Just for information, a concrete pipe (most 18 inch pipes these days are
concrete) will carry less water when it is full that it does when only
three-quarters full. This is due to the friction exerted on the full
circumference of water in the pipe in contact with the pipe wall itself.
At three-quarters full, less water is in contact with the pipe, and the
top of the water experiences virtually no friction from the air above
it. That is why flood waters rise so quickly, but take a lot longer to
drain away.

jim

An interesting point

Graham
Penzance


Yes, I was not aware of that aspect of "Coverack" type flooding
incidents. I've circulated that to the others in our local flood action
group.
There is so much of that sort of stuff that emerges that is not on EA
sites, that is not immediately obvious.
Like spend many thousands on marine flood measures and you compound the
problem/severity of landward intense rainwater flooding from
surrounding high ground, bad storm-drain sea outlet flap-valve design,
cross-coupling of storm-drains and foul-water systems, only a certain
amount of water depth you can keep out of standard brick-built houses
before structural failure etc etc