Coastal Erosion

Coastal Erosion.

Author: Tony.

Abstract

This paper focuses on coastal erosion. The paper analyzes the wave actions that cause coastal erosion. The main wave actions are corrosion, attrition and hydraulic action. Also, the paper analyzes the main factors that influence coastal erosion. Moreover, the paper also analyzes the strategies used to control coastal erosion. Both structural and non-structural strategies would be analyzed.

Introduction.

           Coastal erosion refers to the gradual disintegration and wearing way of rocks and soils, from the shore, due to chemical dissolution, physical breakdown and the transportation of such material by wave actions. Coastal erosion is caused by waves that are generated by windstorms, ocean storms and any speeding motorboats. These waves cause loss of sediment (due to dissolution of soil), gradual disintegration of rocks and transitory sediment redistribution. Waves cause coastal erosion through a combination of the following processes: corrosion, hydraulic action, abrasion and impact processes. Coastal morphodynamics have shown that coastal erosion in a beach causes agglomeration of sediments in the adjacent areas (Dodd et al, 2003).

          Coastal erosion leads to rock formation on non-rocky coastlines. A rock formation is made up of multiple rock layers of varying thickness. These rock layers represent fracture zones which were formed due to variable degrees of erosion. Hard areas are eroded at a slower rate than soft areas with the resultant formation of pillars, tunnels, columns and bridges. Abrasion is the process of erosion caused by the mechanical effects of friction. Abrasion of soft rocks (and loose sand) is caused by strong winds, with the consequent development of a sandblasting effect. Some anthropogenic activities, such as dredging, have promoted coastal erosion. Moreover, climate change has led to increased coastal erosion (Zhang, 2004)

Wave action.

            Waves are created when wind blows over an expansive water surface. The water wave is a product of the friction created by the wind on the air-water interface. Waves are influenced by the fetch and the duration that the wind blows over the water body. Wave energy is directly proportional to the amount of moving water. There are three major wave actions: hydraulic action, attrition and corrosion/abrasion (Hyndman & Hyndman, 2010). The quantity of wave energy significantly influences the wave actions as is discussed below.

            The hydraulic actions of waves occurs when an incoming water wave strikes the face of a cliff; and in the process, the wave energy compresses the gases contained within the cracks in the cliff face. According to Boyle’s Law, the pressure of the compressed gases will subsequently increase. This gradual increase in pressure forces the surrounding rock material to disintegrate and splinter. The water wave removes the splinters, thus causing erosion of the rock. The water wave deposit these rock pieces onto the adjacent shoreline. Continued hydraulic actions cause the crack to develop into a cave (Hyndman & Hyndman, 2010).

            Attrition is a process that occurs when water waves force scree to collide with other rock debris. This collision process allows mechanical forces and friction to grind and chip the scree into small, round and smooth talus. A similar effect is also achieved after collision of the scree onto the cliff face (Hyndman & Hyndman, 2010).

            Corrasion and corrosion usually occur simultaneously. Corrasion (also termed as abrasion) is a process that occurs when high-energy water waves break on the surface of a cliff, thereby eroding it. Moreover, the hydraulic action of such water waves enables it to carry scree which will be subsequently smashed onto the cliff face, thereby causing more erosion on the cliff face. Corrosion occurs when acidified sea water causes chemical erosion (and the dissolution of small rock pieces) of rocks on the face of a cliff. Limestone rocks are particularly vulnerable to corrosion. Hydraulic action and high wave energy speeds up the process of corrosion (Hyndman & Hyndman, 2010).

Factors influencing coastal erosions.

            Costal erosions are determined by the erosion rate. Erosions rates are influenced by a myriad of factors which can be categorized into primary, secondary and tertiary factors.

The primary factors determine the rate of erosion. The principal primary factors are: hardness of rocks, sea levels, hydraulic action, foreshore stability, bathymetry and wave energy. The degree of hardness of ocean-facing rocks is determined by its intrinsic strength, its underlying non-cohesive scaffolding materials, and, the number and location of fissures in the rock. Fissures, low intrinsic rock strength and a weak non-cohesive scaffold reduce the hardness of rocks, and thus predispose them to erosion and disintegration. Usually, hydraulic action removes the scree from the debris lobe and the foreshore. Powerful hydraulic actions increase the debris flow, thereby increasing the rate of erosion of debris lobes. A stable foreshore enables a strong wave to smoothly dissipate its energy without altering the configuration of the foreshore. This reduces the extent of coastal erosion. Up-drift materials increase the stability of the foreshore. Bathymetry determines the energy of the water waves that reach the shoreline, and hence, it influences the degree of erosion of the cliff face. Shoals decrease the rate of erosion, since they dissipate most of the wave energy. Thus, the presence of a shoal determines the extent of coastal erosion. Rising sea levels due to global warming has led to the formation of high-energy water waves and altered bathymetry which has increased the rate of erosion (Gillie, 1997).

            The secondary factors influence the landscape and topography of the shoreline. They include the following: vegetation cover, resistance to attrition, slope hydrology, weathering processes, slope incline, and; erosion and accumulation of sediments at the foot of the cliff (Gillie, 1997).

The main tertiary factors are coastal management and mineral extraction. Appropriate coastal management reduces coastal erosion, and also mitigates the effects caused by coastal erosion. Mineral extraction along shorelines destabilizes the compactness of rocks, thereby increasing the rate of coastal erosion (Gillie, 1997).

Controlling coastal erosion.

            Coastal erosions have led to the destruction of beaches, thereby reducing their commercial value. The resulting economic pressure caused the concerned parties (government agencies, environmentalists and the private sector) to come up with strategies that are aimed at stabilizing the coastline. There are three main approaches used to control of coastal erosion (Clark, 2004). These approaches are described below.

1)      Hard structural stabilization.

            This involves hard structural engineering of structures such as groin, jetty, revetments, sea walls, rock armor and offshore breakwater structures. The construction of these structures is usually undertaken by the county or state government (Clark, 2004).

            Groins are impermeable compact solid structures that are constructed perpendicular to the water surface. They are constructed in collective groups termed as groin fields, which extend from the shore. The groin fields entrap and retain sediments, thus stabilizing the shoreline. Groins are fairly effective against unidirectional longshore currents. However, it alters the aesthetics of the shoreline by creating artificial scallop-shaped shoreline (Clark, 2004).

            Jetties stabilize channels which open into lakes, seas or oceans. A jetty permits ships and boats to enter a water channel. Hence, they are created in pairs, in order to ensure that the entrance into a channel is appropriately stabilized. Moreover, they can be used to stabilize man-made maritime structures such as piers and docks. However, they are prone to blockage caused by sand sedimentation (Clark, 2004).

            Seawalls are hard concrete structures constructed on inland locations of coastlines in order to protect the adjacent populations from coastal erosion and flooding. They reflect the wave power. They can be vertical, inclined or curved. The backwash of water waves removes sediments from the sea walls (Clark, 2004).

            Offshore breakwaters are concrete structures constructed parallel to the shoreline. They change the direction of waves, and reduce the wave energy. They protect an anchorage from the water waves and longshore drift (Clark, 2004).

            Revetments are wooden structures containing rock infill. They are constructed parallel to the shoreline, and they protect the base of a cliff from waves. Rock armor is made up of a pile of rocks placed on the shoreline, and their main functions are to absorb wave energy and retain sediments (Clark, 2004).

2)      Soft structure stabilization.

            It encompasses beach nourishment, breach drainage and sand dune stabilization. Beach nourishment involves deposition of sediments and sand on beaches, in order to replace the sand lost to erosion. Replacement sand is dredged from offshore locations, and transported to the beach. It is a safe method of restoring the aesthetic quality of a beach. However, the process of beach nourishment is relatively expensive. Sand dune stabilization is achieved by introducing a vegetation cover. Plants act as good trap for blown sand. Beach drainage involves lowering the water table, thereby causing an agglomeration of sediments and sand on the beach (Clark, 2004).

3)      Non-structural strategies.

They involve placing legal limitations on land-use and prohibition against development (that is, construction and exploitation of resources). However, most local authorities oppose these non-structural strategies (Clark, 2004).

Conclusion.

            Water waves cause loss of sediment, gradual disintegration of rocks and transitory sediment redistribution. Waves cause coastal erosion through a combination of the following processes: corrosion, hydraulic action, abrasion and impact processes. Costal erosions are determined by the erosion rate. Erosions rates are influenced by a myriad of factors which can be categorized into primary, secondary and tertiary factors. The principal primary factors are: hardness of rocks, sea levels, hydraulic action, foreshore stability, bathymetry and wave energy. The secondary factors are vegetation cover, resistance to attrition, slope hydrology, weathering processes, slope incline, and; erosion and accumulation of sediments at the foot of the cliff. There are three main approaches used to control of coastal erosion: Hard structural stabilization, soft structure stabilization and non-structural strategies.

References.

Clark, J. (2004). Integrated Management of Coastal Zones. Miami, FL: University of Miami

            Press.

Dodd, N; Blondeaux, P; Calvete, D; De Swart, H; Falqués, A; Hulscher, S; Różyński, G &

            Vittori, G. (2003). Understanding Coastal Morphodynamics Using Stability Methods.

            Journal of Coastal Research, 19 (4), 849-865.

Gillie, R. (1997). Causes of Coastal Erosion in Pacific Island Nations. Journal of Coastal

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Hyndman, D & Hyndman, D. (2010). Natural Hazards and Disasters. New York, NY: Brooks

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Zhang, K. (2004). Global Warming and Coastal Erosion. Climate Change, 64, 41-58.