Engineer says let erosion take South Dunedin

Bones McCoy - I agree with your post.  It just seems sensible that if you block the pipe, then what is coming down it will no longer be available. 

What is even more interesting is what plans the Otago Regional Council are making to deal with the vast sedimentation that is occurring now, and will occur thousands of times more seriously when the Alpine Fault finally lets go.  The aggradation of the vast country behind Queenstown drained by the Shotover will pour countless tons of gravel and sedimentation down to Lake Dunstan.  I raised this issue with the ORC some years ago and got told that it was the responsibility of the power companies to deal with it.  Mind you, at the time they were concentrating on how to ignore their own reason to exist and figure out how they could get away with borrowing money to build the stadium.  Maybe their attention was diverted.

I lived in South Dunedin for a number of years until shifting to Christchurch 7 years ago, and have experienced first hand all of the quakes. I remember how sodden the land was in South Dunedin, and you only had to dig barely a foot before hitting water. The ground is very wet, and makes the buildings there dificult to heat, damp, and given most of the houses are weatherboard, subject to often substantial decay.

The land in some of the eastern suburbs in Chch was also quite wet, although I suspect it wasn't as bad as SD, but in a good shake I imagine there would be significant liquefaction, probably worse than what has been experienced up north. A sizeable quake in the South is inevitable, and really it is just a matter of time. If Dunedin is affected (although it is one of the least quake-prone cities in NZ as Cr Vandervis pointed out), I feel there is a serious risk of substantial damage in SD. I think a managed withdrawal just makes sense, and you may be doing the residents a favour - this coming from someone who used to be one.

Russandbev... while I'm far from an expert in this topic I believe you've hit at least one nail on the head, about this matter. You may recall that in the mid-1990s ECNZ - which later morphed into Contact Energy, at least so far as the Roxburgh and Clyde hydro power stations were concerned - initiated a natural sluicing/flushing regime to convey fines/sediment downstream, from behind the Roxburgh dam. That process was triggered when very high water flows (flooding events) made sluicing/flushing a sensible activity for ECNZ/Contact to undertake. And from what I recall that regime was successful in shifting huge quantities of silt back into the Clutha River, to a point. (Of course, as long as flooding episodes provided enough flow to allow ECNZ/Contact to flush).

Unless something similar can be enacted for Lake Dunstan, behind the Clyde Dam, I suspect the bulk of sediment settling onto that lake's bed around Cromwell will remain where it is, rather than migrating 'naturally' downstream to, eventually, provide sand to the beaches north of the Clutha's mouth.   I wouldn't know where to begin to determine how long it would take for sand that would have, pre-Roxburgh dam, been naturally conveyed to a Dunedin beach (and I suspect I am grossly over simplifying the topic).   But, the question you pose seems sensible, at least at a lay-level.

Put very simply, with the advent of the Clyde and Roxburgh dams, we (and by that, I mean the 'Royal we': aka New Zealand's society...) have placed a plug into what was the natural flow of sedimentation down the Clutha River. Where once that sedimentation was - surely - naturally and gradually conveyed down the Clutha to, eventually, exit at Balclutha before it began it's natural drift up the coastline, that "feed stock" no longer exists.

So, perhaps you are spot on: what is now being experienced in Dunedin could, in part, reflect that the sands that once washed onto our city's beaches have lost at least part of their replenishing source of supply.

Hype.O.Thermia has a valid point and there's also the consideration of rising sea levels (whether you believe in global warming or not) and tsunami threat.

Beyond all common sense and supporting research, however, politics will always rule the day.

Which expert has the figures on sand depletion over time I wonder?  I would have thought that a major contributory factor to loss of sand on our beaches would have been the building of Roxburgh dam which, from that time, prevented the natural carriage of fine silts and sand down the Clutha.  My understanding is that the lovely white sands of our coastal beaches and Blueskin Bay have their origins in the quartz sands of Skipper's creek and from there via the Shotover and Clutha rivers.  The northerly coastal "set" then conveys this sand up the coast.  It is easy to see the vast quanities of this sand and silt by merely looking at the amount of siltation behind both Roxburgh and Clyde dams.

About 25 percent of The Netherlands is below sea level and 50 percent is less than 1 metre above it and they seem to have it under contro.

What are the other councillors saying?

Land is not in such short supply that Dunedin needs to engage in a war against water.  I have known people who lived in parts of South Dunedin and St Kilda who talked about the cold and damp that comes from houses - nice houses, not run-down slums - being so close to ground water.  When you can dig a hole and even in the driest summer you hit damp one or two spade depths down it's wet and it's not going to be better in winter.  The amount of heating required to keep houses dry is much greater than where they are built on well-drained ground.  This is personal household expense.  Where it cannot be afforded the housing is cold, damp and unhealthy,  compounding the problem of shortage of money for the people who live there.  A managed retreat is the sensible way to go, sad though it will be to lose the attractive historic area with its streetscapes telling stories of lifestyles and transport use throughout the years the area was settled.

Yes, Councillors are quite right - relinquishing South Dunedin is not politically acceptable at the moment. But insurance companies will call the shots in the end by refusing to insure property at risk.

It's just a waste to spend public money in areas at risk from sea level rise or encroachment (since part of South Dunedin is already at or below sea level). Unfortunately for Dunedin this includes our extremely expensive sewage treatment system.

Politicians tend to think only three years ahead (their term of office). Nowhere near long enough for issues like this.