Wednesday, 31 October 2012

Hurricane Sandy damage

Flooded areasCheck 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!

Tuesday, 30 October 2012

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.

Sunday, 28 October 2012

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.

Wednesday, 24 October 2012

Coastal erosion of rubbish tip

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

Tuesday, 23 October 2012

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.

Friday, 19 October 2012

Ocean wave prediction

Prediction of ocean waves is of vital importance for shipping and designing coastal infrastructure (and for surfing!). The science of wave prediciton started in WW2, when preparations for D-Day landing were made. For a succesful invasion of France, the allied forces had to be sure that the waves on the Normandy beaches were not too large. Two oceanographers, Munk and Sverdrup, came up with a method for wave forecasting using wind speed and fetch length that is still being used today (although mostly more sophisticated methods are being used). For those of you interested in the history of wave forcasting, have a look at this report (read only the first two section - it gets quite technical for the second part). See you at next week's lecture where you will be doing some wave forecasting.

Storm waves at Portleven, Cornwall

Thursday, 11 October 2012

Tidal bores

You would have learned by now that the tide is actually a wave, in fact a shallow water wave, and that the behaviour of the tide near coastline is very much affected by the underwater topography, or bathymetry, and the coastal configuration. The largest tides are generally found in areas with the more complex coastal topographies and gently-sloping continental shelves (like around the UK). The smallest tides are found in the middle of the ocean.

Like ocean waves, tidal waves (I mean here a real tidal wave and not a tsunami) can break and even be surfed. The best example of a tidal wave breaking, which is generally referred to as a tidal bore or river bore, is the Silver (or Black) Dragon in the Qiantang River, China, which can reach a maximum height of 9 m. The Dragon has recently been surfed by some American surfers - see this Utube clip.

Such tidal bores always occur in rivers where the shallowing sea bed and the funnel-shaped river entrance results in enhanced shoaling of the tide, making the tide increasingly asymmetrical and higher. Tidal bores also occur in the UK - the Severn bore is the best known example.

Wednesday, 3 October 2012

Sea-level rise animation

There are many animation of sea-level rise on UTube - this one is quite good and short, so have a look. Then, ask yourself the question, in light of what you already know about coastal morphodynamics, how realistic are these animations, also referred to as bath tub models? Coastal environments have the ability to respond to sea-level rise. Especially environments such as salt marshes, tidal flats, estuaries and coral reefs are depositional environments that build up at rates comparable to rates of sea-level rise (mm's per year). This means that many of the low-lying areas you see being flooded in such animations will actually keep their head above the water. Similarly, sandy and gravel barriers might retreat due to sea-level rise, but as long as they remain more or less intact, they will keep protecting the low-lying hinterland. And what about the Dutch (near the end of the animation)? Most of the country is already below sea level and certainly not underwater. So, coastal protection will also keep the sea at bay. Are these animations therefore useful? Well, they are a great way to scare people and thus provide a very useful argument for people like me to apply for research funding and justify our professional existence. But to be serious, they do present the worse case scenario, and that's how they should be considered. They are not realistic predictions.