Coastal processes

Coastal processes are those processes that bring about various combinations of erosion and deposition along the coastline of an ocean, bay, or lake. These processes are highly unpredictable and depend upon a variety of mechanisms. Given the significant worldwide acreage of coastlines, an understanding of these mechanisms is essential to conservation, water quality efforts, and decisions in intervention.

Introduction

Coastal processes bring about change to the coastline of an ocean, bay, or lake using mechanical processes such as erosion and deposition of sediment. These mechanical processes result from normal cycles, such as the changing of tides or prevailing currents. They also result from weather events that are less predictable and more likely to cause significant disruption. Left alone, coastal processes seek a state of equilibrium. In places where human activity depends upon certain conditions prevailing along a coastline, engineering interventions are often undertaken, causing a permanent disruption to the natural state of equilibrium.

Sediment and Sediment Transport

The material deposited or eroded from a coastline is made of sediment that is, in turn, made of several parts. The heaviest layer, the bed load, is at the bottom and comprises grains that roll, slide, or bounce along the bed of the body of water. The bed load can be made of rocks, shells, and similarly heavy objects. The rate of bed load transport can be estimated using what is known about the slope of the coast, the action of the wave, the weather, and other conditions. Larger matter is moved when there is more energy and smaller matter when less energy is present.

Above the bed load is the suspended load. This layer of deposited or eroded sediment is moving in suspension but lightly enough that it does not come into contact with the bed. It can be made of small shells or pieces of shells and various other types of lightweight debris and objects. The last layer of sediment, the top layer, is the wash load transport. This layer is made of fine particles that are transported in the water. They are fine enough that they are not always visible. Sometimes near the shore, however, they are visible as a “cloud” in the water, a cloud made of grains of sand and similarly small bits of matter.

Sediment is naturally eroded and deposited along the coast in two key ways. Littoral drift (longshore drift) is one significant method. With this phenomenon, the movement of the sediment is parallel to the coast. The larger, heavier parts of the sediment are found at the start of the drift and the smaller, lightest parts are found at the end. With this type of erosion, material from the coastline is literally carried parallel to itself and deposited at various points. Gabions are often used to forestall this type of activity. They trap the longshore drift at a point close to the point of origin or at some other site along the beach where the deposit of sediment is desired.

Transport is the other natural way sediment is eroded and deposited. Offshore and onshore transport carries sediment perpendicular to the shore. To control this activity, an offshore barrier of some sort is required. The barrier can be a barrier island, an object sunken offshore, or some combination of the two. The purpose of the barrier is to keep the sediment from flowing outward, away from the coastline, and past a given point. The barrier also keeps material from being deposited on the coast because it stops wave action and weather-related activity from having a full effect on the coast. It does this by halting that activity in full or in part on the other side of the barrier, protecting the coastline.

Another variable in the movement of sediment is the slope of the coast. If the slope of the coastline is steep, any change in the water level of a lake, bay, or ocean will cause a small shift. If the slope of the coastline is gradual, any change in the water level will cause a more profound change in the coastline. This occurs because more surface area is affected by the action of the waves and other natural coastal processes. The slope of the coast also will make a difference in the action of the tides and currents for the same reasons.

Coastlines also change because of wind and other forces on the beach. Sand grains are blown loose by wind until the grains meet an obstacle. These obstacles include vegetation and fences, which are used to keep sand from moving away from the coastline; the wind itself cannot keep the sediment in motion.

Different layers of sediment are moved by the wind. The heaviest layer is the saltation. These sand grains bounce along in the wind, making up nearly all of the matter moved. The sand does not stop moving until it meets an obstacle. As it moves, creep accounts for the sand that collides with grains of other matter, and causes that matter to move along with the sand. Suspension is made of the lightest sand grains that are blown into the air and then settle.

Without obstacles, the sand will form dunes. Those sand dunes will then move from the coastline, carrying their sand with them. Once the dune reaches the point where it is too tall for conditions on the beach, the dune will collapse and repeat the process unless it is disrupted. Over time and on a beach with strong prevailing winds, a significant amount of sand will move inland through this process.

Chemical Erosion, Mechanical , and Resulting Structures

Rock formations along a coast are also subject to erosion. Chemical erosion, or chemical weathering, causes the rock to break down by altering the internal structure through the presence of chemicals. The chemicals can come from the environment in the form of acid rain or they can be present in the water itself. Either way, the chemical agents cause a change in the composition of the rock that makes it more susceptible to the effects of weather and waves. This, in turn, causes the rock to break down more rapidly than it otherwise would.

Mechanical weathering is another way that a coastline can change. Over time, the material making up the coast will erode from the action of the wind, rain, ice, and other natural processes on the coast. This can be seen in the case of sea-facing rocks or beaches made of lava and other materials, in which the motion of the silt and fine sand moving with the water will wear away the surface. Any fissures or fractures in the surface will be worked upon by changes in temperature, resulting in many striking formations.

Sea Stacks. One such formation is the sea stack, vertical columns of rock that are in the water near a coast but are not attached to the coast itself. These columns result from erosion on the rocks around them. The stacks themselves are composed of erosion-resistant rock and were formed by the effects of wind and water over a long period, often centuries. The rock around it has been eroded completely, so all that remains are pillars rising from the water. The pillars eventually will erode enough so that they will crumble and fall into the sea. Before they do so, however, they exist as commanding features of the coastline.

Sea Caves. Sea caves, also caused by weathering, are formed primarily by the action of the waves upon areas of weakness in coastal cliffs. The weakened area can be caused by a fault in the rock or at a meeting point of several different types of rock, in which one type of rock is not as strong as the other. The weakness or variation in rock strength gives the sand and silt in the water an opportunity to weather the rock. Often, a cave will begin to form as a crack in the rock. The weight of the waves—along with friction from sand and other sediment carried by the waves—acts to wear away the rock around the crack. In time, the wave action hollows out a cave.

Sea Arches. The sea arch is another prominent example of the effect of natural coastal processes. With these formations, the weathering process erodes a portion of the rock and leaves a passage through the rock or cliff. The process may have begun as a sea cave. In time, the rock material will look like an arch that has been intentionally formed. The “legs” of the arch will erode until they are too weak to bear the load of the arch they support. When this occurs, the arch collapses.

Weather

Weather events can have a profound effect on a coastline. Large storms often cause storm surges that bring additional material to the shoreline. This material will then be carried farther up the beach by the winds, along the shoreline by the currents, or between the water and the beach through the changing tides. Hurricanes cause widespread flooding, huge storm surges, and the movement of great amounts of sediment. In the process, significant alteration of a coastline can occur. When the damage includes damage to wetlands and coastal vegetation, it further disrupts the ability of the coastal system to restore equilibrium, increasing the likelihood that some form of intervention will occur.

Weather also affects the coastline of enclosed bodies of water when an unusually large amount of rain falls, causing the water level to rise dramatically, or when a drought occurs, causing the water level to fall dramatically. Because of the changes in water level, some areas of the coastline that were not affected by the action of waves on the lake will now be vulnerable.

Artificial Barriers

As time passes, a coastal system left to itself will reach a state of equilibrium. In this state, the material that is eroded will be deposited again at a point where the conditions at that coast can be sustained. This may not be ideal for humans, especially in areas where the coastline is used for recreational or commercial purposes. In such cases, more aggressive methods are taken to preserve the coastline in the desired state.

The use of gabions and barriers are two ways to preserve the coastline. These methods direct or control the flow of sand along or away from the coastline. To protect the sand on the shoreline, barriers can be erected to keep the sand and sediment on the beach. These barriers can be fences or vegetation, both of which keep the dunes from moving too far inland.

Once the sediment has been kept on the beach, it can be redistributed along the shoreline using a bulldozer or other heavy equipment. This process is not part of natural equilibrium, but it acts as a control within the natural processes that would occur. Redistribution is often done along the shoreline at beaches to maintain them.

Another way to preserve a coastline is to reclaim the sediment that has moved offshore. With this process, known as beach nourishment, sediment is dredged far offshore and returned to the coastline through large underwater pipes or tubes. When the sediment reaches the coastline, it is distributed where it will do the most to restore and protect the coastline.

Modeling Coastal Transport

Several models and approaches make it possible to calculate the amount of material moved along the shoreline. The energy flux model considers the amount of energy available in the waves at the surface as the waves arrive at the shoreline and calculate a rate of energy transfer per unit area. This model does not consider the action of the tide or ocean currents but rather measures the effect of the wind on the water and the energy transfer from the wind to the waves—the windier the day, the larger the wave, until the maximum wave height for the total conditions is met. This model also considers the water’s depth. A stochastic approach recognizes the random aspects of wave behavior. It pays particular attention to the short waves, considering when they break and how the energy is transferred from these waves. Still, other models consider the ratio of wave height to wavelength.

The simple cross-shore transport model, first proposed in 1982, posits that for a uniform sand size across the profile and equilibrium beach, there is a constant energy dissipation rate per unit volume. Any divergence from this equilibrium rate will lead to additional accretion or erosion of sediment. By using this model, it is possible to estimate the effect of predicted new events on a beach site and to estimate the ability of that site to reach equilibrium if no singular events occur.

The sediment transport formulation is designed for a numerical model of shelf-sand transport. This model was used to test the performance of the transport relation given a range of hydrodynamic conditions. It also was used to determine the reliability of field observations. The energetic models were formulated in the late 1980s using the sediment transport relationship for cross-shore flows, along with random wave-breaking models and other random wave activity, to determine the littoral transport rate for any given shoreline. Null point and equilibrium planforms are useful for long shorelines with variations of shoreline orientation. Studies have shown that these variations produce significant changes in the annual location of the null point, the point where no net transport occurs, which corresponds to the variation in the annual littoral drift rates.

Coastal Preservation

Coastlines can be stable or unstable for a variety of reasons. They can be stable because of regular weather patterns and wave-to-beach height ratios that maintain conditions of equilibrium. They can be unstable because of significant weather events, such as hurricanes. They also can be unstable because of human intervention, such as dredging for accessibility to ports or efforts to preserve a beach through artificial barriers and reclamation strategies.

Conserving the coastline in the face of falling sea levels, changing weather patterns, and human activity is challenging. Many mechanisms can be used to intervene when the natural coastal processes are not robust enough to maintain a desired state. Some are relatively passive, as in the case of barriers and gabions, and some are active, like beach nourishment efforts.

As conditions along the coasts change with increased commercial activity along rivers that flow into the ocean and increased population along the coastlines, coastal preservation efforts demand more attention. The wetlands at the coast serve as habitats for migrating species, and beaches serve as recreational areas for people worldwide. The preservation of the coastline is essential to a host of interested parties. Many methods ensure that coastline processes are respected, and other methods exist to estimate the effect of future actions.

Because preserving coastlines is a global endeavor, scientists from many disciplines worldwide are involved in research that will ensure the health of the world's coastlines. By publishing their studies and discussing their work at conferences, these scientists are part of a process that considers global climate change and increased industrialization in their efforts to support and augment natural coastal processes.

Principal Terms

accretion: natural or artificial deposition of sediment at a particular location

beach nourishment: the restoration of a beach by the mechanical placement of sand on the beach for purposes of recreational and shore protection

deposition: accumulation of sediment from natural processes

erosion: the removal of sediment from a particular location by the action of wind or water

gabions: boulders and rocks that are wired into mesh cages and placed in front of areas likely to experience heavy to moderate erosion

littoral drift: displacement of sediment down from and parallel to the shore

mechanical process: the action of forces on matter or material systems

offshore transport: movement of sediment or water perpendicular and away from the shore

onshore transport: movement of sediment or water perpendicular and toward the shore

sand spit: a low tongue of land or relatively long, narrow shoal extending from the land

sediment: the matter that settles to the bottom of a liquid

sediment transport: the movement of sediment from one location to another

tombolo: an offshore rock or island connected to the beach by a sand spit

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