Alas, like a lot of good ideas, the siphon doesn't work. I've just
mocked it up and experimented with it.

It would work in rapid flooding situation but, with slow water rise,
the cofferdam equalizes slowly with too much air left in the top of
the siphon to reverse the flow. If you could arrange the float switch
so that it didn't activate until the water was at the top of the
cofferdam, the siphon would start reliably. But, then you wouldn't
have accomplished anything.

This idea is only going to work with a good, reliable, low head, check
valve.

--

Roger Long