Underwater surge due to waves

In shallow waters, waves create sediment movement felt as a back-and-forth movement of the water at the bottom.

As a wave passes by, each little element of the water undergoes a closed-loop trajectory or orbit motion. In deep-water waves the size of the orbits decreases sharply with increasing depth until they are negligible (4%) at depths greater than 1/2 the wavelength; at lower depths the oscillation interacts with the bed to form an oscillatory boundary layer. 

In shallow water, the orbits becomes more elliptical and become back-and forth straight lines at the bottom, with the speed in the direction of the wave being faster (and for a shorter period of time) compared to the opposite backward movement, with a zero net result in term of movement.
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Refraction

When a wave approaches underwater contours at an angle, the sections of the crest in the deeper parts travel faster than those in the shallower sectors. This causes the wave crest to turn towards the depth contour. This bending effect is called refraction, and is analogous to similar phenomena in physics (light, sound). It takes place in addition to the effects of shoaling and continues up to the shoreline.
The energy of the wave gets dispersed in divergent refraction and concentrated in convergent refraction.  Source

Diffraction

Diffraction occurs when a wave encounters an obstruction in its path and will change direction, or wrap around it. Normally both diffraction and refraction happen at the same time. In ocean waves, we see this occur when a wave encounters an object like a jetty and the wave rotates around it. The ‘wrapping’ or turning potential of a wave is larger in waves with a longer wavelength (i.e. longer period). This is why a long period Groundswell wave can sometime wrap a full 180 degrees around a barrier/jetty, whereas short-period Windswell wave will often shoot straight by it. Diffraction can occur in shallow or deep water and is separate from refraction since it is not a result of a change in ocean depth. However, both refraction and diffraction will involve a change in a wave’s direction. Source

Reflection

Wave reflection is the process of waves bouncing off a hard surface, such in the case of the path between Mnly and Shelly Beach . The angle of incidence and the angle of reflection are equal, based on a perpendicular line to the coast. Generally, the energy, frequency, period and speed of the wave are not impacted by this reflection. Source

Shoaling

Shoaling is the deformation of incident waves on the lower shoreface that starts when the water depth becomes less than about half of the wavelength, causing the waves to become steeper: increase in amplitude and decrease in wavelength. 

For us divers, it means that the waves we face near the access point are higher than what forecasted, and after the break the vertical oscillation is converted into horizontal speed which is our ultimate measure of comfort in entering/exiting the water. Source

Wave interference

When waves from different directions meet, they interfere with one another. Wave interference can be:

• Destructive interference – two waves that cancel each other out, resulting in reduced or no wave. 

• Constructive interference – additive interference that results in waves larger than the original waves. 

• Rogue waves - freak waves that occur due to interference and result in a wave crest higher than the theoretical maximum.
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