3 ms·

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

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...
    - pengaru 3y ago
      > 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... Dude, this has to be one of my favorite and most relevant HN responses ever. Thanks for digging up and sharing that link! "Water Hammer Equation" "Gate Openings 6'x2' (25% of blast area)" this is bonkers.
      - Arrath 3y ago
        My pleasure! Enjoy!
    - phyzome 3y ago
      That sounds intense. :-) And wow, I'd never thought about the effects of a dam on waterway temperature. :-O
  - phyzome 3y ago
    Hah, I guess that makes sense.
gerdesj 3y ago
All dams have spillways. There are various types and they have several functions: avoiding collapse due to say flooding; allowing controlled release - "spate" - handy for, say, helping salmon to run upstream - yes it is a bit counter-intuitive. The river Tamar largely delineates Devon and Cornwall in England, UK. It is mostly run off from Dartmoor and hits the sea at Plymouth (which is named after the river Plym). The Tamar is joined by the river Tavy (hence Tavistock, which is a few miles north of Plymouth). The Tamar and Plym run into Plymouth Sound which is the name for the area of sea just south of Plymouth, inside the breakwater. Right, its sodding complicated! The original dam and reservoir for Plymouth and the surrounding areas was Burrator Reservoir. In the 1980s - Roadford Dam and reservoir was built. I studied Civil Engineering from 1989 to 1991 at Plymouth Polytechnic/Poly South West/Uni of Plymouth - it went through a few changes in a short time! Roadford has a bellmouth spillway. Basically there is a plughole with a tube to the Tamar in the base of the reservoir with a concrete tower with slits at a set level. Water is heavy stuff and needs to be treated with care so the spill way attempts to dump as much energy as possible, whilst still shifting as much water as possible to down stream. I left Plymouth in 1996 but I do recall that in 1994 or 5 when there was a dry summer, that a spate was released from Roadford for the salmon. That spate was equivalent to three Burrator reservoirs capacity and it happened over a few days. Oh and Roadford has the river Wolf downstream 8)
- pengaru 3y ago
  What does spillways have to do with this thread of discussion? They're not generally capable of discharging water down to the original river's level, as they're intended for flood conditions where the reservoir behind the dam is too high. It's the opposite role of what we're talking about. i.e. not a single spillway here[0] is capable of achieving the equivalent of destroying the dam... [0] https://en.wikipedia.org/wiki/Spillway https://en.wikipedia.org/wiki/Spillway
- Arrath 3y ago
  > All dams have spillways. That's a broad statement that doesn't apply as generally as you suggest. For example, low-head dams generally don't have spillways though in essence function as a spillway. They're entirely passive structures with no mechanism for the owner/operator to change the water impoundment or flow rate.

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...
  - pengaru 3y ago
    > 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... Dude, this has to be one of my favorite and most relevant HN responses ever. Thanks for digging up and sharing that link! "Water Hammer Equation" "Gate Openings 6'x2' (25% of blast area)" this is bonkers.
    - Arrath 3y ago
      My pleasure! Enjoy!
  - phyzome 3y ago
    That sounds intense. :-) And wow, I'd never thought about the effects of a dam on waterway temperature. :-O
- phyzome 3y ago
  Hah, I guess that makes sense.

gerdesj 3y ago

All dams have spillways. There are various types and they have several functions: avoiding collapse due to say flooding; allowing controlled release - "spate" - handy for, say, helping salmon to run upstream - yes it is a bit counter-intuitive. The river Tamar largely delineates Devon and Cornwall in England, UK. It is mostly run off from Dartmoor and hits the sea at Plymouth (which is named after the river Plym). The Tamar is joined by the river Tavy (hence Tavistock, which is a few miles north of Plymouth). The Tamar and Plym run into Plymouth Sound which is the name for the area of sea just south of Plymouth, inside the breakwater. Right, its sodding complicated! The original dam and reservoir for Plymouth and the surrounding areas was Burrator Reservoir. In the 1980s - Roadford Dam and reservoir was built. I studied Civil Engineering from 1989 to 1991 at Plymouth Polytechnic/Poly South West/Uni of Plymouth - it went through a few changes in a short time! Roadford has a bellmouth spillway. Basically there is a plughole with a tube to the Tamar in the base of the reservoir with a concrete tower with slits at a set level. Water is heavy stuff and needs to be treated with care so the spill way attempts to dump as much energy as possible, whilst still shifting as much water as possible to down stream. I left Plymouth in 1996 but I do recall that in 1994 or 5 when there was a dry summer, that a spate was released from Roadford for the salmon. That spate was equivalent to three Burrator reservoirs capacity and it happened over a few days. Oh and Roadford has the river Wolf downstream 8)

pengaru 3y ago
What does spillways have to do with this thread of discussion? They're not generally capable of discharging water down to the original river's level, as they're intended for flood conditions where the reservoir behind the dam is too high. It's the opposite role of what we're talking about. i.e. not a single spillway here[0] is capable of achieving the equivalent of destroying the dam... [0] https://en.wikipedia.org/wiki/Spillway https://en.wikipedia.org/wiki/Spillway
Arrath 3y ago
> All dams have spillways. That's a broad statement that doesn't apply as generally as you suggest. For example, low-head dams generally don't have spillways though in essence function as a spillway. They're entirely passive structures with no mechanism for the owner/operator to change the water impoundment or flow rate.