chris wise engineer

Wet behind the ears: how to stop the flooding

Mevagissey flooding: image by Karen Hatt

I have yet to see anyone explain how the recent floods can be tackled technically. The Governemnt’s Eric Pickles laments, “We perhaps relied too much on the Environment Agency’s advice.…we thought we were dealing with experts.”  But how expert do you actually need to be to deal with a flood problem? To find out, we gave our 1st year civil engineering students at UCL the task of solving a typical river flood. We took the Cornish seaside village of Mevagissey as our test case. The village is regularly under a foot of water. The landlord of the Ship Inn quit his pub last year after being flooded out 11 times in just over 2 months. Other river-side communities are similar, so find out how to fix Mevagissey, and you have a pretty good insight into fixing the rest.

But could the students fix Mevagissey, without any prior experience? It turns out they could, in an afternoon, and pretty much on the back of an envelope. Our fledgling flood experts found what went wrong with their forefathers’ cunning plan to build Mevagissey over a river, and how to fix it. It goes like this:

  1. Like other flood areas, Mevagissey’s river picks up rainfall from the surrounding “catchment”. In this case it’s a modest 5.2 km2 of farms, woods,roads and houses, and drains it into the sea. (The Thames picks up rainfall from 16,000km2)
  2. The land slopes at about 1:30, so the water flows down to the sea at about 1m/s, or walking pace.
  3. At that speed, it takes 3,600 seconds for rain falling on the furthest high ground 3.6km away to flow down to the sea.
  4. So a storm lasting 1 hour is enough to fill the whole river system with water.
  5. In that part of Cornwall, weather patterns mean such a storm delivers 21mm of rain in one hour, every five years.
  6. 4.5 of the 5.2km2 is rural, so about a third of the water runs off into the river and the rest is absorbed. The rest of the storm falls on hard urban surfaces and so 90% of it runs off.
  7. Multiplying that out, the students found that the Mevagissey river collected 45,000 m3 of rain during the storm.
  8. The river itself could store some of the rain for a while, 16,000m3, behaving as a giant long pond, leaving 29,000m3 to end up in the town in one hour, flowing down at an average rate of 8m3/s. Once there, unfortunately all the water had to do was squeeze through a 6ft diameter pipe into the harbour.
  9. Because the maximum flow is 2 or 3 times the average, we’d need the 6ft pipe to carry 20m3/s.   That’s 20 tonnes of water every second, travelling at 20mph, quite a lot even for a small and very common situation like Mevagissey.
  10. Using a simple formula called “Mannings equation”, and a calculator, the students found that the maximum capacity of the pipe was 10m3/s, so it could carry only half what was coming down the river. That leaves the rest to flow through the streets, causing the flood.  A situation made worse if rubbish screens upstream aren’t maintained.
  11. With one last simple sum, using Manning’s equation backwards, the 19 year old students estimated a flood depth of about 250mm in the streets, a pretty close match to reality.

From those numbers, it is a small step to design the possible solutions and stop the flooding:

  1. Double up the pipe.
  2. Provide another river channel nearby, the same size as the existing one.
  3. Or store half of the water for an hour every 5 years, 15,000m3 , which could be done by flooding a field (in other places a flood plain) the size of a football pitch to a depth of a couple of metres or so.
  4. And maintain the rubbish screens so they don’t block.

These techniques can provide protection against even the storm-of-the-century. And the methodology works, scaled up, even for big problems like Somers

Mevagissey flooding: image by Karen Hatt

et and the Thames.

Most flooding is avoidable, and it can be fixed. But Mr Pickles has learned nothing from history, because many of the problems have their origins in poorly balanced planning decisions over many centuries. He risks repeating them if he really wants to discount technological advice which, as I have shown, it really isn’t that complicated even for a politician to grasp.

Yes we badly need the land; no we can’t afford to protect all of it. Yet, by demoralising the “experts”, engineers and all, the likes of Mr Pickles risks alienating those who can help us most. Hisculpable political foolishness was wonderfully satirised by Abraham Lincoln who demolished a political dissenter:

“He is the man who murdered both his parents… then pleaded for mercy on the grounds that he was an orphan.”

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