Backwash is a wave that travels seawards after being reflected from a steep shoreline. At some point it then usually meets and interferes with another incoming wave. If the two waves meet in such a way that two crests or two troughs coincide they will add together, but if a crest and a trough coincide they will cancel each other out. The result is a wobbly, bouncy wave, unpredictable and difficult to surf. In some cases, if two crests coincide really well there can be a spectacular up-thrust of water.
Some spectacularly upthrusting water, captured by Andrew Castellano - , http://www.andrewcastellano.com
Backwash can occur anywhere where the shoreline is steep, including a cliff or harbour wall, but the most interesting case is on a sandy beach, which I will describe here. The most favourable configuration for backwash to occur is where you have a beach made of fine grains, a steep high-tide bank and a sandbar not far from the shore. Typically, the waves that break on the bar are surfable until just before high tide. But then, as soon as the water starts to climb up the steep bank, the waves will be reflected off the bank and start to interfere with the incoming waves on the bar.
When waves come out of deep water and start to encounter shallower water, they slow down. This squashes them up together, but it also makes them higher. So, the incoming wave grows in height as it approaches the sandbar from the seaward side. But the reflected wave also grows in height as it approaches the sandbar from the landward side. This can make for quite radical effects when the two waves meet.
For the outgoing and incoming waves to interfere with each other with maximum effect, the two waves must coincide exactly at the position of the sandbar. This, of course, won’t always happen. Sometimes you might just get a bump in the incoming wave as the outgoing wave passes through it. Of course, this bump still makes surfing annoyingly difficult, not just when you are on the wave, but also before you even catch it. The constructive interference between the outgoing and incoming waves makes the wave suddenly bigger and steeper, so you turn your board around and paddle for it. The wave then shrinks again, just when you thought you’d caught it.
Alternatively, you paddle for a wave that you think is just at the right steepness to catch and, suddenly, the backwash hits it and it breaks on your head.
With the incoming wave, some of the energy usually ‘leaks’ over the top of the sandbar so that the wave still has some strength left when it carries on past the bar on its way to the shore. However, if the bar is quite large or shallow, or actually sticks out of the water, the incoming wave might dissipate all its energy on the bar itself. As a result you will have no energy left to propagate to the shore and reflect off the high-tide bank. Also, the bank itself must be steep enough for the wave not to break; otherwise you will get a lot of energy dissipated in the breaking process and not enough left to reflect. This is called a surging wave – a mass of water that surges up and down the shore without breaking.
The ability for water to sink into the beach surface (the permeability) also plays a role in all this. The wave will reflect much more easily on an impermeable beach than on a permeable one. On a highly permeable beach, all the water sinks into the beach face instead of reflecting back out to sea. The downwards motion of the water percolating between the gaps in the grains absorbs all the energy. The permeability of the beach increases with increasing sediment size. So, for the wave to reflect well and produce a good backwash, you need a fine-grained beach.