Rainfall intensity ?

[robgraham]

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

[Roger Chapman]

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

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.

[JimK]

In your calcs did you factor in the ability of the land (assuming it's not paved or otherwise impervious) to absorb rainfall or are you looking at worst case "days & days of rain" scenarios?

It's noticeable here that a cloud burst doesn;t necessarily mean a culvert swamping torrent, it seems to depend on what has been happening over the last week or so.

If the ground is already "moist" the capacity to absorb appears much greater..

Extended periods of dry weather don't seem to mean that lots of rain will be absorbed, quite the opposite, as our regular summer flash floods illustrate! Presumably when dried out a harder "crust" develops which the rain cannot permeate quick enough...

Cheers JimK

[Harry Bloomfield]

Roger Chapman explained on 03/03/2010 :

Apart from the odd obvious glitch in stored data, those are fairly similar to the peaks I have seen.

The peaks do not tend to cause the flooding, it is more a case of lots falling and over a long period to cause the floods. Also the ability of the ability of the ground to absorb the water. Already moist ground soaks up water better than perfectly dry ground, just like an already moist cloth absorbs water more easily than a dry one.

My garage floor has a tendency to suffer the slab flooding, which happens once maybe twice a year. I have tried to correlate data from my rain gauge to predict when it might be liable to have flooded, but there are so many variables it is almost impossible to work out. All I have been able to determine is that if the ground is already sodden and is then hit by a sudden really heavy down pour, it quite possibly will be wet next time I go in there. My occasional flood will be similar to your tank flood.

My solution so far has been to improve the drainage around the garage base as time permits. It used to flood much more frequently, but the fact that it still does rarely flood means I have a little more work to be done.

[The Natural Philosopher]

what the f*ck has rainfall to do with septic tanks? You dont put rain in them. Just shit and piss.

[Roger Chapman]

I took it to mean that the problem was keeping rain out of a septic tank in flood conditions.

[The Natural Philosopher]

Why would it ever get in?

[Roger Chapman]

leaky cover. Down an air vent. It is certainly a possibility.

[The Natural Philosopher]

Both easy and trivial to fix..

The only time I have had probs is when the water table locally rose so high the things wouldn't drain at all.

Then poor flow lead in one case to blocages, and, in te case of teh whole tank being under water, to a period when I used it as little as possible.

But that was the least of my problems. The water was up to the doorstep that time for a day or so.

[robgraham]

Yes but they do have to drain somewhere, and if the area is saturated to the extent that it has flooded then the tank has nowhere to drain. Try using some thinking power before you get abusive - it does tend to make your contribution a bit more respected.

Rob

[The Natural Philosopher]

well if it cant drain then the problem is elsewhere, in the local water table.

Which is simply a mater of clearing ditches and arranging general land drainage . nothing to do with septic tanks.