Episode 2: The London Sewers

Map of the London Sewers
Image credit: MappingLondon.co.uk
Political cartoon about the stench coming from the Thames River
Cheeky-but-apt illustration of how bad the Thames smelled before the sewers were built
Photo Credit: https://wellcomecollection.org/works/c9b26px5?wellcomeImagesUrl=/indexplus/image/M0012507.html

Get ready for a gross one. Believe it or not, the concept of safely transporting and disposing of waste is relatively recent; prior to the innovations discussed in this episode, people would just sort of get it as far away as they could and hope for the best. Needless to say that large cities like London became olfactory affronts on a hot day… Thankfully a brave engineer dared to dream of a better-smelling future, and braved endless political feuds and construction headaches to make it happen!

Join us as we discuss bad smells, good engineers, stubborn leaders, cholera, and the thrilling world of sewage treatment!

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Additional Technical Background:

So I did do some math to validate Bazalgette’s sewer using the Manning Equation for uniform open channel flow. Although the sewer was an enclosed pipe and not open channel, this applies because the sewer is not designed to flow as pressured, which means it flows as more or less an open channel.

In metric: V = (1.00/n)(Rh2/3)(S1/2)

where n = roughness coefficient, Rh = hydraulic radius (cross-sectional area / wetted perimeter), and S = slope.

Roughness coefficient for brick & mortar is 0.015.

Looking at some photographs of construction of the sewer, I estimated the sewer to be at least 1.8m in diameter, which equals 0.9m in radius.

A = 3.14 * 0.9^2

A = 2.54 m^2

Wetted perimeter = 2*pi*r = 2*3.14*0.9 = 5.65m

Rh = 2.54 m^2 / 5.65m = 0.45m

Slope was as mentioned in the podcast, 2 ft / mile which equals to 0.00038 m/m

Put it all together.

V = (1.00/0.015)(0.452/3)( 0.000381/2)

V = 0.76 m/s

(Yes, I know I said 0.77 m/s in the podcast. This is just rounding discrepancy. In case you haven’t noticed so far, us civil engineers aren’t super concerned with decimal places and absolute values.)

Remember that this is compared to a rule of thumb that pipes should be designed to have minimum flow of 0.5 to 0.6 m/s, and this formula I used wasn’t developed until 1890, some 25 years after Bazalgette’s sewers were built!

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