3 ms·

How does removing a dam compare with simply opening the gates and releasing all the water? A dam not holding any water back has far more strength than necessar

How does removing a dam compare with simply opening the gates and releasing all the water? A dam not holding any water back has far more strength than necessary and will probably last thousands of years. It's obviously far cheaper. You get many of the same environmental benefits (lake area no longer flooded, fish can traverse the base of the dam).

mistrial9 3y ago
> imply opening the gates and releasing all the water would that be step 1 perhaps? without much thinking about it, I would expect that the shape of the waterway structure may not really match a natural water course at all. For example, quite a lot of the release mechanisms would be at the top third of the dam wall, etc. Probably very specific to each dam?
markedathome 3y ago
Switzerland are building a dam next to another dam that can no longer be repaired, B1M have a video going over the method - https://www.youtube.com/watch?v=k0vxzYvIHUk https://www.youtube.com/watch?v=k0vxzYvIHUk
- ok_dad 3y ago
  That dam also holds back the largest (as far as I recall) pumped hydro storage system in the world. I’m not sure but I think they talk about it in the video you linked if it’s the same one I watched a while back.
- jart 3y ago
  Thanks for linking that. It was a nice half an hour well spent.
dylan604 3y ago
The spill way being opened won't release all of the water from a dam. Spill ways are just a type of pressure relief valve to keep the water from destroying the damn. So while it might add more water down stream, it will do nothing for restoring the flooded area upstream of the dam.
parl_match 3y ago
First off, that's opening a spillway, and that will still leave a substantial portion of land flooded. > A dam not holding any water back has far more strength than necessary and will probably last thousands of years. And, actually isn't true, there's a lot of complicated factors when building a dam, and having a consistent, low lying waterflow (like a river). Depending on the type of damn, soil conditions, etc, it can actually cause disproportionate erosion that will cause the damn to collapse in on itself (occasionally in a surprisingly short time frame, sometimes under a decade even - or in the case of heavy waterflow, hours or days). So, engineers will sometimes do what you suggested when it's feasible. But, it's not always.
pengaru 3y ago
There aren't gates at the base of any large dam. The only way to discharge the artificial lake behind a large dam is via destruction AIUI. It will deteriorate with age and eventually break in an uncontrolled/unplanned manner anyways. At least this way you tear it down gracefully with a gradual discharge of the water, and enable restoration of the environment/salmon breeding etc.
- phyzome 3y ago
  Wouldn't there still be a diversion tunnel or channel from when the dam was originally constructed? I wonder if those could be used to empty a reservoir. Or maybe the water pressure and flow rate of a full reservoir would be too high...
  - pengaru 3y ago
    I don't think those are sealed upon completion in a gracefully reversible manner. My guess is dynamite blasting and dumping stone to collapse/fill it in until the water stops flowing. This short video[0] describes the process used for the Hoover Dam... sSounds like the blasted diversion tunnels were above the river level, then ad-hoc "cofferdams" were filled in both upstream and downstream of the construction site. So even if you reopened the diversion tunnels, it still wouldn't be down to the original level. It certainly doesn't seem like it'd be a controlled release of the water if you could manage to unseal them. There's no gate... [0] https://www.youtube.com/watch?v=n9Gy_1Ppw5U https://www.youtube.com/watch?v=n9Gy_1Ppw5U
    - Arrath 3y ago
      > My guess is dynamite blasting Exactly right! I was involved in a 'lake tap' project in Oregon where the concrete plug in the diversion tunnel was mechanically demolished down to the minimum safe thickness, then we came in and finished it off with explosives. A gate structure was first built farther down the tunnel, to allow the operators to again close off the tunnel or chose a given flow level and let me tell you, having to sidle sideways through that partially open gate to go place explosives into the remnants of the tunnel plug was an interesting experience. Here is a presentation put together on the project: https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2005/triservice/track1/schlenker.pdf https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2005/tr...
patmorgan23 3y ago
No, most dams are not designed to be able to completely disengage and completely discharge their entire reservoir. Some dams don't even have gates and are entirely passive structures.
- gerdesj 3y ago
  Spillways. I'm not too sure you have any idea about what you are talking about. Anyway, water at scale is a scary thing. A cubic meter of water is a tonne - 1000 Kg. Look at any river or even a stream and try and mentally construct a cross-section. Say 30m wide and say 3m deep, and assume a 75% section - that's 30x3 = 90x0.75 = 67.5m^2. Let's call it 70m^2. If the flow in this river is 1 ms-1 then 70 tonnes of water flows per second. That's roughly one and a bit main battle tanks per second! 1 m/s = 2.2 mi/h Let's set walking speed at 3 mph and a river in spate at say 15mph (6.7m/s). Far worse has been seen. Now you have 70m^2 x 6.7ms-1 = 469 tonnes of water per second. Let's call it 500 tonnes per second.
fckgw 3y ago
Do you think a dam looks like the gates from Jurassic Park?
psc 3y ago
> You get many of the same environmental benefits (lake area no longer flooded, fish can traverse the base of the dam) Besides the fact that the lake area will still be partially flooded, as other replies have said, fish are complicated and there are a lot of dam related issues that affect fish passage. Just a few: - Downstream passage would have to be through spillways. Survival rates are pretty good (95%+) [1], much better than turbine passage, but not as good as not having a dam in the way. - Conventional spillways open from bottom, which fish have a hard time finding. [2] Granted, if the water levels are reduced this could be less of an issue. - Time spent in slow moving water increases predation on salmon. [3] If you visit a dam you'll see ospreys all around. Slow water can also favor some species over others. - There are a bunch of other water quality issues that dams cause, most notably water temperature. The 2002 Klamath fish kill [4] (the largest in history and a major impetus for un-damming the Klamath) was caused by high water temperatures. [5] - Even shadows can cause significant issues, from structures as small as docks or piers. [6] - Upstream passage is not solved by opening the spillway and would still be an issue, requiring fish ladders or transportation. All the existing problems with fish ladders apply. [7] - Besides salmon and trout, other important species get blocked by dams, including sturgeon and lampreys [8], and each are affected by dams in their own way. [1] https://www.salmonrecovery.gov/Hydro/Structuralimprovements/Surfacepassage.aspx https://www.salmonrecovery.gov/Hydro/Structuralimprovements/... [2] https://www.fisheries.noaa.gov/west-coast/endangered-species-conservation/juvenile-downstream-passage-west-coast https://www.fisheries.noaa.gov/west-coast/endangered-species... [3] https://www.fisheries.noaa.gov/west-coast/endangered-species-conservation/how-dams-affect-water-and-habitat-west-coast https://www.fisheries.noaa.gov/west-coast/endangered-species... [4] https://en.wikipedia.org/wiki/2002_Klamath_River_fish_kill https://en.wikipedia.org/wiki/2002_Klamath_River_fish_kill [5] https://www.waterboards.ca.gov/waterrights/water_issues/programs/bay_delta/california_waterfix/exhibits/docs/PCFFA&IGFR/part2/pcffa_155.pdf https://www.waterboards.ca.gov/waterrights/water_issues/prog... [6] https://wdfw.medium.com/casting-shadows-on-the-sound-3a9a55563b9b https://wdfw.medium.com/casting-shadows-on-the-sound-3a9a555... [7] https://medium.com/re-form/whats-the-dam-problem-3b5cd839ae74 https://medium.com/re-form/whats-the-dam-problem-3b5cd839ae7... [8] https://e360.yale.edu/features/sea-lampreys-pacific-lampreys-american-rivers-dams https://e360.yale.edu/features/sea-lampreys-pacific-lampreys...
- 3y ago

mistrial9 3y ago

> imply opening the gates and releasing all the water would that be step 1 perhaps? without much thinking about it, I would expect that the shape of the waterway structure may not really match a natural water course at all. For example, quite a lot of the release mechanisms would be at the top third of the dam wall, etc. Probably very specific to each dam?

markedathome 3y ago

Switzerland are building a dam next to another dam that can no longer be repaired, B1M have a video going over the method - https://www.youtube.com/watch?v=k0vxzYvIHUk https://www.youtube.com/watch?v=k0vxzYvIHUk

ok_dad 3y ago
That dam also holds back the largest (as far as I recall) pumped hydro storage system in the world. I’m not sure but I think they talk about it in the video you linked if it’s the same one I watched a while back.
jart 3y ago
Thanks for linking that. It was a nice half an hour well spent.

dylan604 3y ago

The spill way being opened won't release all of the water from a dam. Spill ways are just a type of pressure relief valve to keep the water from destroying the damn. So while it might add more water down stream, it will do nothing for restoring the flooded area upstream of the dam.

parl_match 3y ago

First off, that's opening a spillway, and that will still leave a substantial portion of land flooded. > A dam not holding any water back has far more strength than necessary and will probably last thousands of years. And, actually isn't true, there's a lot of complicated factors when building a dam, and having a consistent, low lying waterflow (like a river). Depending on the type of damn, soil conditions, etc, it can actually cause disproportionate erosion that will cause the damn to collapse in on itself (occasionally in a surprisingly short time frame, sometimes under a decade even - or in the case of heavy waterflow, hours or days). So, engineers will sometimes do what you suggested when it's feasible. But, it's not always.

pengaru 3y ago

There aren't gates at the base of any large dam. The only way to discharge the artificial lake behind a large dam is via destruction AIUI. It will deteriorate with age and eventually break in an uncontrolled/unplanned manner anyways. At least this way you tear it down gracefully with a gradual discharge of the water, and enable restoration of the environment/salmon breeding etc.

phyzome 3y ago
Wouldn't there still be a diversion tunnel or channel from when the dam was originally constructed? I wonder if those could be used to empty a reservoir. Or maybe the water pressure and flow rate of a full reservoir would be too high...
- pengaru 3y ago
  I don't think those are sealed upon completion in a gracefully reversible manner. My guess is dynamite blasting and dumping stone to collapse/fill it in until the water stops flowing. This short video[0] describes the process used for the Hoover Dam... sSounds like the blasted diversion tunnels were above the river level, then ad-hoc "cofferdams" were filled in both upstream and downstream of the construction site. So even if you reopened the diversion tunnels, it still wouldn't be down to the original level. It certainly doesn't seem like it'd be a controlled release of the water if you could manage to unseal them. There's no gate... [0] https://www.youtube.com/watch?v=n9Gy_1Ppw5U https://www.youtube.com/watch?v=n9Gy_1Ppw5U
  - Arrath 3y ago
    > My guess is dynamite blasting Exactly right! I was involved in a 'lake tap' project in Oregon where the concrete plug in the diversion tunnel was mechanically demolished down to the minimum safe thickness, then we came in and finished it off with explosives. A gate structure was first built farther down the tunnel, to allow the operators to again close off the tunnel or chose a given flow level and let me tell you, having to sidle sideways through that partially open gate to go place explosives into the remnants of the tunnel plug was an interesting experience. Here is a presentation put together on the project: https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2005/triservice/track1/schlenker.pdf https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2005/tr...

patmorgan23 3y ago

No, most dams are not designed to be able to completely disengage and completely discharge their entire reservoir. Some dams don't even have gates and are entirely passive structures.

gerdesj 3y ago
Spillways. I'm not too sure you have any idea about what you are talking about. Anyway, water at scale is a scary thing. A cubic meter of water is a tonne - 1000 Kg. Look at any river or even a stream and try and mentally construct a cross-section. Say 30m wide and say 3m deep, and assume a 75% section - that's 30x3 = 90x0.75 = 67.5m^2. Let's call it 70m^2. If the flow in this river is 1 ms-1 then 70 tonnes of water flows per second. That's roughly one and a bit main battle tanks per second! 1 m/s = 2.2 mi/h Let's set walking speed at 3 mph and a river in spate at say 15mph (6.7m/s). Far worse has been seen. Now you have 70m^2 x 6.7ms-1 = 469 tonnes of water per second. Let's call it 500 tonnes per second.

fckgw 3y ago

Do you think a dam looks like the gates from Jurassic Park?

psc 3y ago

> You get many of the same environmental benefits (lake area no longer flooded, fish can traverse the base of the dam) Besides the fact that the lake area will still be partially flooded, as other replies have said, fish are complicated and there are a lot of dam related issues that affect fish passage. Just a few: - Downstream passage would have to be through spillways. Survival rates are pretty good (95%+) [1], much better than turbine passage, but not as good as not having a dam in the way. - Conventional spillways open from bottom, which fish have a hard time finding. [2] Granted, if the water levels are reduced this could be less of an issue. - Time spent in slow moving water increases predation on salmon. [3] If you visit a dam you'll see ospreys all around. Slow water can also favor some species over others. - There are a bunch of other water quality issues that dams cause, most notably water temperature. The 2002 Klamath fish kill [4] (the largest in history and a major impetus for un-damming the Klamath) was caused by high water temperatures. [5] - Even shadows can cause significant issues, from structures as small as docks or piers. [6] - Upstream passage is not solved by opening the spillway and would still be an issue, requiring fish ladders or transportation. All the existing problems with fish ladders apply. [7] - Besides salmon and trout, other important species get blocked by dams, including sturgeon and lampreys [8], and each are affected by dams in their own way. [1] https://www.salmonrecovery.gov/Hydro/Structuralimprovements/Surfacepassage.aspx https://www.salmonrecovery.gov/Hydro/Structuralimprovements/... [2] https://www.fisheries.noaa.gov/west-coast/endangered-species-conservation/juvenile-downstream-passage-west-coast https://www.fisheries.noaa.gov/west-coast/endangered-species... [3] https://www.fisheries.noaa.gov/west-coast/endangered-species-conservation/how-dams-affect-water-and-habitat-west-coast https://www.fisheries.noaa.gov/west-coast/endangered-species... [4] https://en.wikipedia.org/wiki/2002_Klamath_River_fish_kill https://en.wikipedia.org/wiki/2002_Klamath_River_fish_kill [5] https://www.waterboards.ca.gov/waterrights/water_issues/programs/bay_delta/california_waterfix/exhibits/docs/PCFFA&IGFR/part2/pcffa_155.pdf https://www.waterboards.ca.gov/waterrights/water_issues/prog... [6] https://wdfw.medium.com/casting-shadows-on-the-sound-3a9a55563b9b https://wdfw.medium.com/casting-shadows-on-the-sound-3a9a555... [7] https://medium.com/re-form/whats-the-dam-problem-3b5cd839ae74 https://medium.com/re-form/whats-the-dam-problem-3b5cd839ae7... [8] https://e360.yale.edu/features/sea-lampreys-pacific-lampreys-american-rivers-dams https://e360.yale.edu/features/sea-lampreys-pacific-lampreys...

3y ago