Outside the marginals

a commentary on the politics that followed the UK 2010 & 2015 elections

Water water everywhere

Do we get the environment we deserve? I fear we do. So many public utterances – initially by the public but repeated echo-like by our Lords and masters – seem to illustrate a wilful refusal to engage with science. (And that last word has probably lost me half those who read this blog – and the other half probably know what is coming and will agree anyway!)

So how do I make the accusation of a “wilful refusal” stand up?Let us do it by three examples.

Reporting that flood defences were breached

Throughout yesterday the BBC (and no doubt others) were reporting that flood defences were breached accompanied by pictures of widespread flooding that means widespread heartache and suffering for many thousands of people who have seen their homes flooded.

But no pictures of “breaches” in the flood defences – as we saw in New Orleans when the levees gave way.

They were not breached – they were overwhelmed. The difference is important.

If they were breached, that is structurally failed we have an engineering issue and either the designers or the constructors of the defences should be held to account. But they held.

They were overwhelmed, that is the floods were greater than those for which the defences were specified. This is not a design failure; the designers were asked to provide defences for specified water levels in the rivers. They did so, but the water levels where higher than specified.

This may therefore be a specifying or planning issue – which is essentially political. Building defences to:

  • accommodate water levels x centimetres higher
  • will definitely cost £y millions and
  • will defend property and avoid damage potentially costing £z millions

The politicians have to decide on the balance of cost versus the risk of such a flood ever occurring.

We the general public do not seem to be able to get our minds around this issue. “The defences were breached that is terrible, someone should be strung up” is not an adequate response. Understandable when you have had to leave your home in the middle of the night, but not an acceptable political demand in the cold light of day.

The 1 in 100 years flood

“We had floods 6 years ago and we were promised it was a 1 in 100 years flood and now it has happened again – we were misled”.

The idea that God (who else?) looks down on say Carlisle and says “well, I flooded you recently, so I had better lay off you for another 100 years” is not the way the world works. Climate and weather are not determinant – our forecasts work on probabilities.

But probabilities / risk / chance is something that is not generally understood and some people seem to delight in innumeracy – in a way that they would never delight in illiteracy.

The “1 in 100 years flood” means:

  • based on past experience (which does not necessarily predict the future)
  • our expectations of the future (no one has a reliable crystal ball)
  • we estimate that we may have a flood at this level once every 100 years.

If you are throwing dice, you would expect to “throw a 6” every 6 throws or so – but you would not be that surprised if you threw two consecutive sixes. You would not immediately accuse the dice maker of loading the dice.

Yet if we get two big floods in 6 years, we cry foul. It’s not as if predicting floods is as easy as predicting sixes when you throw dice. (You can count the faces on a die and observe that there are six and only one of the six is marked with a 6 – with flooding it is a bit more difficult.)

So it is quite possible to get four “100 year floods” 10 years apart. (Just as you could throw 4 sixes in a row.)

These sort of events could be used for teaching probabilities and helping people to learn to live in what is an uncertain world. They are dreadful events – but they are real world examples and probably will mean more to school children than artificial “probability problems”.

Uplands and dredging

On the today programme there was a “non dialogue” between a farmer and George Monbiot. The farmer was demanding more dredging to get flood water down to the sea as soon as possible. Monbiot was trying to suggest that allowing some of the water to soak into uplands and be slowly released into water courses would prevent the sort of walls of water hitting towns.

The catchment area of the rivers that flow through say Carlisle are huge – many square kilometres, but the cross-section of the channels under the bridges (the major choke points) are a few square metres.

Let’s create an example that can keep to round numbers so they can be followed:

Imagine a catchment area that is 10 kilometres by 10 kilometres (100 square kilometres) and that 20 cm of rain falls in a day. That is a volume of water 10,000m x 10,000m by 0.2m or 20 Million cubic metres of water (a bath tub uses about 1/6th cubic metre – so 160 Million bathtubs of water).

If that water has to flow, the same day, through a bridge arch that is 5m x 5m, the water has to flow through the bridge at a speed of about 9 metres per second if it is not to overflow. That speed is both frightening and damaging – Usain Bolt goes at about 10 metres per second – and a tree blocking the arch will cause almost instant overflow.

(9 metres of water per second through a hole 5m by 5m is 225 cubic metres per second, which is 810,000 cubic metres per hour or just under 19½ Million cubic metres per day.)

It would seem clear to me that this sort of flow has to be slowed to avoid towns being flooded and bridges and river banks being ripped away.

Dredging a channel by 2.5 metres (an average door is 2m high) so that instead of being 5 metres deep it is 7.5 metres deep will slow the flow by only a third.

But if the uplands could soak up the rainfall so that it took two days to flow down the rivers, you half the speed. If the uplands could hold up the water for longer, say 10 days, you cut the flow by a tenth.

Much of the uplands of areas like the Lake District has little soil cover so cannot soak up much rainfall. More vegetation means more soil – and because of the roots less erosion of that soil. More soil means more rainfall is soaked up and less goes directly into rivers that have been dredged to dump that water lower downstream.

The Lake District used to be heavily forested (including the high fells). OK there would be less upland sheep (and I like my roast lamb), but we would have more native woods (and who knows more wild boar?).

We need to think different and we need our politicians to think different. Heavy rainfall is becoming more frequent, building on lowland flood plains is also more prevalent, and there is a limit to how deep you can or want to dig out the rivers that flow through such areas. And yet we seem to be almost wilful in our refusal to look at any steps to delay the flow  of water off the uplands.

So do we get the environment we deserve?


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