On 4 Aug 2019 19:04:59 GMT, Tim+ wrote:

Why not syphons? I mean, pumps use power, quite a lot of it and are
relatively expensive.

The Fire and Rescue service high volume pumps are part of the
national reslience system. Seems there are 51 dotted about the
country to be called upon locally or nationally as required. Each one
is capable of delivering 7,000 litres per minute (enough to meet the
demand of up to three fire engines all running flat out) over several
km of hose. They are floating "sump pumps", the small red boxes in
the water in some of the pictures. Seen reported that 16 of these are
in use. The same pictures also reveal larger yellow pumps, presumably
commercial pumps of even higher capacity.

The coverage of these "technical" details is sadly lacking. Most
people won't have any idea of the size of pipes and speed of flow
that shifting that volume of water requires.

For a syphon, you just need a suitably sized bit of pipe.

And some means of priming it and some means of keeping the inlet
clear enough of the bottom to stop it sucking that up and some means
of shutting it off without needing to reprime. Shutting off if the
outflow starts to cause damage at the discharge point of whatever is
being used to carry the flow starts to flood further down stream due
to local rain.

--
Cheers
Dave.