Destructive Waves Coastal erosion takes place with destructive waves. These destructive waves are very high in energy and are most powerful in stormy conditions. The swash is when a wave washes up onto the shoreline and the backwash is when the water from a wave retreats back into the sea.
Destructive waves have stronger backwashes than swashes. This strong backwash pulls material away from the shoreline and into the sea resulting in erosion.
Constructive Waves Constructive waves, on the other hand, are low energy waves that result in the build-up of material on the shoreline. Constructive waves are low energy and have stronger swashes than backwashes.
This means that any material being carried by the sea is washed up and begins to build up along the coastline. The material that is deposited by constructive waves can most often be seen by the creation of beaches.
Image credit: Jeff Hansen, U. Geological Survey. Hydraulic Action Hydraulic Action is the sheer force of water crashing against the coastline causing material to be dislodged and carried away by the sea. Compression Compression occurs in rocky areas when air enters into crack in rock. This air is trapped in cracks by the rising tide, as waves crash against the rock the air inside the crack is rapidly compressed and decompressed causing cracks to spread and pieces of rock to break off.
Compression is one of the main processes that result in the creation of caves. Abrasion Abrasion is when rocks and other materials carried by the sea are picked up by strong waves and thrown against the coastline causing more material to be broken off and carried away by the sea.
Attrition Attrition is when material such as rocks and stones carried by waves hit and knock against each other wearing them down. As these materials are worn down sand and rounded beach pebbles are formed. Parallel, ptigmatic or inharmonic folds occur, although without a specific distribution. An obvious structural feature in the area is the presence of a large number of hydrothermal veins, which may present thicknesses in the millimeter to meter scale.
Based on their structural relationships with the host rock, various topologies, mainly of quartz veins, are recognizable at outcrop scale: 1 Millimeter to centimeter-scale quartz veins, which are concordant with the metamorphic foliation developing boudinage.
These veins are contemporaneous with the foliation, preceding the metamorphic rock deformation. Their formation was subsequent to the veins described above.
Field relationships indicate that these are the youngest of the three families of quartz veins observed. Coastal landforms. The coastal landforms correspond to any of the relief features present along the coastal areas as the result of a combination of processes, sediments, and the geology of the coast itself Davis , It is a dominant driving force of change for coastal regions, which is becoming increasingly important Williams , , taking into account the degree to which it will affect the distribution and abundance of coastal landforms Crooks , The geomorphology observed in the study area reveals the occurrence of fragile and unstable zones where several phenomena waves, currents, tides, sediment download, runoff, landslides movements and changes in the landscape due to human action operate.
The high relief hinterland reaches an altitude of m, whereas the lowlands coastal plains truncated in some areas where the rocky littoral extends into the shoreline, and manifests itself in the presence of bays and headlands that emphasize the generation of a variety of landforms that give a great view of the landscape.
Headlands and bays occur frequently together on the same stretch of the coastline. They form on discordant coastlines, where bands of rock of alternating resistance run perpendicular to the coast.
Bays form where weak, less resistant sandy and clay rocks are eroded, leaving bands of stronger, more resistant, rocks such as schist or granite, which form a headland or peninsula. Wave refraction disperses wave energy through the bay and occurs together with the sheltering effect of the headlands, thus protecting bays from storms.
This effect means that the waves reaching the shore in a bay are usually constructive waves, and because of this, many bays feature a beach. A bay may be only meters or hundreds of kilometers across.
Figure 2. Figure 3. Coastal deposition landforms. Landforms of coastal deposition are the result of the material deposited on the coast by the sea, which occurs in areas where there is a large input of sediments from rivers, beaches or cliffs, long shore drift and an irregular coastline Nagle , In addition to beaches, a range of unique coastal deposition landforms exists such as coastal plains, inner lagoons, spikes, beach ridges, low flood zones, flats with sand dunes or cuspate forelands.
Additionally, the formation of these landforms depends on the process of long shore drift, which occurs when waves approach a coastline at an angle, due to the dominant wind. Therefore, there is a sideways component to the swash, which helps to move beach material diagonally up the beach it travels laterally as well as inshore.
Backwash under gravity returns water and beach material directly to the sea perpendicularly to the beach profile. The net result is a zigzag motion that can carry material past the end of a headland. Figure 4 illustrates the coastal deposition landforms in the study area. Figure 4. These coastal deposition landforms occur extensively along the coastal area defining a narrow strip except at some places where they are cut by the Manzanares and Gaira rivers and interrupted by rocky shorelines.
In the Santa Marta, Inca Inca and Gaira bays, as well as in the hotel sector, important beach areas occur. All along the shore, the beach gently slopes. A significant amount of the material forming the beaches comes from erosion of detrital material transported to sea by rivers depositing sediments on the shore.
However, the long shore drift, a stream of water flowing parallel to the beach created by the action of waves breaking at an angle to shore, is responsible for the transport of sediments along the coast. Beaches mostly consist of fine-to medium-grained sandy sediments, with a good selection, subrounded grains, high sphericity, mostly composed by quartz, chert, small fragments of shells and minor biogenic material. Several fine- to medium-grained sandy beaches occur Figures 4 a- 4 e , although gravel beaches e.
A well-defined and continuous berm is seen in the Salguero and hotel sector beaches where accumulations of trunks, wood and coarse fragments of shells or other materials of anthropogenic origin, brought by the sea at high tide or during storms, can be seen. Where there are cliffs, there are no beaches, mainly due to the presence of fallen blocks.
Los Cocos beach tends to increase slightly its size by the sediment contribution brought by the Manzanares River in winter, whereas it significantly decreases marking a surf wave zone in summer.
Beaches in Punta Gaira have a steeper slope, coarser sediments and landforms typical of wind action dunes and rolling. To the north, beaches transitionally show a gentle slope and finer sediments and scarce dunes and rolling. Coastal plains. These coastal landforms represent extensive lowlands that show some slight topographic undulations. The coastal plains end in the shoreline, and hills or high relief hinterlands breaking into the coastal plains in a different direction generally bound them.
In some areas, these are located near current sea level adjacent to the shoreline, and, in others, the beginning of the coastal plains is located several meters above sea level, and they are associated with marine fluvial processes.
Coastal plains can diminish significantly due to land use e. The largest coastal plain occurs near the hotel sector. Santa Marta developed on a coastal plain, which was progressively lost due to the development and spacing of this touristic city. Manzanares River floods also influenced it. Another coastal plain occurs in the Plenomar sector where it occupies a small area bounded on the continent by hills or high relief hinterlands Figure 4 g , and whose origin is related to the floodplain lakes caused by the growing season or storm surges.
Inner lagoons. The inner lagoons have different extensions and lie adjacent to the shoreline. However, some of them are separated from the sea by narrow sand bars, thereby creating a relative influence of the sea on the lagoon.
In contrast, there are lagoons that despite their situation on spikes, are distant from the present shoreline and do not receive the direct influence of the sea. The interior lagoons are present near low flood zones, and some of them resulted from past floods or human action. Examples of this type of landforms occur in La Escollera lagoon and the Plenomar sector Figure 4 h.
The most common coastal landforms are spikes. The longitudinal extent and strength of these may vary according to the contribution of material and other oceanographic and geological factors. Spikes mark the direction of the littoral drift, which is generally SW for the Colombian Caribbean, explaining the overall trend of the headlands toward this direction.
Most of these landforms have a hook showing the greatest erosive effects due to marine dynamics, since the waves struck with greater force on them. Besides this erosive effect, geological features high jointing of the area also accentuate the spike shape and generate the development of stacks as in Punta Gloria Figure 4 i.
Beach ridges. Beach ridges are moderately undulating features of marine depositional type formed parallel to the coastline. Well-developed beach ridges occur in Punta Brava. Each ridge shows gentle longitudinal depressions that run parallel to the ridge.
They may appear as a single groove or a series of parallel grooves. In general, these coastal landforms are less than 20 cm high, and some times it is difficult to recognize them. This type of landforms occur in Punta Brava, located towards the side of the high tidel zone, on a mound of approximately 1 m high, revealing how they are influenced by wind action Figure 4 j. These ridges occur along the beaches at high tidal zones and are important to understand how the wind acts as a changing factor and establishes accretion zones.
Low flood zones. They are flat and low coastal areas whose extension is variable. They are associated with sand spikes. In winter these areas capture rainwater, thus acting as temporary shallow lagoons, while in summer they have clay soils that generally show mud cracks Figure 4 k.
These low flood zones located very near the shoreline are frequently flooded due to high tides or, sometimes, storm surges. Near La Escollera lagoon, desiccation cracks are not well marked, but temporary shallow lagoons with clay soil development are characteristic in the rainy season. In the Plenomar sector, temporary shallow lagoons and desiccation cracks were observed. Flats with sand dunes. These landforms correspond to sandy low-lying coastal areas, which are associated with the formation of spikes.
They extend from the present beach to the edge formed by ancient cliffs, and are extensive areas of unconsolidated material associated with ancient beaches, which now may or may not be vegetated. Often thin sand covers occur on these flats. In other areas, they present scattered sand mounds of low elevation less than 1 m and no preferred orientation, which relate to the dynamic action of wind.
Flats with sand dunes generally occur in all sand spikes in the study area. These landforms were observed in the beach around Punta Gaira, where they manifest as coarse- to very coarse-grained sandy plains whose width varies from 10 to 15 m. They are about 30 m away from the present beach. Due to their morphology and location, they are subject to periodic flooding. Cuspate forelands. Cusp-shaped geoforms are common along several beaches. They grade in size from small beach cusps to large cuspate forelands.
Ideal sites for the formation of these coastal landforms include those locations where a major change in coastline direction occurs. Many cuspate forelands have been built by the progradation of a series of beach and dune ridges as sediments deposited in the slack water zone between two coastal eddies. Coastal erosion landforms. The landforms of coastal erosion are the result of large waves crashing onto a shore with a tremendous amount of energy and a significant eroding effect; several unique erosion features commonly form on rocky shores with strong waves Earle , The most spectacular scenery found along coastlines is the result of the effects of wave erosion Magbagbeola , Landforms of coastal erosion include cliffs, wave-cut platforms, caves, arches, stacks, stumps, and headlands, amongst others.
Figure 5 illustrates the landforms of coastal erosion in the study area. Figure 5. As mentioned above, headlands represent outstanding landforms occurring between two bays and they consist of resistant igneous and metamorphic rocks, which are more resistant to abrasion.
Refraction of waves occurs on headlands concentrating wave energy on them, so many other landforms, such as caves, arches and stacks, can develop. These landforms present strong jointing and surrounding cliffs ranging in size. They are very important due to their substantial influence on the erosion-accretion effect as they modify the wave direction and, in a way, also that of the littoral drift, being decisive in the sedimentology of the study area.
Headlands form when the sea attacks a section of coast with alternating bands of hard and soft rock. They determine the coastal settings as immersion or transgression type coasts that represent abandoned coast-lines or berms revealing the coastline regression. Examples of headlands are shown in Figure 5 a. They are topographic promontories adjacent to the coast, which stand along a wide coastal plain Figure 5 b.
Generally, the hills have a relief with gentle slopes and heights of about m, comprising a variety of sedimentary material. In some sectors, landslides and erosion due to overgrazing and gullies can be observed. The undulating morphology results from the action of endogenous faulting or folding and exogenous climate and erosion processes. The first occur at La Gloria hill and in the coastal cliffs of Punta Gloria, marking a trend of satellite faults.
Exogenous processes, although lesser, also operate, and they have modified hills in the Inca Inca bay. In the first of them, hills present topographic heights between and m and a moderate to high drainage density, while in the latter, they do not exceed m and the density of drainage is low. These hills are part of the SNSM foothills, which reach the littoral, significantly influencing the development of coastal landforms such as headlands, making them more resistant to erosion, and forming a series of cliffs in between which bays have formed.
Sea cliffs. These landforms represent escarpments that mark abrupt changes in slope, formed by marine erosion Figure 5 c. They generally consist of a rocky substrate and their height is variable high and low cliffs. In high cliffs, the coastline has a narrow beach close accompanied by a slope of generally inclined walls. In addition, they limit terraces or shells with heights equal to or greater than 10 m. Along the coast, high cliffs represent the common type, showing convex to vertical profiles with heights of m.
The escarpments caused by the action of the sea are not longer than its direct action and they are separated from the present coastline, developing paleo-cliffs Figures 5 d and 5 e , which are well exposed in Los Nativos and Lipe beaches in the NE side of Punta Gaira. This not only reveals the retreat of the sea but also the development of a process of accretion.
In the study area, cliffs are not related to fluvial-marine terraces as usually occurs on the Caribbean coast. On the contrary, there are cliffs associated with hills and mountains belonging to the SNSM foothills. Wave action generates typical erosional landforms that depend on rock compaction and structure forming these foothills, mainly composed by schists and phyllites, with wave and erosion action through the foliations and joints of these metamorphic rocks.
The erosion on the cliff is shallow and results from rock falls, possible landslides, water action cracks, caves and alveolar or honeycomb shapes and bioerosion, the latter favored by the presence of organisms such as mollusks and plants that adhere to bedrock. On the other hand, in low cliffs Figure 5 f , the height of the coastline does not exceed 10 m.
Generally, the bedrock is undermined and it is common to find separate rock slabs at the base of the cliff. At the foot of the active cliffs accumulations of blocks, whose size can reach a 2 m diameter in average, occur often formed by the material from the cliff itself Figures 5 g and 5 h.
The accumulation of these blocks reflects the instability experienced by the bedrocks by the action of waves. The existence of many of these features depends upon the maintenance of a vertical cliff-face through an on-going cycle of undercutting, collapse and retreat.
When waves break at the foot of a rock face, marine erosion processes cretated a wave-cut notch. Over time, a section of rock removed from the bottom of the cliff face, leading ultimately to the collapse of the un-supported section above, developed a wave-cut platform Figure 5 i. Stacks and stumps. During erosion process caused by sea action over a cliff, waves can attack its base forming a cavern. When two caverns in the opposite sides join, they form coastal arches, which finally collapse due to the gradual fall of their roof, developing isolated remnants of rock as small rocky stacks.
They typically represent more resistant portions of the ancient cliff that survived for a time to form stumps before the sea destroyed them. The Gaira stack Figures 5 e and 5 f is located about m from Punta Burukuka, on the western part of the Gaira bay. Its size reaches up to 30 m high. Some stacks that fail to stand out above the sea level represent a threat to ships moving near the coastline.
These features may be marked in headlands that have no stacks today, as can be seen in Punta Gloria and Punta Gaira. Towards Punta Gloria, a coastal stack no more than 5 m occurs Figure 5 k. Stunning coastal promontories or stumps represent coastal landforms, which form when a stack erodes, leaving a rocky stack usually small enough to be submerged at high tide as shown in Figure 5 k. The erosion and accretion processes.
Shoreline changes occur because of erosion and accretion processes. The erosion is a condition of the coast on the skids because of the activities of seawater, whereas the accretion represents the material addition caused by the deposition of material from a river Prasita , The shoreline represents the area where the ocean and continent interact and where geomorphological processes develop quite fast compared to other oceanic and continental areas.
Waves, sea currents, tides and other ocean phenomena erode the solid material of the coast, promoting its transport and deposit in other areas, and continually modifying the coast morphology as revealed by the occurrence of different landforms as described above.
Rivers, meanwhile, transport continental sediments and accumulate them on the shoreline forming deltas. Erosion and sedimentation coasts can form due to these phenomena. The coastal erosion landforms present steep rocks or cliffs and caves for the sector of headlands. Generally, the active process of erosion caused by waves, tides and sea currents crumbles the base of cliffs and ultimately leads to their collapse and retraction of the coast. In this way, the so-called abrasion shelves formed.
The sedimentation coasts show sandy beaches e. The Manzanares and Gaira rivers supply and transport sediments in a process that can form new beaches. The Gaira River, which covers more territory within the city than the Manzanares, is one of the most polluted rivers in the department of Magdalena. The Gaira inlet extends from Punta Gloria on the south to Punta Gaira on the north, and its coastline is about The beaches in this area have similar characteristics and present gravel sandy marine and clayed sandy fluvial sediments.
In some sectors, particularly at Punta Gloria and the mouth of the Gaira River, carbonate sediments and fragments of igneous and metamorphic rocks occur. However, a third factor that has influenced both the instability of the seabed and the current sedimentary characteristics of the beaches is the dredging for the recovery of El Rodadero beach, which has been necessary, as the coastline had suffered a setback that could cause the entry of sea water into the urban area.
The only direct discharge of fresh waters occurs on the south of the Gaira inlet and comes from the Gaira River, polluted by domestic and industrial waste discharges continually dumped in it. The Manzanares River crosses the city from northeast to southwest; its channel receives the discharge of sewage and liquid wastes, and besides, houses are extremely close to the river.
In the littoral, interaction processes among the lithosphere, hydro-sphere, biosphere and atmosphere are quite marked and rapid. All these factors evidence how coastal areas play an important role in biogeochemical cycles and, in general, in the ecosystem. Generally, the beaches of the Colombian Caribbean region are subject to seasonal weather changes.
The study area presents a predominance of trade winds of N-NE direction. Erosive phenomena have affected beaches causing considerable decreases of the beachfront. On the other hand, in the rainy season, when low intensity and variable direction winds occur, beaches are reconstructed to increase their amplitude. These two situations explain why the coastal boundary configuration and SW coastal drift in its direct relation to wave action influence of hydrodynamics on this sector of the littoral.
Coasts can change quickly due to processes of erosion and sedimentation, and result in new and varied coastal landforms. The main agents contributing to changes in littoral modeling are waves, tidal currents, littoral drift currents coastal currents , fluvial currents, winds, organisms corals and human activity. The main erosion forces among these agents acting on the coastal environment proceed directly from marine activity, waves being the most important agent in the coastal denudation, even if some denudation action can be also attributed to tidal and littoral currents.
With regard to supply or loss of sediments, littoral drift currents somehow influence in the region, creating a SW-striking current that allows the transport of sediments in this direction and generates a significant change in the characteristics of landforms. In relation to fluvial currents, they act mainly in the rainy season when the Manzanares River caudal increases, carrying large amounts of sediment, which influence the direction of the drift for Los Cocos beach.
In general, the accretion landforms on the coast are relictic, due to the lack of input by present rivers. Erosion is primarily caused by the lack of direct accumulation the river load does not reach the coastline and by induced erosion since littoral drift is not saturated from river input, erosive processes operate on the coastal outcrops.
Erosion in the coastal area is evident in the current inability of the Gaira and Manzanares rivers to transport gravel-sized sediment to the coast, at least in large volumes.
A decrease in sediment supply from the upstream drainage basins has created an unstable mass balance, because the quantity of solid discharge is less than that moved by the littoral drift Aiello, et al. Therefore, we could say that some of the existing sediments originated in periods when climatic conditions were different from today, and these rivers had a higher carrying capacity and greater erosive power. The stability and the movement of the coastline represent its behavioral parameters.
Vargas has discussed these aspects in detail. Based on the stability of the coastline in relation to the dynamic action of the sea, the coast can be consolidated or unconsolidated. A consoli-dated coast is a coastline formed by rocky cliffs. Stump formation: Step 4. The arch grows larger and eventually collapses leaving a stack separated from the mainland. Stump formation: Step 5.
The stack erodes and becomes a stump. Landforms created by erosion The coastline is constantly eroding. There are four key types of erosion: Abrasion - waves transport material which hit the cliff and gradually wear it away.
Hydraulic action - as waves approach the coast they trap air and force it into gaps in the cliff. Eventually this weakens the rock.
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