successive incident waves have different amplitudes and phases. In deep water the wave height distribution often follows approximately a Rayleigh distribution. However, this is not the case in the surf zone where the wave height is limited by the water depth. See Statistical description of wave parameters.


For regular sinusoidal waves, the wavelength is the distance between two successive wave crests. Real sea waves are irregular without a well-defined wavelength. The wave field off the coast can be characterized by a wave spectrum. Wavelength is then a statistical parameter, see Statistical description of wave parameters.

Wave propagation

Progression and transformation of waves in time and space.

  • The speed [math]c[/math] (celerity) of a wave propagating without frictional losses in deep water is given by [math]c \approx \sqrt{g/k} = g/\omega[/math] where [math]g[/math] is the gravitational acceleration, [math]k=2 \pi / \lambda[/math] the wave number, [math]\lambda[/math] the wavelength and [math]\omega=kc[/math] the angular frequency. Deep water means: still water depth [math]h[/math] much larger than [math]1/k[/math]. The dependency of the propagation speed on frequency causes wave dispersion.
  • In shallow water (depth [math]h[/math] much smaller than [math]1/k[/math]), the wave propagation speed is proportional to the square root of the water depth [math]h[/math] (formula: [math]c \approx \sqrt{gh}[/math]), thus not depending on the wave frequency.
  • In shallow water, wave propagation is a strongly nonlinear process, leading to wave transformation and breaking.
  • A wave group propagates at a smaller celerity [math]c_g[/math] than the constituent short waves. Wave energy propagates at the celerity of the wave group.

See: Shallow-water wave theory.

Wave run-up

The maximum onshore elevation reached by a wave running on the beach, relative to the water level in the absence of waves. It is the sum of swash uprush and wave set-up. Wave run-up is an important factor in the design of coastal protection structures and is a dominant process leading to the erosion of coastal dunes. See also Wave run-up, Swash zone dynamics, Tsunami.

Wave set-up

Elevation of the mean water level at the shoreline due to wave breaking in the surf zone. The wave set-up is proportional to the wave height at the breaker line. As a rule of thumb, the wave setup is of the order of 20% of the offshore significant wave height. See: Wave set-down and set-up.


  1. Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI
  2. USACE, 2012. Coastal engineering manual. Report No 110-2-1100. Washington DC: US Army Corps of Engineers
  3. Coastal engineering Research Center, Department of the Army, Waterways Experiment Station, 1984. "Shore protection manual".

The main authors of this article are Mangor, Karsten and Job Dronkers
Please note that others may also have edited the contents of this article.

Citation: Mangor, Karsten; Job Dronkers; (2024): Definitions of coastal terms. Available from [accessed on 17-06-2024]