Issues with a lake for Christchurch

May 15th, 2012 at 2:00 pm by David Farrar

I blogged how Sir had proposed a lake in the Chch CBD, with businesses being allocated a shorefront location. At the time I said I liked the proposal, but wasn’t sure it was practical.

A reader e-mailed his brother who is a geological engineer. He commented:

In an area that is prone to liquefaction the last thing you want is have more ponded water in the form of a lake.  Lake edges are typically worst affected by lateral spreading induced by liquefaction, often 10s to 100s m back from the ponded water. To create an artificial lake in the Chch CBD you would have to dig a large hole in the ground to hold the water.  This allows the surrounding lake shore and adjacent land to move (expand) under strong earthquake shaking – to spread laterally into the lake. But If the ground is still in place you get a lot of cracking and water and silt ejection, but not much ground deformation.  Its the ground spreading, often accompanied by subsidence, that is most damaging to buildings.  The best way to stop this from happening is to lower the general ground water table, and create a ‘non-building buffer zone’ (of land) along the banks of rivers, canals, and around the shores of other bodies of water bodies  – not dig more holes in the ground and fill them with water !!

So sadly not practical.

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12 Responses to “Issues with a lake for Christchurch”

  1. kowtow (8,175 comments) says:

    How practical is it tp have a country straddling two tectonic plates?

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  2. mavxp (496 comments) says:

    I said as much in my response to your earlier thread.

    There is the additional problem of cost of digging that much earth out and putting it somewhere (Christchurch residents wont thank you for all the dust from the trucks moving it all along city streets). There will be some surprises in the form of contaminants (chemicals, diesel spills) as well in the old industrial sites that will need to be cleaned up. The whole idea is simply stupid.

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  3. xy (181 comments) says:

    As I said last time, it’s just fantasy. Half of the CBD is still standing – tearing down all those perfectly good buildings would be idiotic.

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  4. Alan Wilkinson (1,867 comments) says:

    The CBD already has the bank subsidence problem with existing Avon and tributary streams winding through it with unbraced banks. Could not the problem be addressed by building up the banks above the existing ground level rather than digging a hole?

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  5. Viking2 (11,367 comments) says:

    This morning their was an article about the research being done by the fellow that told us about the CHCH quakes.( haven’t been able to find it).
    He has dug up his own backyard and found that there has been previous liquifactions. They are looking to determine the quakes that caused them and where those quakes were centered.
    Actually, it’s rocks in the head stuff to rebuild there. Turn the place into a park

    The local land agents tell me that th CHCH money is moving to the BOP. Running out of good properties to sell.
    It will be years before the CBD is worth visiting. Companies have signed longterm leases elsewhere.

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  6. krazykiwi (9,189 comments) says:

    How practical is it tp have a country straddling two tectonic plates?

    Noted. I’ve called the movers. We’ll be gone by lunchtime :)

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  7. Paul Marsden (991 comments) says:

    Perhaps Bob was thinking an extra large Para-Pool…

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  8. Auberon (873 comments) says:

    There’s that, and the fact it was a really pants idea.

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  9. Shunda barunda (2,977 comments) says:

    This is simply not true.

    It is possible to build safely around man made lakes BECAUSE IT IS MAN MADE.

    Half the problem in Chistchurch was that they filled in natural lakes and channels and built on top of them, digging these areas out again is hardly going to make the situation worse than throwing up new buildings on the same sites.
    In effect, if this guy is right then the entire CBD is not safe to build on and should be abandoned.

    It won’t change the water table either because the water table in that area is already very high and the city is very low!

    This guy simply doesn’t like lakes! :)

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  10. wreck1080 (3,866 comments) says:

    oh yes, but, do we trust experts anymore? They get stuff wrong all the time.

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  11. Paulus (2,598 comments) says:

    After yesterday’s quake it should be recognised that YOU CANNOT BUILD IN A POTENTIAL FLOOD PLAIN, which is what liquifaction is.

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  12. flipper (3,948 comments) says:

    Folks need to think about the implications of what was set out in DPF’s post.

    There are experts and experts. Some venture into areas where they are unqualified. Some are simply academic. Some actually have to put their names on the line. Some are recognised as international authorities, and so on….

    It would be most surprising if the scientific data (and the general conclusions as set out in the post) being compiled over the ChCh/Cant quakes does not foreshadow some really difficult rebuild decisions.

    The banks, insurers, re-insurers and developers will be carefully weighing the data and conclusions from ChCh. That data may be throwing a very large spanner in the works!

    Getting Gerry B’s 100 day task force to adopt the ChCh council draft vision may be difficult without Governmernt underwriting. But I doubt there is any appetite for that outside of ChCh.

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