Tuesday, November 16, 2010

Playing with numbers on the New River

When Rick and I were young engineering graduates, we worked together in a research department.  Well, when I say worked I mean played really.  Time pressures were few and we indulged ourselves in fascinating computations such as investigating the claim that the entire human population of the earth could stand together on the Isle of Wight.  Maps would be obtained and areas calculated, average sizes of people estimated and solutions to difficulties overcome (such as dealing with all the obese people and pregnant women by standing them along the coastline facing out so as to save floorspace).  The answer to the problem is of course "No" because population growth is so fast that the Isle of Wight ferry couldn't keep up with the demand.

So you can imagine my delight when I came upon a book "London's New River" by Robert Ward who seems to have the same ridiculous penchant for investigating and reinterpreting obscure statistics.

I walked along part of the new river here,

 quite near the source, when we were moored at Stanstead Abbots recently.  It is a 42 mile aqueduct, built in the early 1600s to carry drinking water from springs near Ware all the way to Sadlers Wells in Islington.

In Mr Ward's rather wonderful book he includes loads of detailed information, but better than that ( to me anyway) he likes to play with the numbers to give them more meaning.  E.g. the difference in height over the 42 miles between the beginning and the end is a mere 17 and a half feet.  He works that out as  a tiny 5 inches a mile or a gradient of 1 in 12500.  Just enough to provide a steady flow. That's no mean feat of surveying and digging for those days!

The cost of digging the cut is worth translating by doing a few sums because it shows how much more expensively we do things today, even after comparing historical money values.  The river, which is 10 feet wide and four feet deep, cost something over £10,000 to dig, ( Mr Ward has it to the nearest shilling somewhere but I forgot to note it down)  and it comes out as as 2s 10d a yard.  I found a web site giving a conversion of what that would be at today's labour cost and it comes out as £286 a yard, still amazingly cheap by our standards.  No elf and safety then to add to the costs.

Quite a lot was spent on the construction or road and foot bridges and our author calculates this to be at a rate of £2 per bridge or £4000 in today's money.   The nice little hump back bridge rebuilt over the Wendover Arm a few years back cost £233,000.

128 men did the digging and achieved a remarkable 90 yards a day over a six day week or at my calculation 83 cubic feet a day per man.  How long do today's restoration groups take to dig 90 yards of canal?

When the water arrived in London, the demand exceeded the supply.  It was distributed to customers along pipes made from hollowed out elm trunks.  Stopcock men would do a weekly round in which they would let water through to particular customers (landowners, street landlords, workshops) for an hour or two, two or three times a week.  I suppose the customers would then have to store what they could to eke out the time until the next dose.

The new river is still all there today, neat and well kept and it still provides water to London.  The original springs remain but they are supplemented with water stolen from the river Lea via pumping houses such as this one I walked past.

If you are ever up at the top of the Lea, it's worth a short walk to see the New River.  The easiest place is at Stanstead Abbots 100yards from the Railway Station.  I believe it has attractive stretches around Enfield too and in Islington you can see the New River Head where it all  gets integrated with the London water system.


Halfie said...

Fascinating, Neil. I'm thinking about the gradient thing: presumably if the gradient was slightly steeper, then the water would want to flow faster. Would this lead to a drop in level at the upper end, and an overflowing at the bottom end? Conversely, if the gradient was too flat, then the water wouldn't want to flow at all. They had to get it spot on! Or was there a little bit of trial and error involved?

Neil Corbett said...

Yes, I suppose they had to be concerned about having it too steep, as it is an open channel. They had to arrive at Islington at the right level. Had it been a pipeline they could have had some sort of water tower or raised reservoir at the southern end.

Halfie said...

And very good joints!