Charged stormwater with insufficient drop - advice?

Hey all - I found this forum while googling for this rainwater/stormwater issue. Hoping someone give us some advice!

Super short version: We have a charged stormwater system. 2x downpipes on the house which run under the driveway and empty into a 60,000(ish) litre tank. But there's only a very small drop in height between the house guttering and the inlets to the tank. And I mean VERY small, to the extent that we have standing water in the gutters. The system does work - the tank fills up nicely when it rains - but we're not at all happy with the gutters holding water all the time.

So a couple of questions:

1. Short of raising the gutters or lowering the tank (neither of which are really viable sadly) is there anything we can do to ameliorate the standing water problem? Some sort of overflow perhaps?

2. Are there any firm legal requirements that apply to tank/stormwater installers with regard to the drop distances?

Inadequate head is a common problem.

To calculate hydraulics, I really need information about the roof areas and slope that drain to each downpipe, the region so I know the 1:20 ARI intensities, the pipe sizes and the length of pipe between the downpipe nearest the tank and the tank but given that you obviously have inadequate head, can you first post a photo that shows the height of the vertical riser above the tank in relation to the overflow pipe thanks.

What size is the riser's pipe and are the subsurface pipes DWV?

Does the tank have a liner?

Hey, thanks for the reply.

The riser/inlet are on the opposite side of the tank to the overflow(s), so I've had to take two photos.





All the pipe seems to be 90mm PVC, and yes the tank is lined.

There are two down pipes (on opposite corners of the house). Roof area is around 270m², region is Southern Tasmania.

Do the two overflow pipes have a direct flow outlet or are they fitted with upturned elbows inside the tank?

How high is the liner on the side wall?

I'm not entirely sure - I'll see if I can get a look through the hatch, although I'm not sure I'll be able to see back to the overflow outlets.

Based on the water level, I'd strongly suspect they're direct (no elbows) but I could be wrong...

EDIT: The liner goes right up to the top corrugation. It's a little hard to see back to the other side of the tank, but I'm 99% sure there are no upturned elbows.

My thoughts.

The bottom of the LHS overflow pipe is at the bottom of the third corrugation but the pipe on the RHS appears to be larger and a bit lower than the LHS.

Two supplementary inlets fitted at the bottom of the second corrugation would gain roughly 300mm head. The problem would be the liner and access which would be via the roof hatch in the photo. Not being on site, I can't advise about this. The new inlets would be plumbed off the existing risers.

If your qualifying figure for a 1:20 ARI is 120 mm/hr, each pipe would transport 270 lpm but you ideally should design for about 330 lpm. The 90mm pvc stormwater pipe has an internal diameter of 86.2 mm.

I went online and found the Hazen-Williams flow rate calculator below and entered a roughness coefficient of 150 (pvc) plus a target flow rate of 5.5 lps (330 lpm). I also entered the pipe's internal diameter plus 100 metres of pipe length. The pipe length is to give me a figure that I will divide by 100 to give the friction loss per metre. As shown, a nominal 90mm pvc stormwater pipe will lose (or will require) 1.01 metres (1010 mm) of head when flowing at 330 lpm over 100 metres. This equates to 10.1 mm head loss per metre which means that a 300mm head can serve no more than 29.7 metres of pipe

When determining the pipe length, you have to include the various fittings (elbows etc) as equivalent pipe length. These figures are easily found online. You can play around on the calculator with a 100mm DWV pipe (the internal diameter is 104mm) and different flow rates.

https://www.engineeringtoolbox.com/haze ... d_797.html

You would need to retain the existing risers because while mandates to mosquito proof tanks are not found in the regulations, most northern councils and health departments do require it plus it should be done anyway. If you divert directly into the tank by overcoming the liner issue, you could fit the new inlets with flip up mozzie stoppas for easy maintenance. If they were to block, you would still have the risers.

https://rainharvesting.com.au/products/ ... -max-flow/

Are there any spare inlets on the tank?

Apart from this suggestion, you would need to divert to a small holding tank and later on pump into the larger tank.

I hope that this has helped.

Thanks that's very helpful indeed. Lots to think about.

No spare inlets on the tank.