Flood Planning Q&A

A flood study is a technical investigation of flood behaviour for a particular catchment, river or creek. The aims of a flood study are to define existing flood behaviour, including depths, extents and velocities, to help inform practices such as building, land use planning, community awareness and disaster management. This allows us to minimise flood risk and enhance flood resilience to people, property and infrastructure.

A flood map visually shows the flood behaviour of a range of flood scenarios. A flood study will usually produce a number of maps for different flood scenarios or simulation of a past event that show (at least) the following:

• flood extent (how far the water spreads);

• an indication of flood depth (how deep the water is across the flooded area); and

• an indication of flood velocity (how fast and in which direction the water is travelling).

Read more about:

• flood studies

• flood maps.

The flood models which created the flood maps were produced by external engineering consultants using industry best practice. The nine flood models that were produced for the ‘Townsville Recalibrated Flood Modelling and Mapping Project (TRFMMP)’ were all peer-reviewed by a different engineering consultant to ensure their accuracy.

While the models and maps were made using best practice, they show likely flooding scenarios. Actual flooding events in the future may be different from what is shown in the maps.

The flood models are based on on-ground data for a particular moment in time. As new development and natural processes change the landscape, this can change the nature of flooding. When planning for flood, larger new developments will have to consider these changes. Council will periodically update its flood studies to reflect these changes.

The flood study areas cover mostly the urban areas of Townsville; however, there are some locations outside of these flood study areas that may still be susceptible to flood. While we have less detailed flood information about these areas, the Queensland Reconstruction Authority has mapped them, and they are also shown in Council’s Flood Information Portal mapping and are shown as ‘Flood Investigation Area’.

In the current Townsville City Plan mapping, these areas are known as ‘Medium hazard – further investigation area’. In these areas, new buildings will generally need to be located on the highest part of the site.

When we talk about a hazard, we are talking about a natural event that has the potential to cause harm to people, property or the environment. For flood, deeper or fast flowing flood waters present a high hazard, whereas shallower and slower moving waters present a low hazard.

Risk is how likely it is a hazard will impact people, property or the environment, as well as the consequence of that impact. For example, an area close to a river which floods regularly might be a high hazard area, but if there are no buildings located here, there are limited consequences from flooding, making it a lower risk. If people live or there are buildings located in these areas, flooding may have greater consequences, making this a higher risk.

Flood risk levels were calculated by assessing the level of hazard under different likelihood flooding events.

Generally, the Very High and High Risk areas experience both more severe and more frequent flooding.

Very Low and Low Risk areas tend to experience less severe and less frequent flooding.

Medium Risk may be caused be rare flooding which is more severe or frequent flooding which is less severe, or moderate flooding with moderate severity.

Read more on flood risk mapping.

A ‘one in one-hundred-year flood’ is a commonly used term; however, this term can be very misleading. We don’t know when or how the next flood will occur, but we can estimate the likelihood (probability or chances) of a certain size flood at a given location during a given period of time. The term Annual Exceedance Probability or AEP is now the preferred terminology. The equivalent of a 1-in-100 year flood is a 1% chance in any one year to see a flood of this size or greater. It is possible for more than one of these floods to occur within a year, or in 5 years, 10 years or it might take more than 100 years for the next one; we just don’t know.

Annual Exceedance Probability (AEP) is used to explain the chance of a flood of a given size (or larger) occurring in any one year, usually expressed as a percentage. For example, the Bureau of Meteorology explains that for each and every year, there is a 1% chance (a 1 in 100 chance) that the defined event will be equalled or exceeded (once or more than once).

The defined flood event is the area shown in the 1% annual exceedance probability (AEP) flood map. The defined flood level is the level that floodwaters are expected to reach during the defined flood event. We use the defined flood level to determine appropriate floor heights for most new development.

The flood mapping will only apply when setting flood resilient standards for new development. If your house met the flood standards at the time it was built then you won’t be required to change to meet the new standards.

As part of our community engagement, we are asking for your feedback about whether extensions to existing houses should be required to meet the new flood standards for floor levels.

We are planning for a Townsville that is resilient now and into the future. Some of the oldest buildings in Townsville are more than one-hundred years old. We need to expect that a building built today might still be being used in the next century. It is also a Queensland state planning requirement that planning schemes consider climate change for hazards.

Climate change projections show an increase in rainfall intensity in the future. As such, our new flood models consider a climate change scenario for the year 2100, and this is shown in our flood depth maps and flood risk maps.

Flood depth maps do not show a specific flooding event. These show the likely extent and depth of flooding in a 1% AEP event.

It is important to remember that all flood events are different and different parts of Townsville can be affected differently. Geographic features like hills and mountains can create big variations in rainfall intensity and duration in different parts of Townsville. The different creek and river catchments may also behave differently and affect the way floodwaters move through the landscape, and tidal areas can also change the impacts of flooding.

Although you didn’t experience flooding in 2019, there is no guarantee it won't happen in the future. While the 2019 event exceeded some of the highest observed rainfall on record, this varied across the city. In some cases, flood impacts were very localised with some areas less affected.

The flood studies show a range of scenarios and therefore provide an estimate of areas that may be affected during different types of flood events.

Significant rain at the top of a hill will flow over the ground and concentrate in low lying gullies, channels, roads and surface depressions. The speed and depth of floodwater may still be significant enough to cause damage.

Council has no control over how insurance providers will respond to new flooding information. Insurance providers may use other non-council information about flooding when making decisions about risks, coverage and premiums.

This updated flood mapping and information is provided so residents, professionals and local government can plan for and mitigate the possible impacts of flooding and improve Townsville’s flood resilience.

Many factors can affect the value of a property, however Council does not control property values and is unable to advise how flood mapping may affect them.

This updated flood mapping and information is provided so residents, professionals and local government can plan for and mitigate the possible impacts of flooding and improve Townsville’s flood resilience.

The modelling and associated mapping provides an indication of flooding from river, creek or stormwater flows during various modelled rainfall events across a number of locations in the Townsville local government area. Some of the studies are also based on actual historic events. It is possible that flooding may affect an area of the region not currently indicated as being at risk. However, flooding in these areas is less likely and considered a much lower risk.

Even if your property is not directly impacted by flooding, it is good to understand the flooding impacts on the city or your local area. Having this broader understanding of potential flood behaviour is important as it allows you to take appropriate precautions to protect yourself, loved ones and property. For example, you should gain an understanding of access issues and if your property becomes isolated (not necessarily flooded), that you are able to evacuate safely.

The floodplain risk assessment has calculated flood risk for the flood study areas. With Council’s proposed policy responses, this will determine whether the level of risk to property is acceptable, tolerable or intolerable in relation to the type of development that may be expected there, e.g. residential, commercial, industrial.

In the most extreme flood risk areas, if the risk is too high, Council may have to consider if the current zoning is still appropriate and fit for purpose. In these cases, Council will need to consider if the zoning is still fit for purpose regarding the types of development that are expected for that zone. Rezoning is a last resort and Council will need to consider all other alternatives before proposing rezoning.

If properties are proposed to be rezoned as part of a future planning scheme amendment, any affected landowners will need to be notified of potential changes and given the opportunity to make a submission prior to any zoning changes being accepted.

The new flood maps and studies only show flooding caused by rainfall and do not include flooding from the sea. However, in coastal areas or places near tidal waterways, the flood models do take into account how high sea levels, could impact floodwaters from rainfall and rivers. This can make it harder for rainwater to drain away quickly, which may mean it takes longer for floodwaters to recede in these areas possibly leading to higher floodwater levels.

The Townsville City Plan has a separate Coastal Inundation map which specifically shows areas that could be affected by sea-related flooding, like storm tides or king tides. Council is working towards new maps for coastal hazards and developing planning policy responses for coastal risk. These will be released when this work is complete.

Townsville City Council is working to improve Townsville’s resilience to coastal hazards also. We are doing very similar work as for flooding which includes a study, a model, maps, a risk assessment and proposed ways to respond to risk in the Townsville City Plan. We are also developing a strategy for how we deal with a changing coast called ‘Adapting to Coastal Change in Townsville (A2CCT)’.

Once all of this information is ready, we will be doing community engagement for coastal hazards like we are doing now for flood.