Low head water tank

Only one downpipe?

How far is the run?

Are you off the water grid?

If I know your 1:20 ARI or your region, the roof slope and see a diagram, I can run a few figures.

You can't use all of the 'available head' below the gutter's sole because of the weir flow that will generate bubbles during heavy rain which lowers the atmospheric pressure plus you also need to operate with a safety reserve of available head. You will also need to use 45 degree junctions, not tees in order to minimise head loss.

Not having leaf diverters restricts you to a standard system because you cannot use a low restriction inlet to supplement the flow rate when the water hasn't passed through a filter screen.

If you are in the Sydney region, you would be looking at around 1,350 lpm during a minimum qualifying 1:20 ARI. I would design for at least 1,700 lpm and this would usually require a 2 pipe system plus some serious overflow design or else some additional serious manual overflow management whenever required. What is the tank's volume?

Pumping out a wet system such as one you would have would result in considerable water loss but you could use a x2 sediment trap system and manually drain to a low point during rain periods when the tank is full to kill 2 birds.

EDITED: Altered "draining" to "and manually drain" for clarity.

The best time to divert first flush is after a dry spell and your idea has merit provided you have the subsurface storage capacity to do what you want. If you have a downhill slope, you could divert the water in the sediment trap to a small (maybe 500 L) tank similar to the photo but obviously away (lower) from the main tanks. The small tank will need to be strong as there will be some reasonable pressure exerted down the vent tube which also needs to be meshed at the top.

Just be aware that the top of the vent has to be higher than the house gutter, not the tank.

The photo also shows a vertical riser with a direct entry discharge into the tank. To do this, you need to have leaf diverters fitted to all applicable downpipes but bizarely, I don't think it is an actual regulation as yet.
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First flush is the polluted roof wash, sediment and grit in the gutter is mostly flushed during heavy rain.

Water diverted to tanks settles in stratified layers with higher density, low oxygen water settling at the bottom and the best quality low density high oxygen level water settling at the top. The surface water is also poor quality and so it would be best to drain settled water from about 300mm above the bottom of the first flush tank for re-use. The best quality water is found 250-300mm below the surface.

When you do your pipe flow calculations, you can use any number of friction loss calculators found online. I prefer to use ones that are based on the Hazen-Williams formula but no calculator used will give an accurate result but they are reliable.

You will need to add friction losses through the wye and any elbow fittings as equivalent pipe lengths.

The 12 downpipes will drain different roof areas and this means different flow rates through different sections of pipe. A 100mm DWV pipe has an actual internal diameter of 104mm (8.5 L/m) but this means that there will be no flushing velocity at the head of the system when the pipes are full. I assume this is why you want to use a dry pipe system that diverts water through a trap and then to a storage area which is a good idea as you will only need one diversion during a prolonged rain period.

Two 100mm pipes will be insufficient. Lets say each pipe is figured to have about 75 m of equivalent pipe length. If your 1:20 ARI is 3 mm/minute based on an average rain intensity over a 5 minute duration, your roof will supply 1,320 lpm or 660 lpm for each pipe.
You have 450mm maximum head.
The (nominal) 100mm DWV pipe holds 8.5 L/m. A velocity of 1 L/s = 510 lpm.

A 75 m (equivalent lenght) 100mm DWV pipe flowing at 510 lpm requires 680mm head! Even if the 100mm DWV equivalent pipe length was 60 m, the required head would be 544 mm but 60m is almost certainly unrealistically short given the number of downpipes.

The pressure drop and recharge along the pipe's length will vary from the hydraulic grade line due to the roof areas etc but with a standard charged system that only diverts to a vertical riser, you need to calculate the required flow rate and pipe size as per the total length.

It must also be understood that a region's 1:20 ARI figure is the minimum intensity needed to qualify as a 1:20 ARI. You could experience a rain intensity that is just short of qualifying as a 1:50 ARI but you will still have a 1:20 ARI. This is why I always advise to have an absolute minimum 20% design flow rate above the 1:20 ARI figure. With the first flush and sediment capture system you propose, you could use it to manually prevent gutter overflows during such times if you don't choose to upsize the pipes close to the tanks but this would of course entail yield loss..

Have the downpipe locations been decided or are the current drawings showing nominal positions which unfortunately is the norm?

The roof slope also has to be factored for wind driven rain. While this doesn't affect the total roof catchment yield, it does determine the volume of water draining to individual pipes during wind driven rain and the greater volume through those pipes at those times has to be calculated to ensure deliverability to the tank by the available head.

The 104mm ID DWV pipe has a cross sectional area of 8,495 sq mm whereas the 150mm pipe has a cross sectional area of 18,050 sq mm. The rule of thumb is that each 1% increase in diameter will see an increased flow rate of nearly 1.5% with the same head but roof drainage isn't quite that simple and you wouldn't be replacing the entire section of 100mm pipe although I have seen it done!

How did you arrive at 450mm head? I have seen numerous failed installations over the years where the head was measured as the distance between the bottom of the gutter and the top of the tank's inlet basket whereas it should be no less than 100mm below the gutter's sole (to allow for air entrainment and characteristic weir flow) to the discharge pipe's overt at the top of the vertical riser but I must admit that I often say 1/2 way between the top of the horizontal elbow at the top of the riser as I have watched risers discharging during heavy rain.

One of the easiest ways to increase a pipe's flow rate is to branch a tee off the vertical riser and connect it to a second discharge point that directly enters the tank. This would obviously have to be no lower than the tank's overflow pipe because of the way you intend to use the sediment trap and the need for a meshed filter at the entry but you can usually gain 200mm or more head depending on the inlet basket's location on the tank's roof, the shape of the roof and the height of the pipe above the basket.

I advise fitting an easy to access/removable meshed filter to an additional inlet, they are available in 100mm and 150mm to fit DWV pipes. A video in the linked page shows how easy they are to maintain.

https://rainharvesting.com.au/products/ ... le-screen/

You can (sort of) see one in the photo that I posted earlier and commented on. If the riser had continued upwards to also discharged into a meshed inlet basket at the top of the tank, you would be seeing a tee diverting water to the inlet on the side of the tank with the elbow then at the top of the riser.

Where did you measure the head to? The top of the discharge pipe?

If you can calculate the reasonable head from let's say 100mm below the gutter to the top of where the vertical riser's horizontal discharge pipe will be, that would be good. Can you get hold of a laser level? Many tradies have them now as small ones are so cheap. Line of sight using a straight edge with a level works for me.

You will most probably need to upsize the horizontal pipes to 150mm before the last one or even two downpipes on each run (depending on roof areas harvested and allowing for wind driven rain) even if you install some meshed side inlets on the tank because you really need to allow for that really big high intensity rain event that sooner or later you will have. Designing for a bare minimum 1:20 ARI just doesn't cut it.

The overflow pipe also needs to be upgraded if you are planning on having just one.

The immediate move would be to remove the flow restricting overflow mesh and fit an air gap on the outside vertical pipe. An air gap has two easily serviced mesh filters, see link below. There is a video at the bottom of the linked page.

https://rainharvesting.com.au/products/ ... s/air-gap/

To increase the overflow outlet's overflow capacity, you can also securely solvent weld a pipe about 150mm long to the overflow fitting inside the tank but drill holes (about 32mm) all over the pipe (but not too close together) to increase the total weir wall length. This will substantially increase the discharge capacity.
NOTE: This must never be done if using flimsy 90mm pvc stormwater pipe! I recommend you having two 100mm overflow pipes on the infeed (settling) tank.

150mm overflow outlets are also available.

There is an online charged pipe flow calculator that you can consult re wet system pipe sizes and their lengths Vs different hydraulic heads. It also has a link to a You Tube instruction video on how to use the calculator. It would be very handy if you could calculate and write the volume of water flowing along each section of pipe during a 1:20 ARI but again, allow for a greater rain intensity event.

https://www.roof-gutter-design.com.au/C ... dPipes.php

The measures can all be done as a straight line with the calculator because a house corner is simply represented by a 90 degree elbow that is factored by the calculator as being an equivalent length of pipe added to the total length.