Flood hazard and flood risk
What is flood hazard?
Flood hazard relates to:
- flood extent - where the water goes
- velocity - how fast the water moves
- depth - how deep the water is
- time of onset - how quickly flooding occurs after rain
- rate of rise - how swiftly the water rises.
Flood hazard varies in severity within a floodplain. The greater a flood hazard, the more significant the consequences are for people, homes, businesses, community facilities and infrastructure.
It is possible to have the same flood hazard, but different consequences. The level of consequence will vary depending on what is being exposed to the flood hazard and how vulnerable it is to flood impact.
Flood hazard categories and their respective consequences table
This table describes what different categories of flood hazard from low risk (H1) to high risk (H5) mean for people, cars and buildings.
Areas affected by still, shallow water (H1) will be unlikely to have any significant consequences. You may see silt or soil and very light debris such as rubbish deposited in your yard from flood water.
Areas affected by very fast, deep water (H4 and H5) could have major to catastrophic consequences. A major flood hazard can result in structural damage to buildings and is a danger for all people. A catastrophic flood hazard involves flood water that causes building failure and is dangerous or even life-threatening.
| Low Risk to life and property |
High risk to life and property |
| H1 |
H2 |
H3 |
H4 |
H5 |
| Insignificant |
Minor |
Moderate |
Major |
Catastrophic |
| Shallow water up to 0.3 metres and still flood water moving slower than 0.5 metres per second. |
Deeper water of up to 0.8 metres and water moving at a rate of up to 2 metres per second. |
Deep water up to 2 metres in depth and water moving at a rate of up to 2 metres per second. |
Deep water of up to 2.5 metres and water moving at a rate of 2.5 metres per second. |
Very deep water that is greater than 2.5 metres in depth and fast flowing flood waters above 2.5 metres per second. |
| No significant life risk. |
Low life risk. |
Moderate life risk. |
Major life risk. |
Extreme life risk |
| Property risk only to items which come in direct contact with floodwaters such as building contents. |
This flood hazard is dangerous to children, the elderly and people who are unsteady on their feet. Able-bodied adults can walk safely. |
This flood hazard is dangerous to children, the elderly, people who are unsteady on their feet and most adults. Able-bodied adults cannot safely walk. |
Light frame buildings such as houses can fail structurally. |
The majority of buildings fail. |
| This hazard has a low danger to personal safety. |
Cars can float and precautions must be followed to keep them out of floodwaters. |
Only large vehicles such as trucks can safely travel. |
This flood hazard is dangerous to all people. |
This flood hazard is dangerous to all people. |
Back to question ⤴
What is the difference between flood hazard and flood risk?
Flood hazard relates to the nature or behaviour of flood water including how fast the water flows, how deep the water is and how fast it rises.
Flood risk is the chance of a flood occurring, combined with the consequence for people, property, infrastructure and the environment.
Find out more about the difference between a flood hazard and flood risk.
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What are the different types of flooding that could affect my property?
Flooding is a natural process. Floods can occur in different ways. You may experience more than one type of flooding at the same time. This is why variations in flood events occur. When planning and preparing for flooding, it is important to know how and where the water might come from. There are four main types of flooding that can occur in the region, these are:
- riverine - flooding that comes from creeks and rivers bursting their banks
- overland flow - water from heavy rain that accumulates and flows across the ground before reaching drains or waterways
- tidal inundation - water coming from tidal water that comes onto the land as part of the regular tidal cycle including very high tides
- storm tide - water coming from tidal waters that comes onto the land associated with the effects of a severe storm.
Learn more about the definitions of these four types of flooding and find out which types may affect your property and surroundings by generating a Flood check property report.
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What other factors contribute to flood risk?
The following types of flood risk and impacts can occur:
Inundation risk
Inundation risk is when flood water inundates a property. Depending on the depth and velocity of the water, this type of flood risk can cause property damage and building failure and impact on life.
Isolation risk
Isolation risk occurs when floodwaters surround people or buildings and they become cut off for some time. This type of flooding can cut off evacuation routes to flood-free land.
Loss of access
Loss of access, similar to isolation risk, results from road closures and loss of evacuation routes by foot, private vehicle and emergency vehicles. This can disrupt transport networks and impact business supply chains. Loss of access makes it difficult for the community to access essentials such as groceries and medical supplies.
Loss and disruption to services and amenities
Access can be restricted to medical facilities and reduce the ability of emergency services to reach isolated areas. Loss of power, water supply, wastewater treatment or loss of access to shops, schools and work can disrupt everyday lives.
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What does it mean to be vulnerable to flooding?
The more vulnerable someone is, or the more sensitive a land use, asset, or property is to negative flood impacts, the greater the risk is to flooding. Areas where people, buildings, infrastructure and assets are concentrated have a greater flood risk as there are greater consequences should these areas get flooded.
The following factors can make you more vulnerable during and after a flood event:
- a lack of flood awareness. If English is your second language or you’ve recently moved to the area, you may be less aware of flood hazards and their consequences
- a lack of access to a personal vehicle limits your ability to evacuate
- if you have multiple dependents
- if you own multiple animals
- if you are a part of a vulnerable population for example elderly, young children and those with a disability
- if you are economically disadvantaged, this may be a barrier to post event recovery and clean-up.
If you are vulnerable to flooding, this means you may need to respond sooner in a flood event. Deciding to evacuate earlier could help protect your life and welfare. Make sure you have an emergency plan in place and know your flood risk.
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My property is not within the mapped flood extent, does this mean I am safe from flooding?
Council is committed to providing accurate flood mapping and Flood check property reports. However, the process of updating Council’s region-wide flood mapping can take a considerable amount of time, meaning the mapped flood extent might not reflect the latest changes in the terrain or land use. Council applies a data reliability rating to the flood information being reported within a Flood check property report. This data reliability rating, as well as other information provided in the reports, can be updated, it is a good idea to re-check your Flood check property report regularly.
Flood mapping can also have limitations. Localised factors, such as large trees, blockages, and impervious fences, can impact overland flow flooding in particular, resulting in unexpected flooding in severe rainfall events.
Flooding can also indirectly impact you, even if your property does not get wet during a flood event. It is important to have an understanding of both the direct and indirect impacts of flooding to prepare yourself when our region experiences flooding.
Learn more about the different types of flooding that could affect your property.
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