Dynamic Coastal Environments Flashcards
- Coastal Processes and Features
Students should be able to:
(i) demonstrate knowledge and understanding of coastal processes - wave action, refraction, erosion, transport, swash and drift-aligned coasts
(ii) demonstrate knowledge and understanding of the formation of landforms at:
- high-energy coasts: headlands, cliffs, arches, stacks and stumps
- low-energy coasts: beaches and dunes, spits, tombolos and bars
The Nature of the Coast
Coastlines can be described as consisting of three zones: the backshore or upper beach (backed by cliffs or sand dunes), the foreshore (uncovered at low tide) and the offshore (covered by water).
Coastal Processes - the Energy from the Sea
Waves, tides, currents and changing sea level provide the flows and forces to modify and alter the shore. Coastal environments vary greatly over time and space.
Twice daily tides, generated by the gravitational pull of the moon and sun, sweep across the coastal zone, constantly altering the point at which waves arrive. The coast of Western Europe has some of the highest tidal ranges in the world, meaning that, at low tide, huge stretches of land may be exposed, only to be covered again in a few hours.
Waves are created by the wind that blows over the water surface and the waves then transfer the energy across the oceans until they reach land. Waves vary in their nature and impact. Steep, plunging waves may cause severe erosion of the coastline, while shallower but powerful surging waves may carry sediment towards the shore.
The Nature of Waves
The Nature of Waves
Sea waves are generated by the friction of winds blowing over the open sea. The power of a wave reflects the speed of the wind and the time and distance (fetch) over which it blows. This is why waves on the west coast of Ireland tend to be powerful, as they can be generated across the width of the Atlantic Ocean, while east coast waves are restricted by the shorter potential fetch of the Irish Sea. Waves in the open sea merely transfer energy as waves of transfer; water particles move in a circular motion as waves pass through.
Swash and Backwash
When waves enter shallow water near the coast the friction with the seabed slows the base of the wave, the circular water movement becomes increasingly elliptical and eventually breaks down forming waves of translation, throwing water forward onto the shore - the swash. The same water then runs back down the shore pulled by gravity - the backwash.
The relative power of the swash and backwash will determine if each wave deposits or erodes material from the shore. The precise nature of the wave varies according to variable of the wind and the shape of the coastline.
Winter Conditions
Winter storms often form high energy plunging waves, termed destructive waves, which commonly remove material from the foreshore, making the beaches steeper and forming a ridge where the wave breaks.
Summer Conditions
Gentle summer winds over the open ocean create low energy swell waves that, when they break, push material onto the shore adding to the beaches. These are termed constructive waves.
Wave Refraction
An important relationship with coastlines is the direction from which waves approach.
At first it would appear that this would be determined by wind direction. For example, to the west of Ireland the most persistent winds are from the south west so waves will commonly come from this direction. However, as waves approach the land and shallower water, the shape of the seabed will alter their form and, as they slow down, they often become increasingly parallel to the shape of the coastline. This important process is known as wave refraction.
Most often waves will approach the shoreline at an oblique angle: at one end of the wave front friction between the wave base in shallow water slows the wave and then the wave front appears to turn towards the shore. If refraction is complete, each wave front will break directly onto the shore and the swash and backwash will run up and down the beach.
Waves are approaching a complex coastline with headlands and bays. As waves approach the headlands they feel the shallower shore and friction slows them down. Further along, the same wave in the bay continues to move onshore, only slowing later. Each wave can change shape to parallel the coastline and break directly onto the shore. At the same time it shows how wave energy will be concentrated onto the headlands, causing erosion.
Wave Refraction Diagram
The wave energy represented by the line a-b becomes focussed on the smaller area a’-b’ Meanwhile, in waves approaching the bay, their energy, represented by the line c-d will be spread across a wider area on the shore, c’-d’, reducing erosion and allowing the deposition of material.
Longshore Drift (LSD)
Despite the process of refraction, waves often reach the coast at an angle to the shore. In this case the swash of each wave will break at an angle onto the shore and carry water and sediment up the beach at the same angle. The subsequent backwash of each wave wil run I directy back down the beach, under gravity to the sea. This initiates a process known as longshore drift (LSD), by which material, such as sand and shingle, is moved along the coastline in a zigzag or saw tooth manner.
Swash and Drift-aligned Coasts
Shorelines that are dominated by the continuous movement of sediment along the coast, due to the local prevailing winds driving waves at an angle to the shore, are known as drift-aligned coasts. These are associated with distinctive deposition features including spits, bay bars, tombolos and cuspate forelands.
The alternative is swash-aligned coasts which, as closed sediment-cell systems, involve limited movement of materials along the coastline. Features of swash-aligned coasts include bay head and barrier beaches (offshore bars). On swash-aligned beaches the swash and backwash of waves move material up and down the beach, sorting it into different sizes of materials and forming long ridges or berms across the beach. In some cases, ridges of pebbles are deposited near the high tide mark, often with a cusp or saw tooth shape along their edge. At low tide, ripple marks stretch across the sandy beach and shallow ponds or runnels lie parallel to the coast.
Wide beaches may form the source area for wind-blown sand to form sand dunes above the high tide mark of the beach.
Coastal Erosion
Both the sea and the atmosphere are at work on the coast to break down the rocks that form the shore. The common sub-aerial processes of weathering - mechanical, biological and chemical - will all operate here.
On rock faces, freeze thaw weathering will cause rocks to fall to the foot of cliffs; plant roots and wildlife can widen cracks and weaknesses in rocks; and the dissolving of calcium carbonate by rainfall will continue in limestone deposits. The coast has some special forms of mechanical and biological weathering processes. The abundance of sodium and magnesium compounds at the coasts produces salt weathering. As these compounds crystallise, they expand within the cracks and joints of the rocks, widening these lines of weakness. Additionally marine organisms such as molluscs, sea urchins, sponges and boring worms biologically attack rock surfaces along the shore.
Marine Erosion
Waves use their energy and chemistry to erode coastlines in four complex and interacting processes: hydraulic action, abrasion, solution and attrition.
• Hydraulic Action occurs when the air in cracks in the rocks is compressed by the force of breaking waves. As the wave subsides, this air expands again and these pressure changes can open the joints and weaken the rock. This process is also known as wave pounding.
• Abrasion is a highly effective form of erosion along exposed coastlines. Waves throw sediment, including sand, pebbles and shingle, against the base of cliffs, wearing them back. Abrasion and hydraulic action are both concentrated between the high and low tide marks.
• Solution results from salts and organic acids in the seawater attacking the chemistry of rocks. It is particularly effective on lime-rich strata and is oin evident in rock pools on rocky shorelines.
• Attrition involves the wearing down of the material eroded from the cliffs. It provides the tools used in abrasion, as well as the wide variety of sediment that forms beaches and other depositional features. The rapid rounding and polishing of beach materials, including broken glass, is evidence of the effectiveness of this process.
Coastal Transport and Deposition
Sources of Material
Sediment on coasts has many sources including eroded cliff material, river sediment, glacial deposits and offshore marine material.
Transportation of Material
The sea moves this material as bedload by traction and saltation (rolling, dragging and bouncing sediment along), or as suspended load (with silt or sand held up by turbulent flow). On the shore, transport is associated with the swash and backwash of waves and the longshore drift process noted earlier.
Sediment Cells
The coastal sediment system (littoral cell system) is a model for the study of processes and patterns on a section of coast. Each cell is treated as a self-contained unit in which inputs and outputs are balanced and sediment is transferred within the cell.
Formation of Coastal Landforms
The geology of the coastline is a key factor in determining the outcome of the interaction betwen land and sea.
At its simplest, rocks may be classified as those that are easily eroded and those that resist erosion. Often it is the arrangement of the rocks that guides the coastline that is formed. Alternating bands of hard and softer rock, running at right angles to the shore can form a headland and bay topography. Differential erosion rates will also be associated with existing weaknesses in geology, such as fault lines or the existence of river or glacier valleys. The distinctive indented coastline of South West Ireland, Western Scotland and Western Norway are al the consequence of the sea invading and eroding valleys that were created by glaciers.
High-energy Coasts
These are coasts that are subject to regular strong wave impact for much of the year. They often face the open sea, where the prevailing wind has a long fetch over which powerful waves can form. The west coasts of Scotland and Ireland are good examples.
High-energy coasts are associated with erosional landforms including cliffs, headland features and coarse sand and shingle beaches in the bay environments.
Cliffs
The primary coastal landform associated with erosion is the cliff or cliff line. This is a break in slope between the land and sea that may be low and gentle, or high and steep. Cliffs are the outcome of sustained erosion along the shore and active cliffs are associated with retreating coasts. Figure C12 illustrates how cliffs are formed and how they can grow in size as the coastline retreats, creating the related feature of the wave-cut platform.
Initially, erosion is focused in the relatively narrow zone between high and low tide. At this point, a small notch will form as material is weakened and Cliff removed. Overfime this wave-cut notch will undermine the rock above and collapse will occur. The fallen material can prevent further erosion for a time along the cliff base but eventually attrition and other marine erosion processes will remove this material. and erosion of the cliff base will be renewed. Gradually, the cliff face retreats and the residual wave-cut platform, often covered by sediment, is widened. The rate of cliff retreat, assuming the wave energy conditions are unchanged, will slow over time as waves will have further to travel across the wave-cut platform before reaching the diff base. Finally, sea erosion of the cliff base may virtually cease and cliffs become degraded by sub-aerial processes.
In the UK, cliff retreat rates of 50 cm per year are not uncommon, while the government regards rates of 100 cm or more per year as a cause for concern.
Headland, Arches and Stacks
Earlier in the discussion of wave refraction it was demonstrated that wave energy can become focused onto headlands and in such settings a series of distinctive erosion landforms is often found.
Example
The north coast of Ireland has many dramatic erosion landscapes. At Fair Head (Benmore), Co Antrim, a band of hard volcanic rock known as a sill juts dramatically out into the North Channel. To the south, along the Co Antrim coast, each of its famous Glens is separated by headlands and promontories, between which lie long curving bays and beaches. Many headlands show the impact of Concentrated wave erosion at their base, where a wave-cut notch is evident, especially at low tide. Where there are weaknesses in the geology, faults, jointing or weaker strata, the sea will widen and open caves. Caves in a headland can be worn backwards, perhaps to meet a similar feature developed along the same line of weakness from the opposite side of the headland. Where this happens, a hole through the headland leaves the upper rock spanning an arch.
Ongoing erosion, both by waves and sub-aerial processes, will widen the arch undermining the spanning rock until it collapses to the sea below. The remaining part of the headland is now separate from the land and often surrounded by the sea as a stack. Even stacks in their turn will be eroded and reduced to stumps of rock that only appear at low tide.
Example
At White Rocks in Co Antrim there are a number of caves, arches and stumps, including the Wishing Arch.
Arch The collapse of an arch is a dramatic event and several have happened in recent gal decades, including the collapse of one of a double arch known as the London Bridge, in a headland in South East Australia. Well known stacks include Dún Briste (The Broken Fort), Co Mayo and the Old Man of Hoy, in the Orkneys (Figures C19 and C20).
Historic paintings reveal that 200 years ago the Old Man of Hoy was an arch, the two legs giving its name. Later its collapse left the 137 m high stack detached from the cliff and the ground between covered by rubble from the arch.
Blowholes and Geos
Other landforms of erosion that can be seen at coastlines include blowholes and geos. These may form in headlands or along cliffs.
A blowhole, as the name might suggest, involves the blowing out of water at the top of a cliff. This is the result of the sea eroding a hole in the roof of a cave. At high tide waves sweep into the cave and force air and water out through this hole. These dramatic features can be short lived as erosion continues.
A geo is a narrow, steep sided inlet that may have been a constricted cave in which the roof has collapsed.
The sediment created by erosion of the coast must be transported and deposited somewhere. While some of it is carried offshore into deeper water, much is moved along the shoreline to lower energy environments, where it is deposited forming other distinctive coastal landforms.
Low Energy Coasts
The landforms of deposition are closely associated with low-energy coastal zones. Low and high-energy zones are frequently close together. An offshore island can shelter a section of otherwise storm torn coast or the change in orientation along a shore, from say north to north-west, may expose the coast to the full force of an ocean wide fetch, creating surfing breakers.
The earthquake induced tsunami wave of Boxing Day 2004 devastated many coastlines. However, in places shores only a few metres from destruction were virtually untouched, as the shape of the land and seabed sheltered them from the enormous waves. In short, it is possible to find erosion and deposition landforms adjacent to each other. Between most headlands bays are found, which are dominated by the most common of all deposited landforms - the bay-head beach. Clay and silt sized particles are often too light to remain on beaches, so the coarser sand particles are left. Beaches can be made of a wide variety of materials, including sand, pebbles and cobbles. These particles in turn are often sorted into size either up and down the beach or along the shore.
Difference between Swash aligned and Drift aligned
Two forms of beach/deposition coastlines can be distinguished: the
swash-aligned and the drift-aligned shore.
In swash-aligned environments the prevailing wind, and thus wave direction, acts at right angles to the shore,
whereas in drift-aligned environments the wind and wave fronts arrive at an angle to the shore, driving water and sediment along the beach.
Beaches
The shape and form of beaches are influenced by many factors and they often change seasonally. Destructive storm waves are more common in winter, with constructive swell waves dominant during summer months thus beaches may become wider and steeper in summer as material is added.
The different areas of the beach
Beaches consist of sediment material that lies between the low and high water, marked
with the upper beach created by sediment thrown up by storm waves. Successive ridges created by high tide lines are known as berms and further down the beach long transverse ridges separate parallel hollows called runnels where water lies at low tide.
Dunes
Strong onshore winds blowing across a wide stretch of beach can lift and transport fine sands onshore in a bouncing motion termed ‘saltation’.
Above the strand line high water mark, organic material and/or litter obstacles can trap this sand, forming small embryo dunes. Specialist plants that are adapted to this salty, windy and arid environment thrive in this shifting sand, including sand couch and marram grasses. The complex root network of the vegetation binds these embryo dunes, allowing them to grow in height and breadth. A cover of marram creates a
‘dead-zone’, some 10 cm above the dune surface, where wind falls to almost nothing and windborne sand is trapped. Over time, assuming a continuous supply of fresh sand, a successive line of dunes can develop with linear low-lying dune slacks running between them. Near the beach, dunes are almost pure sand and are described as mobile yellow dunes. Inland, older fixed dunes have a wider range of vegetation and plant species, and the soil is less alkaline and arid. These are termed the grey dunes. Dunes may be 15 or even 20 m high but are very fragile features. Damage to the vegetation cover or storm waves at high tides can cause them to erode rapidly. Blow-outs are large hollows where exposed loose sand has been swept away by storm winds.
Drift-Aligned Environments
Drift-aligned beaches form where the prevailing onshore wind generates waves that arrive at an angle oblique to the trend of the coast. These carry sediment along the shore in the zigzag process - longshore drift. The beach will be maintained as long as there is a continuous supply of sand available, for example from an eroding cliff line or from sediment brought down to the coast by rivers. If no such supply exists, the beach will gradually be depleted of sand at one end, possibly leaving only coarser pebbles or cobbles which the waves cannot easily remove. In this case, people have often attempted to keep the sand beach in place or increase its depth and width by building barriers across the beach to trap the drifting sediment. These are called groynes. Normally made of wood, concrete or simple boulders these run across beaches at right angles to the shore in groups known as groyne fields.
Spits
In either case, the longshore drift of sand down the coast will continue only until the shape of the coast changes. If the coastline turns away from the direction of drift by 30° or more, such as at an estuary or river mouth, then one of the coast’s most distinctive landforms may develop - a spit. Essentially, a spit is simply a beach that continues to extend out into the sea.
These are stores of sediment that form as drift processes continue at the change of trend of the shoreline and where the tidal range is not too extreme.
Spit formation shown in (a) and (b):
• The dashed line shows the position of the original coastline. In this region the prevailing winds and the direction of maximum fetch are from the south west. As a result, the strongest and most common waves come from this direction, transporting material eastwards by longshore drift along the beach at A.
•Where the trend of the coastline changed direction dramatically, at the headland, the drift of material continued out into the river estuary, and pebbles and cobbles were deposited on the bed at B.
•This material builds up over time to reach the sea surface, initiating the extension of the beach as a spit.
• Deposited materials continue to extend the beach to C. Storm waves and winds add sand to the upper beach.
• The beach continues to grow across the estuary to points D and E. At this end of the spit, the distal end, occasional storm winds or a second common wind/ wave direction can curve the end into a distinctive hook.
•Onshore winds carry sand landward, forming sand dunes above the high tide mark. Behind the spit, the sheltered area of the estuary allows fine silts and mud to settle to form a flat, wet saltmarsh at G.
• The spit reaches point F but deeper water and the scouring effect of the river flowing into the sea prevents further growth and expansion.
•Spits may be breached by winter storms and their position and size can be altered, slowly or dramatically.
Spits are fed by active erosion further back, ‘upstream’, along the coast.
A classic example of such a situation is the Spurn Head Spit, Yorkshire, where erosion from the cliffs along Holderness feeds its growth.
Example
Another example is the spit that is home to Marlough Nature Reserve, Co Down and helps form inner Dundrum Bay. The source of this sediment lies offshore and was deposited during the last Ice Age.
It appears that, in recent decades, this source is declining as the beach is reducing in width and scale, providing less natural protection from storm winds. Remedial action has been taken to protect the foredunes from erosion by waves, especially where Royal Co Down Golf Links is threatened
Spits come in many forms, such as double spits where two extend across from both sides of a bay or estuary.
Tombolos
Where a spit extends from the mainland and links a former island to the shore it is referred to as a ‘tombolo’.
Chesil Beach
The most famous UK example is at Chesil Beach, where a shingle spit, 25 km long, connects Dorset to the Isle or Portland. It varies in height from 5-15 m and in width from 50-200 m. The sediment along the beach is graded in size from pea-sized gravel in the north to potato-sized in the south. There are many theories about its origin and the movement of sediment along the beach. It is widely agreed that Chesil is a relict landform feature, an offshore bar formed after the last Ice Age ended from material deposited on the seabed. As the ice melted, raising world sea levels, this bar was driven onshore to form the tombolo we see today. Chesil Beach protects a 13 km long tidal lagoon called the Fleet, a rare feature on Europes coast and the largest in England. The Fleet has a national conservation designation as a Site of Special Scientific Interest (SSSI), European status as a Special Protection Area (SPA) and internationally it is recognised as a Ramsar site for its wetland bird habitat.
However, tombolos are often much smaller features, for example where an island causes wave refraction to build a spit (causeway) from the land to the island itself. Some tombolos are only evident at low tide when they form a natural causeway giving access to the connected island.
Bars
Ridges of sediment lying offshore but parallel to the coastline are known as bars and form in very distinctive environments.
These are a common feature along the eastern seaboard of the USA, where they form almost 300 separate long islands stretching his Florida in the south to New York State in the north. There are several theories about the formation process of these so-called ‘barrier beaches and it is probable that more than one theory may be correct. Changing sea levels in the post glacial period and the partial drowning of existing sand dunes or beach berm ridges seem to be relevant in some cases. Bars can be formed 20 km offshore from low-hing shallow coasts or be driven onshore to lie within metres of the beaches themselves.
The term bay bar is used to describe the situation where a bar extends across an inlet or bay, cutting off an area of open water from the sea - a lagoon. These may be formed with an offshore bay is driven onto the shore in a swash-aligned coast or alternatively where: spit grows right across a bay or inlet in a drift-aligned coast. One example is found at Slapton Ley in Devon.
- Regional Coastlines
Students should be able to:
(i)demonstrate knowledge and understanding of the processes (eustatic and isostatic) and features (fords, rias, raised beaches and relict landforms) associated with coastlines of submergence and emergence
(ii)demonstrate knowledge and understanding of the threat of rising sea levels due to climate change on the human and physical environment
Eustatic and Isostatic Change
Sea levels are constantly changing, it not only changes with tides and seasonally with winter storms but over the longterm due to the forces of plate tectonics and climate change.
Today world sea level is rising globally, as a consequence of a warming climate, but in the past two million years a series of ice ages has caused repeated changes to ocean levels. Such worldwide sea level adjustments are termed eustatic change.
Eustatic Change
Eustatic change is global and is due to variation in the volume of water in the world’s oceans. This is the result of climate change.
Natural climate change is due to several cycles including:
• variation in the Earth’s orbit, a 400,000 year cycle.
• variation in solar energy output (sun spots), an 11 year cycle.
• variation in the tilt of the Earth’s axis, a 41,000 year cycle.
In addition, periodic major volcanic eruptions can cool the Earth as debris in the upper atmosphere blocks out insolation. Falling world temperatures means more precipitation will fall as snow, which will turn to ice storing water on the land. As a result world sea level will fall.
When global temperatures rise again, the water stored as ice on the land, as glaciers and ice sheets, will thaw and return to the ocean raising the sea level.
Today, the human induced warming of the world’s climate has caused 22 cm rise in global seas level since 1880 and projections suggest a further rise of between 30 and 120 cm before the year 2100.
Isostatic Change
However, not all relative sea level change is worldwide. Locally, vertical changes in land height occur. Isostatic change is the local rise or fall in land level relative to the sea. This is caused by physical processes of plate collision or the addition or removal of the weight of ice during a glacial period. Land buried under deep ice, such as Greenland, is depressed down under the sheer weight of the ice sheet. If and when the ice melts , the burden will be removed and the land will slowly rise upwards. The last glacial era, which covered most of Northern Europe in deep ice, ended around 10,000 years ago. Today, Scotland and the northern coast of Ireland are still slowly recovering, with the land rising upwards through isostatic change by as much as 7 mm year. As a result of this process, around the present day coast former beaches, cliff lines caves and other coastal features can now be seen high and dry above our current sea levels.
Consequences of Rising Sea Levels from the Past
Elsewhere the consequence of rising sea levels in the past can still be observed. In the shallow sea inlet of Strangford Lough the numerous islands and pladdies (drowned drumlins exposed at low tide) are the result of the sea drowning the rolling drumlin hills of that part of Co Down.
During the geological era known as the Quaternary, the dominant pattern has been a series of climate changes producing about 20 separate periods of glaciation. During each of these, world sea level has fallen as water becomes stored as ice on land only to rise again between glaciations during warmer eras known as interglacial periods. During the last glaciation, world sea levels were 100-120 m lower than today so large areas of the continental shelf around Ireland and Great Britain were dry land. During the current interglacial period, the Holocene, which started about 10,000 years ago, the sea level has risen to its present level, flooding much of this land and transforming Western Europe from a large peninsular area of the Atlantic into the numerous islands we see today.
Regional Coastlines of Submergence and Emergence
At a regional scale, such as in the relative height of sea level can create two distinctive coastlines:
- Coasts where the land is falling relative to the sea, due to rising sea level or falling land, or both causing submergence of the shore.
- Coasts where the land is rising relative to the sea, due to falling sea levels or rising land, or both causing emergence of the shore.
While eustatic change in sea level is global, its impact on coasts will be adjusted by any local scale isostatic changes that occur. This helps to explain why parts of the UK and Ireland, the south east coasts in particular, currently display evidence of submergence while western Scotland and the north of Ireland show the features of an emergent coast.
~Submergent Coastlines~
Submergent or drowned coastlines are marked by marine transgression: the sea spreads over the land and the coastline retreats. The landforms created will vary on whether the coast was a lowland or upland area and on the trend of the local geology.
Fjords
In Northern Europe, including the west coasts of Norway and Scotland, during the last Ice Age, glaciers created broad and steep sided valleys.
Post-glaciation these have been drowned by the sea forming dramatic deep fjords running at right angles or discordant to the coast. These have steep sides often with waterfalls plunging from hanging valleys down the sides of their distinctive U-shaped cross-section. The mouth of a fjord is often shallower, possibly due to the material deposition by ice as a moraine or because at that point the sea had caused the valley glacier to float, lifting it from the bed of the valley. Fjords coasts are also seen in the former glaciated regions of West Canada and in Chile and New Zealand in the southern hemisphere.
Rias
Further south, beyond the area affected directly by ice it is river valleys that have been flooded by the rising sea. These are termed rias, and typically show the winding tree-like (dendritic) pattern associated with a river and its network of tributaries. Rivers such as the Exe and leign on the south coast of Devon and Cornwall are good Exemple examples of such rias.
Dalmatian Coastlines
If the trend of the pre-submerged coast was of ridges and valleys running parallel to the shore then the flooding sea forms long narrow inlets as sounds and leaves long islands that lie parallel or concordant to the coast. Named after the on coast that forms the eastern shore of the Adriatic Sea, these are known as Dalmatian coastlines.
Along low lying coasts the ‘invading’ sea creates flooded valleys, wide sandy beaches and large salt marshes. The East Anglian coast including the counties of Norfolk, Suffolk and Essex show these features well and land is continuing to be lost here to the rising sea. Across the North Sea the very flat coastal lands or polders of the Netherlands are in fact below sea level and only inhabited and farmed as they are protected by an extensive and expensive coastal management, a series of dams, dykes and barrages.
~Emergent Coastlines~
Emergent coastlines are distinctive in that landforms of coastal erosion and deposition will be found above the present day sea level. These are a common element in the landscape of Northern Europe and in particular the presence of former beaches and wave-cut platforms as raised beaches.
Raised Beaches
While world sea levels rose by up to 120 m after the end of the last ice age later isostatic uplift raised newly formed beaches up out of the sea to where they now stand several metres or tens of metres above the sea. At least one such raised beachline can be traced around most of the coastline of Northern Ireland. Parts of the coastal road network of Antrim and Down are built onto the conveniently flat raised beachline. Not only do the relict landforms provide evidence for previously higher sea levels, but the raised beaches themsehes are often still covered by marine sands and shell fragments.
Gradual emergence of land allows the formation and extension of depositional features such as salt marshes and mangrove swamp ecosystems. In sheltered bays around Ireland the growth of wide sand beaches has, in turn, encouraged the formation of extensive sand dunes systems above the high tide mark. Many of these have been growing seaward for 5000 years or more and some are backed by the original cliffs of the coast such as the dune system at White Park Bay in Co Antrim. Ireland’s largest deposition landform is the striking cuspate foreland at Magilligan, Co Londonderry.
This low-lying triangular landform feature is backed by the steep cliffs of Benevenagh and extends several kilometres north to Benone beach and the Umbra sand dunes.
Cuspate (tooth-shaped) forelands are complex deposition landforms involving the working of river, beach and wind-blown sediment in a sheltered environment in which both swash and drift alignment processes are incorporated. At Magilligan, the gradual emergence of the seabed due to isostatic rebound after glaciation has facilitated these deposition processes in the overover 32 km2 of land.
Rising Sea levels due to a climate Change
Research suggests that global sea levels have been stable for the last 2000-3000 years, although isostatic changes mean that local or regional change in relative sea level has taken place. However, in the last 200 years a different pattern has emerged with a significant eustatic rise in sea level approaching 30 cm recorded across the planet. Linked to global warming this increase is expected to continue in future. In fact scientists suggest that even if global warming was stopped the worldwide sea level will continue to rise for many years to come. As stated by the 5th report of the Intergovernmental Panel on Climate Change (IPCC):
“It is virtually certain that global mean sea level rise will continue for many centuries beyond 2100, with the amount of rise dependent on future emissions.”
At first the rising global sea level was simply due to thermal expansion - as ocean water warms it expands upwards, its only option. Later some of the sea level rise was due to the melting of mountain glaciers reported from all continents and mountain ranges. Added to this is the melting of major ice sheets in Greenland and Antarctica, currently the least predictable in terms of scale and timing.
In regions where local isostatic change is causing land to sink, the impact of rising eustatic sea levels will be greater.
For example, on the Gulf of Mexico and North Atlantic coasts of the USA a further 30 cm rise in relative sea level is predicted by 2050. Other global warming related changes include the increased violence of storms. In North Carolina, USA, recent studies show that extreme coastal storms that were formerly regarded as one in 100 year events are now expected once in 30 years.
The Threat of Rising Sea Levels on the Human and Physical Environment
Human threats
The degree and the form of the threat posed by rising sea levels will depend on the nature of the coastline in both physical and human terms. Coasts dominated by steep plunging cliff lines are unlikely to change dramatically whereas beaches and low-lying coastlines may be rapidly altered by erosion. Similarly, for the developed nations of Europe the threat may entail cost and inconvenience but some LEDCs may be facing the demise of their nation. It is estimated that globally, one billion people face increased physical hazards from coastal flooding, storm surges and violent storms. Inevitably land will be lost to the sea impacting on agriculture, fishing, aquaculture, tourism and coastal residential settlements. Higher sea levels will cause the destructive erosion of beaches and cliffs producing more sediment, which in turn must be deposited elsewhere: in short the coastal sediment budget becomes highly disrupted. Coastal wetlands will be inundated changing the abiotic conditions for these ecosystems including raised salinity levels in rivers, bays and groundwater. Saltwater penetration of terrestrial groundwater sources impacts on the local supply of fresh water for domestic, industrial and irrigation uses.
Environmental threats
These I nclude the potential loss of the distinctive and globally important coastal coral and mangrove ecosystems.
Mangroves
In the tropics, around 75% of all coastlines are home to tidal forests known as mangroves. Lying between the land and sea these specialist ecosystems formed by salt-tolerant trees provide a vital habitat for many fish, insect, bird and mammal species. Mangroves are recognised as a natural defence from the sea and their loss by human activity in the past has led to increased rates of coastal erosion.Bangladesh is one of the world’s lowest lying nations and also one of the most densely populated. Around 25 million Bangladeshis live on coastal delta land less than 1 m above sea level. Of the nation’s 735 km coastline, 20% is lined and protected by natural mangrove forest, known locally as the Sunderbans.
-Rising sea levels threaten the loss of these forests along with the numerous species that depend on them.
-This in turn will increase the rate of coastal erosion and the loss of rich agricultural land.
The government of Bangladesh has invested huge sums of money to reinforce their coastal defences but many argue that this is a hopeless task in the face of I global sea level rise. Human activity including shrimp and rice farming has already caused the loss of most of the mangrove forests of the Caribbean and Pacific. Global warming and rising sea levels now threatens those that remain.
Coral Reefs
Linked to mangroves is an equally important and threatened ecosystem - coral reefs.
Coral reefs are created by a group of species of soft bodied polyps, known as reef building stony corals that excrete calcium carbonate to build an external skeleton. The polyps surweina symbiotie (mutially beneficial) relationship with algae - a plant that provide oxygen and food. In tropical waters corals grow by 2-60 cm each year and these animals have created some of the largest organic structures on Earth.
The 2600 km long Great Barrier Reef, off Australia’s north east coast, took five million years to form.
Coral reefs only occupy 0.25% of the oceans but they are home to around 25% of all the recorded marine species - polyps, fish, mammals, turtles, molluses and crustaceans. Reef building corals thrive in a very specific range of conditions.
Global warming is causing the oceans to absorb more carbon dioxide increasing its acidity. Higher acidity can chemically dissolve existing coral and reduces the rate at which corals can grow. This combined with sea level rise is threatening many of the factors needed for the healthy development of coral reefs. One major impact on coral reefs is described as coral bleaching: under environmental stress the algae on which polyps depend can abandon the coral leaving it to die.
A 2010 report by the Global Coral Reef Monitoring Network suggested that coral reefs were the first ecosystem to show major damage as a consequence of climate change. A combination of rising sea levels, increased storm strength, higher sea temperatures and acidification has led to the loss of 20% of the planet’s coral reefs, with another 35% under serious threat of destruction. It is estimated that by 2050, 90% of coral reefs will be negatively impacted by local and global threats.
The United Nations states 500 million people depend on coral reefs for food, protection, building materials or for income from the tourist industry. It estimates the net economic worth of coral based goods and services at $100 billion a year.
The threat of global sea level rise to MEDCs includes a threat to many of their large cities.
The large, complex and expensive engineering schemes may only be realistic and practical for such special cases. Elsewhere, even MEDCs will need to accept the real loss of land along with infrastructure and resources. While LEDs have relatively fewer resources with which to address the rising global sea level issue, many are vulnerable both in their location, coastal and low-lying, and in their economic structure, subsistent often farming and fishing base.
- Coastal Management and Sustainability
Students should be able to:
(i) demonstrate knowledge and understanding of the role of Environmental Impact Assessment (EIA), Cost-Benefit Analysis (BA), Sediment Cells and Shoreline
Management Plans (SMP) in coastal management
(ii) evaluate the impact and sustainability of hard engineering (sea walls, revetments, rip-rap, gabions and groynes) and soft engineering (beach nourishment, dune regeneration and managed retreat) strategies on the human and physical environment
Demands on the Coast
In the UK and Ireland one in three people live within the coastal zone. Around the coast dramatic erosion events, such as railway lines collapsing onto beaches and lighthouses having to be jacked-up and moved inland, suggest coastal processes cannot be ignored. There are a range of demands applied to coastlines and the potential outcomes. It is this increasing pressure that has led governments to seek to develop management strategies that are both effective and sustainable.
By the end of the twentieth century it was recognised that the management of coasts in the UK and Ireland was frequently chaotic, with dozens of different government departments and non-government interest groups making decisions and taking action without any meaningful coordination. The threat of sea level rise, as a consequence of global warming, helped to focus attention on the need for a sustainable and integrated approach.
Sediment Cells
The physical division of coasts into littoral or sediment cells was a key starting point for recent management plans. In nature, sediment is transported within sediment cells involving large sections of the coast. In these cells, sand and shingle movements are largely self-contained. Cells are normally separated by major headlands or dramatic changes to the coastline trend. Within each large sediment cell, sand and shingle can move more freely between smaller sub-cells. Eleven sediment cells are identified along the coast of England and Wales, with eight along the Scottish coastline.
Shoreline Management Plans, Cost Benefit Analysis and Environmental Impact Assessment
These sediment cells have formed the basis for an integrated management project for England and Wales known as Shoreline Management Plans
(SMP). Unfortunately to date, the management of Northern Ireland’s coast has not been addressed in this way and so decisions are made locally by whatever government department is most directly concerned.
Shoreline Management Plans were defined as:
“a document which sets out a strategy for coastal defence for a specified length of coast taking account of natural processes and human and other environmental.. needs”
These management plans are created by local authorities and other interested parties (stakeholders) to provide an overview for a region within which local authorities can undertake schemes that fit in with the aims of the strategic plan.
The first SMPs were used in the 1990s but the second generation, known as SMP2, were implemented from 2005.
Plans for the next century were made at three timescales: the next 20 years (2005-2025), the following 30 years (2025-2055) and 50 years after that (2055 - 2105). It is stated that on the longer timescale the majority of plans should be either: 1) No active intervention or 2) Managed realignment.
For any section of coastline the following tests are used to evaluate the most appropriate option selected:
• A Technical Feasibility Study
This concerns the effectiveness of the technical aspects of a proposed option. Given the specific factors and processes, will the approach accomplish the goals for which it was designed?
• A Cost-Benefit Analysis (CBA)
CBA seeks to find the net balance between the cost of the construction (capital) and up-keep (maintenance) of any proposed coastal defence option against the value and income from the property, employment and land that is being protected. This is normally stated as a ratio, with the benefits divided by costs.
Finding accurate up-to-date figures for CB is difficult; for example, property values constantly change and what is the monetary value of business confidence over a ten year period?
• An Environmental Impact Assessment (EIA)
The EIA evaluates the scale and nature of a proposed option’s impact.
Environment is interpreted as both the natural environment, such as the threat to plant and animal habitats and the built environment, constructed features including the loss of sites of heritage such as castles and churches. ElAs are designed to reflect both negative and potentially positive impacts.
Coastal Management Techniques
~Hard Engineering~
As the variety and scale of demand made on coastlines has grown, so the need to manage these environments has increased. In the past this management was accomplished almost exclusively, by hard engineering. Hard engineering involves the construction of structures, often large-scale and intrusive, to control or prevent natural processes that threaten property, harbours and tourist amenities.
Early examples of hard engineering followed the rise of seaside tourism in Victorian Britain. During the nineteenth century the coast became the preferred destination for the earliest mass tourism. The advent of the railways had opened up the possibility of families holidaying away from home. Within reach of all the industrial urban centres, seaside resorts developed. Purpose built coastline walkways called promenades stretched for miles along the shore and entertainment piers ran out to sea. Sea walls were constructed to protect these coastline developments from winter storms and, where beaches were not wide enough or where erosion moved the precious sand away, fields of groynes were built.
These wooden walls were constructed at regular intervals across the beach at right angles to the shore. Some groyne fields consisted of dozens of individual barriers. The aim was to trap sediment on the ‘upstream’ side of the barrier, preventing the loss of sand. These were successful in creating wide beaches for recreation and also helped protect promenades from storm waves. Groynes needed to be maintained and eventually replaced so, while effective, groyne based beach management was not cheap.
The concept of coastal sediment cells states that the stores and flows of sediment along coasts are in a state of dynamic equilibrium, so any interference is likely to have consequences. Sea walls and beach groynes had unintended impacts. In many cases groyne fields that retained sand on the pleasure beaches of Victorian seaside resorts also caused rapid beach and shoreline erosion further along the coast. Sand and pebbles trapped by groynes reduced the sediment carried by longshore drift and starved the beaches beyond the groyne field.
In other cases poorly designed sea walls directed the energy of storm waves down onto the beach, undermining their own foundations and causing collapse. Often the answer to such issues was simply to build more structures, higher and stronger or with variations such as rip-rap or revetments. Coastlines became increasingly artificial in both their nature and appearance.
~Soft Engineering~
The expense and failure of many
hard engineering strategies around the coast encouraged discussion of alternative approaches, in particular soft engineering. A soft engineering approach involves schemes that use and work with natural processes to achieve the desired outcome. Environmentalists have encouraged planners to develop such solutions, if not to replace, then at least to reduce the impact of hard solutions.
Until recently the debate over coastal protection concerned the choice between tons hard or soft engineering, now the issue has shifted to a more fundamental question
-‘Should we protect the coast at all?
Three factors are responsible for this new debate over how coasts should be managed:
1. Rising global sea levels will continue for the foreseeable future.
2. Any engineering schemes to retain current coastlines are very expensive and will become more costly in the future.
3. The many negative environmental impacts of coastal schemes mean they are seen as both unsustainable and unacceptable.
Today, partly from the use of Cost-Benefit Analysis and Environmental Assessments, a new concept has emerged, ‘do nothing’ (no active intervention). In other words, let nature take its course and through natural processes the coast will find a new equilibrium and balance. A slight modification to this view is termed managed retreat. Under this approach any existing coastal defences would be gradually removed allowing the sea to change the morphology of the shore. Some engineering strategies might be used to slow down the rate of change. One example of the new thinking is illustrated by Norfolk County council. In the light of Norfolks rapidly eroding coastline, the council proposes to establish a setback line some 75 m inland from the present shore and to ban the building of new developments on the coast. This means no new houses, roads or other construction can be located within that distance of today’s shoreline.
Nationally, the UK government, faced with an estimated bill of over f5 billion to stop coastal erosion by future rising sea levels, has adopted a plan to stop maintaining coastal defences, except where flooding threatens settlements.
Hard Engineering Table
Soft Engineering Table
