Waves in Northern Morocco.
Wave in Pismo Beach, CA.
Wave in Baja, CA.
Wave in Baja, CA.
- What causes waves?
- What causes waves to break on shore?
- Measuring waves
- Types of waves
- What are long shore currents?
- Rip currents
What causes waves?
Waves are generated at sea by the wind. Small ripples form on the water as the wind blows across the ocean’s surface. The size of waves depends on three things:
- The duration of the wind;
- The strength of the wind, and;
- The fetch, or the distance over water across which the wind blows.
The longer the wind blows the bigger the waves; stronger winds mean higher waves; and the greater the fetch, the bigger the waves. Thus the biggest waves of all occur in the storms that last the longest with the most energetic winds with hundreds of miles between the storm at sea and the beach.
Waves do not actually consist of water traveling from where the wind is blowing all the way to the beach. Instead of moving water, waves are moving energy that was transferred from the wind to the water. This energy propagates, or moves, through the ocean to the beach in the form of a wave. But the water itself is not moving forward as in a current. Instead, the energy rolls through the water in a circular motion called a wave orbital. The crest of a wave is the top of a wave orbital, and the trough of a wave is the bottom of a wave orbital. When the waves reach the shore they expend their energy by breaking and then moving sand and shaping the beach.
What causes waves to break on shore?
As waves move towards shore, they begin to "feel" the ocean floor. In a process known as shoaling, this causes the wave orbitals to flatten as the bottom shoals. When waves feel the bottom they slow down and bunch together (decrease their wavelength); but the time between wave crests (period) does not change. The height of the wave will initially decrease when it feels bottom, but then will steadily increase until the wave becomes unstable and breaks near the beach. The water literally falls over. Waves expend the energy they gained from the wind by transferring that energy to the beach when they break.
The dimensions of a wave are measured both by crest height and the distance between crests. Wave height is the vertical distance from the crest (highest part of the wave) to the trough (the lowest part of the wave). Most untrained observers at sea tend to greatly overestimate wave height, which is quite understandable because they do not have any stationary reference points. And then there is the terror factor. A person holding on for dear life on a rolling, bounding vessel is easily convinced of the giant size of the waves.
Standing on a beach, a good way to estimate wave height is to assume that the surfer out there is 6 feet tall! In many cases, the amplitude is also used as a measurement of the wave’s size; wave amplitude is one-half of the wave height. The wavelength is the distance from one crest to the next crest, or from one trough to the next.
Waves travel at different speeds, and the speed is typically measured as the wave period or wave frequency. Wave period is the number of seconds it takes two successive wave crests to pass a given point. Wave frequency is the inverse of the period, or the number of waves that pass a given point during a given time period. As the length of a wave increases, so does its speed. In a general way, the higher the wave period the greater the wave height. Big storms in North Carolina may produce waves with 12 to 15 second periods, while calm weather wave periods are more likely to be 3 to 5 seconds.
Types of waves
The way each wave breaks depends on the slope and shape of the bottom. In general, the wave will break in one of three ways: as a spilling, plunging, or surging breaker (see Fig. 1.7). Where the beach is relatively flat and wide, spilling breakers will form. Spilling breakers look like they are crumbling as they move along. With a slightly steeper beach slope, the crest curls over and creates a plunging breaker (Fig. 1.8). These breakers actually form a tube of air trapped beneath the curl. The tube of air is forced into the bottom when the wave breaks and this air helps to stir up the bottom sediment. A plunging breaker is the sensational, curling type commonly sought after by surfers. Because the energy of the plunging breaker is concentrated in a small or narrow area of the seafloor, it is able to move large amounts of sand.
On the steepest bottom slopes, the wave often does not break before reaching the beach. Instead, a surging breaker is formed where the wave surges up the beach and is reflected back to sea. These types of breakers may look just like a series of bubbling mounds of water moving ashore. All three wave types may be found on any beach at different times but often a beach has a characteristic or commonly occurring wave type.
Figure 1.8. A surfer catches a calm-weather plunging wave at South Nags Head in September 2002. Waves on the Outer Banks are higher, on average, than those of southern North Carolina because the narrower continental shelf off North Carolina offers less friction to incoming waves from the open Atlantic.
As waves move from deep water and approach the shore at some angle, the part of a wavefront that enters shallow water first will begin to slow as it feels bottom. This portion of the wave slows down while the deep-water part keeps moving at its original higher velocity, causing the wave crest to bend or refract. Along most beaches, by the time the wave breaks, the refraction is so great that the wave crest is typically much closer to paralleling the shore orientation than it was in deep water. Since waves can, in theory, approach the beach from any direction, the amount and type of refraction varies widely. Waves that form offshore may arrive at the beach with an orientation identical to that of the beach. Such waves will undergo little refraction. But waves generated by storms may approach the shore at varying angles and are sometimes strongly refracted. The wave refraction picture is further complicated because shorelines may be oriented in many directions.
Rogue waves are spectacular and dangerous waves. These poorly understood waves are rare on beaches, but they do occur occasionally. A few years ago, a rogue wave struck a south Florida beach, rearranging some cars, but no one was injured because it occurred in the middle of the night. As wave trains travel across the ocean from various storms, they frequently meet each other. When this happens, the waves will either cancel each other out or reinforce each other. If the wave crests coincide with other crests, they will have positive interference, which really means that the two intersecting waves will be one wave, with the combined height of the two. Likewise if the waves of two intersecting trains are out of phase, the troughs of one set can cancel out the crest heights of the other. When several waves intersect at just the right time and phase, a rare rogue wave of immense height can form. Such a wave can bury a ship. Currents can also increase or decrease the heights of a wave train depending on the local conditions. If a wind is blowing against a current, wave height will increase. Experienced sailors fear a strong wind from the north when sailing in the north-flowing Gulf Stream.
Tsunamis, sometimes called tidal waves, are not generated by the wind like other ocean waves. They are caused by sudden underwater disturbances such as earthquakes, volcanic eruptions and submarine landslides. The catastrophic release of energy during any one of these events forms a sudden wave of immense wavelength (sometimes hundreds of miles!). Traveling at very high speed, these waves spread out from their source until they reach an obstacle such as a shoreline
Tsunamis are extremely dangerous because they travel so fast that there is little time to warn people of their impending arrival. A tsunami formed in Hawaii will take only hours to reach Japan, Alaska and Washington. Reaching heights of 100 feet (30 meters), tsunamis can remove all the water away from the beach as they approach the shore. Boats unfortunate enough to be in that water will be grounded by the tsunami wave, and unwary beach dwellers are likely to run out to the newly exposed land to gather stranded fish. When the wave comes ashore, it does so with such force that it literally destroys everything in its path. And people and debris caught in the wave often get transported out to sea when the water returns to the ocean.
Learn how to survive a tsunami. Lessons from Chile, Hawaii, and Japan.
What are long shore currents?
Breaking waves form longshore currents that carry sand grains (and swimmers) along the beach. Longshore currents form in the surf zone because waves approach the shoreline at an angle (Fig. 1.9). When a wave breaks, a portion of the energy is directed laterally along the beach and this forms the current. Even a very gentle current can carry a lot of sand because the breaking waves kick sand up into the water column as evidenced by the discoloration of the water in the surf zone.
For a given wave size, the greater the angle between the waves and the shoreline (up to 45 degrees), the stronger the longshore current. This current, sometimes called littoral drift, is responsible for most of the sand movement along beaches. Other things are involved in the genesis of surf zone currents. For example, winds can either decrease or increase current velocity depending on whether they blow with or against the wave formed current. Tidal currents can also be important.
Figure 1.9. Wave refraction. Wave crests approaching a typical North Carolina shoreline bend or refract, causing the waves to strike the shoreline almost head-on. As the wave breaks, a portion of the energy flows in the direction show by the arrows, forming the longshore current. The longshore current transports sediment. Structures sucn as the one shown here, interrupt sediment transport, causing downdrift erosion. Drawing: Charles Pilkey.
A special type of surf zone current, one that everyone who swims in the ocean should be familiar with is the rip current, sometimes called rip tides. These are strong seaward flowing currents set up by the return flow of water held onshore by the longshore currents, waves and storm surges. Often they flow through gaps in offshore bars and thus they may occur repeatedly at the same location. The details of the genesis of these currents are beyond the scope of this chapter but it is important to note that they are a significant hazard to swimmers. While standing on the beach, watch for the telltale band of seaward flowing water and if you see such – stay out of the water. If you have the misfortune to caught in a current, swim laterally rather than directly toward shore. Be cautious and know what you’re doing when you swim at the beach.