Agnes Water Drainage Study

Overview

Gladstone Regional Council has formally adopted the Agnes Water Drainage Study, delivering improved outcomes for landowners and greater certainty for the building and development industry in the Agnes Water area.

The comprehensive drainage study provides updated flood mapping and clear guidance for residential development applications, replacing previous uncertainty with streamlined processes that benefit both property owners and industry professionals.

Scenario 1 - Site not mapped

Scenario

Dwelling within flood investigation overlay area and

the site is not mapped as including any flood extent or applicable flood level under the Agnes Drainage Study Maps

Example 1- Site not mapped within the flood extent

Council Advice

In this case, Council can issue a written notice confirming, for the purposes of complying with acceptable outcome A01 of the Flood Hazard Overlay Code, that it is satisfied that the performance outcome PO1 would be achieved without the need for a site-specific flood hazard assessment. This is because the extent of the flood has been verified as not impacting this site.

Process/Explanation

Council has issued pre-emptive broadscale written notices for lots that qualify for this requirement. Where applicable, Council will notify landowners by letter.

Once written advice is provided, land owners can use the letter to satisfy AO1 of the Flood Overlay Code.

Scenario 2a - Site partially mapped/ dwelling outside flood extent

Scenario

Dwelling within the Investigation Overlay Area and

Part of the subject site is mapped within the flood extent (which requires a flood height for the subject site)

Example 2 - Site partially mapped with flood extent

The proposal is for a slab on ground dwelling sited wholly outside the flood extent with a only a driveway in the area mapped flood extent.

Scenario 2a.png

Council Advice

Provided the development/dwelling is located outside the mapped flood extent it is considered that the proposal would meet A01 of the Flood Hazard Overlay code being – Development including intensification of an existing use, does not occur on land within a flood hazard area. (In this case the flood hazard area is considered as the mapped flood extent).

Note - Flood hazard areas adopted by Council resolution requires building certifiers to ensure compliance with relevant requirements including QDC MP3.5.

Process/Explanation

Customers should check with their Building Certifier to ensure compliance with QDCMP3.5

Scenario 2b - Site mapped/ dwelling built on poles

Scenario

Dwelling within the Investigation Overlay Area and

• part of the subject site is mapped within the flood extent (which requires a flood height for the subject site – refer to example 2) or

• the entire site is mapped within the flood extent (refer to example 3)

The proposed development is for a dwelling within or partially within the flood extent area that is 100% built on poles at the free board level (ie. flood height plus 1m) allowing for the unobstructed conveyance of flood water under the dwelling.

Example 3 - Entire site is mapped within the flood extents

Example 4 - House on poles

Council Advice

In accordance with the Flood Investigation Overlay Code –

The applicant can either seek to get a flood assessment completed in accordance with A01.

A01- a site specific flood hazard assessment demonstrates that risks associated with the development can be mitigated to an acceptable or tolerable level.

This would not involve new modelling to be required provided an RPEQ consultant had access to Councils flood model data. So that they could certify the development was at the appropriate height and allowed for unobstructed conveyance of flood water within the understorey.

Completing this requirement would allow the development to continue to be accepted, the building certifier to have regard to the RPEQ advice and no MCU would be required.

Alternatively, the accepted development provisions allow:

If the premises is located in a Flood Hazard Investigation Area in the Flood Hazard Overlay mapping, a written notice is issued by Council at its sole discretion, for the purposes of this acceptable outcome, confirms that it is satisfied that the performance outcome P01 would be achieved without the need for a site specific flood hazard assessment and/or a Registered Professional Engineer of Queensland certifying the actual level of flood risk for the site and measures required to ensure the risk associated with the development can be mitigated to an acceptable or tolerable level.

Note - In exercising its discretion, Gladstone Regional Council may, without limitation, have regard to:

the location and characteristics of the site any existing flood studies or work being carried out in the course of undertaking flood studies which may be relevant to the site
any work undertaken by or on behalf of Gladstone Regional Council in the course of locally verifying the extent of flood hazards which may be relevant to the site

In this situation, the customer could submit plans to Council for an Accepted Development Compliance Check.

Council can check to ensure the development complies with height and understorey requirements (described above) and issue advice that the development complies with A01 option 3 and that PO1 is satisfied without the need for a site specific flood study.

Process/Explanation

Customers can choose between engaging an RPEQ qualified consultant or lodging an accepted development compliance check with Council.

Once either process is completed the letter or advice can be forwarded to your building certifier to consider as part of the building works application.

Scenario 3 - Site mapped/ dwelling on slab

Scenario

Dwelling within the Investigation Overlay Area and

Part of the subject site is mapped within the flood extent (which requires a flood height for the subject site) refer to example 2 or
the entire site is mapped within the flood extent refer to example 3

The proposal is for slab on ground construction or any partial slab on ground construction (garage) and the development is wholly or partially within the flood extent area

Example 4 - Slab on ground construction for dwelling house

Council Advice

As this is a slab on ground proposal and doesn’t allow for the unobstructed conveyance of flood water, a flood model will need to be developed by an RPEQ engineer to demonstrate compliance with AO1 that can then ensure the development remains accepted (ie. not needing an MCU).

A01- a site specific flood hazard assessment demonstrates that risks associated with the development can be mitigated to an acceptable or tolerable level.

The key risks for this type of proposal are that proposed slab on ground construction may result in flood water worsening for adjoining site. Ie. flood storage is taken up and therefore the flood water moves to another space or that flood waters moving at over 1m/sec can have a “bounce” when they hit an obstruction.

Note – Councils modelling can be provided on data sharing agreement, please contact Councils Asset Planning team for more information.

Process/Explanation

Customers will need to engage an RPEQ to certify compliance with AO1 to be considered accepted development. Where the RPEQ provides advice that satisfies AO1 this can be considered as part of a building works application.

If AO1 cannot be satisfied an MCU is required to be lodged to Council demonstrating compliance with PO1 of the Flood Hazard Overlay Code, using a site specific flood report.

Scenario 4 - Site now mapped/ not mapped in Planning Scheme

Scenario

Subject Site is not mapped as part of the flood overlay maps, however includes flood mapping adopted by Council (not yet forming part of the planning scheme).

Council Advice

Flood hazard areas adopted by Council resolution requires building certifiers to ensure compliance with relevant requirements including QDC MP3.5.

For other development types ie. Code/ impact assessable - ROL or MCU Council will have regard to updated flood mapping as part of the application process.

Process/Explanation

Check with your building certifier to ensure your development is compliant.

Ensure your development appropriately accounts for the flood characteristics of your site as part of any MCU or ROL lodged with Council.