Drainage issues - Can someone please help....

I should have enlarged the photo that showed the pit's pipe, that would have shown me that it wasn't a thin (1.9mm) walled 90mm stormwater pipe. Aah! The perils of not being on site.
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It looks like water could seep through around the pipe. I would be sealing around it.

It is good that all pipes are 100mm Drain Waste Vent (DWV) pipes which have 4mm thick walls, an internal diameter of 104mm and have a volume of 8.5 litres per metre..

I'll do calculations for the roof area being 230 sq m.

The roof slope uses a slope based multiplier to calculate the individual roof harvest areas draining to each downpipe during wind driven rain but we only need to use the plan area to calculate the rainfall on the total roof area for this exercise.

I surmise that the house was (most probably) handed over after construction with stormwater drainage that didn't include the connecting pipe shown as red in the diagram but which did include the water tank.

Your area probably has a 1:20 Average Recurrence Interval (ARI) of 200 mm/hr. The ARI is based on an average 5 minute rainfall intensity and 200 mm/hr = an average 3.33 mm/min of rain intensity over a five minute duration. A region's 1:20 ARI is used to determine compliance for the eaves gutter cross sectional area, downpipe size, the maximum roof harvest areas and the stormwater pipe sizes.

Downpipes draining a 230 sq m roof during a 200 mm/hr 1:20 ARI would collectively drain 230 x 3.33 = 766 litres per minute (lpm). This is a velocity of 1.5 metres per second through a DN 100mm DWV pipe. AS/NZS 3500.3.5.4.11.2(d) states that the full pipe velocity is recommended to not exceed 1.5 m/s and shall not exceed 2.0 m/s.

The house has 6 downpipes and for this exercise, I will treat all downpipes as harvesting equal roof areas of 38.33 sq m each. This equates to 128 lpm during a 1:20 ARI. In reality, downpipes do not harvest equal roof catchments.

I also surmise that the pit was installed after handover which invariably is the case.

I will refer to the downpipe at the garage as dp1 and so forth.

Using the above and assuming that the diagram is accurate, the 100mm stormwater pipework was originally designed to:

1. Plumb the downpipe connections to merge with and assist the stormwater flow rate.

2. Have four downpipes (dp1, dp2, dp3 & dp4) connected to a charged (wet) system that discharged into the tank via a vertical riser. The downpipes were not fitted with leaf diverters and the available head from the top of the horizontal wet system pipe above the tank to the gutter's sole looks to be at most 0.7 m.

3. The 4 downpipes would have drained 512 lpm (128 L x 4) to the tank during a 1:20 ARI. A 100mm DWV pipe has a volume of 8.5 litres per metre. 512 L/min = 8.53 L/sec or a velocity of fractionally more than 1 m per sec.

4. All fittings, elbows etc create friction loss that is calculated as equivalent pipe lengths, for example, a 100mm 90 degree elbow has an equivalent pipe length of 3.7 metres friction loss and there are two 90 degree elbows and eight 45 degree elbows plus the junctions and pipe length in the wet system fed by dp's 1-4.

It is very difficult to calculate flow rates when there are four charged downpipes at different positions along the wet system. Regardless, I can calculate that a 104mm internal diameter pipe with 0.7 m of head will flow at 512 L per minute with friction losses measured as 77.7 metres of equivalent pipe length.

I am very interested to know the actual head but note that the original drainage calculations also needed to include the roof slope multiplier that allows for additional wind driven rain. For a 23 degree slope, the multiplier is 1.21.

5. Dp 5 was also diverted to the tank through another charged pipe, making a total of 640 lpm diverted to the tank during a 1:20 ARI.

6. The tank has a single high fitted 100mm overflow pipe with in practical terms no mitigation above the top of the overflow.

7. Dp6 connects to the stormwater pipe.

UNDERSTANDING ISSUES THAT WOULD HAVE BEEN FIRST EXPERIENCED

Depending on the actual head and the effect of wind blown rain during a 1:20 ARI, the single nominal diameter 100mm charged DWV pipe may not have been able to cope during a 1:20 ARI, resulting in the gutters overflowing. I consider this scenario to have been most likely.

A mistake often made is to calculate the head as being between the tank's top meshed inlet and the gutter's sole whereas for a pipe diverting flow at a good velocity, it should be from the top of the vertical riser to a minimum 50mm below the gutter's sole to allow for bubbles which lower the water column's atmospheric pressure at the top of the column. For your installation, the difference would have been nearly 200mm optomistic if the distances were incorrectly calculated.

Regardless of the above, the tank's 100mm overflow pipe could not have possibly coped with the inflow during rainfall much heavier than one mm/min.

Flow out of a side hole is calculated by using Torricelli's Theorem but AS/NZS 3500.1 has a convenient table (Figure 8.2) that shows flow rates through a short horizontal outlet at different heights above different size pipe's inverts. While it doesn't show a 104mm internal diameter pipe, it does show that a 100mm pipe with 100mm of water above the invert will discharge at 4.67 L/s or 280 lpm which is a long way short of our example of 640 lpm.

A significant additional point to consider is that Torricelli's Theorem and the figures in Figure 8.2 do not factor a tank having a meshed overflow outlet which presents a flow restriction and which can also clog.
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Because the tank was part of the stormwater system, the installation was not compliant by a huge margin.

FIXING THE TANK OVERTOPPING PROBLEM FOR THE FIRST OWNER

As it wouldn't have taken long for the tank to fill from five downpipes, the new home owners were sure to have experienced overtopping and associated flooding very early and this was almost certainly the reason why the additional (red) connecting pipe was added.

It is really impossible to know how much would discharge into the tank now from dp's 1-4 during different rain intensities but with the significant flow restriction presented by the vertical riser, a lot of water would now drain through the new pipe marked in red as it is the path of least resistance and the reason why the tank does not overflow. I would not expect much if any water to flow into the tank during light to medium rain apart from the diversion from dp5.

CURRENT PLUMBING

A pit has been added to drain surface water but the pit's discharge pipe runs with minimal slope to the junction and the small amount of head provided by a full pit is negative to any head from the tank and dp6 as well as the other pipe connecting to the junction. This makes the pit a new path of least resistance, adding to the saturation in that area. The effect will be much greater during heavier rain than shown in the video.

The very low head of water in the pit will never win the battle against the water flowing from the additional pipe connection which itself is velocity retarded by the direction of slope and intersecting flow from dp's 1-4.

FIXING THE PROBLEM

The stormwater system needs less water flowing through the pipes on the tank side and the water from the pipe currently supplied by dp 1-4 needs to flow faster to the kerb.

Is the following possible with your landscaping?

Connect dp5 to the stormwater pipe with the direction of flow heading downstream.

Have the existing tank's overflow connected to a new detention tank sited inline with the existing tank. A detention tank provides mitigation by discharging a limited flow of water through a small outlet fitted near the bottom of the tank. This will reduce the flow of water to the stormwater pipe when the 3,000 L tank is full. The detention tank would need to be a minimum 2,000 L and have a 32mm bottom outlet connected to the stormwater drain to provide a sufficient high velocity flow into the stormwater pipe.

Only plumb dp 3 & 4 to the tank and cap the pipe at dp3.

Reverse the direction of discharge and slope from dp 1 & 2 towards the kerb so that the stormwater flows faster.

WATER TANK AND FIRST FLUSH DIVERTER
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What you see near the top of the vertical riser is a first flush diverter. First flush is the dirtiest and most polluted water that first comes off the roof.

The original installation had sealed wet systems which means that the downpipes remained full of water to the overflow level into the first flush diverters.

There is an Inspection Outlet (IO) at the bottom of one vertical riser, this has been installed to flush built up crud from the underground pipe but it cannot drain the pipe.

Whether it rained after the water in the downpipes and the vertical risers was lowered to the bottom of the IO or whether the pipes were full to the level of the discharge into the first flush, the first flush diverters will not fill with the first flush. If by some miracle the first flush from all five downpipes managed to get to the head of the queue, the amount of water collected by each first flush diverter would be maybe 8 litres each but miracles like water jumping the queue don't happen.

The first flush diverters also have a dripper which usually wastes 4-5 litres each per hour.

Fitting the first flush diverters at the top of the vertical risers was imbicilic and a waste of money and yield for the original owner.

Current installation.

Because the pipe (dp 1-4) now drains through the additional pipe connection after every rainfall, each downpipe's first flush will now drain to the kerb. The first flush diverters (still) serve no purpose.

Keeping first flush diverters fitted to vertical risers on 'proposed' installation.

No purpose.

VERTICAL RISER.

The top of the vertical riser should have the elbow fitted lower to increase the head and it should also discharge through a flap valve to prevent mosquitoes and other pests from accessing the standing water. This is not compulsory in your area like it is further north but it is good practice. From egg to adult mosquito takes about 12 days.

A late note about the vertical riser on the downpipe 5 wet system.

None of the downpipes have leaf diverters and the underground section of dp5's wet system has been slowly building up crud. The 104mm internal diameter pipe has a volume of 8.5 litres per metre. Even when diverting 128 lpm to the tank during a 1:20 ARI, the velocity up the vertical riser will only be 0.25 metres per second (128 litres only fills 15 metres of pipe).

This is insufficient velocity to flush/carry anything except light suspended sediment. This wet system diversion is very much sub standard.
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LEAF DIVERTERS.
Wet (charged) systems should be fitted with leaf diverters at the top of the downpipes but again, they are not compulsory in your area like it is further north but it is good practice. The ICON Leaf and Debris Controller is the best of the current offerings.

SUBMERSIBLE PUMPS.

Many submersible pumps draw water from the bottom (see first photo below) and it will be a term and condition of warranty to have the pump positioned above the sediment layer but this is rarely done. The result is dirty water being vacuumed from the tank's floor and supplied to cisterns and washing machines.

You should check your pump to see whether it is sitting on the tank's floor and whether the floor around the pump is clean. If the floor surface around the pump is dirty, then the water pick up will be higher up. If it is clean, then the pump will be vacuuming the floor like in the photo below.
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I know that this hasn't answered your question about additional drainage but you cannot move to that stage until the stormwater system has the capacity to exceed anticipated demands.

An additional fact that you need to know to understand the system is that the friction loss expressed as equivalent pipe length of the pipe between the pit and the intersecting junction is about 46m. A 250 mm water level above the pipe's invert in the pit will provide a maximum flow rate of about 390 lpm provided that the intersecting water through the junction has good velocity and (I think) not act as a flow restriction that the current slow flow from dp 1, 2, 3 & 4 does.

The real life pit to kerb figure will be less than this because of the flow restricting and volume reducing entrained air produced from jetting water in communication from dp5 and dp6 to the stormwater pipe (the same cause of the bubbling you have seen in the pit) and also from the additional intersecting water in the junction and the remaining pipe to kerb, the reason why achieving good stratified flow velocity from dp1, dp2 & dp3 will be vital.

There will be some minor pipe slope between the junction and the kerb.

There would be no air entrainment generated by the 32mm valve because it would operate under full flow conditions but fitting an air release valve further along the storm water pipe will need consideration.

There are also a couple of other but similar variations to my initial suggestion.

NOTE: The surface level at the junction is 200mm lower than at the pit but we don’t know what the pipe’s depth at the junction is. The plumber however has said that the cam showed ponding in the pipe and it would be expected that the pipe depth at the junction was not as deep as the pipe level in the pit. No allowance for any additional (unknown) pipe slope was made in the head/flow calculations but some minor additional head should be reasonably expected.