Benmore Dam spilling

[Okey Dokey]

It has been about 5 years since the lake levels have been high enough to require the spillways to be opened. I rode up to have a look yesterday and thought I would post a couple photos.

Benmore is the largest earth dam in NZ and one of the largest in the southern hemispere. Construction began in 1958 and it was commisioned in 1965. The water in the lake is equal to 1.5 times the water in Wellington Harbour!

As you can see, there are 4 spillways, but only 2 were operating. There was a man on site and he seemed to be controlling the spilling. This surprised me; I guess I assumed it would all be done remotely. There is a raised lip, or ridge, at the base of the spillway to prevent the water scouring out the river. That provides the nice splash at the base.

[Pussy]

Cool pics, OD!
Bet there was a rumble coming from that

[Okey Dokey]

Yes, it was thunderous! (is that a word?)

Really impressive. I can imagine what it would be like if all 4 were open.

[Jantar]

..... There was a man on site and he seemed to be controlling the spilling. This surprised me; I guess I assumed it would all be done remotely. ....

Spill is controlled remotely, but like all remote operations, local control must be tested from time to time. The guy operating the spill from on site will be doing it under instruction from moonbase in Twizel.

We have been spilling from both Clyde and Roxburgh stations since mid December. The problem isn't so much that there has been too much water, its just that the transmission system is so downgraded that the power can't be moved to where its needed. So Benmore, Tekapo, Hawea, Clyde, and Roxburgh are all spilling water, and gas fired power stations are going tits out in the North Island.

[Trumpess]

They are awesome to watch!

Many moons ago my dad worked for Ministry of Works in Twizel (helped electrify Ohau B) and he took us around the dams at the time they were spilled. The noise and vibrations scared the crappers out of me at the time, a kid of 12 lol. Something I will always remember.

[Okey Dokey]

Spill is controlled remotely, but like all remote operations, local control must be tested from time to time. The guy operating the spill from on site will be doing it under instruction from moonbase in Twizel.

Aaaah, thanks for that, Jantar. It's great how on kb there is always someone who knows the answers!

[Okey Dokey]

They are awesome to watch!
The noise and vibrations scared the crappers out of me at the time

Yes, it is really very big and loud and exciting and not something you get to experience everyday.

[Winston001]

We have been spilling from both Clyde and Roxburgh stations since mid December. The problem isn't so much that there has been too much water, its just that the transmission system is so downgraded that the power can't be moved to where its needed. So Benmore, Tekapo, Hawea, Clyde, and Roxburgh are all spilling water, and gas fired power stations are going tits out in the North Island.

Groan. This is precisely what I hoped wasn't happening - spilling water in the SI while the NI gas generators are spinning. What a bloody shambles. Not enough electricity in the winter, natural gas supplies running out, sod all major new generation.

Has any journalist picked up on this?

On another matter - Jantar - is the Roxburgh dam releasing sediment from the bottom gates? Seems like a good time to do it. Especially at night.

[Jantar]

No. We never release sediment from the bottom gates.

What you are reffering to is called "flushing" and it is only ever done via the main spillway gates. The lake level is lowered at times of very high flow and parts of the river with a high sediment build-up are turned back into a river. this mixes the fine sediment with the water and is then discharged via the spillway gates.

There isn't enough water in the river for that, but there's too much to generate and still be able to get the power to the consumer. Right now Clyde is on 33% generation and spilling 480 cumecs. Roxburgh is on 40% generation and spilling 440 cumecs.

[Winston001]

Most interesting. Fortunately I'm no engineer but the simple way of removing the sediment (which is a problem for Lake Roxburgh) would be to allow a great splodge of toothpasty mud to squeeze out the bottom of the dam. The fish might be a bit annoyed but hey - they'd get over it.

Otherwise it'll just fill the lake up.

[Jantar]

Most interesting. Fortunately I'm no engineer but the simple way of removing the sediment (which is a problem for Lake Roxburgh) would be to allow a great splodge of toothpasty mud to squeeze out the bottom of the dam. The fish might be a bit annoyed but hey - they'd get over it.

Otherwise it'll just fill the lake up.

That would probably work if there was a build up at the dam face, but the siltation is happening many kms upstream from the dam. Now with Clyde dam upstream of Roxburgh, there is very little extra silt reaching Roxburgh, but Clyde will need to start flushing in around 130 years time.

[FJRider]

Otherwise it'll just fill the lake up.

Thats exactly what happens...

Fortunately, such dams do have a life expectancy... Roxburgh I think (Jantar may correct me) has passed its original period of expectancy. Even with upgrades, its still a 50 year old dam.

And downstream of the dam, they don't want the silt either... It tends to block the outlet of the river and cause flooding...

[FJRider]

That would probably work if there was a build up at the dam face, but the siltation is happening many kms upstream from the dam. Now with Clyde dam upstream of Roxburgh, there is very little extra silt reaching Roxburgh, but Clyde will need to start flushing in around 130 years time.

Have you looked at the sandbars in the Kawarau arm lately... ??? I reckon in 50 years, water will flow over the Bannockburn bridge...Upstream of there its only safe for jet boats.

[Jantar]

Thats exactly what happens...

Fortunately, such dams do have a life expectancy... Roxburgh I think (Jantar may correct me) has passed its original period of expectancy. Even with upgrades, its still a 50 year old dam.

And downstream of the dam, they don't want the silt either... It tends to block the outlet of the river and cause flooding...

The plant has a design life of 30 - 40 years, which is why Roxburgh has recently gone through its 1st life refurbishment. ie all the stators have been replaced, rotor poles rewound and turbines reshaped. The dam does not have a fixed life expectancy. The concrete in that environment will last indefinitely.

Downstream of the dam some people do want the silt (spawing beds for fish) and others don't want it (orchidists don't want silt in their irrigation systems). But no, it doesn't block the river outlet. It is only carried during times of high flow, and deposited when flows are low.

[Jantar]

Have you looked at the sandbars in the Kawarau arm lately... ??? I reckon in 50 years, water will flow over the Bannockburn bridge...Upstream of there its only safe for jet boats.

Part of the water right when Clyde dam was built required the Crown to purchase all land upstream of the confluence of the Clutha and Kawarau rivers to height that the river is expected to rise to plus an allowance for floods. When Contact energy was formed this land was passed to Contact. The Kawerau from around the goldfields area to just below the Bannockburn arm will become a braided river just like the Waitaki. The tipping face of the silt was originally expected to reach the confluence 20 - 30 years after the dam started filling, then take another 100 - 150 years to reach the Basin imediately upstream of the Clyde Dam. Filling started 18 years ago, and the tipping face is now around 4 years from reaching the confluence. During those 18 years we have seen the 3 largest floods since records began, so those early engineers look to have got it right.