What should I be doing to fix this flooding

Hi all,

There's a huge downpour in the SE suburbs of Melbourne at the moment.
There's water coming into the garage and water pooling on the side of the house. I noticed that water is spilling out of the downpipes into the floor as well.

I'm wondering if anyone has any suggestions on how to resolve/make things better in the event of another downpour. I believe that pooled water in this area is not going to be a good thing. I just want to get this fixed properly.

I've included photos to better illustrate what I've failed to do with words.

http://imgur.com/a/XSV92

Thank you so much for taking the time.

The main stormwater system most probably overloaded and is why the pipes backed up.

As for the surface water, if you installed ag pipes and silt pits, it would encounter the same problem that the downpipes had if the above was the case.

It is also possible but unlikely that the mains stormwater has not overloaded and that the problem is with your property's subsurface stormwater pipe being undersized. For now, I will assume that your sub surface stormwater pipe is ‘90mm’ pvc but the following should also interest others anyway.

Study time!

If a single dwelling is on an urban allotment that is less than 1,000 sq m, a plumber is allowed to 'design' the subsurface stormwater pipe by what is referred to in AS/NZS 3500.3 5.5 as a "NOMINAL METHOD". The NOMINAL METHOD allows a plumber to design the stormwater system "according to local practice and experience without specific design calculations, and according to the minimum diameter (Clause 7.3.4), cover (Clause 7.2.6, gradient (Clause 7.4.6) and other relevant criteria of this Standard." Once 'designed' and installed, a plumber then signs off on his own work because the industry is now self regulated.

Clause 7.3.4 states "Minimum diameters- (a) for single dwellings in rural areas and residential buildings on urban allotments with areas less than 1,000 m2, shall be DN90." Most plumbers (and the VBA!!!) wrongly take this as meaning that a DN90 pvc pipe is all that is required.

An additional problem is that AS2200 shows calculations for specific internal diameter size pipes whereas clause 7.3.4 refers to DN (Nominal Diameter). 90 mm pvc stormwater pipe refers to the outside diameter, the common use DN90 SN2 pvc pipe has an internal diameter of 86.2 mm and the stronger SN4 pipe has an internal diameter of 85mm. IMHO, the flow rates for all pipes should be calculated as per their true internal diameter, not their DN.

What this all means is that the AS2200 tables show the 90mm pipe having a flow rate of 360 litres per minute (lpm) at the required minimum 1:100 slope but your pipe will undoubtedly have an internal diameter of 86.2mm. The internal volume of a true 90mm internal diameter pipe is 6.36 litres per metre which equates to a flow rate of .94 metres per second when flowing at 360 litres per minute.The 86.2mm internal diameter pipe has an internal volume of 5.84 litres per metre. If the DN90 mm (86.2 mm) stormwater pipe was also mandated for a maximum flow rate of .94 metres per second, the flow rate would be 5.84 x .94 x 60 = 329 litres per minute, not 360!

Many areas of the Melbourne metropolitan area have just had a 1:20 Average Recurrence Interval (ARI). The 1:20 ARI is used to determine eaves gutter and downpipe compliance and the Melbourne ARI is 130 mm/hr, based on a 5 minute average rain intensity of a minimum 2.1666 mm/min. Even if we use 360 lpm as our hydraulic calculation as per AS2200, to calculate your maximum compliant roof area, we divide 360 by 2.1666 = 166 sq m.

Do you know your roof area?

BTW, your weep holes are also a problem, the height above the landscaped surface is not compliant and certainly not good when you have flooding.

Firstly, thanks heaps again SaveH2O for the detailed response. And now on to the main event.



What is the best way to determine if the main stormwater system is overloaded or not? If it isn't, installing ag pipes (what are these?) and slit pits would surely help my situation yes?




Again, how do I go about determining if the property's subsurface stormwater pipe is undersized?


This section was crazily detailed and informative. But took a little time for me to digest. Thanks again. I will need to find out the roof area from the documents.



Could you elaborate more on this? If I understand it better I could go back to the builders but I doubt they will do anything. I had cracks on the garage wall/door and internal walls on this part of the house previously. The builders fixed it up but refused to say install the slit pits etc as the landscaping was not done by them.

Eventually I gave up as it was taking too much time and now I just want to resolve this issue ASAP for fear that this side of the house is going to sink into the ground.

Thanks again.

You could ask your neighbours about their experience.

It is not unusual for a main stormwater drain to not cope during heavy rain. One of the photos show water spurting out of one downpipe at the top of the rain head. That downpipe would have been full of water to do that and that strongly indicates that the main drain was overtaxed because even if your subsurface stormwater pipe is undersized, it would not resist that much hydraulic head if there was no system restriction.

Ag pipes are slotted agricultural drain pipes. Just google it. Ag pipes must drain into a silt pit before the water drains into the stormwater system. I can't say what you should do because I am not on site, you need to talk to someone who does drainage.


You need to uncover the pipe and measure the outside diameter, it will be either 90mm or 110mm. The top of the pipe will be a minimum 100mm below the surface but probably more because of the landscaping.


It had to be. Once you understand a few pointers, it is easy.



Your weep holes (and DPC) are very low...or rather, your landscaping is too high. Non compliant landscaping is a builder's Get Out Of Jail Free Card when there are structural or heave issues.

The BCA states:

"(c) The height of a DPC, or flashing serving as a DPC, (see Figure 3.3.4.1), must be not less than—
(i) 150 mm above the adjacent ground level; or
(ii) 75 mm above the finished surface level of adjacent paved, concreted or landscaped areas that slope away from the wall (see Figure 3.3.4.1); or

Explanatory information:
150 mm clearance between the DPC and adjacent ground level reduces the risk of the effectiveness of the DPC being affected by changes in the surface level. Where changes in surface level are less likely to occur, such as where the adjacent surface is finished with paving, concreting or landscaping, the height of the DPC above that surface may be reduced to 75 mm. When also protected from the weather by a carport, verandah or the like the height of the DPC may be reduced to 50 mm.

Further reductions in the height of a DPC are permitted in low rainfall intensity areas."


A concrete path and French drain could be your best bet but again, I am not in a position to say.

If the stormwater drains were overtaxed, then you could put another tank in to mitigate the drainage but that only holds until the tanks fill. If you are able to see on the BOM radar that a lot of rain is coming, you always have the option to empty your current tank for mitigation but the effectiveness is dependent on the % of roof area harvested.

One other thing, you urgently need to get a cover for your WaterSwitch and pump.

I have a similar problem in my place once every four or five years when heavy rain over several days can cause water from a neighbouring battle-axe driveway to flow over the retaining wall to my yard.
I just keep a $100 Bunnings fountain pump in the cupboard for such occasions. It can easily shift floodwater at a similar depth to yours back over the retaining wall in a few hours.

I know installing more ag pipes is a better solution but my house is surrounded by concrete paths and the builder put the drain at the higher corner of the yard.