Working plans allow for some 4 million m3 to build up in the reservoir without affecting the quantity of water available for the irrigation scheme to be viable - a built-in 20-year shingle accretion "buffer".
Of course the shingle won't stop fetching up against the dam just because 20 years is all they've allowed for. There'll be 7 million m3 built up after 35 years.
Thirty-five years is a significant timeframe: it's the length of the initial consents, and the contracts water-users sign up for. Until then, the consultants insist, the amount of water available and the financial viability of the scheme still stack up.
But after that, shingle will have to be removed or the scheme degrades - and they have no idea how to ameliorate this.
Because - and this is not a joke - HBRC/HBRIC are not required to begin to devise a "sediment management plan" until the dam is already built and the reservoir is almost full.
So they simply haven't bothered to project what it will cost to stop shingle filling up the lake until it eventually becomes a 100 million tonne rockpile.
Their public response to this? It won't be a problem, because shingle is an economic resource. Bull.
A chap who has spent his life doing exactly this work - removing shingle from behind dams - tells me the minimum cost for dredging (in relatively deep water, at the dam face) is $50/m3, up to around $100.
Trucking it off-site is at least $20/m3, and because it will need considerable rinsing and grading, add another $20 for that. Total, minimum, $90 per metre.
You can buy clean shingle for $20/m3. So that's a loss of $70 each metre, times the annual accretion equals a $14 million loss per year. Just to tread water.
Put that against the $6 million allowed as an ongoing annual total scheme operating cost, and you can see they haven't begun to add this in.
Bottom line, arguably the scheme is only viable for that first 35-year period. Anyone claiming it will have an economic life thereafter is at best ill-informed - at worst, deliberately disingenuous.
Yet all the economic modelling has been done on the basis of an assumed life for the scheme of 70 years. That does not compute.
The second reason, which I'm out of space to detail, is the cost of decommissioning the dam when it's no longer viable. That's anywhere from 50 per cent to 150 per cent of the build cost (which is $330+ million).
You do the sums. Because they haven't; that's also not part of their "economic modelling".
Who will pay these "extras"? Bet it won't be the farmers being subsidised to use this "boom" water.
