Rainwater tank overflow

[Spencer74]

I am putting in a 9000 litre water tank to catch all the rainwater possible to assist with watering my impending new garden.

The tank will be located in the lowest point of my block and I am struggling to know what to do for the overflow. My only real option for the is to lay pipe to join back into the existing stormwater pipe that empties into the street.

My question relates to the ability for the overflow water to make it that far. Please see the attached diagram - there is a fair distance that includes an incline towards the end.

I was assuming that as long as the final exit point was below the height of the overflow outlet at the top of the tank wall then I would be right?

Any advice out there? Any way to test it short of laying the pipe and running a hose through it?

[Planned LScape]

Can it be connected to another existing stormwater pipe coming off gutters from the house?

[bricks]

Where in adelaide are you?,
How much water will be trapped in the system- Ie 1m of rise, 2m of rise???

All you need to do is upsize the pipe a bit prolly to at least 100mm if not 150.
I,d put a sump type arrangement in at the lowest point to facilitate cleaning.
Put an IO point at the start and end,
put a secondary overflow at the tank just above the water level

What you want to do is not that uncommon, the water won't flow but will displace out of the other end. To test it I'd fill it up with water and see if it leaks.

Once you have it installed and it is operating, you need to keep it clean, blockages wont flow out the other end, install a leaf guard on the inlet to your tank too.

[Border boy]

Hi Spencer,
as your tank is at the lowest point in your block, I'm assuming that you are also installing a pump to direct water to your garden. Simple solution, keep your new garden well watered - make sure that your tank never overflows - then you don't have to worry about it!! You could also consider a length of 50mm corrugated hose hooked up to your overflow to direct to any garden that needs a bit of H2O - I I guess that means gravity needs to be involved so maybe not. I'm not sure of the regs in your area but I think that installing a tank means that you have to take a hands-on approach & manage the resource that you are creating in a creative way. I've just put a 10000 litre tank in our yard & just hooked up the plumbing to the shed that is going to feed it - 108 sq metre roof area. I don't have your probs with fall as the tank is at the highest point on our block. My issue with the overflow is deciding if it's worth doing this http://www.rainharvesting.com.au/auto_tank_vacuum_system_1.asp & how to fit it - any ideas??
Good luck with your rain harvesting!!
Cheers!!

[Spencer74]

Thanks Planned LScape - I am directing all existing stormwater (5 drainpipes!) to the tank - it makes sense to do it now cos the entire garden has been ripped up so now or never. I am redirecting an existing underground stormwater pipe to the tank and planning to hook the tank overflow up to the using the remainder of that pipe (which goes to street) So, I am actually using the existing stormwater drain for the overflow. Cheers.

[Spencer74]

Thanksa Bricks. The rise would be less than 1 metre and I was going to use standard 90mm pipe. Good call re sump point and/or IO (inspection outlet right?) Don't care if it doesn't flow - just don't the tank overflowing on some cold winters night when my neighbour will come knocking on my door and complaining. His living area is just over the fence adjoining tank which is why I want to get this right. Cheers mate. P S I am in Torrens Park.

[Spencer74]

Thansk Border boy. Correct re the pump. I did consider having no overflow but I understand it is a legal requirement (not comething I always worry about). Main reason is that Murphys Law says that the next time I go away for a week it will rain steadily and the tank will overflow into my next door neighburs patio area! Knowing him he woudl find out where I was and call me demanding my immediate return!

[Border boy]

Yeah, damn Murphy & the horse he rode in on!!

[Jacksin]

Having a sump at the lower end will see it flood unless the top of it is higher than the outlet end to the street. An IO would be better as it can be capped off.

Have you ever considered you have too much stormwater directed into your tank, creating problems with overflow in winter?

Perhaps you could direct half your stormwater into the tank and connect the tank overflow to the remaining stormwater drain to the street?

[bricks]

If I were you spencer Id include an over flow junction at every down pipe connected to the tank, this would be so water doesnt flow into your eaves should the line blockup.

[Spencer74]

Hi Bricks. I have got those Leaf Eater rainheads installed on the downpipes - this will ensure the water doesn't flow back up the downpipe to the eaves if the pipe to the tank ever blocks up. They seem to work pretty well - not cheap but I like the idea of seeing the water go into the downpipe when it is raining!. I also have some pretty decent gutter guard - so the water quality into the tank will be decent enough.

[Spencer74]

Hi Jacksin. Never thought that I could be harnessing too much water - doesn't seem possible in these days of water shortages. I still remember playing under the sprinkler as a kid - wonder if my kids ever will!

I need to keep 120 sq m of lawn going through our Adelaide summers and keep the pool topped up - and I thought I should get all the water that still falls and can whack another 9000l in before next winter if I feel like it.

If I get the overflow right then the tank should never overflow - which is my objective atm ......

[notenoughtoys]

I faced a similar problem when I set mine up (if I understand correctly). Spoke to a drainer and his advice was to simply run the pipe to the street which is higher then the tank, when it overflows it will run up to the street and and when it stops raining it will find it's level and there will be an amount trapped in the pipe till it evaporates??? He said it was as simple as that.

Bob K

Mine hasn't filled yet by the way

[Spencer74]

I have been told that 90mm stormwater pipe shoudl be avoided at all costs for underground systems. 100mm sewerage pipe should be used cos it is so much stronger and the joins are better. There is a cost involved but the peace of mind might be worth it - especially is the pipe is running under some very expensive paving! Thoughts on this?

[Big Shed]

When I built my place I was told by my plumber to use 100mm sewage pipe for stormwater, for the reasons you pointed out. I did so and so far (7 years) it has stood the test of time.

As you are in Adelaide, chances are you have reactive clay soil (Bay of Biscay clay), the expansion and contraction of this soil is staggering. I know, I used to live in Wattle Park.

For the small saving involved in using 90mm pipe, I think you would be ill-advised to use it under ground.


You only have to look at the relative wall thickess of the 2 pipes, as well as the quality of the fittings.

[Spencer74]

When I built my place I was told by my plumber to use 100mm sewage pipe for stormwater, for the reasons you pointed out. I did so and so far (7 years) it has stood the test of time.

As you are in Adelaide, chances are you have reactive clay soil (Bay of Biscay clay), the expansion and contraction of this soil is staggering. I know, I used to live in Wattle Park.

For the small saving involved in using 90mm pipe, I think you would be ill-advised to use it under ground.


You only have to look at the relative wall thickess of the 2 pipes, as well as the quality of the fittings.
SOLD! Thanks Big Shed. Just ?????? that the plumber that did my extension used 90mm stormwater pipe for the pipe that is already underground (and I'm not digging it up!)

[Planned LScape]

Your tank will fill pretty quick if there's a decent downpour...I think if it's 100m² of roof and there is 1mm of rain it equals 1000lt's?

Be wary though it empties as quick as it fills. If you have a 30lt/min pump, and water your garden for 15mins, thats 450lt of water gone. It pays to get the biggest tank your budget and yard can afford.

[rsser]

Yes, sounds like a bit of overkill to me.

One day of showers yielded around 1000l from a 50 sq m garage roof over here recently.

[Border boy]

Your tank will fill pretty quick if there's a decent downpour...I think if it's 100m² of roof and there is 1mm of rain it equals 1000lt's?
I think it's more like 100m2 of roof and 10mm rain equals 1000l of water

Cheers!