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Coastal Geography - Part 1
1.0 INTRODUCTION
Coastal Geography is a study of the interface between land and oceans. It encompasses both human and physical aspects. This interface is studied both in respect of geological factors and human factors. The coastlines of the world's continents measure about 3,12,000 km (1,93,000 miles). They have undergone shifts in position over geologic time because of substantial changes in the relative levels of land and sea. The changes in sea level over large periods of time have played an important role in shaping the coasts. Studies of glaciations during the Pleistocene Epoch (2.6 million to 11,700 years ago) indicate that drops in sea level caused by the removal of water from the oceans during glacial advances affected all coastal areas. During the last Pleistocene glacial period, the sea level is thought to have been almost 122 m (400 feet) lower than it is today, resulting in the exposure of large portions of what is now the continental shelf. Some coasts are the result of near equal balance between tide and wave processes. As a consequence, investigators speak of wave-dominated coasts, tide-dominated coasts, and mixed coasts.
A wave-dominated coast is one that is characterized by well-developed sand beaches typically formed on long barrier islands with a few widely spaced tidal inlets. The barrier islands tend to be narrow and rather low in elevation. Longshore transport is extensive, and the inlets are often small and unstable. Jetties are commonly placed along the inlet mouths to stabilize them and keep them open for navigation. The Texas and North Carolina coasts of the United States are excellent examples of this coastal type.
Tide-dominated coasts are not as widespread as those dominated by waves. They tend to develop where tidal range is high or where wave energy is low. The result is a coastal morphology that is dominated by funnel-shaped embayments and long sediment bodies oriented essentially perpendicular to the overall coastal trend. Tidal flats, salt marshes, and tidal creeks are extensive. The West German coast of the North Sea is a good example of such a coast.
Mixed coasts are those where both tidal and wave processes exert considerable influence. These coasts characteristically have short stubby barrier islands and numerous tidal inlets. The barriers commonly are wide at one end and narrow at the other. Inlets are fairly stable and have large sediment bodies on both their landward and seaward sides. The Georgia and South Carolina coasts of the United States typify a mixed coast.
2.0 COASTAL LANDFORMS
The two major categories of coastal landforms are erosional and depositional. The formation of these landforms is a result of various processes acting on the coastal sediments and rocks. Wave currents and tides are the most important processes.
2.1 Erosional landforms
Erosional coasts typically exhibit high relief and rugged topography. They tend to occur on the leading edge of lithospheric plates, the west coasts of both North and South America being excellent examples. Glacial activity also may give rise to erosional coasts, as in northern New England and in the Scandinavian countries. Typically, these coasts are dominated by exposed bedrock with steep slopes and high elevations adjacent to the shore. Although these coasts are erosional, the rate of shoreline retreat is slow due to the resistance of bedrock to erosion. The type of rock and its lithification are important factors in the rate of erosion.
Formation of headlands and bays: Coastlines in most parts of the world are replete with evidence of erosion. Bays and headlands are formed where the rock type runs perpendicular to the coastline. Headlands are composed of a harder, more resistant rock and the inlets - the bays - formed of a softer rock. Hydraulic action, abrasion and corrosion are more effective at eroding the softer rock, particularly during storms, and this will erode further inland than the harder rock. During periods when there are no storms, the hard rock absorbs a lot of the wave energy and refract or bend the waves into the area whereas the softer rock allows sediment to be deposited and accumulate as beaches. When this happens over a long period of time, the hard rock is left jutting out to sea as a headland, and the softer rock is eroded into curved sand filled bays.
Cliffs and cliff retreats: A cliff is an almost vertical wall of rock or sediment that borders the sea. The angle of the slope differs due to rock structure and geology. Marine erosion processes which happen at the foot of the cliff causes fragments to break of. This traps air in pores and crevices causing the rock to break further. This process is known as hydraulic action. Corrosion occurs when sediment and rocks at the sea level are hurled against the cliff face. Oxidation and carbonation weaken the structure of the rock, and depending upon the climate physical processes such as freeze thaw and water layer weathering can take effect. Over time this weakens the structure of the cliff face, and coupled with the erosion of the wave cut notch at a critical point this cliff face will succumb to the influence of gravity and collapse in a process of mass movement. This material will then be carried away by the sea in the process of long shore drift by the transportation process of solution, suspension, saltation and traction (depending upon the particle sizes). The most widespread landforms of erosional coasts are sea cliffs. These very steep to vertical bedrock cliffs range from only a few metres high to hundreds of metres above sea level. Their vertical nature is the result of wave-induced erosion near sea level and the subsequent collapse of rocks at higher elevation. Cliffs that extend to the shoreline commonly have a notch cut into them where waves have battered the bedrock surface.
Coves: Coves form where rock runs in bands horizontal to the direction of wave attack. There is a band of resistant rock closest to the sea and a band of less resistant rock inland. The waves seek out faults in the hard rock and erodes using the processes of abrasion/corrasion and hydraulic action through to the soft rock behind. Wave processes erode the softer rock faster and this leaves a circular cove with a narrow entrance where the sea enters. The waves are also refracted within the cove, spreading out to erode in all directions.
Wave-cut platforms: At the base of most cliffs along a rocky coast one finds a flat surface at about the mid-tide elevation. This is a benchlike feature called a wave-cut platform, or wave-cut bench. Such surfaces may measure from a few metres to hundreds of metres wide and extend to the base of the adjacent cliff. They are formed by wave action on the bedrock along the coast. The formation process can take a long time, depending on the type of rock present. The existence of extensive wave-cut platforms thus implies that sea level did not fluctuate during the periods of formation. Multiple platforms of this type along a given reach of coast indicate various positions of sea level.
Sea stacks: Erosion along rocky coasts occurs at various rates and is dependent both on the rock type and on the wave energy at a particular site. As a result of the above-mentioned conditions, wave-cut platforms may be incomplete, with erosional remnants on the horizontal wave-cut surface. These remnants are called sea stacks, and they provide a spectacular type of coastal landform. Some are many metres high and form isolated pinnacles on the otherwise smooth wave-cut surface. Because erosion is a continual process, these features are not permanent and will eventually be eroded, leaving no trace of their existence.
Sea arches: Sea arches are formed as a result of different rates of erosion typically due to the varied resistance of bedrock. These archways usually have a rectangular shape, with the opening extending below water level. The height of an arch can be up to tens of metres above sea level.
It is common for sea arches to form when a rocky coast undergoes erosion and a wave-cut platform develops. Continued erosion can result in the collapse of an arch, leaving an isolated sea stack on the platform. Still further erosion removes the stack, and eventually only the wave-cut platform remains adjacent to the eroding coastal cliff.
2.2 Depositional landforms
Continuous weathering of rocks and cliffs generate a lot of material which needs to be deposited on some surface. When ocean currents slow down due to frictional forces such as sea beds, counter currents and vegetation depositional landforms such as beaches, spits and bars are formed. Coasts adjacent to the trailing edge of lithospheric plates tend to have widespread coastal plains and low relief. The Atlantic and Gulf coasts of the United States are representative. Such coasts may have numerous estuaries and lagoons with barrier islands or may develop river deltas. They are characterized by an accumulation of a wide range of sediment types and by many varied coastal environments. The sediment is dominated by mud and sand; however, some gravel may be present, especially in the form of shell material.
Beaches: Beaches are areas of sand, pebbles and shingle that are formed by deposition produced by wave processes. Beaches are by no means uniform and contain a huge variety of sediment types and sizes, and have many different shapes. The type of wave influences the slope of the beach. Gently sloping beaches are formed by strong destructive waves that backwash more material away from the beach that they swash up the beach. Steeply sloping beaches occur by constructive waves that swash more material up the beach than they backwash away, building up a steep beach gradient.
The two distinct parts of a beach profile are:
- The seaward and relatively steep sloping foreshore, which is essentially the intertidal beach, and
- The landward, nearly horizontal backshore.
Beach profiles take on two different appearances, depending on conditions at any given time. During calm wave conditions, the beach is said to be accretional, and both the foreshore and backshore are present. During storm conditions, however, the beach experiences erosion, and the result is typically a profile that shows only the seaward sloping foreshore. Because the beach tends to repair itself during nonstorm periods, a cyclic pattern of profile shapes is common.
At the nearshore zone waves steepen and break, and then re-form in their passage to the beach, where they break for the last time and surge up the foreshore. Much sediment is transported in this zone, both along the shore and perpendicular to it. During storms the waves tend to be steep, and erosion of the beach occurs with sediment transported offshore. The intervening calmer conditions permit sediment to be transported landward and rebuild the beach. Because wave conditions may change daily, the nature of the profile and the sediment on the foreshore portion of the beach may also change daily. This is the zone of continual change on the beach.
The backshore of the beach is not subjected to wave activity except during storm conditions. It is actually in the supra-tidal zone-i.e., the zone above high tide where inundation by water is caused not by regular astronomical tides but rather by storm-generated tides. During nonstorm conditions the back-beach is relatively inactive except for wind action, which may move sediment. In most cases, there is an onshore component to the wind, and sediment is carried from the back-beach landward, typically forming dunes. Any obstruction on the back-beach, such as vegetation, pieces of driftwood, fences, or even trash discarded by people, results in wind-blown sand accumulation.
Spits: Spits are caused by a process called longshore drift. Some eroded material gets caught up in within the waves and is carried by the sea along the coastline in cells known as littoral cells. Due to the prevailing wind patterns, the eroded material called swash material is carried along the sea in a zigzag manner. The angle of swash is determined by the prevailing wind. If there is a break in the coastline (e.g. across a river or a change in coastline direction) then material is deposited closest to the shore. This is because there are often counter currents and a loss in velocity, so material is dropped or deposited. Eventually this material builds up out into sea to form a spit.
Bars, Tombolo and Dunes: A bar is a spit that joins together two headlands. Bars are particularly obvious at low tide when they become exposed. At high tide bars make the water shallow which often causes waves to break early. A lagoon may be formed within a bay as the result of a bar. Where a spit links the mainland and an island a tombolo is formed. Dunes are landforms formed from sand deposits that have been blown off the beach. Where sufficient sand is deposited and dries in the intertidal zone (foreshore - area between the high and low tide marks) it is then transported by saltation by the blowing wind. Sand dunes only form where the rate of beach deposition is greater than erosion (positive sediment budget).
Deltas: Deltas are accumulation of sediments at the mouth of a river extending beyond the trend of the adjacent coast. Deltas vary greatly in both size and shape. The size of a delta is typically related to the size of the river, specifically to its discharge. The shape of a delta, on the other hand, is a result of the interaction of the river with tidal and wave processes along the coast. River-dominated deltas are those where both wave and tidal current energy on the coast is low and the discharge of water and sediment are little affected by them. The result is an irregularly shaped delta with numerous distributaries (eg. Mississipi delta).
Continuous action of waves removes much of the fine deltaic sediment and smoothens the outer margin of the delta landform. This results in a smooth, cuspate delta that has few distributaries.
Tide-dominated deltas tend to be developed in wide, funnel-shaped configurations with long sand bodies that fan out from the coast. These sand bodies are oriented with the strong tidal currents of the delta. Tidal flats and salt marshes also are common. The Ganges-Brahmaputra Delta in Bangladesh are representative of such a deltaic type.
Barrier island/estuarine systems: Embayments of irregular coasts fed by streams are called estuaries. Estuaries receive sediment due to runoffs from adjacent coasts. Seaward of the estuaries are elongate barrier islands that generally parallel the shore. Consisting mostly of sand, they are formed primarily by waves and longshore currents. These barrier islands are typically separated from the mainland and may have lagoons, which are long, narrow, coastal bodies of water situated between the barrier and the mainland.
Barrier islands contain well-developed beaches, coastal dunes, and various environments on their landward side, including tidal flats, marshes, or washover fans. They are typically interrupted by tidal inlets, which provide circulation between the various coastal bays and the open marine environment. These inlets also are important pathways for organisms that migrate between coastal and open marine areas as well as for pleasure and commercial boat traffic.
3.0 FACTORS AFFECTING COASTAL LANDFORMS
3.1 Waves
The continual motion of the waves moving toward the beach is one of the major factors influencing the formation of coastal landforms. There is a lot of variation in waves according to their size and geographical location. As waves travel towards the ocean bottom, they cause sediment to become temporarily suspended and available for movement by coastal currents. The larger the wave, the deeper the water in which this process takes place and the larger the particle that can be moved. Even small waves that are only a few tens of centimetres high can pick up sand as they reach the shore. Larger waves can move cobbles and rock material as large as boulders.
Generally, small waves cause sediment-usually sand-to be transported toward the coast and to become deposited on the beach. Larger waves, typically during storms, are responsible for the removal of sediment from the coast and its conveyance out into relatively deep water.
Waves erode the bedrock along the coast largely by abrasion. The suspended sediment particles in waves, especially pebbles and larger rock debris, have much the same effect on a surface as sandpaper does. Waves have considerable force and so may break up bedrock simply by impact.
3.2 Longshore currents
A longshore current is caused due to the waves approaching the coast at an acute angle rather than approaching the coast parallel to the coastline. Due to the acute angle, waves are bent (or refracted) as they enter shallow water, which generates a current along the shore and parallel to it. Such a current is called a longshore current. It extends from the shoreline out through the zone of breaking waves. The speed of the current is related to the size of the waves and to their angle of approach. In calm conditions, longshore currents move only about 10-30 centimetres per second; however, under stormy conditions they may exceed one metre per second. The combination of waves and longshore currents transport large quantities of sediment along the shallow zone adjacent to the shoreline.
Depending upon the direction of wave approach longshore currents may move in either direction along the coast, depending on the direction of wave approach. This direction of approach is a result of the wind direction, which is therefore the ultimate factor in determining the direction of longshore currents and the transport of sediment along the shoreline.
Waves typically cause sediment to be picked up from the bottom, and the longshore current transports it along the coast. In some locations there is quite a large volume of net sediment transport along the coast because of a dominance of one wind direction-and therefore wave direction-over another. This volume may be on the order of 100,000 cubic metres per year. Other locations may experience more of a balance in wave approach, which causes the longshore current and sediment transport in one direction to be nearly balanced by the same process in the other direction.
3.3 Rip currents
Rip current or rip tide as it is called is another type of coastal current caused by wave activity. As waves move toward the beach, there is some net shoreward transport of water which causes a slight but important upward slope of the water level (setup), so that the absolute water level at the shoreline is a few centimetres higher than it is beyond the surf zone. This situation is an unstable one, and water moves seaward through the surf zone in an effort to relieve the instability of the sloping water. The seaward movement is typically confined to narrow pathways. In most cases, rip currents are regularly spaced and flow at speeds of up to several tens of centimetres per second. They can carry sediment and often are recognized by the plume of suspended sediment moving out through the surf zone. In some localities rip currents persist for months at the same site, whereas in others they are quite ephemeral.
3.4 Tides
The rise and fall of sea level caused by astronomical conditions is regular and predictable. There is a great range in the magnitude of this daily or semi-daily change in water level. Along some coasts the tidal range is less than 0.5 metre, whereas in the Bay of Fundy in South Eastern Canada the maximum tidal range is just over 16 metres. Bases on tidal range, coasts are classified into three categories which are; micro-tidal (less than two metres), meso-tidal (two to four metres), and macro-tidal (more than four metres). Micro-tidal coasts constitute the largest percentage of the world's coasts, but the other two categories also are widespread.Tidal currents transport large quantities of sediment and may erode bedrock. The rise and fall of the tide distributes wave energy across a shore zone by changing the depth of water and the position of the shoreline.
The process of transportation of sediment is similar to that of the process of longshore currents. The speeds necessary to transport the sediment (typically sand) are generated only under certain conditions-usually in inlets, at the mouths of estuaries, or any other place where there is a constriction in the coast through which tidal exchange must take place. Tidal currents on the open coast are not swift enough to transport sediment. The ebb-tide cycle may be either six or 12 hours in duration, depending on whether the local situation is semidiurnal (12-hour cycle) or diurnal (24-hour cycle). Tidal prism is the product of the tidal range and the area of the coastal bay being served by the inlet. This means that though there may be a direct relationship between tidal range and tidal-current speed, it is also possible to have very swift tidal currents on a coast where the tidal range is low if the bay being served by the inlet is quite large. This is a very common situation along the coast of the Gulf of Mexico where the range is typically less than one metre but where there are many large coastal bays.
4.0 OTHER FACTORS
Climate is an extremely important factor in the development of coastal landforms. The elements of climate include rainfall, temperature, and wind.
Rainfall is important because it provides runoff in the form of streams and also is a factor in producing and transporting sediment to the coast. This fact gives rise to a marked contrast between the volume and type of sediment carried to the coast in a tropical environment and those in a desert environment.
Temperature: Temperature influences physical weathering of sediments and rocks along the coast and in the adjacent drainage basins. In cold regions the freezing of water within cracks in rocks causes the rocks to fragment and thereby yield sediment. In some temperate and arctic regions due to the presence of shore ice for several months in a year, there is no wave impact and the coast is essentially static. The ice thaws or breaks up during severe storms which causes a change in coastal landforms for a period of three to four months.
Winds: Winds and waves are closely interrelated. Coasts that experience prolonged and intense winds also experience high wave-energy conditions. Seasonal patterns of winds directly influence wave conditions. Wind also influence formation of coastal landforms directly, particularly coastal dunes. As onshore winds are present over much of the world's coast, sand dunes are prevalent in all places where enough sediment is available and place for accumulation exists.
Gravity: Gravity causes the downslope movement of sediment and rock as well. It also indirectly influences processes associated with wind and waves. Along shoreline cliffs waves attack the base of the cliffs and undercut the slope, resulting in the eventual collapse of rocks into the sea or their accumulation as debris at the base of the cliffs.
5.0 General coastal morphology
Depositional coasts can be described in terms of three primary large-scale types: (1) deltas, (2) barrier island/estuarine systems, and (3) strand-plain coasts. The latter two have numerous features in common.
Although there is a common trend to the beach profile, some variation exists both because of energy conditions and because of the material making up the beach. Generally speaking, a beach that is accumulating sediment and experiencing low-energy conditions tends to have a steep foreshore, whereas the same beach would have a relatively gentle foreshore during storm conditions when erosion is prevalent. The grain size of beach sediment also is an important factor in the slope of the foreshore. In general, the coarser the constituent grains, the steeper the foreshore. Examples include the gravel beaches of New England, as contrasted to the gently sloping sand beaches of the Texas coast
6.0 COASTAL EROSION
Coastal erosion involves the breaking down and removal of material along a coastline by the movement of wind & water. It leads to the formation of many landforms and, combined with deposition, plays an important role in shaping the coastline.
Methods of Erosion
Hydraulic Action: When a wave impacts a cliff face, air is forced into cracks under high pressure, widening them. Over long periods of time, the growing cracks destabilise the cliff and fragments of rock break off of it.
Corrasion/Abrasion: The repeated action of waves breaking on a cliff is enough to remove material from it over time. If sand & shingle are present in the water, it will act like sandpaper and erosion will take place faster.
Attrition: Beach material is knocked together in water reducing their size and increasing their roundness & smoothness.
Corrosion: Carbon dioxide in the atmosphere is dissolved into water turning it into a weak carbonic acid. Several rocks (e.g., Limestone) are vulnerable to this acidic water and will dissolve into it. The rate of dissolution is affected by the concentration of carbonates & other minerals in the water. As it increases, dissolution becomes slower.
Factors Affecting the Rate of Erosion
The biggest factor affecting coastal erosion is the strength of the waves breaking along the coastline. A wave's strength is controlled by its fetch and the wind speed. Longer fetches & stronger winds create bigger, more powerful waves that have more erosive power. As waves approach a coastline they lose energy though because friction with the seabed increases. This means that the bathymetry (the underwater elevation) of the ocean or sea bed also impacts the strength of waves.
Certain landforms further reduce wave's erosive power. Beaches increase the distance a wave travels before it reaches the coastline's cliffs and so reduces its energy. Headlands refract waves around them, reducing their erosive power at one location while increasing it at another.
Weathering also plays a role in the rate of erosion by creating weaknesses in rocks that are exploited by the processes of erosion. Freeze-thaw weathering, for example, creates cracks in rocks, increasing the rock's susceptibility to hydraulic action.
As always, humans have an impact on coastal erosion. Human activities have a variety of complex effects on coastal erosion but most commonly the activities increase the strength of waves. One activity, dredging, is commonly carried out to improve shipping capacities but it reduces the amount of energy dissipated from incoming waves and so increases erosion.
Lithology: Lithology refers to the physical properties of a rock such as its resistance to erosion. The lithology of a coastline affects how quickly it's eroded. Hard rocks (e.g., Gabbro) are resistant to weathering & erosion so a coastline made of granite (e.g., Land's End) will change slowly. Soft rocks (e.g., Limestone) are more susceptible to weathering & erosion so a coastline made of chalk (e.g., Dorset) will change relatively quickly.
If you looked down on a coastline from above and saw the geology of the area, you'd be able to see that the rock type changes as you approach the coastline and that the different rocks are arranged in bands. The angle these bands make with the coastline makes it either a concordant or discordant coastline.
Concordant coasts have alternating layers of hard and soft rock that run parallel to the coast. The hard rock acts as a protective barrier to the softer rock behind it preventing erosion. If the hard rock is breached though, the softer rock is exposed and a cove can form (e.g., Lulworth Cove).
On a discordant coastline, alternating layers of hard and soft rock are perpendicular to the coast. Because the soft rock is exposed, it is eroded faster than the hard rock. This differential erosion creates headlands and bays along discordant coastlines.
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