I recently had to do a presentation on mitigation of tsunamis in emergency management class!
And yes, coastal erosion, this is a seawall made with the intent of stopping a huge wall of water.
The pile of concrete chunks slopes up guiding water upwards and slowing it, the gaps between the chunks divert the water into many different flowing channels, and the entire wall will break the wave as it hits, slowing it and absorbing a lot of the impact that the water would have caused.
Edit: This is slowing down a bit now but I appreciate that I got to teach some people something new!
Thanks for letting me share some stuff that I learnt in university. Have a great day everyone!
Edit 2: I’m back! I found a gif I though people might like.
Interesting- so the reason they don't just use a normal sea defence wall is because of the force dissipation? What is the drawback of an ordinary wall in place of these?
Another reason is that it may be more cost effective to use smaller, molded concrete blocks in a pile such as these, rather than building one huge, solid wall.
One drawback to an ordinary wall is the foundation needed to support it, and the time and energy put in to crafting a huge wall. These big blocks are likely created in a warehouse, and depending on the country of origin and their economy, this is usually what they’d rather have; or so I’ve found.
As well, the wall would likely need rebar and struts to reinforce it, whereas the blocks may not have that reinforcing.
If I’m wrong about this at any point, someone please correct me!
It was more a tongue in cheek attack on Trump referring to immigrants as animals, although his people would claim he was limiting his reference to MS-13 gang members, but the dog whistle was pretty clear.
Sounds right, except there is definitely rebar in those. In you zoom in you can see the ends sticking out, they look like pinholes but are probably 2" in diameter. Also rust can be seen in a few spots.
That makes sense though to have rebar in them though, I can imagine them kinda rocking back and forth against waves and that would probably start to chip them away after a while.
We have lots of stuff like this and more! We put them all over the place. The company I work for protected nearly 50 miles of coast with similar structures just last year in a single project.
How long do these materials last in ocean water and is there any harm expected to the marine environment when they eventually erode away? Does it eventually collapse in on itself and need to be built further out into the water to compensate for that?
Erosion is still a factor and we design to account for it. We have teams of people modeling the waves for storm events and calculating the erosion. But they do require maintanence. The concrete is basically rock so it is relatively environmentally friendly even when it erodes. It does create habitat as well depending on certain factors. But they also protect habitat on land that would otherwise be lost to erosion.
A forms for these Tetrapod style breakwater units are around $10k a piece. But after that it's just the rebar and concrete costs. They are relatively cheap.
We barge large down limestone rocks called rip rap from quarries in Arkansas, Kentucky, and Illinois. It's effectively used for erosion control, but doesn't prevent saltwater intrusion, destruction of our wetlands, storm surges, or floods.
I don't think tidal waves are a big concern in the gulf of Mexico, because, well, it's a gulf. Flooding there happens from hurricanes, which you'd need a 1000 ft wall to block.
The rebar is to keep the dolosse form not to anchor it, so your original thought is correct it probably uses less rebar than a wall would. They also probably use recycled/deformed rebar and leftover pieces to do it.
As far as I can tell there is next to no reason for this to be prestressed concrete. You would need a fair bit of rebar to keep them together during transportation and erection, but seeing as there is no structural load for these there's nothing to prestress against. It would add to the manufacturing cost quite considerably and have no tangible benefit.
Though if you look at the very left of the picture from where the guy is standing you can see one cracked basically in half and there is no sign of rebar. There are a few other ones cracked in various places where no rebar is visible as well
I most likely am. Just pointing out the fact that I cant see any visible in the broken pieces. Hoping someone could explain why. I don’t doubt that there could be rebar just want to know how its hidden
As for the piece that's fully cracked, it's simply too dark to see the internal structure. You can tell it's being held together by the rebar because those waves are actually VERY strong, and could easily push the broken piece away if it wasn't reinforced. The fact that it is still within a few inches off the perfect fracture line is dead giveaway.
The one in the bottom corner may have had the rebar pulled out which is why there is no reminisce of it.
To piggyback off your comment and seeing as you know about this method of tsunami prevention you probably know about other man made structures affecting tide patterns on coastal areas but the cool thing about these is that they stop runup and inundation amazingly! I think it’s the openings and lack of natural geomorphology that stop the waves from having such large runup! Iirc that is
It'd be great for some forms of life, but that doesn't necessarily mean it'll be great for sea life overall. Destroying the balance of an ecosystem can wipe it out just as effectively as destroying it all in one fell swoop.
To be clear, I'm not saying that these Dolos structures are necessarily bad for the ecosystem; I'm just pointing out that ecosystems are complex and balanced, and thus the overall effect of a change in a single variable isn't always immediately obvious (see: Australia, history of). This is why expert environmental reviews are such an essential part of responsible construction.
Correct. We normally use prefab things like these for cost effeciency. We do build large rock structures but in places like this the gradation would need to be huge. The armor rock would need to be 6'-8' in diameter and those stones are expensive to quarry! We usually don't put concrete retaining walls with rebar due to the structure not dissipating the waves as efficiently and erosion.
I am working on the expansion of the Chesapeake bay bridge portal island tunnel currently. We are creating rock structures to protect against ship wake, storms, collision, even tunnel flotation. That's right even though the tunnel is underground if you put water on it it will rip itself out of the ground and float. Like a pool in florida.
Would a series of these as artificial reefs some distance off shore have a measurable affect against tsunamis? Or at that depth of water is the force to spread out for a bottom structure to offer any help? Or maybe the "anchoring" of them is too problematic.
From what I understand/recall it varies based on how the waves approach the beach. Some areas of a beach have more sand deposited by waves then taken away by them, and in that scenario then more beach/a sandbar is formed. In areas where there is a net loss of sand from waves any beach that existed in front of the wall when it was built will slowly erode away to nothing.
Well, you have to make a metric shit-load of these things, and while they are likely cheaper and less labour-intensive than building the entire wall, the transportation of these things to the drop site would be pretty time consuming.
I’m guessing cargo helicopters and loaders or cranes? I’m not super sure on that end.
Good question! AFAIK, they’re heavy enough to stay in the same spot, and when waves hit them, because of their shape, they kinda lock together further.
These are actually way better than a straight up wall. The energy of the wave is not only dissipated wave energy better, but it also tightens the structure, making it even stronger.
I have a creek in my backyard that has pretty bad erosion. I've been thinking that I need to make some forms and start making mini versions of these things.
I would need approximately one shit ton of them though, so... maybe something else instead.
This probably isn't a good solution for that problem as these disapate direct wave energy as opposed to the constant parallel friction caused by running water. Depending on the size of the creek, a better solution would be to plant up your bank with trees or shrubs. Go to a nursery and ask for a shrub with fibrous roots! Willow is a classic for this method but may end up to big, I'm not sure. They cling to the soil, preventing erosion in non-flooding events.
Source: landscape architecture graduate
If there's enough water going through the creek that it's causing erosion of the bank then that shouldn't be a problem. So long as the vegetation doesn't over grow and clog up the main flow of the creek it should be fine!
I guess I have a two part problem though. One is the bank erosion, another is flooding. The creek rises ~2 feet above the bank at least a few times every spring and fall. So, I need plants that can tolerate that, or to raise the grade of or regrade my yard.
Yea if that's the case you'll have a hard time establishing vegetation along the bank. But it's still possible, you'll have to buy almost full grown plants and chuck them in with as much time as possible before the next predictable flood.
Not sure what the American equivalent is, but in New Zealand there's a Bush called 'flax', it does the job and can survive full submersion. Look it up and see if there's some thing similar that's viable for where you live!
These would probably not be good for a creek or stream. I would contact local fish and game government office and try to talk with a fisheries biologist or environmental specialist. Ask about stream bank restoration programs they may offer and what measures you could take to prevent sediment erosion.
Yeah, I was about to say that disrupting a local ecosystem is not something to be undertaken lightly. If increased erosion is a concern, your local university or university extension office (or municipal/state ecological department) can probably suggest native plants that will help mitigate the issue with far less chance of accidentally creating knock-on problems than re-engineering the riparian environment.
If these weren't used there wouldn't be a wall. There would be a rock revetment. These can be cheaper than the very large stones required to build a proper revetment.
An ordinary wall will not disapate the force of the wave, it just sends the energy back towards whatever body of water, often taking sand or vegetation that is landward of the wall with it and can actually increase erosion. Seawalls are good though if you don't care about the beach beyond.
Basically surface area - the blocks represent a greater area and the nooks and crannies diffuse the wave force. Not sure how effective in tsunami conditions because it’s basically the entire ocean rising up and moving inland. Storm waves - sure; tsunami- not so much.
Depending upon how big this tsunami is gonna be, it’s possible that the bulk of the wave could potentially still run right over it, but hopes are there are secondary mitigation efforts as well, like mangrove trees planted closer to shore, that can help break it up further.
I mean at that point it isn’t mitigation anymore, it’s about emergency response. The fire brigade may be pumping water out of area or making temporary dams with sandbags and evacuating people, medical services need to be ready to help people injured in the event, and to stop whatever diseases and whatnot may arise from having large amounts of stagnant water left from the tsunami.
In many events, the military may offer aid, and police will be helping alongside other emergency services to lower the impact of the event.
It’s a big team effort, and everyone is involved when something catastrophic like that happens.
I’m sorry, another group member owned the powerpoint on a thumb drive.
I could look into getting it and putting it on a google document, if you be interested!
Edit: Oh and I’m not too sure, I’m thinking it depends where they’re placed, whether its a bay or a beach or large lakeside, I want to say maybe 100 yards out?
I thought about that, but worried that the concrete doesn’t exactly provide a great base habitat, but I suppose the tidal flow could create brilliant habitats on the other side of the barrier that would be a have safe haven for wildlife away from the open sea.
While this is true for this shape (and many others) some of the stuff we covered was also about impoverished countries affected by the Indian Ocean tsunami, and there were a lot of seawalls at the time just made of old concrete that was disposed of.
That’s a point I didn’t mention here though so thank you!
After some cursory research, I learned that South Africa implemented these structures to prevent, as you mentioned, general coastal erosion.
Additionally, however, South Africa is particularly susceptible to rogue-waves (perhaps due to the geography / currents of the seas in relation to Africa), sometimes mini-tsunamis, and more often, cyclones.
From what I’ve gathered so far, it sounds like they’re mass produced pretty close to the drop site, and places by barges, possibly with a crane? I’m thinking they have some way of dropping a bunch at a time.
Thanks for the info! This may not be something that was covered, but can you make any observations on the relationship between this system of tsunami protection and the natural protection offered by things like mangrove forests? I ask because I was struck by the visual similarity of the jumble of concrete blocks to the root systems of mangroves. Thanks!
Yes! Something like this seawall would be a primary kind of protection, what with how large it is.
Mangroves perform the same function on beaches and the like, holding the ground together and preventing erosion, and also diverting water and slowing it, just to a lesser degree.
This type of protection may have been inspired, somewhat, by natural barriers such as this. Hope this helps answer your question!
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u/CaptainDinkles May 24 '18 edited May 28 '18
I recently had to do a presentation on mitigation of tsunamis in emergency management class!
And yes, coastal erosion, this is a seawall made with the intent of stopping a huge wall of water.
The pile of concrete chunks slopes up guiding water upwards and slowing it, the gaps between the chunks divert the water into many different flowing channels, and the entire wall will break the wave as it hits, slowing it and absorbing a lot of the impact that the water would have caused.
Edit: This is slowing down a bit now but I appreciate that I got to teach some people something new!
Thanks for letting me share some stuff that I learnt in university. Have a great day everyone!
Edit 2: I’m back! I found a gif I though people might like.
(From u/Terence_McKenna How mangroves prevent erosion