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 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 !!

I've obviously got my google parameters wrong because I can't find
what rainfall rates are. Can anyone help please?

Thanks
Rob