Subdividing large block with overland flow and services

S

Steel Concrete

Hi All,

I recently bought a big block (1250m2) PPR in a good Brisbane suburb and knew of its deficiencies during due dilligence including 900 dia concrete stormwater pipe collecting stormwater / overland flow drainage and a sewer through the back yard! The white ants and dodgy reno / extension construction don't help either. But the price was right and the wife is happy...

The future plan is to tidy the place up, live in the house for 15 - 20 years and the bonus upside is to subdivide, (by possibly building a new house) before selling.

Survey plan attached showing overland flow collected at top left of the block and piped diagonally to the right when rainfall less than ARI20 and when a big storm hits...flowing overground along the same path.

Does anyone have any experence - and ideally success - with Brisbane City Council in subdividing a block with overland flow across it?

any and all comments appreciated.... with thanks,

SC
 

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Have you witnessed the over land flows? It looks as though it flows from right to left on the survey??

go to the dial before you digg plans and get more plans for surrounding your block, mainly regarding the sewer. I'd imagine it runs along the rear boundary so shouldn't be to much of an issue. The storm water could be a problem. It's a decent sized pipe, the dbyd plans might show additional info. Council may have issues with a pipe of that size even though storm water is the neglected service by council from a maintenance point of view. if there is over ground flows they may require modelling and investigation to the current pipe size plus additional pits to collect and dump into if you are to build at the rear.

For starters it may be worth along council about the over land flows and what can be done before building?? As in a collection put on the high side of your block to prevent run off.
 
Thanks Bob,

I've watched three recent big storms (up to 80mm rainfall) and the collection pit in the top left corner of block (collecting the upstream overland flow) only flowed half full at the 900dia pipe.

A previous hydraulic model shows about 0.5m depth of overland flow across 10m width of the northern site during the maximum design storm event of circa 250mm rainfall; this would be about 8 m3/sec of water! So when I get a proper big storm the garden is going to be a deep river!

It doesnt seem viable to build a big enough pipe to catch the design overland flow as it would need to be 2+m in diameter.

I'm thinking more about planning a 300m2 house pad in the top right corner of the block which doesnt obstruct overland flow and constructing a pole house way above the maximum flood level.

I've 'dialed before digging' and there is a 200dia approx sewer running below and parrallel to the 900 storm pipe. Its appears to be the storm pipe easement and overland flow that would be the main show stoppers from getting a subdivison approved.

I guess I'm looking for developers or consultants who have had recent or any success in this sort of problem with the BCC as council appear to be very cautious about approving such type of subdivisions.

I don't want to spend $20k on fees/consultants on a subdivision application if there is effectively no chance of success, BUT I'd bet similar money if there was fair odds of success to get a new block of land. :)

I'll take your good advice Bob and arrange a chat with council....

Any and all comments appreciated.

Cheers,

SC
 
I've had recent experience with a subdivision application for a block of land affected by overland flow, unfortunately I haven't had recent success.

You mentioned subdividing and building a house on poles. Just keep in mind that the new block cannot have a house on poles as it has to meet flood immunity so you will have to fill the block to make it higher. That will in turn affect neighboring properties as overland flow would be diverted into their properties increasing the flood risk for those neighbors. Council will reject it based on this unless there is an engineering solution. We had a nearby waterway, the engineering solution proposed to us was to spend $20 000+ making the waterway wider, of course that would require council permission to dig up the park across the road to make that waterway wider...not very likely.

Obviously your block of land isn't my block of land, so there may be differences that allow yours to get passed. So you never know. However I would recommend stocking up on lots of panadol because you're going to be in for some headaches.
 
That's a lot of water!! Is it a river bed dressed as a residential block??

Can you upload the dbyd? Drawings are easier than words for me! Even a photo of the block

I was also going to suggest moving the storm water pipe with done additional pits to go around the proposed area and follow your boundaries but a pipe that size and the amount of water becomes a costly exercise!

Could be tricky getting anything through! Could you down size to a granny flat that would avoid the pipes??
 
Tim86 said:
I've had recent experience with a subdivision application for a block of land affected by overland flow, unfortunately I haven't had recent success.

You mentioned subdividing and building a house on poles. Just keep in mind that the new block cannot have a house on poles as it has to meet flood immunity so you will have to fill the block to make it higher. That will in turn affect neighboring properties as overland flow would be diverted into their properties increasing the flood risk for those neighbors. Council will reject it based on this unless there is an engineering solution. We had a nearby waterway, the engineering solution proposed to us was to spend $20 000+ making the waterway wider, of course that would require council permission to dig up the park across the road to make that waterway wider...not very likely.

Obviously your block of land isn't my block of land, so there may be differences that allow yours to get passed. So you never know. However I would recommend stocking up on lots of panadol because you're going to be in for some headaches.
Click to expand...

https://pdonline.brisbane.qld.gov.a...ymaster/default.aspx?page=wrapper&key=1121304

No reason why you cant raise it on steel posts to gain immunity, they will actually not allow fill which changes the overland flow.

Cheers
 
Overland flow can be pretty tricky and it is difficult to provide advice as every single lot affected by overland flow is different and depends on the extent of the overland flow path, catchment size, flow velocity, flood depth etc.

How much of the new lot at the rear would be outside of the mapped overland flow path?

I have seen a number of recent subdivision applications (affected by overland flow) that have been refused by Council. However, I have also seen one recent example that was approved by Council (under City Plan 2014) over a site that had significant overland flow (i.e 75% of the site). In this example, the applicant was able to demonstrate that the site had flood free access and they lodged plans with the subdivision to demonstrate that the proposed houses were able to meet the minimum habitable floor level and result in no worsening to neighbouring properties.
 
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