Waves

We have seen them before, crashing against the beach, carrying a surfer-dude as he hangs ten or roll by bobbing a buoy. We are already somewhat familiar with waves, but now we will look a bit closer and study the ins and outs of an ocean wave. What is a wave? An ocean wave is a rhythmic movement that carries energy throughout sea water. It is important to understand that water is not carried in the wave – it is only energy. The crest is the highest point of a wave and the trough is the lowest point. Amplitude is the vertical distance between a crest and a trough. Wavelength is the horizontal distance between crest and equilibrium or trough and equilibrium. The frequency is how often the crest or trough of a wave passes a certain point in a given period of time. There is a relationship between the wavelength and the frequency of a wave which is presented in the formula below. With this formula, one can determine the speed or velocity of a wave.

Speed = frequency X wavelengthv = fλ

What causes waves?

Day 51.1

Waves are caused by winds or a disturbance in the water such as an earthquake. The size of a wave depends on three factors:  the distance along open water – which is known as the fetch, the strength of the blowing wind and the length of time the wind blows. The biggest waves on the planet are where the wind blows consistently – typically south of the Indian Ocean. Wind waves are caused as the wind blows across a body of water and the resulting friction causes the water to move. The wind transmits energy to water to create a wave. As the wind continues to blow, the wave increases in height.

Earthquakes can cause large ocean waves by transmitting the energy from the moving earth to the water. The resulting wave is called a tsunami. A tsunami in deep water may have a very small wavelength, but when the wave reaches the shore, the friction from the ocean floor causes the tsunami to grow suddenly and crash on the beach at heights reaching 20 meters.

How do waves affect the shore? Waves usually approach the shore at an angle and thereby shape a beach by eroding the shore at one end and building it up at the other end. The breaker of a wave is the growing swell that approaches the shore, surf zone, and begins to break. Every time a wave breaks on the shore of a beach it carries sand to and from the beach causing different patterns and shapes unique to each shore. Longshore drift is caused when sand is carried by the water along the beach to create a sandbar.

Day 51.2

What are the different types of waves? There are several different types of waves. Seiches are standing waves that slosh to and fro in an enclosed or partially enclosed body of water such as deep lakes reservoirs and seas. A swell is a long stable surface wave formed by distant tropical storms and stable wind systems. A short steep wave with a high frequency is refereed to as a chop. A capillary wave is a wave that travels between two fluids and are most commonly observed as ripples. A tsunami is a wave that is created when there is a major disturbance in the open ocean. Tides are the waves that are generated by the gravitational pull of the sun and the moon. The chart below outlines the major characteristic of each wave type.

Wave Typical Wavelength Disturbing Force
Capillary Up to 1.73 cm Usually wind
Chop 60 – 150 m Wind over the ocean
Seiche Large variable; a function of ocean basin size Change in atmospheric pressure, or storm surge
Tsunami 200km Faulting of seafloor, volcanic eruption, landslide
Tide Half of Earth’s circumference Gravitational attraction between the Earth, Sun and Moon; rotation of the Earth

 

(source)