Stormwater - minimum fall?

[gsouth]

All,

After searching the net for about an hour I thought I'd post the question here....

I am looking to install a water tank under the deck and connect to the 2 downpipes at either end of the deck.

Currently the pipes run thru the deck and into the ground.
My plan is to cut them off and run them across under the bearers to the top of the tank (about 9m away).
What is the minimum fall I need to have on the pipes to ensure a good flow?

Cheers
Geoff

[echnidna]

As long as there is slope you will have flow.

[Wood Butcher]

When I did the shed at my last house I was advised to have at least 5mm of fall for every metre of pipe/gutter.

[Termite]

Irrigation ditches in cotton fields have a fall of 1 foot to the mile.

[Iain]

Currently having a sh!te fite with our council as our land is 3 metres lower than the drain, they still seem to think that water runs uphill, saga continues...

[ozwinner]

Love the new avatar.
Horse stops mid air, must be the council horse?

Al

[Iain]

Few years ago, it did, I didn't, 8 cracked ribs, 1 broken wrist.
Horse suffered sore bum when I got home from hospital, re admitted, 1 broken foot.
(last one was a lie)

[ozwinner]

Is that you on the horse?
I would have stopped too if I was the horse.

Al

[echnidna]

Is that why yer ride donkeys now.

[elphingirl]

A fall of 1 in 100 is adequate in a closed pipe situation, in a larger pipe (250mm diameter) you could go to 1 in 200. In an open gutter, where litter such as leaves and dirt could get trapped it would be safe to allow more like 3 in 100.

Cheers
Justine

[maglite]

Have you given any thought to disposal if/when the tank fills up?

[gsouth]

maglite,
The overflow is directed back into the stormwater channel...
Geoff

[TassieKiwi]

People get carried away with this. If this section of pipe is below downpipes etc the 'head' of this water will push it through this section, even with reverse fall. To illustrate this better our last house had a syphon system: The kerb on the roadside was only about 1.5m below the guttering, but the pipe had to be about 1m below the kerb outlet as it was buried under the driveway. I sealed all joints, had an overflow on the downpipe, and it worked a treat. Note that the system is permanently full when not running.

That said, most inspectors like to see 'normal' pipes sloping towards the outlet. Put a 'turn of the bubble' on it if you can.

[soundman]

The other answer is...... as much fall as you can practicaly get.
If thats all the fall you have....

[Iain]

If it's likely to build up a retarding basin may be order of the day to save any problems.
Thanks Al, just because I'm 5'2" and 55kg you have to make fun of me