Electric future

[Snoopy]

Siltation is an issue in Lake Roxburgh, and yes, it did happen faster than expected. However since 1994 there has been a flushing regime during large floods that keeps that silt moving downstream. It is effective in that the volume of silt in Lake Roxburgh has been reduced over the past 25 years.

When Clyde was built that also had the effect of reducing the amount of silt entering Lake Roxburgh. It was expected that it would take 150 years from dam completion before any action would be needed. That time frame has now been reduced to 125 years as there is less sedimentation transported down to Roxburgh than what was expected

I am not following you on the effect of Clyde on Roxburgh Jantar. You say that less silt is now coming down to Roxburgh and so the time frame to take any action against silting has been reduced from 150 years to 125 years? I would have thought that with less silt coming down the river, then that would mean the 'silting up requires remedial action' time would increase, not decrease?

However if the first paragraph I quoted from you is correct, then there is no silting problem at all because you tell us that 'flushing events' have reduced the silt in Lake Roxburgh. So if sediment in Roxburgh can be made to go down over 25 years, then there is no silting problem at Roxburgh at all, is there?

Whether the silting up time frame was 125 years or 150 years though, that is long enough to make sure the engineers that designed and commissioned the dam would be long since dead! Even a junior engineer starting at Clyde today would be retired by the time the 125 year Roxburgh countdown was up. So with both Roxburgh and Clyde running on the short term corporate profit model, I imagine no-one at Contact worries much about long term silting up?

SNOOPY

[Jantar]

I am not following you on the effect of Clyde on Roxburgh Jantar. You say that less silt is now coming down to Roxburgh and so the time frame to take any action against silting has been reduced from 150 years to 125 years? I would have thought that with less silt coming down the river, then that would mean the 'silting up requires remedial action' time would increase, not decrease?

However if the first paragraph I quoted from you is correct, then there is no silting problem at all because you tell us that 'flushing events' have reduced the silt in Lake Roxburgh. So if sediment in Roxburgh can be made to go down over 25 years, then there is no silting problem at Roxburgh at all, is there?

Whether the silting up time frame was 125 years or 150 years though, that is long enough to make sure the engineers that designed and commissioned the dam would be long since dead! Even a junior engineer starting at Clyde today would be retired by the time the 125 year Roxburgh countdown was up. So with both Roxburgh and Clyde running on the short term corporate profit model, I imagine no-one at Contact worries much about long term silting up?

SNOOPY

The major source of siltation is the Shotover river, and it was always known that the Clyde dam would halt this source. Using data available in 1979 when the Clyde dam was designed it was thought at that time that it would take 150 years before the tipping front of the silt layer would reach the base of the Clyde dam, but that very fine silt suspended in the water would continue to pass through Clyde and on to Roxburgh. With experience, it is now known that not as much suspended silt is passing through as expected and hence the rate of siltation of Clyde is slightly faster than expected.

The long term effects are not just a concern for the long term operation of the two dams, but are also part of the resource consent conditions. Flushing is working at Roxburgh, just as it does at many other large dam sites around the world. When the silt tipping face reaches Clyde, then a flushing procedure will be refined for that dam as well.

[Snoopy]

The major source of siltation is the Shotover river, and it was always known that the Clyde dam would halt this source. Using data available in 1979 when the Clyde dam was designed it was thought at that time that it would take 150 years before the tipping front of the silt layer would reach the base of the Clyde dam, but that very fine silt suspended in the water would continue to pass through Clyde and on to Roxburgh. With experience, it is now known that not as much suspended silt is passing through as expected and hence the rate of siltation of Clyde is slightly faster than expected.

OK, thank you for qualifying that. I thought that in the two paragraphs below, you were talking entirely about Roxburgh. But now I see that when you were talking about the silting time going down from 150 years to 125 years, you were actually talking about Clyde. It makes sense now.

Siltation is an issue in Lake Roxburgh, and yes, it did happen faster than expected. However since 1994 there has been a flushing regime during large floods that keeps that silt moving downstream. It is effective in that the volume of silt in Lake Roxburgh has been reduced over the past 25 years.

When Clyde was built that also had the effect of reducing the amount of silt entering Lake Roxburgh. It was expected that it would take 150 years from dam completion before any action would be needed. That time frame has now been reduced to 125 years as there is less sedimentation transported down to Roxburgh than what was expected

The long term effects are not just a concern for the long term operation of the two dams, but are also part of the resource consent conditions. Flushing is working at Roxburgh, just as it does at many other large dam sites around the world. When the silt tipping face reaches Clyde, then a flushing procedure will be refined for that dam as well.

In my ignorance I hadn't thought much about silting before. Partly because I have never seen the issue raised in any annual reports or presentations. And also, in the case of Contact, that I had assumed that each of the Clyde and Roxburgh dams had weeks or months of water stored in them. Not that they were operating on storage values measured in days and hours. The point being that if the norm is for a lake to hold weeks and months of water supply, then it is difficult to imagine that the turbulence of soil laced water, replenishing that reservoir, would have a major effect on water and suspended soil in the entire lake. Now I know that both Clyde and Roxburgh are 'run of the river' dams, the silting issue makes more sense.

I also had the impression that any silt on the bottom of Lake Dunstan and Lake Roxburgh, if it was an issue, would be more or less be distributed evenly. But from what you are saying Jantar, the silt builds up at the back and gradually creeps forward to the dam face. You imply that this has already happened at Roxburgh, and that the effects of that are already being well controlled by flushing. But, of course, with Lake Roxburgh being "filled from the back" and reducing the lakes reservoir capacity, eventually that may make the flooding risk upstream at Alexandra untenable?

Did you read JBMurc's link on the future of the Clutha?

http://mightyclutha.blogspot.com/201...burgh-dam.html

That paints a very different future, outlining what should happen when, not if, the Roxburgh Dam is decommissioned and deconstructed.

"The reservoir has highly unfavourable impoundment geomorphology, having both a river confluence and a severe natural constriction at its head. Gold-miners knew this narrow section, some 675 metres below Alexandra, as the “Gates of the Gorge.” "

I don't understand the context of the above quote from the Clutha blogspot article. It sounds to me as though the 'Gates of the Gorge' area is an almost ideal natural flushing feature - not a negative.

If silting is really a potential terminal risk to Roxburgh, it strikes me that the best solution would be to construct a temporary by pass for the dam and simply dig the seventy odd years of silt accumulation out of the bottom of Lake Roxburgh. Then the Roxburgh dam would be 'fit for propose' just as it was in 1956.

Contrary to your claim of flushing reducing 'silt build up' at Roxburgh, the blogspot article disagrees:
"bedload re-distribution tends to decrease with each subsequent flushing cycle, and although some sediment is moved into deeper parts of the reservoir nearer the dam, very little of this achieves sufficient suspension to be washed out into the lower river. Flushing is simply a way of “buying time.” "

SNOOPY

[Jantar]

…..

Did you read JBMurc's link on the future of the Clutha?

http://mightyclutha.blogspot.com/201...burgh-dam.html ......

SNOOPY

Yes, that website was first produced during the Clutha river re-consenting. It was taken down, then re-published. However it is absolutely full of errors and incorrect claims. I wont try and detail them here as they involved many weeks of testimony in the consent hearings and environment court.

Silting is not a terminal risk to Roxburgh. It could have been if flushing wasn't carried out, but now that flushing is being done, siltation is controlled. And yes, the Narrows (or Gates of the Gorge) is where flushing has its most effect. It is also the siltation above this point that contributed to the flooding of Alexandra in 1995 and 1999.

The claim of bedload redistribution actually comes from the consent hearings, and was a way of buying time until the heavier bedload reached the dam face. That happened in 2017 when the process of using the low level sluices to move the heavier sediment downs stream was instated. The low level sluices do not require the material be in suspension.

As for decommissioning and removing Roxburgh dam, that would be an environmental disaster, if it could even be done.

The largest dam removal project in history is the Elwha ecosystem project in USA. Both the Elwha and Glines Canyon dams are very small compared to Roxburgh, and were not huge gravity dams.