Dolosse – a South African invention used over the world | Hacker News Reader

Dolosse – a South African invention used over the world

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141 pointsandsoitis40 comments

Finnally something I have knowledge of. Dolosse were invent in South Africa many years ago, but they are only one type of concrete block for wave dissipation and protection. Many others exist and some of them are patented and require the payment of royalties to be used. Anyway, in Portugal we do not use dolosse ever. We used it once in Sines deepwater breakwater, which suffered a catastrophic collapse in 1979. The promise of dolosse is that the blocks work together because of their shape and thus the individual block can be lighter and cheaper. This does not work in practice, at least in high energy seas. The ocean is unforgiven. What happens is that if only a few blocks break the whole structure will quickly collapse during a storm. And things can break due to a series of problems and risks. Concrete is only good at resist compression stresses and reinforcement with rebar is almost always a bad idea in ocean conditions, so complex shapes are hard to make and to move without introducing unwanted stresses that reduce the design strength conditions. Other types of blocks rely mostly on their design weight and density, and the shape is just used for increasing energy dissipation and overtopping. For example, tetrapods can be moved with only compression forces on the block.

> reinforcement with rebar is almost always a bad idea in ocean conditions

While a little more costly, 316 stainless steel should work just fine.

Or composite rebar. Basalt fiber is sometimes used. Way cheaper than stainless and just as rust proof

‘a little more costly?’ Lulz.

316 is an order of magnitude more expensive than the crap used in rebar.

In project budgeting the material cost difference may well be relatively insignificant

Sure, but that’s not what was said.

Fiberglass rebar is comparable in cost, stronger, and not subject to water/salt damage. Rusting steel expands and cracks concrete from the inside, id want to avoid that. I’m actually building a pool right now using it instead of steel.

Thanks for this background! I encountered dolosse or something dolosse-like at Praia da Ilha de Tavira, in the Algarve, here: https://www.google.com/maps/place/Farol+de+Tavira+Molhe+Oest...

Those are tetrapods. In Portugal we only use tetrapods, Antifer blocks and rock slabs if avaiable with the right size and density. The weight of the blocks is a function of the design wave height and period, so we have blocks with different sizes but not shapes. Sometimes we use blocks with high density concrete, cause higher density works both ways, it inscreases weight and reduces volume. You can see this in Sines, the blocks change colour on the tip of the breakwater, because the concrete has a higher iron content. https://www.google.com/maps/place/37%C2%B056'28.2%22N+8%C2%B...

There is one exception. In the Azores, there are core-locs because the breakwater was designed by the US Military. https://www.google.com/maps/place/38%C2%B043'36.6%22N+27%C2%...

Interestingly the author shows a photo of a spot in Cape Town where there are two distinct structures in use - close up, in the photo, are actually Tetrapods - as pointed out in other comments, these are an earlier invention from France and also used quite extensively throughout many parts of the world (including... right here where the Dolosse were invented).

Further down that jetty are structures with the actual Dolos geometry. It would be interesting to know if, specifically, those geometries were chosen specifically to be placed in those two parts of the wall as they are for specific properties, or if came down to having certain quantities made on hand and their distributions matched the shape of the wall required, or what.

You can see what I mean, this distinction in the exact location where that photo is taken on google maps, here:

https://maps.app.goo.gl/Vn3tGhM81oMqPVpm9

-33.899126, 18.412751 Mouille Point, Cape Town, 8005, South Africa

Mathematically, I think they both may be tetrahedons. The Tetrapod models it by 4 lines connecting each of its vertices to its center, the Dolosse by two opposite edges connected by a line through the middle.

I’m saying “may be” because it is possible that the center “arm” of the Dolosse is too large to make a true tetrahedon. That’s a degree of freedom in the design.

> It would be interesting to know if, specifically, those geometries were chosen specifically to be placed in those two parts of the wall

My guess would be that Dolosses interlock better than Tetrapods, allowing for steeper inclines on stacks of them.

That connecting "spine" is there to improve the interlocking of the tetrapods. They can "hook" better to each other. That's the theory at least, I don't know the if it's better in practice.

The Wikipedia article:

> A dolos ... is a type of tetrapod

but also, the Wikipedia article for tetrapod:

> One of the earliest designs is the Tetrapod, invented in 1950. Other proprietary designs include the Modified Cube (United States, 1959), the Stabit (United Kingdom, 1961), the Akmon[W] (The Netherlands, 1962), the Dolos (South Africa, 1963), the Stabilopod [ro] (Romania, 1969),[7] the Seabee (Australia, 1978), the Accropode[W] (France, 1981), the Hollow Cube (Germany, 1991), the A-jack[W] (United States, 1998), the Xbloc[W] (The Netherlands, 2001) and KOLOS[W] (India, 2010) among others.

I have put Ws next to ones with their own Wikipedia page.

Interesting. Been to South Africa a bunch but never clocked that they were different from the more common antecedent, the tetrapod[1].

1: https://en.wikipedia.org/wiki/Tetrapod_(structure)

“Hast thou considered the tetrapod?” can finally be answered positively on HackerNews. Hail Satan.

In Germany (on the island Sylt) they recently removed all of these structures again after decades of them being a staple on Sylt's beaches. They were found to have no positive effect on protecting their shores.

I'm having a hard time to find a good source to quote but look for: "sylt tetrapoden" if you want to dive deeper.

Ok I stand corrected these are NOT tetrapods. Sorry for the confusion

Well, confusingly, the last picture on the blogpost look (to my untrained eye) to be tetrapods. Those are 'tetrahedral' (ish) shapes.

In contrast, the https://en.wikipedia.org/wiki/Dolos (plural 'Dolosse') are two tapering cylinders at right angles to each other. They have the same symmetry, but a different structure. Does that make any difference to how effective they are? No idea!

edit: When I said the "same symmetry" of course I should have said the dolos has some of the symmetries of the tetrapod (subgroup?).

Maybe the form variations were just for patent circumvention.

I find it hard to believe they have not protective effect. You can throw pretty much anything hard with an irregular shape down and it’ll help scatter energy

They don’t prevent erosion though which is the primary issue

Save yourself the blogspam, go directly to https://en.wikipedia.org/wiki/Wave-dissipating_concrete_bloc... and enjoy the whole collection !

A bit rude, this doesn't come across as spam but a genuine human sharing something interesting they came across. Peak old internet (2000s) content. The wiki article is very short and doesn't actually have more or better pictures.

Some people here think if it's on Wikipedia it's authoritative. Yours truly has written many Wikipedia articles, some translated into ten or more languages. I've seen how ugly it is behind the scenes there.

I miss blogs. They are omitted from searches now but are among the best internet search content. Threadinburgh is one of the best online resources about Edinburgh by far.

> They are omitted from searches now but are among the best internet search content

I use google search with &udm=14 to remove all AI elements in the results presentation.

Searching for "dolosse", the HN item is the 3rd link, and TFA is the 4th. They are preceded by Wikipedia, and a dolosse-manufacturer's page.

Not sure why you think "blogs are omitted from searches now".

Maybe because I do searches for specific topics and no blogs appear in the results. This is even though I know certain blogs contain information on the topics I'm interested in, and instead I get "trusted media" which has information often only loosely connected to what I'm after.

Bizarre hate towards someone's little blog. And it's not even LLM spam, just someone running their own WP site. By your logic I should save myself the Wikipedia verbosity and just ingest LLM output for any query I have forever.

Not sure why Dolosse are so special?

There's lots of different forms and shapes of wave breakers, like the Tetrapods [1] that you can find over here across pretty much every coast in Europe.

Maybe I'm just not getting it why the Dolosse shape is superior?

[1] https://en.wikipedia.org/wiki/Tetrapod_(structure)

Apparently the Dolosse works a lot better for high energy wave environments like coastline of SA etc. They are used here, and I am sure they considered Tetrapods and there was a reason they weren't suitable.

ITT the consensus is they don't actually work better, but, were royalty free.

No idea about either, myself.

Another thing is that you need a lot fewer of them to stack to get the same effect. So cost saving.

These are cool shaped but they don’t actually work; coastal armoring in general does not work as it does not prevent erosion. Sand replenishment is increasingly the path taken. Australia has an amazing pumping system (a jetty blocked longshore sand transport so they pump it around to the other side).

We could just stop building so close to shore…

> coastal armoring in general does not work as it does not prevent erosion.

Not that simple. Example: east of Lauwersoog (north shore of the Netherlands) there's a dyke reinforcement project underway. Roughly 2/3 of the dyke's seaside is covered with regular rows of interlocking concrete blocks (~25 x 25 cm top view). At the base of the dyke, there's stone rubble to dissipate wave energy. These are heavy stones: 100s of kg each on average. But loose - some even wobble if you step on them! There's probably some other things in there like layer(s) of geotextile & such.

I'm pretty sure if those rubble stones weren't there, waves would 'eat' some rows of the smaller blocks, wash away a few metres of sand, then rows above would collapse, expose more sand, etc.

So it's not like those stones aren't doing anything.

In other places on the Dutch coast, it's more common to pump sand onto the beach every X year to replace what the sea eroded. 'Must read' in this context:

https://en.wikipedia.org/wiki/Sand_engine

TLDR; wave patterns, coastlines & what's behind them are different. So are strategies to protect them.

Sorry I was unclear. By erosion I meant beach erosion. I do agree that armoring can help preserve a structure like a dyke. The issue is the sand as you mention with the sand engine.

In my neck of the woods armoring is falling out of favor as sand replenishment takes over as the primary strategy.

Neat didn't know that's what they're called you see em in Japan too "over the world" yeah

Invented by Eric Mowbray Merrifield and his team, which included Piet Grobbelaar and Jack Badham-Thornhill.

Dolos are neither the first nor best ocean… rock… shape, apparently.

What's the difference between these and KOLOS?

The article is embarassing. Basically it says nothing but links to wikipedia. I’m saving you a click: https://en.wikipedia.org/wiki/Dolos

The blog article mentions 80 tons while the wiki article mentions 8 tons. Are they using the same definition of a ton ?

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