National Single Points of Failure

28/08/2011 — Leave a comment

The Mississippi River’s Old River Control Structure, a National Single Point of Failure?

Given the recent events in Fukushima and our subsequent western cultural obsession with the radiological consequences, perhaps it’s appropriate to reflect on other non-nuclear vulnerabilities.

As a case in point what about the Old River Control Structure erected by those busy chaps the US Army Corp of Engineers to control the path of the Mississippi to the sea? Yes, well as it turns out maybe trapping the Mississippi wasn’t really such a good idea…

As the name implies the structure diverts the Mississippi from where it wants to go, down the steeper Atchafalaya river, back to it’s traditional channel. This desire to go ambling off in another direction is not unusual for a river such as the Mississippi and it’s been meandering across the basin for millennia.

The control structure was constructed in the 1950s to redirect the Mississippi back into it’s original channel and has been tested severely by flood twice; once in 1973 in which the structure was significantly damaged (but held with emergency repairs) and again in May of 2011 which required opening of the Morganza spill way to take pressure of the control structure and the New Orleans levee’s.

Of course were the control structure to fail the Mississippi would do what it wants to do and carve a new channel to the sea. And once it it did so it’s also difficult to see how you could turn the river back, short of a trillion dollar multi decade 5 gorges style project.

The problem from a design perspective is that as a barrier defence with a multi billion dollar price tag you simply can’t provide system level redundancy. So the Corp of Engineers falls back on design margin and building in ‘soft’ failure modes, e.g. spill ways to take pressure of the control structure.

However the structure is only really tested to it’s design limits under very rare circumstances, so no one really knows whether the system will perform adequately under maximum service loads. Likewise the Control structure sits within a larger system of systems of levee’s and weir’s so a major failure of a levee could also spell disaster.

But if the structure were to fail the consequences would be incalculable, in the precise meaning of that term, as the mouth of the Mississippi has become the oil and petrochemical heartland of America. Then of course there’s the loss of fresh water supply for Baton Rouge and New Orleans and the death by thirst of both those cities would make Hurricane Katrina look like a sunday school picnic.

So how likely is an overtopping event? Well the bad news is that with climate change and glacial retreat more winter snow will end up as spring melt and then as spring flood water increasing flood peaks. Of course as we’ve seen in 2011 arctic ice pack melt encourages colder winters in the short term with greater snow fall, which further increases the likelihood of larger floods. All this of course means that a 1 in 500 year flood for which the structure is designed would become much more probable as would a larger and overtopping 1 in 1000 year flood.

And of course there’s aways the nightmarish prospect of a terrorist act to destroy or weaken the control structure or a critical weir or levee, in practice the system is so large as to be un-defendable, and as the Barnes Wallis bouncing bomb raids on the Ruhr proved, dams are uniquely vulnerable.

In the mean time the relentless pressure of the Mississippi’s fluvial processes build the Mississippi’s bed higher and higher making the risk of a shift more likely every year.

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