17.3: Waves
Waves
Waves form on the ocean and on lakes because energy from wind is transferred to the water. The stronger the wind, the longer it blows; the larger the area of water over which it blows, referred to as the wind’s fetch Links to an external site., the larger the waves are likely to be. The important parameters of a wave are 1) its wavelength Links to an external site., the horizontal distance between two crests or two troughs, 2) its amplitude Links to an external site., the vertical distance between a trough and a crest, and 3) its velocity, the speed at which wave crests move across the water. (Figure 17.5).
Figure 17.5: The parameters of water waves. (CC-BY 4.0; Steven Earle )
As a wave moves across the surface of the water, the water itself mostly just moves up and down and only moves a small amount in the direction of wave motion. As this happens, a point on the water surface circumscribes a circle with a diameter that is equal to the wave amplitude (Figure 17.6). This motion is also transmitted to the water underneath, and the water is disturbed by a wave to a depth of approximately one-half of the wavelength. Wave motion is illustrated quite clearly on the Wikipedia “ Links to an external site.Wind Links to an external site. wave” site Links to an external site.. If you look carefully at that animation and focus on the small white dots in the water, you should be able to see how the amount that they move decreases with depth.
Figure 17.6: The orbital motion of a parcel of water (black dot) as a wave moves across the surface. (CC-BY 4.0; Steven Earle )
The one-half wavelength depth of disturbance of the water beneath a wave is known as the wave base Links to an external site.. Since ocean waves rarely have wavelengths greater than 650 feet, and the open ocean is several thousand feet deep, the wave base does not normally interact with the bottom of the ocean. However, as waves approach the much shallower water near the shore, they start to “feel” the bottom and are affected by this interaction. The wave “orbits” are both flattened and slowed by dragging, with the implication that the wave amplitude (height) increases and the wavelength decreases (the waves become much steeper). The ultimate result of this is that the waves lean forward, and eventually break.
Waves normally approach the shore at an angle, which means that one part of the wave feels the bottom sooner than the rest of it; the part that feels the bottom first will slow down first. Even though they bend and become nearly parallel to shore, most waves still reach the shore at a small angle, and as each one arrives, it pushes water along the shore, creating what is known as a longshore current Links to an external site. within the surf zone. Another important effect of waves reaching the shore at an angle is that when they wash up onto the beach, they do so at an angle, but when that same wave water flows back down the beach, it moves straight down the slope of the beach. The upward-moving water, known as the swash Links to an external site., pushes sediment particles along the beach, while the downward-moving water, the backwash, brings them straight back. With every wave that washes up and then down the beach, particles of sediment are moved along the beach in a zigzag pattern. The combined effects of sediment transport within the surf zone by the longshore current and sediment movement along the beach by swash and backwash is known as longshore drift (Figure 17.7). Longshore drift Links to an external site. moves a tremendous amount of sediment along coasts of oceans and large lakes around the world and it is responsible for creating a variety of depositional features.
Figure 17.7: The longshore current moves sediment down the beach. (CC-BY 4.0, Emily Haddad, own work )
A rip current Links to an external site. is another type of current that develops in the nearshore area and has the effect of returning water that has been pushed up to the shore by incoming waves. Rip currents flow straight out from the shore and are fed by the longshore currents. They die out quickly just outside the surf zone but can be dangerous to swimmers who get caught in them. If part of a beach does not have a strong unidirectional longshore current, the rip currents may be fed by longshore currents going in both directions.
Tides Links to an external site. are very long-wavelength but low-amplitude waves on the ocean surface that are caused by variations in the gravitational effects of the Sun and Moon. Tide amplitudes in shoreline areas vary quite dramatically from place to place. As the tides rise and fall they push and pull a large volume of water in and out of bays and inlets and around islands. They do not have as significant an impact on coastal erosion and deposition as wind waves do, but they have an important influence on the formation of features within the intertidal zone Links to an external site..