FAQs

A flood study is a comprehensive technical investigation of flood behaviour within a catchment.. It uses sophisticated computer models to simulate flooding, defining the nature of flood hazards across the floodplain and provide information on the extent, level and velocity of floodwaters and distribution of flood flows.

Flood studies produce flood models which are important to understand the flood risk for an area, rather than relying only on historical events (which may not be applicable in current day). 

A flood study will usually include several maps for different flood scenario which may show:

• flood extent (how far the water spreads)
• an indication of flood depth (how deep the water is across the flooded area)
• an indication of flood velocity (how fast and in which direction the water is travelling) 
• flood hazard (low, medium, high and extreme areas of hazard based on factors including velocity of the water and whether this poses a threat)
• a maximum probable flood event.

Flood studies produce maps that define flooding characteristics across the floodplain. These are used for community flood awareness, flood risk planning, development controls and emergency management.

Council’s Flood Study Program seeks to improve our understanding of the flood risks to our communities in order to manage risk and support emergency preparedness and infrastructure planning.

Council engaged engineering consultancies KBR and GHD to undertake the studies on behalf of Council. These consultancies are recognized as industry leaders in the flood field and are highly experienced and appropriately qualified to develop the studies.

Upon completion of the public consultation process, the project team will review and consider all feedback received. Where appropriate, the study will be amended to address any valid issues identified. The flood studies will then be presented to Council for adoption and will be used for defining the minimum floor levels for building approvals and development purposes and for future infrastructure planning and emergency preparedness. At a later date Council will seek to amend the Isaac Regional Planning Scheme flood hazard overlay to align with those areas in the flood studies impacted by 1% AEP (+climate change) flood events.

Council is providing information which helps all landowners and residents understand the possible impacts of flood on a property and make plans to build their resilience. If your property is included in the flood risk map it means there is an existing risk – low, significant, high or extreme of flood. Being included in the flood risk mapping area doesn’t impact your existing development rights, however it may mean that any future development needs to meet prescribed minimum flood levels.

A Q100 is a flood event that has on average, a 1 in 100 chance (1% probability) of being equaled or exceeded in any given year. The Q100 flood, otherwise known as the 1% AEP flood or the 100 year ARI flood, (the terms are interchangeable), is sometimes called the '100 year flood'

 A common misunderstanding is that a 100-year flood is likely to occur only once in a 100-year period. This does not imply that 100 years elapses between such floods. While this is true in terms of long-term averages (over several thousand years), it is not true in terms of the immediate future. Thus, it is possible (but unlikely) for '100 year floods' to occur in consecutive years or within relatively short periods. Certainly, there is no guarantee that a 100 year period will elapse between such floods. Even if the 1% AEP floods occurred last year, there is a 1% chance that this flood will occur this year - and in each and every future year.

 The 1% AEP flood events are what are commonly used for establishing minimum floor levels for building and development purposes in accordance with the Building Act 1975 and Planning Act 2016. 

The PMF is the worst case scenario for flooding that shows the maximum extent of the flood plain. Most planning decisions are based on the 1% AEP flood and an allowance for uncertainty and the potential effects of climate change, with some key community facilities, such as hospitals being required to locate above a 0.2 % AEP.

 The PMF is a theoretical flood resulting from the largest rainfall that could possibly occur within the catchment and therefore is extremely rare and unlikely. It is also used to determine which areas are potentially flood prone (in the floodplain) and which areas are flood free (only land outside the PMF extent is truly ‘flood free’).

The Building Act 1975 and the Building Regulations 2021 requires council to declare a flood hazard area for building regulation purposes (i.e. determining minimum floor levels).

Historically, the planning scheme’s flood hazard overlay map has been used for the purpose of determining the flood hazard area. To this regard, if you are currently undertaking development over a site in the flood hazard area (as identified by the Flood Hazard overlay in the Planning Scheme), you are required to undertake an independent flood study to determine the minimum habitable flood level for your development.

Following adoption of the draft flood studies, Council will adopt their modelled 1% AEP flood events (with an allowance for climate change), as the defined flood hazard areas. This will mean that new development located in these defined flood hazard areas will need to meet minimum building levels based off the defined flood levels of the flood studies and the building regulation requirements of the Queensland Development Code.

Council’s flood studies have generated several defined flood events, as per below:

•        Q2: 1 in 2-year flood event, or 50% AEP

•        Q5: 1 in 5-year flood event, or 20% AEP

•        Q20:1 in 20-year flood event, or 5% AEP

•        Q50:1 in 50-year flood event, or 2% AEP

•        Q100: 1 in 100-year flood event, or 1% AEP

•        Q100: 1 in 100-year flood event plus Climate Change or 1 % AEP plus Climate Change

•         Q500: 1 in 200-year flood event, or 0.5 % AEP

•        1 in 500 year, or 0.2% AEP

•        Probable Maximum Flood (PMF).

*AEP is Annual Exceedance Probability

The Isaac Regional Planning Scheme (Planning Scheme) currently contains a Flood Hazard Overlay Map which identifies a flood hazards areas.  This current mapping relies on broad-scale State Government flood mapping which is not as accurate as local flood studies can provide. If you are currently undertaking development on a property located in the Flood Hazard Overlay Map, you are required to undertake a site specific flood study (at your own cost) to determine the extent of onsite flood impacts and minimum habitable floor levels. 

Following the adoption of the flood studies, Council and private certifiers will be able to utilize the flood study data to better understand the impacts of flood during the building and development process and to determine minimum habitable floor levels. This will remove the need for individual landowners and developers to prepare their own costly site specific flood studies.     

In mid to late 2024, Council will commence the process to amend the Planning Scheme to incorporate the findings from the completed flood study to replace the current State mapping for areas in which localized flood studies have been completed. This process will involve further public notification and direct engagement with impacted land owners. 

The effect of this amendment will be that the majority of land that is identified in the current Flood Hazard Overlay will remain as flood affected.  However it is noted that some properties that were identified as flood prone will no longer be impacted and others that were not previously identified as flood prone will now be identified as such. 

If you are currently located in the Flood Hazard Overlay in the Planning Scheme but not located in the study area of one of the new flood studies, you will not be impacted by the flood studies and future Planning Scheme amendment

The Isaac Regional Planning Scheme (Planning Scheme) currently contains a Flood Hazard Overlay Map which identifies flood hazard areas.  This current mapping relies on broad-scale State Government flood mapping which is not as accurate as local flood studies can provide.  

Following the finalization of the flood study, Council will seek to amend the Planning Scheme to incorporate the findings from the completed flood study to replace the current State mapping with the revised flood hazard areas. 

The 1% AEP (+ climate change) flood event will be used as the basis for the revised Flood Hazard Overlay Map in the Planning Scheme. Follow the below steps to view these areas in Council's online mapping:

• Open Council’s interactive flood mapping program - Online mapping - Isaac Regional Council 
• Select Flood Study Consultation
• Agree to terms and conditions
• Click on 'Lotplan Search’ or ‘Address Search’ on the ribbon towards the bottom of the page and type your relevant property details
• Select the relevant flood studies from the Layers section on the left hand side of the page.
• Select the desired flood modelling events available for your property on the left hand side of the page.. 

Note: Council’s interactive online mapping also allows you to view the properties which are impacted by the existing Flood hazard overlay code in the Isaac Regional Planning Scheme. To do this select the ‘Flood Hazard Overlay’ from the Layers section on the left hand side of the page.   

The flood study is complementary to the Coastal Hazard Adaptation Strategy that is focused on impacts from the sea (erosion and storm surge). The coastal hazards and development controls identified in the Planning Scheme with relation to Coastal Hazards will not be impacted by the flood study.

Flood insurance terms, conditions and premiums are set by the insurance industry. Insurers may base property assessments on Council’s flood information however, Council has no influence over flood insurance products or premiums.

Isaac Regional Council in conjunction with the Bureau of Meteorology and neighbouring Councils operates a flood warning system for the main river catchments through river and rain gauges that enables accurate and timely flood warnings and forecasts.  When issuing warning the Bureau of Meteorology description of severity of flooding is termed as minor, moderate or major according to the effects felt in the local area.

Minor flooding causes inconvenience. Low-lying areas next to water courses are inundated. Minor roads may be closed and low-level bridges submerged. In urban areas inundation may affect some backyards and buildings below the floor level as well as bicycle and pedestrian paths. In rural areas, removal of stock and equipment may be required.

In a moderate flood, in addition to the above, the area of inundation is more substantial. Main traffic routes may be affected. Some buildings may be affected above the floor level. Evacuation of flood affected areas may be required. In rural areas, removal of stock is required.

Major flooding will also cause extensive inundation of rural areas and/or urban areas. Many buildings may be affected above the floor level. Properties and towns are likely to be isolated and major rail and traffic routes closed. Evacuation of flood affected areas may be required. Utilities, such as power, phone and water services, may be impacted.

You should become familiar with your catchment and the flood classification levels for that catchment.  To find out more, view the following Bureau of Meteorology videos:

Understanding Floods   https://www.youtube.com/watch?v=ivUKLr8q4sE

Understanding Flood Classification https://www.youtube.com/watch?v=gKDPgp5Ds9s   

Following the current consultation process, Council will progress with the adoption of the flood studies which will result in the adoption of revised ‘flood hazard areas’. Flood hazard areas will include areas impacted by a ‘1% Annual Exceedance Probability (AEP) + Climate Change’ modelled flood event (as can be viewed via council’s online mapping). This will not affect existing development, however new development located in a flood hazard area will be required to meet minimum building floor levels to ensure that the risk of flood hazards is reduced. Existing flood hazard areas are currently identified in the Isaac Regional Planning Scheme as detailed below.