Waves
Waves that batter the coast are generated by offshore storms, sometimes thousands of kilometres from the shoreline where they will expend their energy. Wind blowing over the water produces friction along the air-water boundary. Since the air is moving much faster than the water, the moving air transfers some of its energy to the water, resulting in waves. The waves, in turn, eventually expend their energy at the shoreline.
The size of the waves produced depends on the following:
- The velocity, or speed, of the wind. The greater the wind velocity, the larger the waves.
- The duration of the wind. Storms of longer duration have more time to impart energy to the water, producing larger waves.
- The distance that the wind blows across the surface, or fetch. The longer the fetch, the larger the waves.
Within the area of the storm, the ocean waves have a variety of sizes and shapes, but, as they move away from their place of origin, they become sorted out into groups of similar waves. These groups of waves may travel for long distances across the ocean and arrive at distant shores with very little energy loss. The important parameters are wave height(H), which is the difference in height between the waves trough and its peak, and wave length (L), the distance between successive peaks. The wave period (T) is the time in seconds for successive waves to pass a reference point. If you were floating with a life preserver in deep water and could record your motion as waves moved through your area, you would find that you bob up, down, forward, and back in a circular orbit, returning to about the same place. If you were below the surface with a breathing apparatus, you would still move in circles, but the circle would be smaller. That is, you would move up, down, forward, and back in a circular orbit that would remain in the same place while the waves travelled through. When waves enter shallow water at a depth of less than about one-half their wavelength (L), they feel bottom. The circular orbits change to become ellipses; the motion at the bottom may be a very narrow ellipse, or essentially horizontal, that is, forward and back. You may have experienced this phenomenon if you have stood or have swum in relatively shallow water on a beach and felt the water repeatedly push you toward the shore and then back out toward the sea. The wave groups generated by storms far out at sea are called swell.As the swell enters shallower and shallower water, transformations take place that eventually lead to the waves breaking on the shore. For deep-water conditions, there are equations to predict wave height, period, and velocity, based on the fetch, wind velocity, and length of time that the wind blows over the water. This information has important environmental consequences: By predicting the velocity and height of the waves, we can estimate when waves with a particular erosive capability generated by a distant storm will strike the shoreline. We have said that waves expend their energy when they reach the coastline. But just how much energy are we talking about? The amount is surprisingly large. For example, the energy expended on a 400 km (250 mi) length of open coastline by waves with a height of about 1 m (3.3 ft) over a given period of time is approximately equivalent to the energy produced by one average-sized nuclear power plant over the same time period. Wave energy is approximately proportional to the square of the wave height. Thus, if wave height increases to 2 m (6.6 ft), the wave energy increases by a factor 4. If wave height increases to 5 m (16 ft), which is typical for large storms, then the energy expended, or wave power, increases 52, or 25 times over that of waves with a height of 1 m (3.3 ft).
When waves enter the coastal zone and shallow water, they impinge on the bottom and become steeper. Wave steepness is the ratio of wave height to wave length. Waves are unstable when the wave height is greater than about 10 percent (0.1) of the wave length. As waves move into shallow water, the wave period remains constant, but wave length and velocity decrease and wave height increases. The waves change shape from the rounded crests and troughs in deep water to peaked crests with relatively flat troughs in shallow water close to shore. Perhaps the most dramatic feature of waves entering shallow water is their rapid increase in height. The height of waves in shallow water, where they break, may be as much as twice their deep-water height. Waves near the shoreline, just outside the surf zone, reach a wave steepness that is unstable. The instability causes the waves to break and expend their energy on the shoreline. Although wave heights offshore are relatively constant, the local wave height may increase or decrease when the wave front reaches the near-shore environment. This change can be attributed to irregularities in the offshore topography and the shape of the coastline. The offshore topography is similar to that of the coastline. As a wave front approaches the coastline, the shape of the front changes and becomes more parallel to the coastline. This change occurs because, as the waves enter shallow water, they slow down first where the water is shallowest, that is, off the rocky point. The result is a bending, or refraction, of the wave front. Owing to the bending of the wave fronts by refraction, there is a convergence of the wave normals at the headland, or rocky point, and a divergence of the wave normals at the beaches, or embayments. Where wave normals converge, wave height increases; as a result, wave energy expenditure at the shoreline also increases. The long-term effect of greater energy expenditure on protruding areas is that wave erosion tends to straighten the shoreline. The total energy from waves reaching a coastline during a particular time interval may be fairly constant, but there may be considerable local variability of energy expenditure when the waves break on the shoreline. In addition, breaking waves may peak up quickly and plunge or surge; or they may gently spill, depending on local conditions, such as the steepness of the shoreline and the height and length of waves arriving at the shoreline from a distant storm. Plunging breakers tend to be highly erosive at the shoreline, whereas spilling breakers are more gentle and may facilitate the deposition of sand on beaches. The large plunging breakers that occur during storms cause much of the coastal erosion we observe.
Beach Form and Beach Processes
A beach is a land form consisting of loose material, such as sand or gravel, that has accumulated by wave action at the shoreline. Beaches may be composed of a variety of loose material in the shore zone, the composition of which depends on the environment. For example, many Pacific island beaches include broken bits of shell and coral; Hawaiis black sand beaches are composed of volcanic rock; and grains of quartz and feldspar are found on the beaches of southern California. The landward extension of the beach terminates at a natural topographic and morphologic change, such as a sea cliff or a line of sand dunes. The berms are flat back shore areas on beaches formed by deposition of sediment as waves rush up and expend the last of their energy. Berms are where you will find people sunbathing. The beach face is the sloping portion of the beach below the berm, and the part of the beach face that is exposed by the up rush and backwash of waves is called the swash zone. The surf zone is that portion of the seashore environment where turbulent translational waves move toward the shore after the incoming waves break; the breaker zone is the area where the incoming waves become unstable, peak, and break. The long shore trough and long shore bar are an elongated depression and adjacent ridge of sand produced by wave action. A particular beach, especially if it is wide and gently sloping, may have a series of long shore bars, long shore troughs, and breaker zones.
Transport of Sand
The sand on beaches is not static; wave action keeps the sand moving along the beach in the surf and swash zones. A long shore current is produced by incoming waves striking the coast at an angle. Because the waves strike the coast at an angle, a component of wave energy is directed along the shore. If waves arrive at a beach perfectly parallel to the beach, then no long shore current is generated. The long shore current is a stream of water flowing parallel to the shore in the surf zone. This current can be surprisingly strong. If you are swimming on a beach and wading in and out of the surf zone, you may notice that the longer you go in and out through the surf zone, the further away you are from where you started and left your beach towel and umbrella. As you move in and out through the surf and swash zone, the current will move you along the coast, and the sand is doing exactly the same thing. The process that transports sand along the beach, called long shore sediment transport, has two components: (1) Sand is transported along the coast with the long shore current in the surf zone; and (2) the up-and-back movement of beach sand in the swash zone causes the sand to move along the beach in a zigzag path. Most of the sand is transported in the surf zone by the long shore current. The direction of transport of sand along beaches in the United States is generally from the north to the south for beaches on both the East and West Coasts of the country. Although most of the transport is to the south, it can be variable and depends upon the wave action and in which direction they strike the shore. The amount of sand transported along a beach, whether we are talking about Long Island, New York, or Los Angeles, California, is surprisingly large, at several hundred thousand cubic meters of sediment per year. Having said this, the amount of sand transported on a given day or period of days is extremely variable. On many days, little sand is being transported, and on others the amount is much larger. Most of the sediment is transported during storms by the larger waves.
Rip Currents
When a series of large waves arrives at a coastline and breaks on the beach, the water tends to pile up on the shore. The water does not return as it came in, along the entire shoreline, but is concentrated in narrow zones known as rip currents. Beach goers and lifeguards call them rip tides or undertow. They certainly are not tides, and they do not pull people under the water, but they can pull people offshore. In the United States, up to 200 people are killed and 20,000 people are rescued from rip currents each year. Therefore, rip currents constitute a serious coastal hazard to swimmers, killing more people in the United States on an annual basis than do hurricanes or earthquakes; the number of deaths caused by rip currents is equivalent to the number caused by river flooding. People drown in rip currents because they do not know how to swim or because they panic and fight the current by trying to swim directly back to shore. Winning a fight with a rip current is nearly impossible because the current can exceed 6 km per hour (4 mi per hour), a speed that even strong swimmers cannot maintain for long. A swimmer trying to fight a rip current soon becomes exhausted and may not have the energy to keep swimming. Fortunately, rip currents are usually relatively narrow, a few meters to a few tens of meters wide, and they dissipate outside of the surf zone, within tens to hundreds of meters offshore. To safely escape a rip current, a swimmer must first recognize the current and then swim parallel to the shore until he or she is outside the current. Only then should the swimmer attempt to swim back to shore. The key to survival is not to panic. When you swim in the ocean, watch the waves for a few minutes before entering the water and note the surf beat, the regularly arriving sets of small and larger waves. Rip currents can form quickly after the arrival of a set of large waves. They can be recognized as a relatively quiet area in the surf zone where fewer incoming waves break. You may see the current as a mass of water moving out through the surf zone. The water in the current may also be darker, carrying suspended sediment. Remember, if you do get caught in a rip current, do not panic; swim parallel to the shore until you are outside the current, then head back to the beach.