Barriers, spit and longshore drift

Next week's lecture represents the start of part 2 of this module with a lecture on barriers. Here is a useful clip on the formation of barrier and spits by longshore drift to get you in the mood. Note the description of Slapton Sands at the end of the clip: this is the end destination of our field trip.

Hurricane Sandy - before and after shots

Here is a great web site showing before and after picture of Hurricane Sandy - just move the slider across these images and you will see exactly what damage Sandy has caused. Apart from the flooding and destruction that is immediately apparent, from a coastal geomorphological point of view it is of interest to note that, in general, the actual beach has not undergone much change, but that there was signifcant onshore transport of sand onto roads, car parks, etc. We'll go into this in more detail during the barrier lecture after the class test, but the key message here is that big storms move sediment onshore, rather than offshore.

Hurricane Sandy damage

Check out this link for many good pictures of New York post-Sandy. Quite amazing to think that the most important island in the world - Manhattan - got flooded!

Summary of wave lectures



Plunging breaker on gravel beach during storm, Cornwall.

Here's an On the Road style summary of the wave lectures. Waves have a 'height', a 'length' and a 'period'. Ocean waves are generated by wind - the stronger the wind and the larger the area over which the wind blows, knows as 'fetch', the higher the waves and the longer their periods. Waves cannot grow beyond a certain point and we speak of a 'fully arisen sea' when waves have attained their maximum height. The behaviour of waves can be decribed by 'linear wave theory'. The universal wave equations are rather complicated, but in deep and shallow water they simplify. In deep water, the wave speed is only a function of the wave period, and such waves are called 'dispersive' waves: the longer the period, the faster they travel. In shallow water, waves are 'non-dispersive' and their travel speed only depends on the local water depth: the shallower the water, the slower the waves travel. Waves represents 'energy', and the amount of energy per wave increases with wave height. In deep water, wave energy travels at half the speed of individual waves; in fact, the wave energy travels in 'wave groups' and while groups retain their identity during propagation, individual waves travel through a group and then lose their identity. In shallow water, individual waves travel at the same speed as the wave groups. When waves enter a water depth of about half their wave length, they start feeling the bed and their behaviour starts to change. Specifically, as the wave travel increasingly in shallower water, their wave length decreases and travel speed also reduces. As a result, three important processes are initiated: 'shoaling', 'refraction' and development of 'wave asymmetry'. In very shallow water, when water depth is only slightly larger than the wave height, wave will 'break' and disintegrate in bubbles and foam. Waves can break in different ways, and the controlling variables are wave height, wave period and beach gradient. After wave breaking, waves continue to lose their energy and this is referred to as 'dissipation'. On gently sloping beaches, waves dissipate practically all their energy and have zero wave height when they reach the shoreline. On steep beaches, waves can bounce off the beach, light light of a mirror, and energy will travel back out to sea - this is referred to as 'reflection'. The type of surf zone and breaker can be predicted using the 'Irribarren Number' and the 'surf scaling parameter'. Finally, at the shoreline, a number of other processes are important. 'Infragravity waves' refers to the motion of the water level at very low frequencies, or long periods (more than 30 seconds). Infragravity waves are especially important under storm conditions, because the amount of energy they represent increases with the ocean wave height and their importance increases towards the shoreline. 'Wave set-up' is the super-elevation of the surf zone water level due to the presence of waves. Near the shoreline the extra rise in water level due to set-up can be 30% of the wave height. Wave set-up and infragravity wave motion together are responsible for beach erosion, dune scarping and overtopping during energetic wave conditions.

Hurricane Sandy

Last week we discussed hurricanes and storm surges, and, right on cue, Hurricane Sandy is just about to make landfall in New York. upto 400,000 people will e evacuated and the storm surge is predicted to be up to 11 feet. For the latest updates, see this Link.

Coastal erosion of rubbish tip

An interesting problem posed here by coastal erosion exposing a former landfill site reported by the BBC.

Linear wave theory

As mentioned during the lecture, it is quite insighful to play around with the linear wave equation using Dalrymple's applets. Try and produce a table and/or plot of how wave properties changes with water depth. Assume a wave height H of 1 m and a period T of 5 s, and compute for example the wave length L for different water depths h (40, 35, 30, 25, 20, 15, 10, 8, 6, 4 and 2 m). Wave length L decreases in shallower water, but wave period T stays the same. Therefore the wave speed C must decrease. This lies at the heart of causing the waves to shoal and refract.

I have also uploaded the answers to today's exercise and also the mock test for those who want to see what it looks like.