Wave tank demonstration showing the impact of coastal defences on flood risk

hi i'm dan from jba and what we've got here today are the j ba trusts physical models so we're going to be looking at some of these physical models that are used for training and education in coastal and flood risk management the one that we've got here today is the wave tank and the wave tank is usually used because in these situations in the coastal environment it gets really complicated and we can't always solve this through analytical equations and whilst it's very easy to predict how big waves are going to be in the offshore when they start to move towards the coastline all sorts of things happen to them they start to interact with the bathymetry coastal reefs and headlands we get processes like shoaling and a refraction and diffraction we get depth induced breaking and that means by the time our waves actually get to the coastline they're quite different to what they were offshore and it's these near shore waves that we're really interested in because it's these initial waves that are going to interact with our coastal structures they're going to lead to erosion they're going to lead to wave overtopping and eventually inundation or flooding of the communities that are sitting behind them so what we've got here today with the wave tank we're going to demonstrate the different defenses that we can use we're going to propagate five waves towards each of the defenses we're going to calculate how much waves are going to overtop the defense's we're going to capture this we're going to measure it we're going to see the different performance levels of all of the different structures right now our first situation we're going to represent is our beach environment and normally when waves arrive at a beach they're going to break and they're going to roll up and down the beach face and there's not going to be any problem now today though we're going to we've filled up the tank a little bit more and we're representing a storm surge and this means that there's deeper water bigger waves are going to be arriving at our coast and they've got the ability to roll up and down the beach but they can also go over the dunes and inundated communities behind so the first thing we're going to do we're going to send our five waves down we're going to have a look what happens and what we saw is that even though the waves were breaking there was still enough energy for them to roll up over the dunes in this case fill up our reservoir but in real life that would have meant that they had any houses or properties or communities behind the defense would have been completely flooded and really that isn't an acceptable situation for us so this is around the time where we bring in our coastal engineers all right now we've just reset our tank and now we can start testing some of our different defenses if there's only a little bit of wave overtopping we can usually get away with just a vertical wall if it's small in nature it can easily just be set behind the dunes like we've just done there or even incorporate it into some dunes and then that will essentially just provide another impermeable barrier to wave overtopping now I'll have a look how it goes now we did get still some overtopping there was a lot less than before and we'll have a look how much we've actually got now what we've got here we'll call that about 50 mil and while it's only a little amount in our physical tank this could easily be say 50 litres per second in real life this is the sort of a volume that will start leading to some problems but still it's better than if there was nothing behind there at all and what we want to do now is see if we can try and improve the situation even more so now we're ready to start designing some of our defences now the first thing we have to think about is what is the beach going to be like in the future unfortunately when we're designing our structures they have to last 50 100 years from now we have to assume a lot of the time that the beach is going to be very eroded or receded in those times we don't actually have any sand left so we designed to assume the worst case conditions so our small wall has now turned into a really large wall the crest height is the same but all of the sand or the sediment all the beach in front of it has now gone and this is what we're going to test first how is that small wall going to perform under the same waves that we've just run so we'll send our five waves down now this time we saw larger waves coming in that's because they weren't breaking on the beach they've actually hit the wall under really impulsive conditions now what we can do now is actually see how much wave overtopping happened and how much extra water that we've got now by removing the beach we've now got about there's 70 mils of overtopping now this is the exact same structural conditions but we've just now got no way of breaking in front of the structure so this is really important for our designs but now we can use this as a base case to imagine this is what the conditions are going to be like a hundred years from now and now we can start changing the structural conditions to see if we can improve this situation now the next defense we're going to check and test is our slope for revetment now this is about a 1 and 2 also known as a dike and what we're going to do is we're going to put it in front of our defense now the argument is that we're going to now induce more braking against it the waves are then going to be smaller when out they hit our defense and it's going to have a better performance all right now we'll run our five waves through the tank and what we've actually seen there is by putting the slope in front of the structure we've actually made the overtopping worse where we thought the waves would break and dissipate their energy they're actually hitting the structure breaking right on it and using it as a ramp to get over that the defence itself now we'll just see how much water that we've got now what we've got is a completely full beaker so we've made it consistently worse I'll get that one back now this is one of the common myths that we see the people think putting small slopes in front of the vertical wall is going to improve the situation and what we find is about a 1 into one and one and a half is one of the worst conditions that we can get in relation to overtopping now there are a few ways that we can improve this coastal defence without changing any of the sediment properties or the slope in front of it the best way is to use a recurve wall now the beauty of this is it's not any higher or larger in the structure but it's got this unique shape and this unique shape allows the water to come up actually get captured in here and then shot back up to sea because of the nature of this there's going to be less waves coming over the defence and we can show this by simply removing our vertical wall clipping in our recurve and what we're going to do now is run our five waves again and see if there's any difference and this time what we've seen and the ways are actually getting shot back out to see is probably the same amount of way of energy going up but it's all returning out so in terms of our safety and the inundation behind the defenses it's a lot better and in fact there was no water that actually came over at all so something as simple as a recurve wall is a great way a very cost-efficient way to improve a coastal flooding situation now the next defense we're going to be testing is our step two revetment and this is at the same slope as our other event except the only difference is this is essentially it's got steps we're going to see if these steps are going to change the performance of the defense I've got that locked in and we'll run our five waves now there was still a lot of overtopping that's because we've still got that same gradient that same slope that's letting the waves run up and hit the structure with a little bit more power so this time we see that we've got about a hundred mils that's come over so it's actually less than the sloped revetment but still it's not as good as the the recurved wall but now we're going to have a look if there's any other options that we can do now our last defense we're going to test today is our rock armor this is all scaled down to fit into our model but essentially it's representing a rubble mound structure that can go in in place of the the vertical structures all together now we'll put it in our tank now we're going to run our five waves and what we've found is actually the rock Armour is a really good defense it allows the wave energy to be absorbed when the waves come up they'll hit a rock they'll go left right up down they'll hit another rock they'll have to squeeze through gaps and change direction and all the time they're really dissipating their energy and when we have a look at the performance we can see we've got very little maybe about five mil Rock arm is really effective now it's also cheap but it has its drawbacks now it covers up the beach we get oyster growth so it's always not the best option so out of all of these we've tested the rock Armour the step revetment the sloped revetment in the recurve wall we found that all of these will have different performances they can be handy in different situations and it's always up to these physical tanks to test these environments and understand how each of them are going to react it's only when we actually get them tested in these situations that we are going to be able to do a cost-benefit analysis or an economic appraisal to find the best way that we're going to protect our coastline now another thing we can look at is going further offshore in this situation we've gone for initial break order now in reality we can build a breakwater as high as we want but it's going to be very expensive we can completely block the waves if we want to but it's going to also be unsightly so in this case we've got a submerged breakwater and this one will actually sit under the water I'll just put her in now this way we can actually start to try and stop the wave energy before it hits our defense so now again we'll run our five waves and even though the the vertical wall before was one of the worst performing by doing something offshore we've now prevented all the wave overtopping and the benefits of this is mean we've still got our Beach we've still got an area where we can go and enjoy walk our dogs but we're not having any of the drawbacks of having the rock Armour in there now building this offshore looks like a really good idea but of course it has a large cost associated with it as well so this is when we have to start thinking about the economics is it worth spending more money in an offshore environment or a little bit less money on the beach environment so it's been a pleasure today showing you the JPA Trustwave tank and how it can be used to demonstrate wave processes and wave overtopping and if you want to see the tank in person or for any more information just get in touch with us via our website you

Channel: JBA Trust

Views: 8,776,278

Keywords:

Id: 3yNoy4H2Z-o

Length: 12min 22sec (742 seconds)

Published: Fri Oct 21 2016

Reddit Comments

[u/[deleted]]

/r/videoswhenyoushouldbesleeping

[u/Stimpers]

The water being thrown back was immensely satisfying.

[u/juleswells]

Reminds me of this gifv showing how mangrove forests protect coastlines from erosion.

[u/EliTheCoder]

I wish he would’ve done stepped revetment, curved wall, and the off-shore breakage all at once.

[u/palordrolap]

Most of the beaches that I've been to in the UK have the recurved wall along some stretch or another, and I've seen the more expensive rocks solution in places (sometimes not far along the coast from a recurved wall) where further coastal erosion would be a serious problem.

I also remember one beach I visited as a child that had giant stairs down to the beach, and now I realise they were more than just a way down.

I never was able to dig down to the lowest stair beneath the sand though!

[u/Limitedcomments]

Beaches' love recurved walls.

[u/draginator]

What a professional, not a single drop of water on that dress shirt.

[u/Rebellion111]

Why is oyster growth on the rocks bad?

[u/Aetrion]

I am a wizard and I assure you this man's critique of rock armor is nothing but a politically motivated lie by the cleric lobby. Rock armor is in fact a classic elemental spell, a most useful defense against piercing weapons and goblin bites, and is not at all unsightly!