What is the purpose of these things on this dam?

[DDTJRAC]

Hello people!

What were the purpose of these things on this dam?

LOC

Thanks!

 

[Baluncore]

What were the purpose of these things on this dam?

I do not know, but would hazard a guess, that the ridges, (strakes?), prevent the accumulation of debris at the foot of the wall, while directing the water flow to remain perpendicular to the face.

The falling water would press objects against the flat foot of the wall, trapping them. The strakes allow water to flow under those larger objects, reducing the pressure difference, and so releasing them.

A circular rolling wave can form across the width of the dam. Without strakes, that would transport debris to collect at one end of the dam, and selectively erode a narrower channel below the dam.

There must be some interesting literature on the subject, somewhere.
Can anyone find it?

What are they really called?

 

[Lnewqban]

Perhaps two of their functions are to reduce the kinetic energy of the flow and to protect the concrete at the base.

Please, see:
https://en.wikipedia.org/wiki/Spillway

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

It would be good to see what is under the surface.

 

[DaveC426913]

I know explicit AI is not allowed, but are Google results disallowed, even though they're essentialy AI?

"Dams have strakes, which are essentially vertical fins on their surfaces, to help manage the flow of water by creating small vortices that disrupt larger turbulent currents, thus reducing the erosive forces on the dam's structure and promoting more even water distribution over its spillway; essentially acting as a flow control mechanism.

These points seem pretty plausible to me:

"Key points about strakes on dams:
Flow control:
By creating small vortices, strakes can help break up large turbulent water flows, preventing excessive erosion and damage to the dam's foundation.
Stability enhancement:
Strakes can improve the stability of the water flow over the spillway, especially during high water levels, by directing the water more evenly.
Reduced cavitation:
By manipulating the water flow, strakes can help minimize the formation of cavitation bubbles, which can cause damage to the dam's surface."

 

[fresh_42]

The essential point of such a cascade is energy conversion. The construction cannot be seen. The question is reduced to the architecture of the stilling basin or why there are many short ones instead of a single long one.

I assume that they installed dozens of small stilling basins. This could have had practical reasons, like prefabrication. Another reason is energy reduction. The more elements are placed in the stilling basin, the more energy is lost. It also reduces the risk of underwashing: a single failure won't lead to a collapse of the entire system.

Sources:
https://izw.baw.de/publikationen/mi...teilungen_105_Pfrommer_Energiedissipation.pdf
https://www.hydro.uni-wuppertal.de/...o/downloads/Studienarbeit_Daniela_Henning.pdf
https://tuprints.ulb.tu-darmstadt.de/8507/1/Bachelorarbeit_AnnikaScheidler.pdf
https://de.wikipedia.org/wiki/Tosbecken

 

[Baluncore]

I know explicit AI is not allowed, but are Google results disallowed, evn though they're essentialy AI?

I introduced the word "strake" in post #2. I used Google search and was fed results by AI, but I found only references to boat hull "strakes" in my searches. I suspect Google AI has learned from my searches and post, and accepted the new term, being applied to dams. Can you find an original text that uses the term "strakes" that Google AI supplied as the introduction to its search results?

The OP picture, from between 1880 and 1899, is of the Fox River where it flows through Appleton, Wisconsin, USA. That weir was built during the early days of reinforced concrete, and has since been replaced. The massive concrete was pored in blocks. That left a gap between the blocks at the foot of the wall. The strakes may have been cast in place later, to cover the gaps between the blocks, to protect them from the damaging water, falling from above. I would expect blocks cast more recently to not require that protection.

Terminology: Since water flows over the crest, that is not really a dam, it is a barrage or a weir. A barrage has gates to regulate the flow, which reduces that structure to identify as a weir.

The crest height of a weir is used to regulate the level of the river above the weir. A river used for navigation will have locks to raise and lower vessels travelling along the river. The weir ensures the river is navigable, without rapids between locks. Since weirs are specific to navigable rivers, the technology will not be widely used, nor will it apply to higher dam spillways.

 

[DaveC426913]

I introduced the word "strake" in post #2. I used Google search and was fed results by AI, but I found only references to boat hull "strakes" in my searches. I suspect Google AI has learned from my searches and post, and accepted the new term, being applied to dams. Can you find an original text that uses the term "strakes" that Google AI supplied as the introduction to its search results?

I searched "what are strakes in dams for?" so it was pretty explicit.

 

[Baluncore]

I searched "what are strakes in dams for?" so it was pretty explicit.

Did you notice that there were no hits and no references?
" No results found for "what are strakes in dams for?". "
The only Google hit is this thread.

You were explicit in your challenge, so it conjured up something explicit that sounded good, by using key phrases. On analysis, Google's AI response does not really make sense. It is all platitudinal motherhood and apple pie.

The challenge is still to find, an older document that discusses strakes at the foot of weirs or dams.

 

[DrGreg]

I searched "what are strakes in dams for?" so it was pretty explicit.

I searched for "Dams have strakes" (the first three words from Google's AI response you quoted) and it linked to this thread and nothing else. So Google made it up.

 

[russ_watters]

I searched for "Dams have strakes" (the first three words from Google's AI response you quoted) and it linked to this thread and nothing else. So Google made it up.

Or rather, Google AI regurgitated it back. You/Dave/Ba made it up.

Interesting case study on how impressionable "AI" is.

 

[Rive]

I believe the correct term is 'weir' and 'chute block' or 'chute dentate block': though the last two is not a 100% fit.

 

[fresh_42]

Weir is very general. There are different types of weirs with different names. I had to translate the German names and found cascade (Sohlstufen). There are still three types of cascades: stairs (Treppen), glides (Gleitung), and drops (Abstürze), for which I am not sure about their technical correctness in English. However, we are talking about the construction of the sole (Sohle), and in this case, about the building of the stilling basin (Tosbecken). These structures have two main goals: preventing underwashing (Auskolkung) and energy conversion. The falling waters have a kinetic energy that has to be converted into a uniform flow as quickly as possible, i.e. the sole drum (?) (Sohlwalze) has to be handled. Additional building elements cause additional swirls and friction, which destroys a part of the kinetic energy. They would have also allowed prefabrication, which we do not have any information about. However, it reduces the risk of underwashing. The entire construction remains intact even if a single chamber fails. An additional advantage is that these walls make any maintenance easier.

Sources: see above, post #5. No AI, but university texts and building authorities.

 

[Lnewqban]

For methods of calculation of those things, look for “19.4 STILLING BASINS AND ENERGY DISSIPATORS” in this document:
https://www.academia.edu/8814428/Design_of_weirs_and_spillways

 

[Klystron]

I do not know, but would hazard a guess, that the ridges, (strakes?), prevent the accumulation of debris at the foot of the wall, while directing the water flow to remain perpendicular to the face.

Use of the term strake in this context of fluid flow makes sense and has precedent.

Consider an early improvised dam on a freshwater stream to create a pond in order to power water wheels and simple hoists. Medieval northern European (Nordic) settlers might invert boat hulls secured by stones to dam a stream and induce laminar flow downstream to power a wooden water wheel.

I have seen illustrations of woodcuts from a Cistercian monastery from 8th Century Ireland depicting similar weirs in old manuscripts, exactly the type of sources that are, at best, poorly digitized thus likely unavailable to Google online.

Certainly the term strake appears in contemporary aerodynamic literature such as this NASA wind tunnel test of various strakes on F-16 fighter jets from 1984. The term was also occasionally used for various guide ridges mounted on the walls of recirculating wind tunnels to reduce fluid (air) turbulence and induce laminar flow anterior to the test chamber.

Note the word strake has considerably more usage in British English than American, perhaps due to British English predating American by many centuries. Toggle between American and British on the dictionary.com page to illustrate this.

 

[DaveC426913]

Interesting how pernicious AI is. A further cautionary tale.

 

[Baluncore]

I believe the correct term is 'weir' and 'chute block' or 'chute dentate block': though the last two is not a 100% fit.

A dentate block is used to destroy the laminar flow in water where it is flowing as a thick sheet. In the OP picture, the strakes lie flat at the foot of the vertical weir. They have no obstructive tooth projection, as they are followed immediately by the flat toe of the wall.

Strakes are not present at the foot of modern dams or weirs. Indeed, dentate blocks are designed to break the linear flow during the fall, not to preserve it as the strakes do at the foot of the wall. The strakes are directing water flow away from the wall, while reducing flow along the front of the weir.

In the picture, the strakes are spaced apart by the same separation as the concrete modules that were cast in place. That suggests to me that the strakes cover and protect the joints between blocks, from the dynamic pressure of the falling water.

I see no way that strakes might be an advantage when ice forms in winter, or breaks up in spring.

 

[alexwriter]

I am curious to know about these things, but once I went to visit a dam. The local guide told us they are called "bheta" in local language which translates as to enforce the boats when they water level is down!

 

[DaveC426913]

boats??

 

[russ_watters]

As in, they keep boats away?

 

[berkeman]

Welcome to PF, @alexwriter

"bheta" in local language which translates as to enforce the boats when they water level is down!

What was that local language? I'm not having any luck with that translation using Google Translate so far.

 

[Rive]

I think those waters there are too shallow for boats... Feels like that...

 

[russ_watters]

I think those waters there are too shallow for boats... Feels like that...

Boats need what, 5cm of depth? I think the water in the photo is deeper than that.