robgraham wrote:

We have a drainage problem locally which is causing a problem with the
septic tank in one particular property.- we're all on tanks.

The last thing you want in a septic tank is rain water.

The problem is created by the fact that an area which is nominally
200m x 500m (and possibly bigger) drains under a road in an 18" pipe.
I've found a website that shows that the flow capacity of this pipe at
5deg slope is around 200l/sec. Working backwards I've calculated that
this is equivalent over the area of less than 10mm/m^2 rain per hour.
Anything more than this it would seem is therefore likely to cause a
back-up - in simplistic terms !!

It depends where you are but I would have thought that 10mm (0.4 inches)
of rain in a single hour happens more than occasionally. To find out
local information you could track down a local weather station, there
are a lot of amateur stations about these days, or failing that try your
local TV weatherman. Our local weatherman, Paul Hudson, is quite
approachable, but it doesn't follow that all are.

FWIW I have just checked back through my records for the last few months
and we had quite a bit of rain in November. The largest 15 minute record
was 0.16 inches and the biggest total over an hour 0.43 inches which
included 0.14 inches in 15 minutes. However we had a cloudburst some
time ago and IIRC three quarters of an inch fell in under 10 minutes and
almost an inch in 15. The road turned into a raging river part of which
overflowed onto my neighbours drive washing away the gravel surface and
flooding his garage.