Storm water retention Perth - council condition

[CharlieT]

Hi all,

We're building our PPOR on a clay site in Perth's eastern suburbs. We've received conditional building approval from our council subject to the on site retention of stormwater. Our builder had assumed that we could dispose of our storm water via the council drains.

We've been advised that there is limited capacity in the street network and it's simply not possible to connect to this network.

Our problem is that we've designed our entire build around the future subdivision of our block. I think our builder solution is to dump massive soak wells on the future subdivided land. I don't suppose anyone has experience with challenging a condition such as this? Are we any chance of convincing the council?

Also does anyone have any experience with containing stormwater on clay sites? Are there any reasonable options outside of massive soak wells? Also, am I right in assuming that in a strata subdivision that drains from either property could be located on a common driveway?

 

[westminster]

Lots and lots of soak wells are unfortunately a part of life. With clay soil it makes it worse though as more or bigger ones are needed I believe.
You can't put them all on the other land because the other land will never get approval either.
How many will you need?
Get the good ones which are trafficable and stick them under your driveway, in your front garden and rear garden.

Yes in a strata division you can both put them under the driveway if you have the trafficable ones. I have 5 of 1500mm ones under the driveway in my Gwelup development.

 

[sanj]

Your builder is a bit of an idiot if this is the case, which company is it?

 

[Aaron Sice]

You can use 150dia pipes to increase holding capacity.

Look at an atlantis cell setup under the common drive.....if there is one?

 

[CharlieT]

Thanks Westminster. Our design only permits the use of the future common driveway and the front of our property. The councils condition suggests that the soak wells be placed at a distance equal to the depth of the soak well away from the house and any boundary. Given we've only left with a 4m common driveway and a 5m distance from the front boundary, I don't see how any massive soakwells are going to comply with the condition.

Sanj - I won't comment on the builder yet, as we're still finding a solution to the problem. I think they were essentially going to ignore the fact that we were planning on a future subdivision. I'm hopeful we'll find a solution without too much heartache.

Aaron - Thanks for the info. I'll have a good look at these solutions.

 

[GreenMedallion]

This is a common requirement these days, especially in older areas where the stormwater assets often are already over capacity (or non existent).

The underground storage tank mentioned above is a decent suggestion. What I would be trying to do first however is a simple rain water tank. You probably already have one for potable water for BASIX requirements although you could either add a second one or increase the size of the currently planned rainwater tank (e.g. 2000 litres for potable water; 8000 litres for stormwater capture etc).

The idea being that the rainwater/stormwater tank is sufficiently large to capture the 1% AEP storm. The outflow/overflow outlet from the tank is nominally limited so that the discharge from your roof occurs slowly and mostly after the storm has passed and the peak levels in the Council stormwater system have decreased. The Council should be amicable to this idea.

N.b. the 1% AEP storm (also called the 1 in 100 year storm, 100 year ARI event) is a storm equivalent to one that will occur on average every 100 years. However, it may occur this year AND again next year AND the year after etc. It is not once every 100 years.

To clarify you want stormwater detention, not retention. Retention implies you hold the water permanently (e.g. a pond) and it disappears via evaporation/infiltration. Detention means you detain the water during the storm and release it slowly over time after the storm has passed. This reduces the peak flows experienced downstream of you.

 

[Doovalacky]

The soakwell issue has gone well overboard these days but any builder should know to plan for it. Its got to the point were I had to provide a soakwell plan just for a shed.

Without seeing the plan it is hard to say what you could do. I had a 6mx12m turning area (common) in front of 2 garages I used.
How wide is the driveway? Can you get a row of 900x900 wells down it?

5m to play with out front should be plenty however. 1800 diameter x1200 height holds ~3000L. You should have been given a spec such as 2000L for every 65m2 of roof space. From memory there was quite a jump in cost over 1200 diameter especially if required trafficable lids.

 

[CharlieT]

Thanks for the additional responses. Rainwater tanks are quite uncommon in WA, and there's not too many places for one on the property. The idea of releasing water over time sounds reasonable. I'm not sure what level of conversation our builder has had with council but it's sounding like we need to get involved. I'm actually skeptical that such a conversation has taken place.

The idea of soak well on a clay site is pretty crazy. It's not like the water can go anywhere. The front driveway area will be the most likely place for drainage, with an area of about 10m by 5m. That's Assuming they can take all of the water here, as we've got about 350sqm under the roof.

We're still waiting on the builders response, but it's annoying that they seemed to forget/ignore the subdivision plan, and more annoying they didn't include a soak well set up in the building contract. They have supposedly built numerous homes in the street previously.

 

[Jake D]

Which council? (Gosnells?)

Are you higher or lower than the street?

Along with other suggestions, you can also use sometimes use above ground storage on paved areas. And can build big soakwells under your garage (I'm assuming a 6x6 double)

Depending one how much you want to spend, you could also use piling on your footings to allow your soakwells to be located closer to the house (and/or upgraded slab)

 

[Jake D]

The idea of soak well on a clay site is pretty crazy.

How much sand pad are you going to have?

 

[CharlieT]

Basso, finished house will be about 800mm above street level and we'll have a 900mm sand pad. It looks like there is an old connection to the street drainage at the front of the property.

I think we'll have to upgrade our footings to include pilings. Would be much simpler to connect to the street drains.

 

[jim1964]

Would be much simpler to connect to the street drains.

The soakwell issue has gone well overboard these days

Water retention has become a real PITA for anyone wanting to do anything in terms of development,warranted yes in certain circumstances,but in a lot of cases well overdone in terms of engineering.The old days were a bit simpler IMHO.