overland flow in brisbane ?

[eyespy]

hi there,

can anyone explain the affect on developing properties which are affected by overland flow ?

some agents have said you cannot subdivide the property or build on it bc of overland flow.

i spoke to town planner today and he said can still build but just need to build higher to mitigate for flooding.(so same effect as if at risk of direct flooding)

 

[gaiusb]

You can absolutely build within an overland flow area - not sure about subdividing but I imagine it's the same.

Brisbane's planning scheme requires the habitable floor area of a building to be 500mm above the overland flow. The question is what is the actual overland flood height is, which cannot be found by council. You need to engage a hydraulics engineer to get an "overland flow report" to determine how high you need to build. The engineer might tell you that you need to build 1m out of the ground, they might say it is fine where it is for example. You won't know that until you engage an engineer though, and you're looking at between $500-$2500 for such a report.

 

[eyespy]

Thanks .....developing is so hard !

 

[RPI]

Sometimes the overland flow can be too expensive to deal with, sometimes it is next to nothing. Need to get a good relationship going with a hydraulic engineer.

 

[hpresident]

the property you are talking about is not on 8 olivella st. mansfield right?

if so I had got a prelimanary hydraulic report done already.

 

[eyespy]

Not that property , thanks anyhow.

 

[stevannt]

Overland flow

Does anyone have more information on the impact of overland flow on subdivision applications?

After some research, I've noticed there are additional requirements to address namely:

http://www.brisbane.qld.gov.au/CP/FloodPSP#RequirementsReconfiguringLot

http://www.brisbane.qld.gov.au/CP/FloodOC#Table82113c

I'm particularly interested in Table 8.2.11.3.J Flood planning levels for reconfiguring a lot, referred to in AO 18.1 below:

Additional criteria for reconfiguring a lot
PO17
Development locates and designs all lots resulting from reconfiguring a lot to:
(a) minimise the risk to people from flood hazard;
(b) minimise damage to property from flood hazard;
(c) facilitate safe and efficient evacuation.
Note?

Consideration of all floods up to the probably maximum flood is relevant to minimising the risk to people.
Flood warning time is not considered sufficient in the Creek/waterway planning area sub-categories or the Overland flow flood planning area sub-category.
Filling above the flood planning level for a flood event greater than the defined flood event cannot be assumed to mitigate the flood hazard.

AO17.1
Development creating new lots is to comply with Table 8.2.11.3.I.

AO17.2
Development provides for reconfiguring a lot design that achieves a road and lot layout which:
(a) provides trafficable vehicular egress for evacuation during a defined flood event;
(b) optimises hazard-free movement away from sources of flood hazard within the development.
Note?Further advice on road and lot layout is contained in the Flood planning scheme policy.

AO17.3
Development which creates a new residential lot in an area subject to Brisbane River flooding, if the residential flood level is greater than 12.8m AHD is not subject to a hydraulic hazard greater than 0.6m2/s DV or 0.6m deep in a 0.2% AEP flood.
Note?Refer to the Flood planning scheme policy for further explanation on the 0.2% AEP flood.

PO18
Development involving reconfiguring a lot:
(a) minimises the risk to people from flood hazard;
(b) creates safe evacuation routes or avoids isolation of the development during a flood greater than the defined flood event;
(c) minimises damage to property and services;
(d) provides lots and roads that are not frequently flooded or subject to nuisance ponding or seepage;
(e) ensures lots created for park or private open space minimise the risk to people from flood hazard and are fit for purpose;
(f) provides a lot that is not substantially burdened by flood mitigation infrastructure.

AO18.1
Development involving reconfiguring a lot ensures:
(a) all lots with the flood planning levels in Table 8.2.11.3.J;
(b) a new road complies with the flood planning levels in Table 8.2.11.3.F.

AO18.2
Development involving reconfiguring a lot creating more than 6 residential lots or a lot for industry ensures the flood planning levels of a dedicated road fronting the development or providing primary access within 200m of the development is:
(a) complies with Table 8.2.11.3.K; or
(b) has acceptable trafficability in accordance with the requirements in the Flood planning scheme policy and the Queensland Urban Drainage Manual.
Note?The Flood planning scheme policy contains supporting information about trafficability on existing roads and serviceability during floods.

AO18.3
Development protects the conveyance of flood hazard area by providing an easement over the:
(a) 2% AEP flood extent for overland flow flooding;
(b) 1% AEP flood extent for creek/waterway flooding.

A flood risk assessment is required for residential reconfiguring a lot development where creating more than two new lots in the overland flow flood planning area sub-category.
Additional requirements apply for the Brisbane River flood planning area sub-category if the residential flood level is greater than 12.8m AHD.
Minimum site levels, requirements for no adverse off site impacts and other provisions in the planning scheme still apply.

From what I'm reading you need to mitigate against floods 2 per cent AEP plus 300mm. How does one mitigate the risk of overland flow at the subdivision stage as opposed to the building stage, especially for flat land.

 

[RPI]

From what I'm reading you need to mitigate against floods 2 per cent AEP plus 300mm. How does one mitigate the risk of overland flow at the subdivision stage as opposed to the building stage, especially for flat land.

It depends on the level of Overland flow. The access to the block (ie the crossover and the road leading to it) needs to not get to a depth greater than 1m, otherwise you can have a real issue with any approval at all.

Hydraulic engineer is the one to deal with this sort of thing.

At the building stage most Overland Flow Paths are dealt with by building on stumps or angling solid walls so as to not cause any damming of the flow.

regards

 

[LJW]

Stevannt, overland flow can be quite a significant issue when it comes to subdividing a site. If the lot is existing and you are simply proposing a new house on the site, it's a relatively straight forward process.

However, as a general rule, Council are not supportive of the creation of new lots on floodable land. Under Council's Subdivision and Development Guidelines, where the site is subject to local flooding (i.e. overland flow), the minimum lot levels are to be above 50y ARI + 0.3m. It is required that 60% of the new lot size or 300 sqm rectangular shaped area (whichever is the greater area), achieves the flood immunity standards specified above. See link below.

http://www.brisbane.qld.gov.au/docu..._ed_part a_chapter 01_flood_affected_land.pdf

As RPI mentioned, you will need to engage a hydraulic engineer to determine the extent of the overland flow (depth, flow etc). If the site cannot meet the requirements of the subdivision and development guidelines, you may be able to look at alternative measures (i.e. compensatory cutting/filling) but this needs to be discussed with a hydraulic engineer.

Cheers
Liam

 

[MiQu]

You can absolutely build within an overland flow area - not sure about subdividing but I imagine it's the same.

Brisbane's planning scheme requires the habitable floor area of a building to be 500mm above the overland flow. The question is what is the actual overland flood height is, which cannot be found by council. You need to engage a hydraulics engineer to get an "overland flow report" to determine how high you need to build. The engineer might tell you that you need to build 1m out of the ground, they might say it is fine where it is for example. You won't know that until you engage an engineer though, and you're looking at between $500-$2500 for such a report.

I am currently considering a property with overland flow, affecting a small corner (5-8%) of total land area. Does not involve current building footprint...

How likely is this going to affect any future developments (given that its only a small corner, which may not be affect the development / building footprint)?

My strategy is a medium to long term hold, but may consider redeveloping or sale in future.

Thank you.

 

[Tim86]

I am currently considering a property with overland flow, affecting a small corner (5-8%) of total land area. Does not involve current building footprint...

How likely is this going to affect any future developments (given that its only a small corner, which may not be affect the development / building footprint)?

My strategy is a medium to long term hold, but may consider redeveloping or sale in future.

Thank you.

Have a look at my BCC post in the legal forum.

Granted my block was much more affected by overland flow. But what makes you say it's only a small corner of your block that's affected. Have you paid $3000 - $6000 for a hydrologist report?

If it were me I would run for the hills. Figuratively and literally.

 

[MiQu]

Have a look at my BCC post in the legal forum.

Granted my block was much more affected by overland flow. But what makes you say it's only a small corner of your block that's affected. Have you paid $3000 - $6000 for a hydrologist report?

If it were me I would run for the hills. Figuratively and literally.

No reports...Looked like a small corner on the city plan maps...

 

[gaiusb]

No reports...Looked like a small corner on the city plan maps...

If you build outside the mapped overland flow area then you can pretty much ignore the overland flow. if you want to do other things in the overland flow like putting a shed, pool or retaining wall and fill then it can get messy, as the intent is to not redirect overland flow to affect your neighbours.

If you sought a building approval for a new house or a reno on a house or whatever, the certifier would see that you're outside of the overland flow area and won't require a hydraulics report.

 

[MiQu]

If you build outside the mapped overland flow area then you can pretty much ignore the overland flow. if you want to do other things in the overland flow like putting a shed, pool or retaining wall and fill then it can get messy, as the intent is to not redirect overland flow to affect your neighbours.

If you sought a building approval for a new house or a reno on a house or whatever, the certifier would see that you're outside of the overland flow area and won't require a hydraulics report.

That makes sense. Thank you. Much appreciated.

 

[quoc]

Hi Sorry to reopen an old thread over overland flow but how accurate would you take brisbane city council's mapping system for overland flow found on
http://cityplan2014maps.brisbane.qld.gov.au/CityPlan/

The subject land has overflow issues at the back of the property but nothing encroaching the actual land and is in an area in Moorooka

Would this affect development potential and if anyone has had any reports for overland flow in moorooka that would be helpful!