Wednesday, 9 March 2011

Waves - the basics

We have just started our final module of AS Geography - Coastal Environments - and started off by looking at waves and how they form. I have got a feeling that waves are going to be an important part of this module as the characteristic of a wave will determine whether or not erosion or deposition occurs and these processes contribute greatly to the shaping of the coastline.

So, how do waves form? There seemed to be a bit of confusion as to how excatly waves form. Waves are created by the transfer of energy from the wind to the water - not anything to do with the moon! The wind blowing over the surface of the sea creates friction between the two and this causes the water to begin to move in a circular orbit. When the wave reaches shallow waters, friction with the seabed begins to slow the speed at the base of the wave whilst the top of the wave stays the same. This causes the wave to become higher and steeper until it eventually breaks. If you are more of a visual learner the short clip mentioned in the module book is well worth watching - Understanding wave formation . The characteristics of waves differ and waves are often described as either being constructive or destructive......

Constructive Waves: Constructive waves are often created in calmer weather and have less energy than destructive waves. Constructive waves tend to be lower waves with a low frequency but long wave length. As the wave approaches the beach it gets steeper and so when it breaks it gives a gentle spill on to the beach. As the water percolates through the beach material the swash loses energy but remains stronger the backwash. The backwash is often very weak and has insufficient force to pull sediment off the beach. This results in material being slowly, but constantly, moved up the beach which leads to the formation of berms (berms mark the location of the spring high tide)

Destructive waves: Destructive waves are often created in stormy conditions and are a lot more powerful than constructive waves. Destructive waves are high, steep waves with a high frequency but short wave length. As a destructive wave approaches the beach it gets steeper and therefore, when it breaks, it plunges down with quite a bit of force. Due to the fact that there is little swash (forward movement of water up the beach) the backwash becomes dominant and is often very strong. This results is little material being moved up the beach as, instead, it is pulled away. Sometimes destructive waves are known as erosional waves and are often associated with steeper beach profiles. If the force of the wave is great enough it may well project some shingle and other material towards the rear of the beach and this can result in the formation of large ridges known as the storm beach.
Swell Waves: Swell waves are formed by distant storms. These waves travel long distances and are less steep whilst having both a longer wave length (the horizontal difference between successive crests or successive troughs) and wave period (wave period = the time between one crest passing and the next). Swell waves are usually constructive waves.
Sea Waves: Sea waves are formed by local winds and are effectively the opposite to swell waves. Sea waves do not travel very far and whilst being steeper have both a shorter wave length and wave period compared to swell waves. Sea waves are usually destructive waves.

As well as knowing the key characteristcs of waves there also seems quite a few key terms to learn. Most of them seem quite self-explanatory but after I discovered the definition of wave steepness last lesson (the ratio of wave height to wave length) I was quite interested in understanding why this cannot exceed 1:7. So, because I clearly didn't have anything better to do this afternoon and because the prospect of doing a bit of extra Geography seemed so much more interesting than my maths homework, I decided to try and find out why. As waves approach landmasses they get steeper and change shape. This is because the friction that occurs between the seabed and the water causes the circular orbital motion to slow at the base of the wave whilst, at the top of the wave, it continues at its orginial speed. This causes the wave to get steeper as it begins to lean forward. When it reaches the point that its wave steepness is at 1:7 the wave becomes unstable and so collapses on top of itself which forms a breaker. As far as I know, I think that there are two forms of breakers - spilling and plunging. A spilling breaker is a rolling wave that travels gradually up often sloping sandy beaches. The long incline drains that wave of its energy and I am guessing (this is just a gut feeling and so I wouldn't trust me on this) that spilling breakers are therefore most commonly associated with constructive waves as, as the water percolates through the beach material the swash loses energy. A plunging breaker is most commonly found approaching a steeper beach and forms a curling crest, due to the fact that the curling water is travelling faster, which travels over a pocket of air. Again this is a total guess, but I think perhaps plunging breakers are  more associated with destructive waves.

Unfortunately I really should get back to my maths homework and so this will have to be it from me tonight - I hope this is useful for some of you. I am going to try really hard over the next few weeks to primilarily write about what I learn in lessons and not get too distracted by other geographical things in the news that interest me - should be interesting to see how long this lasts!!!

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