Tricky Business

Image-Tacoma Narrows Bridge1.gif

The poster child for resonance (well, not exactly), the Tacoma Narrows Bridge.  Even your physics teachers can pass on erroneous information!  See below for more…

In a conversation this week with a friend discussing the craziness of our world, they brought up something that shocked me a bit, and given that my reading on the topic of wild and crazy futures is broad, it surprised me even more.  The implications are pretty staggering, because it is one of those things that shows how out of control things can get when you start fiddling with things on a large enough scale.

The issue is at hand is the Bay of Fundy, and the possibility of extracting energy from the large variations in the tides there.   The Bay of Fundy lies on the East Coast of Canada, and tides there can typically range up to 15 meters, and have even gone higher than that.  A bit of research into why the tides are this way shows that they are based on tidal resonance. Resonance is what you use when you push someone on a swing (or kick with your feet) to get to large oscillations (motion).    In that conversation, my esteemed investigator into troubled futures mentioned that by tapping the Bay of Fundy, we might actually stop the resonance from happening, and make the entire project worthless.  In trying to validate that claim, however, another more troubling possibility came about.

A seemingly obscure paper, by McMillan and Lickley in the undergraduate SIAM journal (Society for Industrial and Applied Mathematics), in its inaugural issue a few years ago (2008) points out a serious issue in trying to tap the power of the Bay of Fundy. The upshot of trying to tap the tides is that it might generate a lot of power, but at the same time, it might also start to radically affect the tides in the surrounding region.  As far away as Boston, tides could be raised by at least 15 cm, and even if only a fraction of the power of the Bay of Fundy was realized (2.5 to 6.9 GW or so; note that an average nuclear plant puts out 800 MW (0.8 GW), and the largest tidal renewable energy project in Scotland is scheduled for 400 MW (0.4 GW) ). Now, this may not seem a great deal, but a 15 cm rise in tides is huge.  Given that ocean levels are rising at 3.2 mm/year (0.32 cm/year), this would be like having a time machine catapult us 45+ years into the future as soon as this system was activated.   According to the paper, if a barrier for extracting power was put across the Minas Passage, that change in tidal height in Boston (and other places) could be as large as 45 cm.

The original paper is in a journal based on the work of undergraduates, but the critical elements (discussed below) are still being promulgated. Also, just because the work was done by undergrads, doesn’t necessarily diminish the validity of the work.  The work is still reviewed by practitioners in the field, and so these results shouldn’t be dismissed out of hand.

The classic case of resonance gone wild that comes to mind for most technically minded folks, shown in many civil and mechanical engineering classes around the world, is the Tacoma Bridge failure, which was captured in film (and never fails to bring many an engineer to tears on seeing it).   Alas, it wasn’t resonance (vortex shedding wasn’t the culprit), but “the failure of the bridge was related to a wind-driven amplification of the torsional oscillation.”  This is an important technical detail for those of us who care about such things, but the end result was that amplification of a natural phenomenon, via a man made structure, caused a catastrophe.

Now, things have come a long way since the 1930s when the Tacoma Narrows Bridge was built, and as a result, modern bridges go through a far more rigorous design process.  But the end issue still remains – sometimes, we don’t catch all the bugs in a physical system, and sometimes, these errors and influences are not linear, but can cause non-linear and far reaching effects.

This interesting research topic has far ranging implications, both with respect to renewable energy, and with respect to other large scale engineering projects that might be used to ameliorate the injection of CO2 into the atmosphere.   We have already seen what fracking can do to create earthquakes; the Three Gorges Dam is another large scale project that is causing tremors, and may be another huge disaster in the making.

Who knows what sort of effects a large scale windfarm, solar power station, or other project may cause?   This isn’t to say, “stop all (large) renewable projects!”, but it does bring up an issue of unintended consequences that seem to be ignored in our world all too often.   Non-linearities and positive feedbacks do exist, and we ignore them at our peril.

Questions for this week:

  • The other elephant in the room regarding large projects is that of geoengineering.  Forget about clean energy projects, just trying to replace fossil fuels with large scale renewable energy sources; what happens when we try to cool the earth, or sequester large amounts of CO2 in the earth?  One wrong move could cause massive problems, and is a good reason for treading very carefully in this area.
  • If Canada gets mad at us, do you think they’d ever build a dam across the Minas Passage to flood Boston?   There is a concept called ‘asymmetric warfare’, where a smaller force goes against a much larger one.   Perhaps these non-linearity effects will be harnessed by some “bad actors” for not so good purposes.
  • What other environmental and energy projects with good intentions have caused massive problems (besides the obvious nuclear projects such as Three Mile Island, Chernobyl, Fukushima…)?








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