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Sea States

The content and activities in this topic will work towards building an understanding of factors that affect wave formation and size.

Wind-generated Waves

Winds are the primary source of energy for waves. As wind begins to blow over calm water, it starts to ruffle the surface of the water. As the wind continues to blow, ripples begin to appear and grow into waves. Waves eventually group together in sets of waves that are traveling at similar speeds.

 

Factors that affect the force of the wind, and therefore influence the formation of waves, include duration, fetch, and speed.

  • Duration is the length of time the wind has been blowing. With longer duration of wind, waves have more time to build energy and grow larger.
  • Fetch is the distance over which the wind is blowing. To consider the influence of fetch on wave formation, think about the waves formed when a strong wind gusts over a small pond or puddle compared to the waves formed when strong winds blow over the ocean or a large lake. When wind travels over larger distances, there is more opportunity for the wind to transfer its energy into waves
  • The speed of the wind is a way of measuring its strength. For wind with the same fetch and duration, faster wind is stronger, has more energy, and can produce larger waves than slower wind. The larger and faster the wind is, the more energy or capacity it has to move things. The same is true of waves; larger, faster waves have more energy and a larger capacity to move other objects.

 

<p><strong>Fig. 4.7.</strong> A stormy sea state in the North Pacific basin has noticeable whitecaps.</p><br />

Sea is a term used to describe the mixture of waves often observed from ships. Seas may include ripples, chop, wind waves, storm waves, and swells. Seas are characterized by waves of differing heights coming from many directions (Fig. 4.7).

 

Swells are waves that have obtained enough energy to travel beyond their generation area or fetch. They no longer need additional wind to push them onward. Swells may be generated by prevailing winds, such as tradewinds, which blow over long fetches for long durations. Swells may also be generated by storms. Once formed, swells may carry the energy of storms thousands of kilometers. For example, swells from winter storms in Alaska supply the big waves surfed on the North Shore beaches in Hawai‘i. Swells formed in the Antarctic can travel all the way to Alaska.

 

Sea States

Wave conditions at sea are called sea states. Sea state terminology is sometimes applied to sailing conditions in places that are not seas. For example, sailors on the Great Lakes of North America, although not technically at sea, still refer to sea states. Sea states are closely linked with wind. Table 4.5 describes the characteristics of the different sea states, including the official measure of wind strength as defined by the Beaufort scale, the wind speed, the wave height, and the visual characteristics of the sea as observed from both land and water. Britain’s Admiral Sir Francis Beaufort created the Beaufort scale in 1805 to help sailors estimate the winds using visual observations. The Beaufort scale starts with 0 and goes to a force of 12. In Table 4.5, wind speeds are listed in knots. A knot is a unit of speed used in maritime references. At knot is equal to 1 nautical mile per hour or 1.85 kilometers per hour (0.51 meters per second). Although knots and nautical miles are units of measurement used by navigators at sea, most scientific measurements of wind speed use metric lengths (e.g., meters per second).

 

 

<p><strong>Fig. 4.8.</strong> Photograph of a ship in bad weather on the Georges Bank in the North Atlantic ocean basin</p><br />

When light winds start to blow across an undisturbed surface of water, they produce friction that drags the water into ripples. Ripples are small wavelets averaging about 8 cm high. Continuous energy is needed to form ripples and push them to the next stage of wave formation, because the surface tension of the water and the gravitational pull of the earth act as restorative forces that pull water back into a flat state. When winds push ripples into waves about 1 m high, the sea state develops into chop. As the wind increases, chop builds into wind waves. Storm waves begin to form when there are winds of gale-force intensity, which is more than 70 km/hr (Fig. 4.8). Storm waves may not be very high if the storm is of short duration or if the fetch is short. Large storm waves are generally formed when the fetch is long, the storm is intense, and the storm duration is long. Several short-duration storms following each other and blowing in the same direction may also increase wave height dramatically.

 

A developing sea means that waves are growing larger. However, waves do not continue to grow indefinitely. When the waves have stopped growing, the sea state is described as fully developed. A fully developed sea is one where the energy supplied by the wind is equal to the energy lost in breaking waves. Table 4.6 shows properties characteristic of fully developed seas in winds of 10, 20, 30, 40, and 50 knots. In Table 4.6 and 4.7, significant wave height is defined as the average of the highest one-third of all wave heights over a period of time.

 

 

Wind speed influences wave size, but it takes time and space to make fully developed waves. To become fully developed, a wave must be pushed by the wind over a long fetch. Probable wave size for winds of a given speed, duration, and length of fetch have been determined by ocean engineers from simulations in very long wave tanks. For example, if winds of 25 knots (12.86 m/s) blow for 60 hours over a fetch of 510 nautical miles (944.52 km), the significant wave height would be 4.3 m with a wave period of 8.2 seconds.

 

 

Ocean Wave Organization

Waves in open water are organized in a variety of ways, including wave sets, cross seas, developing seas, and fully developed seas. A wave set, also known as a wave train, is a group of progressing waves of about the same wavelength moving in the same direction at about the same speed. Ocean swells that travel at relatively evenly spaced distances are examples of wave sets (Fig. 4.9 A).

 

<p><strong>Fig. 4.9.</strong> (<strong>A</strong>) Ocean swells in wave sets outside Lyttelton Harbour, New Zealand</p><br />
<p><strong>Fig. 4.9.</strong>&nbsp;(<strong>B</strong>) Cross sea swells, Phares des Baleines (Lighthouse of the Whales) on Île de Ré, France</p><br />


<p><strong>Fig. 4.10.</strong> Inference patterns are the sum of the intersecting waves. These diagrams show (<strong>A</strong>) constructive and (<strong>B</strong>) destructive interference of two waves.</p><br />

Wave sets come from many directions in the ocean. Two or more wave sets intersecting are called cross seas (Fig. 4.9 B). Waves may cross each other at different angles, or they may come from the same direction, but have different wavelengths and periods. This phenomenon causes wave interference. Constructive interference occurs when wave crests from two different wave sets meet, and the amplitude of the combined wave crest equals the sum of the wave sets that interfered (Fig. 4.10 A). Destructive interference occurs if the crests from one wave set and the troughs from another wave set meet, and the waves cancel each other out such that the combined wave at that point has an amplitude of zero (Fig. 4.10 B). In both constructive and destructive interference, the waves return to their original wave height after the waves have passed each other.

 

<p><strong>Fig. 4.11.</strong> Two sets of generated waves in a ripple tank. Notice the patterns of interference where wave sets meet.</p><br />

Fig. 4.11 shows a ripple tank with two wave sets. The light bands represent wave crests; the dark bands represent wave troughs. Constructive interference produces very bright spots on Fig. 4.11, indicating the two wave crests meeting. Destructive inference ocurs when the waves cancel each other out. In Fig. 4.11, this is shown by a decrease in in tensity of the light bands.

 

Activity

Activity: Wave Patterns in a Ripple Tank

Use a ripple tank to observe various properties of propagating waves including interference, reflection, refraction, and diffraction.

Activity

Activity: Wave Interference

Use a long wave tank to observe the properties of wave interference.

 

Storms

Atmospheric disturbances accompanied by strong winds and rain, thunder, lightning, or snow are known as storms. Storm waves are caused by strong winds blowing for a long duration over a long fetch. In the North Atlantic ocean basin, the Caribbean Sea, the Gulf of Mexico, and the Northeast Pacific basin, these waves are formed during marine storms called hurricanes. The same kinds of storms are called typhoons if they occur in the Northwest Pacific basin, west of the International Date Line. These storms are called cyclones if they occur near Australia and in the Indian ocean basin. Despite the different names, storm waves are all formed in the same way, and all can be devastating.

 

<p><strong>Fig. 4.14.</strong> Profile of a hurricane</p><br />

Hurricanes, typhoons, and cyclones form when heat energy stored in the surface water of equatorial oceans heats the air above it. The heated air rises, causing a low-pressure center to form. Winds blow inward over the ocean, heating, then rising and flowing outward at upper levels, creating the cloud formation shown in the cross-section in Fig. 4.14. In the center, cool air moves downward, forming the eye of the hurricane. In the northern hemisphere, hurricane winds flow in a counterclockwise direction; in the southern hemisphere, they flow in a clockwise direction.

 

A mature hurricane is characterized by very low atmospheric pressure. The water level of the ocean rises around the hurricane because it is pushed toward the low-pressure storm center by the surrounding high-pressure areas. This high water level in the middle of the hurricane is called a storm surge, and it can cause extensive flooding when a hurricane crosses a coastline, particularly if it moves inland during a high tide. Hurricane-generated breaking waves pile even more water ashore, adding to the coastal destruction from high water and winds.

 

In a hurricane, winds blow more than 120 km/hr (64 knots/hr) over fetches greater than 300 km. Hurricane winds may persist for several days. This combination of conditions can generate large and destructive storm waves. In low-lying coastal areas, like Bangladesh, where most dwellings are only a meter or two above sea level, storm waves can do substantial damage. Flooding during a cyclone in Bangladesh in 1970 led to approximately 300,000 deaths, the highest marine storm-related death toll ever recorded. Catastrophic damage was also experienced in low-lying areas following Hurricane Iniki in Hawai‘i in 1992 (Fig. 4.15 A), Hurricane Katrina in the southeastern United States in 2005 (Fig. 4.15 B), and Hurricane Sandy in the northeastern United States in 2012 (Fig. 4.15 C).

<p><strong>Fig. 4.15.</strong> (<strong>A</strong>) Storm surge from Hurricane Iniki in September 1992 in Kaua‘i, Hawai‘i</p><br />
<p><strong>Fig. 4.15.</strong>&nbsp;(<strong>B</strong>) Flooding in Venice, Louisiana, after Hurricane Katrina in August 2005</p><br />
<p>Fig. 4.15.&nbsp;(<strong>C</strong>) Heavy damage to Casino Pier in Seaside Heights, New Jersey, from storm waves generated by Hurricane Sandy in October 2012</p><br />


 

Not all destructive storms are hurricane strength. Smaller storms can also form destructive waves. Storm waves begin as turbulent, rough seas near a storm site, but become swells as they move away from the storm center. Waves move away from the storm center in sets. As the waves travel, their wavelengths increase. Thus, the farther a storm is from land, the longer the wavelength of the resulting high waves that reach nearby coasts. In Fig. 4.16, the storm waves have transitioned from unorganized seas to organized, deep-water swells with a long period.

 

<p><strong>Fig. 4.16.</strong> Transition from unorganized seas to swells</p><br />


Deep-water waves change into breaking, shallow-water waves as they approach shore and become surf (Fig. 4.16). High surf can thrust great volumes of water on shore; high surf during high tide can be particularly destructive, causing localized shoreline flooding and damaging of shorefront property. If high surf is caused by localized storms, heavy rains may also flood low-lying coastal areas.

 

Since many storms are seasonal in nature, their impacts on shorelines can often be predicted. For example, when they arrive in Hawai‘i, winter waves pry sand and rocks from the shore and carry them into deeper waters. In contrast, in the summer on the northern Hawaiian shores, the waves are gentle and help to return the rocks to shore. The opposite is true of southern shores in Hawai‘i, which tend to have higher swells in the summer than in the winter.

 

Question Set

Question Set: Storms

Further Investigations: Sea States

Table of Contents:

Representative Image: 
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.