Wet charged systen

OMG!!!

The centre of the 90mm inlets looks to be about 800 mm from the top of the tank. Are you sure it is 1 m?

You're spot on, it's about 800mm

I'll do a summary.

OUTLET DISCHARGE.

Already covered but to recap:-

150mm outlet with no restrictor plate: no council PSD mandate? The outlet also appears to be unmeshed.

Street discharge level (zero head) is 400mm above the bottom of the tank.

The 150mm pipe's invert appears to be about 80mm above the bottom of the tank. This means that the discharge pipe will be submerged with about 70mm of water in the detention compartment and there will be 200mm head with a tank water depth of 600mm.

As per AS/NZS 3500.1 Section 8 WATER STORAGE TANKS Figure 8.2, a horizontal 150mm pipe will have an overflow rate of 16.6 litres per second (996 litres per minute) with 200mm of water above the invert. The flooded pipe will be more efficient but we will use the lesser figure and there will be some minor friction loss through the length of the 150mm discharge pipe but not worthy of consideration here. The discharge tank is extremely unlikely to fill above the 600mm mark and the two 90mm inlet pipes could be fitted at any level above this height but obviously lower is more efficient and neater.

There is a 150mm short balance pipe near the bottom connecting both tanks. With two infeed pipes discharging into separate tanks, there will be minimal variance in both tank's water levels.

There is another 150mm short balance line fitted high up on both tanks. This is ridiculous and a waste of time and money.

4 DOWNPIPES ON THE SHORT RUN FROM THE FRONT.

The average roof area to all 10 downpipes is 25 sq m. Using the ballpark average, the 4 downpipes will be draining a roof area of about 100 sq m. During a minimum 1:20 ARI intensity of 2.33mm/min over a 5 minute duration, we arrive at a figure of 233 lpm but note that a safety margin should be built in plus the roof catchment area also includes the gutter perimeter...something that isn't commonly done.

Regardless, this pipe will have a head of + 1 metre which will give a huge reserve flow capacity.

6 DOWNPIPES ON THE LONG RUN FROM THE REAR.

3 of these downpipes drain a lower gutter that is the same height as the tank.

There is 850m head above the 90mm pipe's invert.

There is a 40 metre pipe run + friction losses.

Using our ball park average roof catchment area of 25 sq m per downpipe, 150 sq m of roof is drained to the (nominal) 90mm PVC stormwater pipe. This equates to a minimum ball park 350 lpm during a minimum intensity 1:20 ARI, or does it?

Roof stormwater drainage compliance factors the effect of the roof slope on wind driven rain, for example, a standard 22.5 degree slope is factored as x 1.21. The 40 metre long pipe (as per this example) should therefore be calculated to potentially divert a minimum of 350 lpm x 1.21 = 424 lpm plus you need a safety margin for heavier rain that exceeds a minimal 1:20 ARI.

Considering that the tank will never fill higher than 600mm, it is 'somewhat strange' that they have installed the inlet 1.45 metres high when it could have been fitted 850mm lower. My ball park calculations tell me that the current 850mm head is inadequate and of interest, the suggested 1.6 m head is ideal according to my calculations.