Rainwater Tanks not filling in light rain

Hi Jeremy,

It is certain to be leakage.

After it rains, the downpipes will/should be full of water to the same level as at the elbow's invert at the top of the riser. There will be some evaporation from the riser and the downpipes during dry periods but not a lot due to the small air-water interface surface areas. You can do some calculations to see what the total volume of water there should be in the downpipes and the riser after it rains and then comparere it against the amount of water that would be collected from just 1-2 mm of rain. You can halve the first one mm's yield to allow for evaporation.

First step is to tap on the riser to see where the water level is.

Second step is to walk above where you think the wet system pipes are subsurface and look for any unusual surface greenery.

Do vehicles drive over any areas where there are subsurface pipes?

It is more than possible that there has been ground movement and a fitting has cracked (it is usually a fitting and not a pipe).

The bottom of the riser is a common leakage area because pvc expands and contracts with the weather, the reason I advise to fit an expanding coupling to an overflow pipe and to also fit a flexible UV stabilised hose between the tank and the pump.

Third step is to excavate around the bottom of the riser.

Storm Plastics manufacture a range of high quality flexible pvc fittings.

https://www.stormplastics.com.au/

Fourth step is to monitor the riser's water level after the next rain.

Hi Jsamways,

Just to add to SaveHSO's excellent response. There is some more trouble shooting you can do before excavating. Your idea to fill with a hose should determine if it's a leak of a blockage. The riser should will to the same levels as your downpipes. The water will equalise, it's not a pressure issue.
If you fill using your hose and the level in your downpipe doesnt rise then you have a significant leak. If the level in your downpipe rises or overflows but your riser doesnt flow into the tank then you could have a blockage.
Might be worth trying before getting out the shovels

Cheers

It's a fairly large house - I am attaching a drone shot so you can see. Yes, there is a significant amount of underground pipe to get to the tanks. Gutters and drains on both sides of the house. I have found one spot between on the north west corner by the trampoline where the ground seems squelchy. Like  ⋅  Add a comment  ⋅  Pin to Ideaboard  ⋅  Share

The riser would be a 100mm DWV pipe with a 104mm inside diameter and a 110mm outside diameter. This pipe holds 8.5 litres per meter. 1 m/s = 510 lpm.

Do all of the downpipes supply the single riser? During a minimum qualifying 1:20 ARI, the 100mm DWV pipe would be subject to a flow rate in excess of 1,000 lpm which is a velocity of +2 m/s. This is very fast. Is the squelchy spot a candidate for locating a likely elbow?

High velocity flows that meet an elbow exert centrifugal force that pushes the water towards the outer side of the bend, creating higher pressure on the outer curve while the inner curve experiences low pressure. This results in the elbow experiencing both a change in pressure and a force acting outward which can result in structural stress.

If you find a damaged 90 degree elbow, I suggest replacing it with two 45 degree elbows.

'neath yon tiles exists but a singular path to verity.

EDIT: The above is a bit of a cheeky play on General Relativity stating that a singular path cannot exist.

Well done!

90mm u-PVC stormwater pipe has 1.9mm thick walls and bends easily. This is claimed to be an advantage as it will deform rather than break but I don't like it much.

Is there any indication, either past or present, of hydrostatic uplift anywhere above ground?

Is the downpipe exposed to UV and is it damaged?

Use some fishing line but let the plumber do it.

How much room is there above the elbow that will allow a straight cut?

Your plumber mate might be able to cut above the elbow's socket and solvent weld a short split 90mm pvc stormwater pipe half inside the (cleaned) vertical pipe and the other half inside a short intact 90mm pipe. A solvent welded coupling would then connect the two intact outer pipes, allowing a new elbow to be fitted. The fly in the ointment will be fitting a coupling if there isn't enough room.

Also look at the possibility of using a 90mm pvc stormwater M+F 90 degree elbow.