Stormwater drainage issue due to fall of land

Hi all,

My block slopes ever so slightly from front to back, and I though that a flat block was fantastic for building, however it is not for stormwater.

My issue is that my stormwater runs to the kerb and channel, and that sits at 59.17. My stormwater pipes need to generate some fall from back to front, and i gather that with a 1:100 fall ratio, and needing to span about 30 meters, than I need a 30cm gradual fall.

The issue is that the house floor height is at 59.70, and the garage floor height is at 59.53 (pipe must pass under this), the plumber is telling me that he cannot get the fall and keep the pipes underground. Current natural ground level runs from 59.18 through to 59.40 at the property boundary, where I tap into the kerb drainage pipe.

I have 3 options I believe:
1) Install a sump and pump solution (which allows me to create my own fall)
2) Implement a charge/sealed system (meaning that water always remains in part of the pipes)
3) Raise the soil around the house and place the stormwater pipes in the fill

If there are other solutions, then I am haopoy to listen to them.

Keep in mind that the pipes need to be at least 100mm underground and if I lay paving next to the house, I still need to leave at least 170mm in order to not have a step at the back of the garage (which also has a 2nd roller door).

Can someone let me now of any issues they may have had with options 1 or 2 Or any issue with raising the ground level next to brickwork. I am building on a slab.

Thanks,
Baz

You will need hydraulics engineer to design your storm water. I did pay for one to design mine. My land is similar to yours. Total cost including design and construction costs me about $5k. I don't think mine is a charged line however it is laid from one side of the property to the other overflowing onto the street.

You can reduce your span from 30 metres with careful placement of downpipes(max gutter length 12m for down pipe) or using box gutters or running pipes in the roof space or walls (depending on design). Raising ground levels does not matter if you maintain required distance from DPC.
There are options but as Ca6leguy says you should invest in experienced designer.

So who is the knucklehead that designed the house without taking that into consideration ?
You should also find out from the council whether they allow either of 1 or 2 - ours won't allow either except in exceptional circumstances.

Stewie

It helps if you post your location.

It sounds like you haven't approached your council. Establishing any applicable requirements, particularly in relation to an onsite detention system or rubble pits (in some States), should be a prerequisite.

1) Have a read of the link below for information about pumped systems.

viewtopic.php?f=1&t=67051

2) The council would possibly/probably require a detention system with a maximum discharge rate measured as lps. If they do, then they will also require a hydraulic assessment. This will be expensive but you are paying for the professional's expertise rather than the 10 minutes of calculations and the hour taken to standardise the report.

3) Don't forget that you are required to have a 1:20 fall away from the house over the first metre.

Option 2 is your best solution. If the council does not require a detention system but you decide to have a detention system rather than a standard tank overflow set up, I can supply the hydraulic calculations for the detention system. I just need to know the roof (plan) area and your region. I am assuming that you are having eaves and not box gutters.

You could also have a simple charged system without tanks if the council did not require a detention system but again...check your council's requirements. Many areas also require mosquito proof downpipe leaf diverters on charged downpipes.

It is always best to have a flush valve fitted to the ground level wet pipe at the pipe's lowest point or to an extended branch off the lowest point.

A detention system entails having a reduced size DWV drain pipe to restrict the flow rate to requirement. This has the added advantage of an increased flush velocity.

Brisbane City Council will let just about anything go through. We have a simple charged/sealed system (option 2) which required 2 x 100mm lines out to the street - works well (I think), we haven't had any real rain for months now!!!!!

Wow! That was my first post and am very happy with everyone's suggestions and advice.

Here is some additional info for some of your comments:
- I am in Melbourne, City of Moreland (Pascoe Vale South)
- I spoke to the city engineer and his response was , "Do what ever you need to get the storm water out", they have no restrictions on the various methods.
- The architect said that the design for the storm water was to be determined on site, after the site cut/levelling was completed. (This is when the problem arose)
- I am building a flat roof home, with box gutters. And double storey, so all the upstairs falls down to the downstairs roof, before going to the down pipes.
- Now I understand why the have placed the downpipes at those positions. The length of my home is 26metres. There are 3 DP on each side.

After all your replies, I have gone back to the architect one more time to put pressure on him to suggest a preferred system. That's why I paid home the cash:)

Thanks SaveH2O, I'll look at your links.

I never cease to be amazed at the different attitudes of councils.

Your simplest method is to have an uncomplicated charged system.

As you will have box gutters, the roof stormwater drainage has to be designed for a 1:100 Average Recurrence Interval (ARI). For Melbourne, this is 180 mm/hr, based on a 5 minute intensity of an average 3 mm /min. You then multiply the roof area by 3 mm/min to determine the amount of rain drained during a 1:100 ARI.

As you can see, if you have let's say a 200 sq m roof catchment (that has to also include the area of any gutter that protrudes past the roof line), our roof drainage example (1:100 ARI as applicable to box gutters) has to be designed for a minimum 600 lpm. This in turn governs the size and capacity of a pipe that diverts storm water to the gutter.

PVC DN 90 laid at 1/100 shall not carry more than 360 lpm and PVC DN 100 laid at 1/100 shall not carry more than 450 lpm as per AS 2200. The velocity, volume of water and pipe size is why most councils have storm water discharge regulations.

You are lucky that the council allows a free rein as it will stop you being ripped off but the above still needs to be addressed when deciding your final design. BTW; Victorian regs do not require leaf diverters or IOs on charged systems...but they should!!!

Note that I will check (time permitting) to see if AS 2200 has being altered in recent years. Your architect should know. Also note that charged drainage systems cannot facilitate surface drainage. Also, the previous link posted only covered the (most important) basics of pumped systems.

Are you really using an Architect or are you applying the term loosely for a draftie? There is a big difference.

Hi building-expert, I am using an architect, however after the initial concept of the house, he passed on the work to his side-kick for finalisation of any minor changes. In regards to the fall, we raised the levels early, however we were informed to wait until the demolition and site-cut was complete to determine if the fall would be satisfactory for a gravity solution.

Anyhow, I informed the plumber about a charged solution, however he is very reluctant to put one in. He even made a comment that if I insisted, that he wouldn't sign off on it. When I quized him as to why, he said he was worried that if the pipes cracked or leaked, that it may affect the foundations. Part of the stormwater runs beneath the garage slab as well.

Secondly, if I had driveway grate and agi piping going into the stormwater, would this cause any issues on a sealed/charge stormwater solution.

I am happy to consult a civil engineer / hydraulics engineer who would give him some further confidence on such systems.
Any recommendations from anyone.

[quote="Bazman"]Hi building-expert, In regards to the fall, we raised the levels early, however we were informed to wait until the demolition and site-cut was complete to determine if the fall would be satisfactory for a gravity solution.

In my view this is entirely unsatisfactory, it's designer's art and skill to make sure it all fits together properly. I don't get the logic of waiting for site cut when the floor levels are set at the design stage and you can work out drainage from the design levels (or do something about it).

Observing planning regulations, site boundaries, building envelope, regulations and attention to levels and services are the most basic design skills. Your architect has them, why are they not applied?

In any case, if there is a problem requiring engineering of the levels and drainage then it's your designer's job to solve it by consulting with appropriate professionals. Why should that now be your job is that not what they are paid for?

In my view recipe should be written before cooking starts or else you might not like the dish.

Many plumbers use flimsy (and cheap) 90 mm stormwater pipe that have walls less than 2 mm thick. PVC stormwater pipe is not pressure rated. It is also manufactured with a percentage of reground PVC.

One problem that occurs with this pipe is when a plumber is over generous with the solvent cement. Solvent cement melts the pipe surface and melds the two surfaces together. If excess solvent pools on a PVC surface after the pipes and fittings are pushed together, it can eat into the pipe or fitting and weaken it.

The two surfaces joined must also be pre wiped with primer to clean the surfaces and remove the glaze from the pipes and fittings before applying solvent cement to both surfaces.

Even if the pipes are properly joined, I would never recommend running PVC stormwater pipe under a slab. It is simply not worth the risk. I also don't recommend using PVC stormwater pipe for rainwater tank wet systems.

DWV (Drain Waste Vent) PVC pipe is thicker and uses a lower percentage of reground material. DWV fittings are also thicker and stronger.

If DWV pipe is used, then there should be no issues with burying it under a slab. Many houses are built with DWV (and stormwater) pipes buried under the slab.

I am wondering if the plumber was thinking of using stormwater pipe!

A 1 metre head of water exerts a pressure of 9.8 kPa. Many people, including some plumbers, confuse the weight of water with the pressure exerted in a charged system. If you have a charged system, it will not be subject to much pressure. Maybe the plumber thinks that the downpipes will be full of water every time it rains! That just won't happen as a .5 metre head would be more than sufficient.

A 100 mm PVC DWV pipe has an internal diameter (ID) of 104 mm. When doing pipe length friction loss calculations, the additional friction losses caused by the fittings must be added to the pipe length as an additional measure of pipe length. A (nominal ID) 100 mm DWV PVC pipe with a calculated friction loss length of 50 metres that is subjected to a .5 metre head will flow at over 500 lpm.

As per my earlier reference to AS 2200 for surface drainage systems when laid at 1:100, PVC DN 100 shall not carry more than 450 L/min and PVC DN 90 shall not carry more than 360 L/min.

Any possible leakage will not be a result of hydraulic pressure.

If you don't have a charged system , then you are restricted to using a very expensive pumped system (that also necessitates having a detention system). This will use PVC pressure pipe. PVC pressure pipes and PVC DWV pipes are made from the same material except that pressure pipe does not have regrind added during manufacture.

In Victoria, new homes are required to have either a solar HWS or a 2,000 litre RWT connected to sanitary flushing. Just about everyone gets ripped off with new home RWT installs but I can advise on the best and most cost effective way to have this done. You could satisfy both requirements by having a detention tank with a minimum 2,000 litres retention and this could be sited anywhere.

Hi Bazman,

Would you be ok to share what you ended up doing ?

Thanks!