On 02/05/2021 18:03, alan_m wrote:
On 01/05/2021 12:24, nightjar wrote:

I like the way they clear the sewer under the river in Paris. It is
circular and, every so often, using water pressure, they send through
a large wooden ball, fitted with steel cleaning strips around the
outside.


Many sewers are built egg shaped, with the pointy bit at the bottom, so
when there is little sewerage the head of liquid keeps the flow from
stagnating.


The sewer tunnel under the Seine is like a giant U bend. It is always
full of liquid.

--
Colin Bignell