Definitions of coastal terms

To ensure sound communication it is important to define the coastal terms used in coastal engineering and shoreline management. Therefore, definitions of terms for coastal features, processes and management issues are given in the following, see also the coastal profile below.

Definition of coastal terms, mainly from Shore Protection Manual, 1984

Definition of coastal terms:

Note: Under construction: Mathematical formulas have not yet been added

ANGLE OF INCIDENCE

(α): The angle between the wave propagation direction and the normal to the coastline or the angle between the wave front and the coastline. The deep water angle of incidence is denoted α₀

BACKSHORE

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

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. Bars are very mobile formations, which tend to be in mobile equilibrium with the presently occurring 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 bars formed by the rip currents, see under: RIP CURRENTS.

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, 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.

BEACH BERM

A nearly horizontal shore parallel berm 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. During dry periods berms are often formed across openings to minor streams and lagoons, such blocking are also referred to as bar formations.

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. The artificial beaches shall be exposed to wave action and shall have a stable plan and profile shape.

BREAKER-ZONE or SURF-ZONE

There is no clear definition of the breaker-zone, but it can be defined as the zone extending seaward from the shoreline that is exposed to depth-limited breaking waves. The outer limit of the breaker-zone is called the BREAKER-LINE. However, the instantaneous width of the surf-zone varies with the instantaneous wave conditions. In this context we define the surf-zone as the zone valid for the yearly wave climate defined by the significant wave height H_S,12h/y, which is the wave exceeded 12 hours per year. The width of the breaker/surf-zone can thus be defined as the width of the zone within which H_S,12h/y breaks. The breaker/surf-zone is somewhat narrower than the littoral zone. It is evaluated that 80 to 90% of the yearly littoral transport takes place within the breaker or surf-zone.

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. According to () the closure depth can be calculated using the expression:

d₁ = 2.28H_S,12h/y - 68.5 (H²_s,12h/y) / (gT²_s)

where d_l is the closure depth relative to mean low water-level, H_S,12h/y is the nearshore significant wave height exceeded 12 hours per year, and T_s is the corresponding significant wave period. This definition is valid for "normal" sandy coastal profiles.

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.

COAST EROSION

Erosion in the coastal profile. This is taking place in the form of scouring in the foot of the cliffs or in the foot of the dunes. Coast erosion takes place mainly during strong winds, high waves and high tides and storm surge conditions. Coast erosion results in coastline retreat. The rate of erosion is correctly expressed in volume/length/time, e.g. in m³/m/year, but erosion rate is often used synonymously with coastline retreat, and thus expressed in m/year.

COAST PROTECTION/DEFENCE

Three different protection/defence definitions are used as follows:

COAST PROTECTION

Measures aimed at protecting the coast against coastline retreat, thus protecting housing, infrastructure, the coast and the hinterland from erosion often at the expense of losing the beach and the dynamic coastal landscape. Coast protection often consists of hard structures such as revetments or groynes.

SEA DEFENCE

Measures aiming at protecting low-lying coast and coastal hinterland against flooding caused by the combined effect of storm surge and extreme astronomical tides. Sea defence often consists of dikes or seawalls of some kind, or in the form of artificial dunes.

SHORE PROTECTION

Measures aiming at protecting, preserving or restoring the shore and the dynamic coastal landscape as well as protecting against coastline retreat to the extent possible.

COASTAL AREA

The land and sea areas bordering the shoreline.

COASTAL HINTERLAND

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

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.

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.

COASTLINE RETREAT

Coast erosion causes the coastline to retreat.

DEVELOPMENT ACTIVITY

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.

DUNE

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.

ENVIRONMENTAL IMPACT ASSESSMENT (EIA)

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

1. a description of the avoidable and unavoidable adverse environmental effects
2. a description of alternatives to the activity which might be less harmful to the environment, together with the reasons why such alternatives were rejected; and
3. a description of any required irreversible or irretrievable commitments of resources required by the proposed development activity

EROSION or PROFILE EROSION

The process of wearing away material from the coastal profile due to imbalance in the supply and export of material from a certain section. Erosion will take place on the shoreface and on the beach if the export is greater than the supply of material, this means that the level of the seabed and the beach will decrease. The deficit can be due to both cross-shore processes and longshore processes. Erosion due to cross-shore processes mainly occurs during extreme events associated with storm surge, which partially is a reversible process. The most important reason for long-term erosion is a deficit in the littoral drift budget, which is often caused by a deficit in supply of sand to the area in question.

FORESHORE or BEACH FACE

The zone between MLW and the seaward berm, which is equivalent to the upper limit of wave uprush at high tide. The latter is identical to the seaward beach berm. The foreshore can be said to be 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 MHW and the coastal profile (MHW line). However, for practical reasons the administrative upper delineation of the foreshore/beachface is defined as the intersection between the MHW line and the coastal profile, which is identical to the definition of the Shoreline.

LAND

The area located landward the shoreline, which is identical to the area landward of the MHW line. This means that the land consists of the backshore, the coast and the coastal hinterland. This definition is identical to the one used on international sea charts.

LITTORAL TRANSPORT

Littoral transport is the term used for the transport of non-cohesive sediments, i.e. mainly sand, along the foreshore and the shoreface due to the action of the breaking waves and the longshore current. The littoral transport is also called the longshore transport or the littoral drift.

LONGSHORE CURRENT or NEARSHORE CURRENT

The longshore current is the dominating current in the nearshore zone, it 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.

MANAGEMENT UNIT (MU)

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

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 seawards than the littoral zone.

OFFSHORE ZONE

The offshore zone is not well defined. In relation to beach terminology, it is thus not clear if it starts from the littoral zone, from the breaking or from the nearshore zone. In the present context, the offshore zone is defined as the zone off the nearshore zone.

RIP CURRENTS

At certain intervals along the shoreline, the longshore current will form a rip current. It is a local current directed away from the shore, bringing the surplus water carried over the bars in the breaking process, back into deep water. The rip opening in the 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.

SEA

The open coastal waters located seawards of the shoreline. The seawater is saline. This definition is identical with the definition of the sea in most nautical maps. The sea extends into major bays, but not into channels, creeks, rivers, estuaries and lagoons. These internal waters are characterised by having brackish to fresh water.

SEA DEFENCE

See COAST PROTECTION/DEFENCE

SEA LEVEL RISE

The so-called greenhouse effect or global warming may cause a Sea Level Rise, which will have a great impact on the long-term coastal morphology. The possible and gradual Sea Level Rise will cause a general shoreline retreat and an increased flooding risk and has to be handled according to the local conditions.

SETBACK AREA

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

SHORE PROTECTION

See COAST PROTECTION/DEFENCE

SHOREFACE or LITTORAL ZONE

The active littoral zone off 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 the littoral processes take place; these are mainly the long-shore transport, also referred to as the littoral drift, and the cross-shore transport. The width of the instantaneous littoral zone varies dependent of the wave conditions. In the general context, we will define the littoral zone as the zone corresponding to the yearly wave climate. The width of the littoral zone can thus be defined as the width of the transport zone for the significant wave height, which is exceeded 12 hours per year, H_S,12h/y.

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:

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

SHORELINE RETREAT

Shore erosion causes the shoreline to retreat.

STORM SURGE

Is 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, decreasing water depth and the horizontal boundaries of the adjacent water. The storm surge does not include the effect of the astronomical tide. The storm surge at a location is inversely proportional with the water depth in the offshore area off the shoreline. This means that shores out to deep oceans will only be exposed to relatively small surge where as shores out to shallow seas can be exposed to high surge.

TIDAL FLAT

Shallow, often muddy, part of foreshore, which are covered and uncovered by the rise and fall of the tide. 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 HS, is higher than 15.

TIDAL WAVE

Is the combined effect of astronomical and meteorological surges - the popular expression for an unusually high and destructive water level along a shore. The expression tidal wave also includes the influence of the associated waves.

TIDE or ASTRONOMICAL TIDE

The astronomical tide is generated by the rotation of the earth in combination with the varying gravitational impact on the water body 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.

References

• Shore practical manual 1984 (coastal profile image)

Links to definitions

Definition: BEACH or SHORE

• Definition: COAST PROTECTION
• Definition: SEA DEFENCE
• Definition: SHORE PROTECTION

Definition: SHORE PROTECTION: See COAST PROTECTION/DEFENCE

References

Author: Karsten Mangor, DHI, km@dhi.dk