Definitions of coastal terms

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This article gives an overview of terminology frequently used in the Coastal Wiki. Many definitions are derived from Mangor et al. 2017 [1]. Terms related to the coastal profile are illustrated in the figure below.



Fig. 1. Definition of coastal terms, mainly from Shore Protection Manual, 1984.[2]


Accretion or Aggradation

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

Active coastal zone

The part of the coastal zone which is highly dynamic 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.

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 [math]\alpha[/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].

Armouring

  1. Manmade structures such as seawalls, revetments, bulkheads, geotubes, etc. intended to prevent damage of properties situated on the coast;
  2. The natural process of removing fine sediment from the beach or seabed top layer, leaving coarse material behind, thus leading to a more erosion-resistant residual top layer. This process may occur at places where beach or seabed material is a mixture of fine and coarse sediments (so-called graded sediment).

Avulsion

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

Backshore

The part of the beach lying between the foreshore and coastline. The backshore is dry under normal conditions; it is generally without vegetation and often characterised by berms. The backshore is only exposed to waves under extreme events with high tide and storm surge.

Bar

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.

Bathymetry

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.

Beach or Shore

The 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 foreshore and the backshore. Often a distinction is made between dissipative and reflective beaches.

Beach berms and swash bars

A beach berm (or beach ridge) 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 foreshore, and over the backshore during severe events.

Berms can also form on the higher intertidal zone of a beach or a tidal flat; these berms are generally called "swash bars". They may be related to onshore moving breaker bars.

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 fill

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

Beach park

A beach park is a scheme which consists of new artificial beaches, stabilising coastal structures and filling/reclamation, which in combination provides new recreational facilities. Artificial beaches will be exposed to wave action and will evolve to a stable plan and profile shape.

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.

Benthos

Organisms living on or in the seabed, including vegetation.

Boundary layer (turbulent) and momentum dissipation

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, momentum is transferred 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.

Breaker zone or Surf zone

The zone extending seaward from the shoreline that is exposed to depth-limited breaking waves. The outer limit of the breaker zone (or surf zone) is called the breaker line. However, the instantaneous width of the breaker zone varies with the instantaneous wave conditions and water level. In the general context, the surf zone width is defined in relation to the yearly wave climate defined by the significant wave height [math]H_{s,12h/y}[/math], which is the wave height exceeded 12 hours per year. The surf zone thus corresponds to the zone where waves of height [math]H_{s,12h/y}[/math] break. The surf zone is somewhat narrower than the littoral zone. It is estimated that 80 to 90% of the yearly littoral transport takes place within the breaker or surf zone.

Breaker bar

A submerged shore parallel embankment of sand or gravel built in the breaker 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 actual wave 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.

Breakwater

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.

Buoyancy

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

Chenier

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.

Closure depth

The depth beyond which no significant longshore or cross-shore transports take place due to littoral transport processes. The closure depth can thus be defined as the depth at the seaward boundary of the littoral zone, see: Closure depth.

Coast

The strip of land that extends from the coastline inland to the first major change in the terrain features, which are not influenced by the 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

See sediment cell.

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

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 morphology

Coastal morphology or coastal geomorphology is the science of natural morphodynamic processes, which are responsible for shaping the coastal zone. The meaning of the Greek word "morphè" is form or shape. These natural processes, which involve wind, tides, currents, waves, biota, soil and sea-level changes, interact with the materials from which the coastal zone is built. Coastal zones around the world are shaped in highly diverse ways. The term coastal morphology is also used for the description of these very diverse coastal forms. See also: Coastal zone characteristics, Classification of coastlines, Classification of coastal profiles.

Coastal profile

The cross-shore profile of the active coastal zone. See also: Parametric equilibrium models.

Coastal protection

Three different protection/defence definitions are used as follows:

Coastal zone

General, wide planning-oriented characterisation: The interface between land and sea, delineated as the part of the land affected by its proximity to the sea, and the part of the sea affected by its proximity to the land. See also: Coastal zone delimitation.

Coastal zone management (CZM)

The aim of coastal zone management is to balance the different interests of coastal areas, which are related to: safety, environment, economy, human activities and esthetic and cultural values, see also: Some definitions of Integrated Coastal Zone Management (ICZM). 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. Coastal engineering (hard or soft measures) and coastal area planning are major instruments for coastal zone management. See: The Integrated approach to Coastal Zone Management (ICZM), Spatial Planning and Integrated Coastal Zone Management, Policy instruments for integrated coastal zone management.

Coastline

Technically the line that forms the boundary between the coast and the shore, i.e. the foot of the cliff or the foot of the dunes. Commonly, the line that forms the boundary between the land and the water. See also: Classification of coastlines.

Coastline retreat

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

Continental shelf or Shelf sea

Part of a continent that 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.

The legal definition of a continental shelf is different than 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.

Coriolis

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

Creek

A tidal channel that drains a salt marsh.

Deep water

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

Delta

The fan-shaped mouth of a river or a tidal basin, formed by several distributary channels. 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 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.

Dispersion

  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.

Dissipative and reflective beaches

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].

Dunes

  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 eroding coasts they are moving backwards in parallel with the erosion process. Dunes act as a kind of flexible natural protection against erosion and flooding. 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 frequently called sandwaves. Subaqueous dunes are bed forms induced by the interaction of the seabed with 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 (generally 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.

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.

Estuary

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.

Fetch

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

Friction

Shear stresses generated by the seabed 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

Foreshore or Beach face

The zone between mean low water (MLW) and the seaward beach berm, which is equivalent to the upper limit of wave run-up at high tide. The foreshore is the part of the shore/beach which is wet due to the varying tide and wave run-up under normal conditions, i.e., excluding the impact of extreme storm waves and storm surge. This means that the foreshore 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 administrative upper delineation of the foreshore is often defined as the intersection between the MHW line and the coastal profile, which is identical to the definition of the shoreline.

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). 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.

Headland

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.

Infragravity waves

Ocean surface waves with a period of typicallly 25-250 s. They arise in particular through non-linear interactions within wave groups. 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 foreshore, tidal flats and (parts of) salt marshes are intertidal zones.

Jetty

  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.

Lagoon

Area of relatively shallow water situated in a coastal environment, separated from the open marine conditions by a natural barrier (a sandbar, 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.

Littoral drift or Longshore sediment transport

Littoral drift is the term used for the longshore transport of non-cohesive sediments, i.e. mainly sand, along the foreshore and the shoreface due to the action of breaking waves and longshore current. The littoral dift is also called longshore sediment transport. Formulas for the longshore sediment transport are given in: Long-term modelling using 1-line models - GENESIS and new extensions.

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, and by the surplus water which is carried across the breaker zone towards the coastline. See: Shallow-water wave theory.

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.

Meandering

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

Mechanical removal of consolidated soil or unconsolidated material (aggregates like sand, gravel, shells) from seabed, beach or dunes.

Mitigation

Action (or series of actions) aimed at (partly) neutralising the adverse impacts of past or planned human interventions.

Model

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.
  • 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 modelling, 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); see Stochastic and fractal methods in coastal morphodynamics. 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. 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.

Nearshore zone

The zone extending seaward from the low water line well beyond the breaker zone; it defines the area influenced by the nearshore currents. The nearshore zone extends somewhat further seaward than the littoral zone.

Nourishment or Sand nourishment

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, Artificial nourishment.

Mud

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, a mud bed can become highly resistant to erosion by currents. See: Dynamics of mud transport , Sediment deposition and erosion processes, Sediment transport formulas for the coastal environment, Characteristics of muddy coasts.

Overwash

Wave uprush over a natural or artificial coastal barrier. The term overwash is also used for the resulting sand deposit at the leeside of the barrier.

Offshore zone

The offshore zone is not clearly delimited. In relation to beach terminology, the offshore zone is often defined as the zone off the nearshore zone.

Progradation

Coastal extension into the sea due to natural aggradation. Coastal progradation occurs specifically where rivers supply large amounts of sediment. Coastal extension due to fall of the relative sea level is generally termed Marine regression.

Propagation speed

The speed at which a wave propagates. The speed [math]c[/math] of a wave propagating without frictional losses in shallow water is proportional to the square root of the water depth [math]h[/math] (formula: [math]c=\sqrt{gh}[/math]). A wave group propagates at a smaller speed [math]c_g[/math] than the constituent short waves. Wave energy propagates at the speed of the wave group. See: Shallow-water wave theory.

Reclamation

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

Reef

A ridge of material at or near the surface of the ocean. Natural reefs are made of rocks or the skeletons of small animals called 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: Natural barriers, Artificial reefs.

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

A local current directed away from the shore, bringing the surplus water carried over breaker bars back into deeper water. Rip currents may occur at certain intervals along the shoreline as a result of the interaction between breaking waves and the nearshore bathymetry, see: Rhythmic shoreline features, Stability models. The rip opening in the breaker bars will often form the lowest section of the coastal profile; 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.

Runnel

Small beach channel draining a tidal pool left on the beach after high water.

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 sometimes referred to as schorre or kwelder. The soil may be composed of deep mud and peat. Salt marshes usually form in areas that are well sheltered, such as lagoons and estuaries where fine sediments can be deposited. They also form behind spits and artificial sea defences where tidal waters can flow gently and deposit fine sediments. See also: Salt marshes.

Sand bank

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

Sand wave

  1. A subaqueous dune;
  2. A 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. 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.

Sediment

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 2.6 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: Sediment deposition and erosion processes, Gravel Beaches.

Sediment cell, Littoral cell or Coastal cell

A coastal compartment that contains a complete 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.

Sediment transport

The capacity of currents and waves to move sediments. 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.

Seiche

Oscillation of a semi-enclosed water body caused by incoming long-period waves (periods typically in the range 200-2000 s). Seiches can be strongly amplified in harbour basins if the period is close to the harbour resonance period, causing damage to ships and moorings.

Setback area

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

Shelf sea

See Continental shelf.

Shoaling

Amplification of incident waves in the nearshore zone before breaking. Amplification is due to (approximate) continuity of the wave energy flux [math]F=c_g E[/math] seaward of the breaker zone, where [math]E[/math] is the wave energy and [math]c_g[/math] the wave group propagation speed. The landward decrease of [math]c_g[/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.

Shore protection

See Coastal protection.

Shoreface or Littoral zone

The shoreface or littoral zone is part of the active coastal zone situated seaward of the low water line. This zone extends seaward from the foreshore to some distance beyond the breaker zone. The littoral zone is the zone in which littoral morphodynamic processes take place, related mainly to longshore sediment transport and cross-shore sediment transport. The width of the instantaneous littoral zone varies dependent of the wave conditions. In the general context, the littoral zone is defined as the zone corresponding to the yearly wave climate. The width of the littoral zone can thus be defined as the width of the sediment transport zone for the significant wave height, which is exceeded 12 hours per year, [math]H_{s,12h/y}[/math].

Shoreline

The intersection between the mean high water line and the shore. The line delineating the shoreline on Nautical Charts (Sea Maps) approximates this Mean High Water Line. The shoreline is not easy to identify in the nature in contrast to the coastline, which is based on a clear morphological shift between the shore and the coast.

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. 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.

Siltation

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 tide

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

Spit

An accretionary feature formed by littoral drift, consisting of a long narrow accumulation of sand or gravel, lying generally in line with the 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.

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 surge at a location is approximately 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.

Stratification

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: Salt wedge estuaries.

Surf beat

Oscillation of the wave set-up on the foreshore. The oscillation is related to the breaking of wave groups in the surf zone. See: Shallow-water wave theory.

Surf zone

See breaker zone.

Swash

Wave propagation on the beach, consisting of a decelerating onshore phase (swash or uprush) and an accelerating offshore phase (backwash or downrush). On dissipative coasts swash processes are dominated by infragravity waves. See: Swash zone dynamics.

Tidal bore

Breaking tidal flood wave. Tidal bores occur in shallow funnel-shaped estuaries with a large tidal range (generally more than 6 m).

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

Drainage channel of a salt marsh.

Tidal flat

Shallow, sandy or muddy part of the foreshore, 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, a tidal flat normally develops when 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 higher than 15.

Tidal inlet

The entrance to estuaries on sandy shores that have formed where sand barriers or spits enclose bays. They comprise a flood and ebb tidal delta, and a deep narrow throat through which strong currents flow.

Tidal prism

Volume of water flowing during flood through a tidal inlet, or through a certain 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.

Tidal ridge

Sand ridge with a length of several tens of kilometers and height of a few tens of meters, produced by tidal currents on a 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.

Tide or Astronomical tide

The astronomical tide is generated by the rotation of the earth in combination with the varying gravitational impact on the ocean of the sun, the moon and the planets. These phenomena cause predictable and regular oscillations in the water level, which is referred to as the tide. The astronomical tide at a specific location can be predicted and is published in Tidal Tables. See: Tidal motion in shelf seas.

Transgression

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

Tsunami

Wave (or group of waves) generated by a subsea earthquake. Tsunamis propagate very fast over the ocean with very few energy loss due to the great ocean depth (see: propagation speed). In nearshore waters the amplitude increases dramatically (up to a factor 10) due to shoaling.

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: Dynamics of mud transport.

Turbulence

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.

Wave

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, which are generated far away at the ocean and propagate to the coast with minor energy losses. Coasts situated on inland seas are mainly subjected to locally generated short-crested waves, also called "sea", see Waves.

Wave breaking

Waves start breaking (by spilling or plunging) when entering shallow water, where the 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: 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]).

Wave group

Incident waves generally arrive in groups, corresponding to the superposition of short-wave trains with slightly differing wave lengths and frequencies. 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.

Wave run-up

The sum of swash 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.

Wave set-up

Water level elevation at the foreshore related to wave breaking in the surf zone. Wave set-up is proportional to the wave height at the breaker line. As a rule of thumb, wave set-up is of the order of 20% of the offshore significant wave height. See: Shallow-water wave theory.


References

  1. ↑ Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines
  2. ↑ Coastal engineering Research Center, Department of the Army, Waterways Experiment Station, 1984. "Shore protection manual".



The main author of this article is Mangor, Karsten
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