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Coastal dynamics course summary
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lOMoARcPSD|2406970Summary Coastal Dynamics
Chapter 2: Large-scale geographical variation of coasts
1. Introduction
2. Cumulative evolution of coastal systems
2.1. Geological timescale
The timescale is divided in mane periods and subdivided in many sub-periods. The sub-periods
are becoming shorter due to our knowledge and records of the later periods. The Current
Geological period Quaternary can be divided into the Pleistocene and the Holocene. During
this period continental separation determined the geological initial state of the boundaries.
Secondly, ice-sheets retracted, causing sea level to rise significantly.
2.2. Continental drift
Around 200 million years ago a single super continent existed (Pangaea). Land masses that
formed Pangaea started drifting apart. The theory of the drifting continents has been fully
developed by Wegener around 1912-1915. It was until the 1960s this was proven by geological
evidence. The movement of plates has governed the formation of coastlines and determines
the basic features of coasts.
2.3. Pleistocene inheritance
During the Pleistocene, earth experienced several periods of cooling and heating up. The vast
amount of water trapped in ice caused severe water level fluctuations (up to 100m). The
inheritance is greatest in the rocks formed by ice that have been deposited in coastal systems.
The so called Pleistocene layers is the layer on which most building foundations in the
Netherlands have been constructed.
3. Tectonic control of coasts
3.1. Plate tectonic theory
The continents are part of the lithosphere. This sphere is divided in 12 large and tightly fitting
plates and several small ones. These plates are in permanent motion moving up to 10 cm per
year. Where plates meet, either mountains or oceanic trenches are formed.
3.2. Tectonic plate setting of coasts
Broad coastal characteristics are related to the position of these coasts o the moving tectonic
plates. We recognize 3 types:
Leading edge or collision coasts: Coasts near converging plates. Usually rugged, cliffed
coastlines in tectonically unstable regions.
Trailing edge coasts: Coasts located away from plate boundaries in tectonically stable
areas because the continent and ocean floor are the same plate. This category of coasts
can be divided into Neo/Amero/Afro. The difference between these types lays in the
state of the developed ocean and the sediment that is available in the coastal area.
Marginal sea coasts: Tectonically stable coasts protected from the open ocean by
island arcs at converging plate boundaries.
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Summary Coastal Dynamics
3.3. First-order coastal sedimentary features – Factors important to coastal sedimentary features
are:
Continental shelf width
Wide and flat shelves facilitate the development of extensive sedimentary features
whereas narrow and steep shelves facilitate the opposite. This causes large deltas and
barrier islands systems to easier develop on coasts with wide shelves. Hence, leading-
edge coasts are dominated by sea cliffs and rocky headlands whereas trailing-edge
coasts are dominated by depositional features like barrier islands.
The shelf width also has hydrodynamic effects. Narrow shelves lack the inability to
dampen wave energy due to lack of a long gentile slope inducing bottom friction. This
causes the waves to be more powerful near narrow shelf coasts. Wider shelves do have
this frictional dampening mechanism and thus wave energy is lower. However, tidal
amplitudes are generally higher at wider shelves because of resonance of the (by
friction) slowed tidal waves.
Marginal sea coasts also have the potential to develop large sedimentary features due
to the presence of a gentle slope and shallow waters of the continental shelves in these
areas, the reduction of wave energy due to the sheltering by nearby objects as island
arcs or land masses and the restricted size of these seas limiting fetch. This all results in
low wave energy and therefore little erosion and a relative high deposit of sediment.
Sediment availability
As said before, sediment supply determines the type of trailing edge coast. American
trailing edge coasts receive large supplies of sediment due to the mountainous origin of
the rivers. These trailing edge coasts are therefor called Amero-trailing edge coasts. In
Africa, both sides of the continent have developed trailing edge coasts. Due to the lack
of mountainous regions and a much less erosive climate, these trailing edge coasts
typically lack sedimentary features as Deltaic areas along the coast. The coasts are
therefore called Afro-trailing edge coasts.
In Marginal sea coasts, sedimentary features can easily develop due to the low-energy
coastal conditions and the large amount sediment. Some of the world largest delta’s
have developed in these regions (for e.a. the Mississippi Delta)
Leading edge coasts are usually fed by short, steep and straight rivers. These may
transport large amounts of sediment. However, large sedimentary features rarely
develop due to a narrow and steep shelf. Most of this sediment is therefore lost to
deep sea.
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Summary Coastal Dynamics
4. Pleistocene inheritance of cliffed coasts
Roughly 75% of continental and island margins is lined with cliffs. These cliffs are commonly
related to tectonics. There are however cliffs, that where formed under different conditions.
Glaciers from the Pleistocene are known to have created the fjords by moving ice carving out
these steep valleys.
Cliffs can also be formed by the deposition of sediment by the ice. This has formed a thick layer
called a till. The layer usually forms near the edge or beneath the ice sheet and contains a wide
variety sediment sizes. Large parts of the British & Danish coast where formed in this manner.
A third variety of rocky cliffed coast is associated with areas where carbonate sediments from
skeletal remains and coral debris accumulated. Lithification of the sediment caused the it to
cement together.
5. (Holocene) transgression versus regression
5.1. Geological sea level changes
Local mean sea level height with respect to land. Local changes to the perceived mean sea
level can be either relative (for ex. Land subsidence) or absolute sea level rise. The absolute
sea level rise can also be called eustatic and can be caused by:
Changes in volume of the ocean water
Changes in the volume of the ocean basis due to tectonic plate divergence
Changes in the distribution of water due to differences in earth’s gravity field
Of the eustatic sea level rise, volume change of the ocean water has been the most
important in earth’s history. The alternating glacial and interglacial periods caused the sea
level to vary significantly (more than 100m). The eustatic sea level changes cause
deformations in earth’s crust. These deformations are caused by loading and unloading of
ice during a Glacial period (Glacio-isostasy), and loading and unloading of water during an
interglacial period (Hydro-isotasy)
In addition to eustatic & isostatic effects, regional effects are also of importance:
Regional subsidence: subsidence caused by compaction of sediments or withdrawal of
resources.
Tectonic activity: movement related due to tectonic movement.
In the past century, global sea level rise has accelerated. Global warming has been blamed
for this.
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Summary Coastal Dynamics
5.2. Role of sea-level rise in Holocene coastal evolution
When sea-level rises, the local equilibrium of the coast is distorted and the accommodation
space enlarges. If there is no source of sediment the land will retreat (transgression). If there is
a source of sediment the coast can advance (regression). During the Holocene coasts in the
Mediterranean and the Caribbean have transgressed due to sea relative sea level rise while
parts of Asia, Southern parts of Africa, South America and Oceania have seen regression of the
coastal areas.
Figuur 1; Curray’s diagram : shoreline migrational factors
5.3. More recent coastal development.
Around 70% of the world’s sandy coasts has shown retreat in the last century; about 10% had
propagation, while the remaining 20% has been stable. The sea level rise is seen as the cause
of this. Sea level is expected to rise even further the coming century. Especially low lying
countries with poor coastal defenses are threatened.
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