Brisbane is a city built along the banks of the Brisbane River. It is known as the “River City” for a reason. But being a River City, it is also a city that is exposed to flood risk, and this is what this article is going to focus on to ensure you can assess the flood impacts in Brisbane before you buy a home or an investment property.
Brisbane has experienced many significant flood events over the past two centuries. Back in February 1893, the first big river flooding event occurred where water levels were recorded at 8.35 meters above the low tide level in the Brisbane CBD, the second highest flood event ever recorded at the City gauge. This event was termed the Great Flood and that month was then referred to as Black February. The flooding was caused by a huge rain event associated with a tropical cyclone. This caused the Brisbane River banks to burst and the water flooded into the surrounding areas. This flood resulted in 11 deaths and about 190 people were hospitalised.
Fast forward to January 1974 and our city experienced the largest flood to affect Brisbane in the twentieth century. Once again, this was caused by a cyclone where 642 millimeters of rain fell within the space of 36 hours and the river system simply could not cope.
At that time the water levels peaked at 6.6 meters at the City gauge, but because development was a lot more advanced than it was back in 1893, 8,500 homes were completely inundated with floodwaters on this occasion. Brisbane was an inland sea during this flood event and 14 people lost their lives as a result, mostly in the inner city suburbs.
Off the back of these flood events, the Wivenhoe Dam was constructed to provide flood mitigation control for the City. This is located about 80km by road from the Brisbane CBD. Residents in Brisbane became more optimistic that Brisbane would never flood again and development was fast tracked throughout the city. No one thought that a river flooding event would impact our lives, or our homes again.
Then in 2011, the flood that was never meant to happen happened. After days of rain, the Wivenhoe Dam was over its capacity and the flood gates had to be opened to release some of the water. Brisbane was in for a shock.
On 11 January 2011, the Brisbane River broke its banks and by 13 January 2011, the river was raging. This time 20,000 residential homes were affected by flood waters across 94 suburbs throughout the city. It was the flood that was never meant to happen, but it highlighted the fact that our River City may never be immune from future flood events.
Understanding how to assess the flood risk associated with a property is therefore important when you are looking to buy a property in Brisbane. So here are some steps you can follow that will help you to assess this risk.
Downloading a FloodWise Report to assess the flood impacts in Brisbane
The Brisbane City Council provides predictions for the potential for flood risk for most properties around the City. A FloodWise Property Report can be completed HERE. You will be required to enter the property address, click Search and then select the way you would like to view the report before downloading.
This search will provide one of four types of reports, depending on the site.
The first type of Report is issued when there is NO known FLOOD Impact across a site. You will see this note on Page 1 of the FloodWise Report if this applies to your property search:
Obviously, this is the best possible outcome as it means the property is not going to be impacted at all by any type of flooding event.
The second type of report provides a warning that the property has Flood and Planning Development Flags, but there will be no visible graphs. The alert on Page 1 of the FloodWise Report will look like this:
This usually means that the property is impacted by overland flow flooding. Brisbane City Council does not have publicly available information on the overland flow modeling. Whilst we can get an understanding of the overland flow pathways, we don’t have details and the onus is on a buyer to engage a hydraulics engineer to complete an assessment to ascertain what that impact actually is. For development, this becomes much more important compared with just buying a residential site.
The third type of report that might be produced happens when there is a known flood impact on a site, but the information is not complete enough to determine what the minimum habitable floor level must be for flood immunity. There will be a graph on the first page of the report that will look something like this:
This usually happens when a block of land is too large for the council to have complete clarity of what the flood impact is likely to be at every point on the site and therefore the minimum habitable floor level is not noted on the report. In this instance, the onus is again on the property buyer to confirm what the minimum habitable floor levels might need to be to achieve flood immunity across the site.
The fourth and final type of report is produced when there is a known flood impact on the site AND there is sufficient information in council’s database to also determine what the minimum habitable floor level must be for a dwelling property to achieve flood immunity on that site. The graph on this report looks something like this:
You can see in this example, there is a dotted line that shows the minimum habitable floor level. This is the most comprehensive report out of all of the possible options.
Now let’s look at how to interpret the rest of the information in these reports.
The Green line on the right-hand side of the graph represents the contours on the property, or the ground levels of the site, based on the Australian Height Datum in meters m(AHD). A level of 0.0 AHD is considered sea level.
The highest and lowest point on the site are noted on this report through the lowest and highest points on the Green line. Obviously, suburbs closer to the bay in Brisbane will be closer to zero, whereas more elevated suburbs will be much higher.
On the left-hand side of the report, you will see some bars on the chart. These bars represent the annual probability of a flood event occurring for that particular property. These bars also show the magnitude of the associated risk for any flood event. This is measured using the Annual Expedient Probability (AEP), in other words, the chances that a property will flood in any year.
Usually, the higher the probability (ie: the higher the AEP expressed as a percentage), the lower the flood level will be. The same holds true in that the lower the AEP, the higher the flood level will be.
Some reports, but not all, will also include a flood level as recorded during the Floods of January 2011. And finally, some reports, but not all, will include a Defined Flood Level (DFL) as well which is a measure used for Brisbane river flooding whereby the flood level of 3.7M AHD at the Brisbane City Gauge and a river flow of 6,800m3/s is the flow. This gets a bit complex, but for those who understand hydraulics, it may be useful.
If the bars on the left-hand side of the chart are higher than the points in the Green line on the right-hand side of the chart, then you can expect that during a flood event, water is likely to cover part or all of that land. You can then calculate the DIFFERENCE between those two levels to get an indication of the likely flood LEVEL for that property at its highest and lowest point.
When it comes to new approvals for dwellings, or renovations on properties, council are focused on the 1% AEP levels. For any non-habitable spaces within a home (eg: garages & laundries) council requires those floor levels to be 300mm ABOVE the 1% AEP level for a site.
For any Habitable spaces (eg bedrooms, living rooms dining rooms) these need to be at least 500mm ABOVE the 1% AEP level for a site.
For any existing dwellings that may not already achieve flood immunity, a calculation between the ground levels and the ACTUAL floor level can often determine what level of flood inundation could be expected in the event of a significant flood event on that site.
If a property ALSO falls within a Creek/Waterway Flooding overlay Category 1, 2 or 3 OR in a mapped overland flow path, you may also need to account for an undercroft area in the event you are looking to complete any future renovation works. This can get quite complex, and we recommend if this applies to seek help from professionals such as architects, certifiers, and town planners.
On Page 2 of the Property FloodWise Report a Technical summary will be provided. This information is predominately for builders and architects who are completing renovation or building works on a property. Basically, if you understand how to read the graphs on page 1, you will not need to understand this more detailed technical information.
Checking the FloodWise Report for a property is an important part of the due diligence process that should be completed prior to a property purchase so that you understand the flood impacts in Brisbane and how they may impact on a specific property. Understanding how to interpret the reports is also essential. Finally, applying the information from the report to the specific property, and understanding what it means in relation to the existing property on a site is invaluable. This ensures any future compliance requirements in relation to renovations or improvement works are understood upfront.
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