Low flood risks occur mostly on the highest land inundated by a very large, very unlikely flood event where the depth and speed of the flood waters ranges from slow and shallow to moderately fast and deep.

Summary of planning and development responses for low flood risk:

• When constructing a new building or extending an existing building:
  • the finished floor level should be at least 300mm above the level of the Defined Flood Event;
  • residential buildings are not slab on ground;
  • associated earthworks don’t worsen flood risk on adjacent properties;
  • compliance with the relevant building assessment provisions is required.
• When filling land, no more than 50 cubic metres of material is added.
• When subdividing land, new lots should have a finished ground level above the identified flood level.
• Locate/design essential infrastructure so it is not impacted by flooding.

Medium flood risks occur across the range of flood events – from moderate floods that are likely to occur, to very large and very unlikely flood events.

For the likely flood events, the hazards tend to be low, however as the floods become less likely, the hazards increase in their impact on life and property.

Summary of planning and development responses for medium flood risk:

• When constructing a new building or extending an existing building:
  • the finished floor level should be at least 500mm above the level of the Defined Flood Event;
  • residential buildings are not slab on ground;
  • associated earthworks don’t worsen flood risk on adjacent properties;
  • compliance with the relevant building assessment provisions is required.
• When filling land, no more than 50 cubic metres of material is added.
• When subdividing land, new lots should have a finished ground level above the identified flood level.
• When land in greenfield areas is subdivided, parts of the land subject to this flood risk is protected from future development.
• Locate/design essential infrastructure so they are not impacted by flooding.

High flood risks at a location arise from the significant hazard areas of the moderate floods and the high hazard areas of the larger, rarer floods.

Summary of planning and development responses for high flood risk:

• No new urban activities occur within this area.
• No new buildings are constructed in this area.
• No filling of land occurs in this area except where a floodplain management plan shows that it is acceptable.
• When subdividing land, new lots should have a finished ground level above the identified flood level.
• When land in greenfield areas is subdivided, parts of the land subject to this flood risk is protected from future development.
• No essential infrastructure is located in this area.

Extreme flood risks at a location are defined for the high-to-extreme hazard areas of the moderate floods and the extreme hazard areas of large floods.

Summary of planning and development responses for extreme flood risk:

• No new urban activities occur within this area.
• No new buildings are constructed in this area.
• No filling of land occurs in this area except where a floodplain management plan shows that it is acceptable.
• When subdividing land, new lots should have a finished ground level above the identified flood level.
• When subdividing land in greenfield areas, the land subject to this flood risk is protected from future development.
• No essential infrastructure is located in this area.

Applicable to land affected by the 1% AEP event in primarily rural areas where flood hazard is known to exist but detailed studies and mapping are not available to define the risk for a particular area.

Summary of planning and development responses for Balance (Mixed) flood risk:

• All development is encouraged to occur on that part of the land which does not have a designated flood risk.
• Where development is proposed within that part of the land which has a designated flood risk, a site based assessment must be undertaken to show that what is proposed is appropriate given the level of flood risk.

Areas of overland flow where the depth is shallow (between 70mm and 300mm) but the speed of the water can be from slow to moderately fast.

Summary of planning and development responses for low hazard overland flow:

• When constructing a new building or extending an existing building:
  • the finished floor level should be at least 600mm above the natural ground level; and
  • associated earthworks don’t worsen the flood risk on adjacent properties.
• When filling land, no more than 50 cubic metres of material is added.
• Vulnerable uses are avoided.
• Locate / design essential infrastructure so they are not impacted by flooding.

Areas of overland flow where the water can be fast flowing or deep (greater than 300mm) or a combination of both.

Summary of planning and development responses for high hazard overland flow:

• When constructing a new building or extending an existing building:
  • the finished floor level should be at least 300mm above the level of the overland flow event; and
  • associated earthworks don’t worsen the flood risk on adjacent properties.
• No filling of land occurs in this area.
• When subdividing land, new lots should have a finished ground level above the identified flood level.
• When subdividing land in greenfield areas, the land subject to this flood risk is protected from future development.
• Vulnerable uses are avoided.
• No essential infrastructure is located in this area.

This designation applies to all flood affected land, including land only inundated during the probable maximum flood (PMF), excluding land only subject to the Overland Flow (Low) designation.

This designation is being used to keep land uses that would be particularly vulnerable in a flood from establishing within an area flood risk area. This includes uses such as child care centres, aged care facilities, hospitals and schools. A full list is included within the definition 'Vulnerable Use'.

Summary of planning and development responses for Vulnerable Use Restriction Areas:

• Vulnerable uses are avoided.

Land isolated but not inundated in a flood.

Summary of planning and development responses for Flood Islands:

Vulnerable uses are avoided.

Specialist engineers were engaged to determine what Council needed to undertake in order to meet its legislative requirement. These scoping studies are publicly available.

The engineers looked at a range of inputs to determine the type of study relevant to a particular location. This included elements such as flood history, type of flooding experienced (i.e. overland
flow/creek/river), existing flood data (i.e. flood gauges), population size, catchment profile (i.e. town on bank of a creek) and importantly, whether LiDAR (very accurate survey data) was available for the location.

This scoping study then determined whether an overland flow path, 2D flood study or historical study would be undertaken. In some areas such as Millmerran two different types of studies have been undertaken. Council has developed a map showing the flood study areas.

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, with the aim of minimising risk and protecting people, property and infrastructure.

A flood map visually shows the flood behaviour of a certain historic event or a range of flood scenarios. A flood study will usually produce a number of maps for each flood scenario 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);
• an indication of flood velocity (how fast and in which direction the water is travelling); and
• flood hazard (low, medium, high and extreme areas of hazard based on factors including velocity of the water and whether this poses a threat).

The extent, depth, velocity and hazard maps provide a good visual basis for understanding the potential flooding that may affect your property.

Council welcomes any information that helps build our knowledge of flood history and flood behaviour in the region. If you have verifiable information such as photos, records, maps or flood level/marks, you are encouraged to come to a dedicated session or use online engagement via our website.  Stories of flood levels need to be supported by photos or flood levels/marks in order to be used to validate the existing flood studies.

Council has been undertaking flood studies for many years. This particular initiative has been described by stakeholders such as the Floodplain Management Association, as the largest single program of studies undertaken at one time in Australia.

Following Council's endorsement, these studies need to be validated by the community. If credible evidence is provided suggesting the study can be improved, then Council will revisit these studies. If the community believes there are other areas worthy of more detailed investigation, Council will investigate what further studies may be required. In addition to this program, Council is currently carrying out the Toowoomba Overland Flow Path Mapping Study which will develop overland flow maps based on feedback from Toowoomba residents and previous flood modelling.

The flood studies found that flood behaviour in the region can be complex and vary significantly between locations, depending on topography, infrastructure and rainfall pattern. Past flooding was caused by high-intensity local rain creating runoff in backyards and streets, and the overflowing of creeks and the Condamine River. These types of flooding can occur separately or together and therefore have varying impacts on properties. Your property may be subject to one or a combination of flooding types.

There are obviously different types of floods. Very intense rainfall can occur suddenly, and the resultant quickly rising flood waters are known as flash floods. This is what much of the region experienced in January 2011.

In larger catchment areas, rainfall can build up over hours, days or, in some cases, weeks. The runoff from this rainfall flows across land and may create significant inundation of large areas of land for long periods of time. In our region, it is the rising waters within the river basins that isolate a number of communities. For instance, Cecil Plains, Quinalow, Maclagan, Clifton and Jondaryan can be isolated by rising waters in the Condamine River basin.

Flood warnings are more effective for these types of floods as there is more time to react. In some cases, a flash flood in the upper reaches of a river system can evolve into a more general river flood when it joins with other inflows and spreads out as it travels downstream. Many locations in our region can be affected by both flash floods and the more general river/creek flooding.

Historic flood studies utilise local knowledge, historic information and analysis of stream flow in a study area to understand the nature and extent of flood behaviour, such as depth and hazard from a historic event. Historic studies usually include a hydraulic model to reproduce a specific flood event experienced in a catchment using the best available data (e.g. surveyed flood levels and anecdotal information).  Its results may be compared to a range of simulated flood scenarios to determine a probability rating for the historic event (e.g. was it a relatively small, frequent flood or somewhere between that and a large and rare event).

Two-dimensional or 2D studies are a comprehensive type of flood study and are generally considered to provide the greatest level of certainty.  The 2D study uses hydrologic and hydraulic modelling tools and techniques and includes detailed analysis of results. These types of flood studies identify comprehensive flood information, such as flood discharges, levels, depths, velocities and hazards under existing catchment conditions for a range of design rainfall events, from small frequent events to large and rare rainfall events.

There is no such flood as a ‘one hundred year flood’, despite it being a commonly used term. All floods are different. While we don’t know when or how the next flood will occur, we can estimate the likelihood (probability or chances) of a certain size flood at a given location during a given period of time. The use of 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.

Average 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 chances of experiencing different sized flood events in a given period of time can be estimated mathematically. If you lived for 70 years in a location that had a 1% chance of flooding in any one year, then there would actually be a 50% chance of you experiencing at least one flood during that 70 year period.

However, the chance that you will be affected by a flood is not only dependent on the likelihood of your own property flooding. Floods can disrupt transport networks, impact tourist destinations and prevent food from reaching markets. With more than 100 rivers and creeks in Queensland, the chances are high that when flooding occurs, many people will be either directly or indirectly affected.

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 region. 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 a risk. However flooding in these areas is less likely and considered a much lower risk.

Nevertheless, even if your property is not directly affected by flooding, it is beneficial for you to understand the flooding impacts on your town or surroundings. 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.

Although you have never been flooded before, there is no guarantee it won't happen in the future. 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. Across a number of years, Council has been undertaking studies to better understand rainfall patterns and topography so we can better predict where water may flow.

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. Your home could also become isolated with access roads cut by rising flood waters. Your safety and wellbeing is paramount, particularly if this is for a significant period of time.

Hazard relates to the type of flood, and varies with severity and location in the floodplain. Hazard is characterised by the velocity and depth, rate-of-rise and the timeframe from rainfall to flooding, as well as the interaction of these factors with the topography of the floodplain. The faster or the deeper the water, the greater the hazard. Our studies have adopted the Queensland Reconstruction Authority’s criteria using four categories for preliminary assessment of flood hazard from Low to Extreme. The table below describes the hazard categories.

Flood risk is a combination of the chances of a flood occurring and the consequences for people, property and infrastructure. The consequences depend on the community’s exposure to flooding and how vulnerable its people, property and infrastructure are to flood impacts. For many parts of Queensland and in our region, areas of natural floodplain have a lower flood risk than areas where people and buildings are concentrated.

Flood risk is harder to manage where development, or the right to develop, already exists. Flood risk to existing infrastructure is usually reduced through improvements to protection as part of any upgrade. However, there are basically three ways of managing flood risk to reduce the consequences of flooding: by modifying flood behaviour, property modification or community response. None of these measures is a stand-alone solution for addressing flood issues. The preferred option is usually a combination of flood, response and property modification measures to reduce risk to an acceptable level and to manage the remaining risk appropriately.

It is important to make a distinction between flood hazard and flood risk. A flood hazard exists whether or not it poses the risk of impacting people.

The below diagram developed by the Queensland Reconstruction Authority further explains the difference between risk and hazard. The flood hazard depicted is the same in each example. However the risk will change depending on the land use exposed to that hazard.

Flood models do not provide detailed information on the extent of potential inundation to physical structures such as homes, sheds and garages. This is because Council does not hold information on floor levels of all properties across the region. Buildings would need to be
individually surveyed against the flood model data to determine this level of detail. A property identified as ‘at risk’ means it sits within the area mapped as being affected by water in a modelled flood event.

The nature of the property (slope, structures, drainage, gardens etc) will determine the presence or depth of floodwater across the property.

The modelling and associated mapping provide an indication of flooding from river, creek or stormwater flows during various modelled rainfall events across a number of locations in the region. Some of the studies are also based on actual historic events. There are areas between communities that include flow paths, creeks or rivers that, while not included in a more detailed study, are still subject to flooding. It is always possible that flooding may affect an area of the region not currently identified in the flood mapping.

Climate change impacts, such as increased rain intensity, have been included in the flood scenarios prepared in a number of the flood studies. Council recognises that the climate is changing and, based on good engineering practice, it is important the flood scenarios investigate possible climate change impacts for creek and river flooding.