Understanding flood hazard and your risk

A flood hazard is a natural event that has the potential to cause harm to people, property, infrastructure, and the environment. We measure flood hazard by how deep the water is (depth) and how fast it flows (velocity). 

The faster or deeper floodwaters are, the greater the hazard, and the more dangerous they become. Floodwater can cause serious damage to homes, businesses, and community buildings. It also threatens people's lives, infrastructure, and the environment. 

Flood risk considers how likely it is that a flood will impact people and infrastructure, and how great the impact will be. For example, an area near a river or creek might flood often with fast-flowing water. If there are no buildings in that area, the risk is lower, but if people live or work there, the risk is higher. This is because there is more potential for property damage or risk to life. 

Types of flooding in City of Moreton Bay  

Four main types of flooding occur naturally in our city.  

• Flooding can occur when heavy rainfall causes the water levels in a river, creeks or urban drainage system to rise and exceed the capacity of the main channel or pipe network.
• Overland flow is excess rainfall runoff that can cause flooding in gullies and depressions located upstream of rivers and creeks and also in areas where the runoff exceeds the capacity of urban drainage systems.
• Tidal inundation can occur on very low-lying coastal land during naturally occurring large high tides. Highest Astronomical Tides (also known as 'king tides') typically occur 2-3 times a year.
• Storm tide inundation can occur on low-lying coastal land when extreme weather conditions produce a storm surge resulting in sea levels above the normal tide levels.

Properties can be affected by more than one type of flooding. Knowing where water comes from and how will help you to understand your flood risk so you can prepare. You can find out if more than one type of flooding affects your property with a flood check property report.  

Likelihood and probability of flooding 

You may have heard the term ‘one in a hundred-year flood’. This is a misleading term and does not mean that a flood will only occur once every 100 years. It refers to a 1 in 100 chance that an event of a particular size (or larger) could occur in any one year.

We use Annual Exceedance Probability (AEP) to describe the likelihood of flood events. AEP describes the likelihood of an event of a particular size (or larger) occurring in any given year and location. This is usually expressed as a percentage. The smaller the AEP, the larger and rarer the event. For example:

• 5% AEP is a frequent flood event with a high likelihood of occurring or a 1 in 20 chance of occurring in any given year.
• 1% AEP is a rare flood event with a medium likelihood of occurring or a 1 in 100 chance of occurring in any given year.
• 0.1% AEP is an extreme flood event with a low likelihood of occurring or a 1 in 1000 chance of occurring in any given year.

This table illustrates the chances of experiencing a flood of a certain size over a lifetime (80 years) and a 30-year mortgage. It is provided to highlight that people can experience certain types of flooding more frequently than expected. The first column lists a flood's AEP. The second column lists the chance of a flood occurring during a 30-year mortgage. The third column lists the chance of a flood occurring over 80 years.

Flood hazard categories  

Flood hazard categories reflect different levels of exposure to the depth and velocity of flood waters. Our Planning Scheme considers 5 flood hazard categories. They include insignificant, minor, moderate, major and catastrophic.  

Insignificant flood hazard 

An insignificant flood hazard generally indicates that floodwaters are shallow (up to 25 centimetres deep) and slow-moving. They may be polluted or contaminated, so it is important to avoid contact with them.  

Minor and moderate flood hazard 

Minor and moderate flood hazards can threaten people's safety. They can make it difficult for children, adults, and the elderly to move around on foot. Floodwaters can range from shallow and fast-moving to up to 2 meters deep and slow-moving. Under these conditions, vulnerable people such as children and the elderly can be swept away, and cars may float or be washed away.  

Major and catastrophic flood hazard 

Major and catastrophic flood hazards can result in structural damage to buildings. This can cause them to fail and collapse. The risk to life is high and extreme.  

Flood depth is always measured relative to ground level. The depth and velocity of floodwaters can change with your location. For example, a 5% AEP (a smaller and more common event) in one part of the city might cause minimal flood hazards, with shallow and slow-moving water. However, in a different area, the same event could lead to severe or catastrophic flooding, with deep and fast-moving water. 

If your property is on low-lying land, you may be exposed to frequent insignificant flood hazards. If you live on raised or filled land, you may still be exposed to moderate or catastrophic flood hazards.  

Suburban roads are designed to be lower than the level of your home so while your house might be safe, the road could be exposed to any level of flood hazard. As the flood hazard increases, so does the potential for loss, harm or injury. 

Factors that can increase flood risk

Vulnerabilities are factors that can make flood impacts worse for people living in flood hazard areas. For example, the elderly, people with young children and pets, and people with a disability may find it more difficult to evacuate. So, they are more vulnerable if areas become isolated during a flood.

You may also be vulnerable if you lack flood awareness or are unable to evacuate quickly because:  

• English is your second language  
• you have recently moved to the area  
• you lack access to a vehicle.  

Higher vulnerability leads to greater consequences, which increases flood risk. 

Being vulnerable to flooding means you may need to respond sooner in a flood event. Being prepared and flood-aware can reduce your vulnerability. Evacuating earlier could help protect your life and welfare. Everyone should have an emergency plan in place. 

Learn more about preparing for a disaster.

Existing homes and businesses in floodplains 

Before city-wide flood modelling was available, floodplains were an attractive area for settlements because of fertile land and access to water. The waterways also provided transport routes for commerce and community. As a result, many Queensland towns have been established on floodplains and in low-lying coastal areas.  

Over time, our technology, flood modelling and understanding of flood behaviour and risk have improved, allowing us to make informed decisions about how we manage that risk.

Flood risk is harder to manage in existing developments in flood-prone areas. However, understanding your flood risk can help you manage your risk and plan for emergencies.

Some homes, buildings and infrastructure may benefit from flood-smart building design. This can be achieved through retrofitting, adapting and modifying existing structures. Flood smart building design aims to reduce flood damages and make cleaning up after a flood easier, so people can recover quicker.

Find out more in the flood smart buildings guidelines.