A definition of coastline was given by Luis de Camões who, in addition to being a great poet, was a man of many knowledge, with emphasis on geography. In Canto III, of “Os Lusíadas”, it can be read: “Where the land ends and the sea begins”.
The term coastline, brought into the geographic and geological lexicons, began as a common vocabulary word. As a noun, it refers to the strip of land next to the sea or seaside, as it is also said. As an adjective, it qualifies everything related to this same track. Noun or adjective, it is opposed to the word “interior”, which is also a noun and adjective.
Do not confuse coastline with coast or coastline. This should be understood as the line that, on the coast, separates the sea from the land or, in other words, that marks the border between the emerging continent and the ocean, as it is drawn on maps, respecting the range of high tide.
In political-administrative language, coastline designates the strip of land next to the coast, covering a width of around 50 km inland, which varies from country to country.
In biology, coastline refers to the set of ecosystems located on the land/sea border subject to the influence of tides, and in this biome we can distinguish three units:
1 – supralittoral, supratidal or supratidal zone (an Anglicism coming from the English “tide”, tide) – above the line of the highest high tide;
2 – mesolitoral, intertidal or intertidal zone – between low tide and high tide;
3 – infralittoral, subtidal or subtidal zone – immediately below low tide.
Towards the sea, with the depth increasing progressively and gently, there is the circalitoral zone, adjacent to the coast, already on the continental shelf (or neritic zone, from “Nerita”, a gastropod identified by Linnaeus, in 1758) which does not suffer the influence of the tides, penetrated by sunlight and, hence, the name photic zone.
For the geologist or geographer, in a view very close to that of the biologist, coastline is not only:
1 – the strip emerging from the coastline limited superiorly (on the landward side) by a physiographic accident (a cliff or a simple break in slope) or by the permanent occupation of vegetation not tolerant to salt, but also
2 – the immersed strip, limited inferiorly by a line below which the seabed is not significantly disturbed by the usual swell in the region.
On our west coast, this lower limit is around 10 m deep, while on average it is 6 m on the south coast (Algarve).
The adulteration of the physical landscape, in the name of development, whether inland or on the coast, is a fact that is reaching worrying proportions.
Regarding the coast, the effects of human intervention are now clearly visible and the solutions found to minimize or eliminate them are not always the best.
The conclusion to be drawn from this reality is: “we cannot continue planning the coast with our backs turned to the knowledge that science is already able to provide”.
It is therefore necessary to know how to live with the sea and respect its codes, which we already know in reasonable detail. We know today that the geometry and dynamic characteristics of this fringe “where the land ends and the sea begins” result from natural oscillations in sea level, which are too slow and can only be referenced on a geological scale, which can be:
1 – eustatic – rise or fall in water level;
2 – epeirogenic – deformations of the crust, whether epeirogenic or orogenic.
Leaving aside a set of factors and conditions typical of civilization that, because they take place on the temporal scale of man and society and/or because they are more visible, and it is not insignificant to know better, interfere in the configuration of the coast:
1 – the nature and structure of the rocks (and their greater or lesser vulnerability to alteration and erosion);
2 – the climate, especially with regard to rainfall, temperature, winds and ice;
3 – vacancies (intensity and orientation),
4 – tides and marine currents.
5 – those resulting from chemical alteration and/or dissolution that sea water exerts on coastal rocks, with varying effects depending on their respective nature.
In conclusion, it can be said that the coastline is defined by natural laws, that is, by the laws of physics and chemistry, always underlying geological and biological processes.
And those responsible for “public matters” or the technicians at their service cannot ignore them.
In order to minimize the inconveniences caused by the aforementioned interventions, tests have been carried out in special tanks, where, in reduced models, they seek to simulate natural conditions and the changes to be introduced, in order to study their effects. Modernly, with the development of computer technology, mathematical models are being used for the same purposes.
We know today that the retention, in the large reservoirs of hydroelectric dams, of most of the inert materials in transit through the rivers, is one of the causes of the retreats observed on certain coastlines, particularly on beaches.
Another cause lies in the industrial extraction of aggregates (sand and gravel) from beaches, dunes and rivers, including estuaries, in the order of many, many millions of tons per year. The desilting of ports and bars is another cause of the aforementioned declines.
The construction of rocks, such as jetties and groins, with the aim of protecting certain sectors of the coast, always ends up transferring the same type of problems downstream and, generally, in an aggravated way.
The waves, triggered by the action of the wind, transmit the energy they receive from it to the coast and their erosive action is greatly enhanced by the abrasive effect of the materials (sand, pebbles, blocks) that they set in motion. As a result of this action, erosion or catamorphic coastlines are formed, characterized by cliffs, or steep cliffs, which retreat as the coastal or marine abrasion platform increases.
The continental shelf is the continuation of this same surface, destroyed in the recent geological past (Quaternary) and today submerged following the rise in sea level in the approximately 20 years following the last glaciation (Würm, in Europe, Wisconsin, in North America) referred to as the Flandrian transgression (described in Flanders, in northern Belgium).
We have examples of catamorphic coastlines on the Costa Vicentina and the one that extends north of the mouth of the Douro. The remains of this retreat, as evidence, are rocky points, such as capes or promontories, often extended into the sea by equally rocky points (islets, shoals, rocks, cinders, pebbles, rocks, etc., in the different local ways of saying), with emphasis on the capes of S. Vicente, Sagres, Espichel, Roca and Carvoeiro, with the well-known and elegant Nau dos Corvos.
When the sea recedes, the coastline is said to be anamorphic or accumulating. The beach, generally sandy (but sometimes gravel), and the dunes are located here. Following this regression of the sea, the cliff is freed from wave erosion, starting to evolve in a subaerial environment, until it acquires a balanced profile dictated by its nature and environmental climatic conditions. Easily recognizable in the coastal landscape, these testimonies of ancient coastlines are considered fossil cliffs.
The beach is, in most cases, an unstable accumulation of sand and sometimes gravel, pebbles or pebbles (three ways of referring to the coarser clasts), generally rounded by abrasion.
It represents an environment where the morphology-sedimentation binomial is characterized by great instability. Any natural or artificial modification introduced to the morphology of the beach or its sedimentary content (sand and, possibly, gravel) has an impact on the erosion-sedimentation balance.
All of our beaches are examples of anamorphic coastlines, from north to south, from west to east (in the Algarve).
NOTES:
Eustatic – This refers to slow variations in sea level caused by changes in the volume of water (ice and glacial melt) or by deformations of the seabed.
Epirogenic – Refers to slow upward or downward movements of the continental crust, responsible for regressions (fall in sea level) and transgressions (rise in sea level).
Orogenic – Said to be a set of internal geodynamic processes that generate mountain chains or orogens.
Inert – Technical term, among engineers, to refer to terrigenous sediments, especially sand and gravel.
Downstream – From the French “downstream”, from the old adverb “jus” (below). This term, initially used in relation to river flows and the like, in which the direction of movement is, naturally, from top to bottom, (from upstream to downstream), ended up entering current language with the meaning of “where to ” (downstream) as opposed to “from where” (upstream). On the Portuguese west coast, where sand is transported from north to south, upstream is to the north and downstream is to the south.
Author AM Galopim de Carvalho is a Geologist
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