Waves are a captivating and powerful natural phenomena. The creation of waves begins with any force that disrupts water. These forces can be gravitational effects from the sun and the moon, wakes from vessels or swimmers, or even underwater disturbances from earthquakes and volcanoes. Once waves are formed they travel across all bodies of water and carry massive amounts of energy. The impact of wave formation is extensive and effects a wide variety of applications including commercial shipping, offshore energy development, coastal protection, and navigation.
Wave characteristics
To better understand characteristics of waves it is helpful to consider the waves in basic mathematical terms. To do this, scientists and researchers use simple sinusoidal waves to describe the surface (Figure 1). By doing this, waves can be easily studied because the characteristics of these functions are uniform and constant.
| Still water level | The level of the sea surface if there were no waves present. |
| Crest | The highest point of a wave |
| Trough | The lowest point of a wave. |
| Wave height | The vertical distance between the crest and the trough. |
| Wavelength | The length of one complete cycle of a wave. This is also the distance between two successive peaks. |
| Amplitude | The distance to the crest from the still water level or one half of the wave height. |
| Direction of propagation | The direction in which a wave is propagating through space. |
| Wave period | The time interval between two successive peaks. This is also how long the wave takes to travel one wavelength in distance |
| Wave frequency | The number of peaks that pass a fixed point per second. |
Note that the wave frequency is the inverse of the wave period, so that:
| \( Frequency = \frac{1}{Period}\) | (1) |
Types of surface waves
All bodies of water experience waves and the forces behind the waves vary. Storm surges can be created by strong wind and pressure that can induce long waves in deep water that gain strength as they propagate towards the shore. Underwater disturbances such as earthquakes, or volcanic eruptions can create increasingly long waves that form into Tsunamis. The gravitational forcing of the sun and moon can also create tidal waves. If you look at the distribution of energy for waves, you can see considerable variability ranging from 12 hours to 0.5 seconds. A significant contribution of this energy is found in the band from 0.5 to 30 seconds and is commonly referred to as wind waves (Figure 2).
Waves generated by the wind are the most common type of ocean waves and are the primary focus for many engineers and scientists. When wind blows across the surface of the water, waves are created by the energy transferred to the water from the friction between the wind and water surface. The energy flows through the water causing it to move in a circular motion, but the water itself does not actually advance with the wave. This is commonly observed when viewing a buoy floating at the surface come in contact with a wave in open water. The object may bob up and down and even appear to have a circular pattern, but it returns very close to its original position.
Wind waves have a variability that makes characterizing waves non-trivial. Waves start out as both small in height and short in length, created by local winds and grow as a function of wind strength, duration of wind, and distance. As a result, the wave environment at a particular location may be composed of a combination of local wind waves from a sea breeze and long waves (swell) generated by storm events hundreds or thousands of kilometers away. When measuring waves we therefore need to take into account that the local sea state is composed of waves with different amplitudes, periods, and directions. Understanding this is the first step towards making accurate wave measurements.
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