A flood map is a map which shows the area of land which could be flooded in a specific flood scenario.

A number of flood maps have been published for different areas in South Australia, but there are not yet maps for every area in the state. This does not mean that there is no flood risk in these areas; just that a flood study hasn’t been done in that location. In South Australia, flood maps vary depending on a number of factors including the reason the flood study was undertaken, the technology used, the time they were made and the author of the flood map.

Reasons for producing flood maps

Flood maps are produced for a variety of reasons including:

• to identify which areas are at high risk of flooding in order to plan emergency response; 
• to determine areas suitable for future development;         to assess the adequacy of existing drainage infrastructure;
• to assist in the design of flood mitigation strategies such as detention dams, levees and pipes;
• to explore a specific scenario that may result in a flood such as a dam break;
• to provide a guide for long-term planning against increased flooding as a result of sea level rise;
• to look at the impact of future development within a catchment or floodplain on the flood risk; and
• to examine the impact of a potential stormwater harvesting project on flood risk.

Each of these purposes can result in a different flood map, so it is important to be aware of why the flood map was made when looking at them.
 

All flood maps are different

Flood maps are different due to:

• The type of flooding considered;
• the purpose they were made for;
• when they were made;
• the technology used to make them; and
• who they were made by.

The date a flood map was published is especially important to keep in mind when using this website. This is because catchment environments and the technology used to produce flood maps have both changed over time. Newer maps may be able to better represent current catchment conditions and will include recent development such as housing, roads and water management infrastructure which can alter the characteristics of a flood.

As technology has improved, newer maps can also provide other relevant information in addition to the location and depth of potential inundation. This may include water velocity, number of properties inundated and the hazard which a flood presents to people (a factor of water velocity and depth). Information can now also be depicted in finer detail than was possible in older studies. Despite these differences, all flood maps are relevant and useful as they represent a point-in-time analysis of flood risk from the flood scenario of interest.

South Australia’s flood maps

A number of flood maps from across the state have been collated and are accessible to view on this website.

These have been commissioned by different organisations including local and state governments. Different companies have produced the flood maps using different technology. Some are coastal flood studies, some investigate flood risk from rainfall events and others examine flood mitigation options. Some of these maps were made recently, while others are older.

If you are interested in a flood map, information about the author, published date, commissioning authority and details about the flood scenario are shown in the information box when you click the information button next to where the flood map is displayed. For more detailed information, it is worth obtaining the relevant flood study of interest.

How are flood maps made?

Flood maps used to be created by hand by plotting a predicted flood level onto a contour map of ground levels.   Nowadays, flood mapping is done using a range of complex computer programs which model how a volume of water will travel across a landscape.

The process of completing a flood map is very complex, expensive and can take a long time.  Before even beginning to model a flood, lots of data has to be obtained about various aspects of the catchment and the types of events that will cause flooding. This background knowledge is vital to ensure that the flood model is appropriate and the finished flood map will be able to provide information relevant to the purpose for making the flood map.

Using this knowledge, flood modellers select inputs for the specific flood and area which they wish to model and prepare and input the data. Data selected for entry into a flood model are based on best available information at that time. It is important to realise, though, that these data are still from a very big range of possible data.

The types of data that are often used to develop a flood map include:

• rainfall data and river data, which can be from either historical records or synthetic rainfall events developed by the Bureau of Meteorology;
• the development conditions of the catchment – roads, roofs and other impermeable surfaces lead to a greater volume of runoff;
• soil moisture and the capacity of the soil type  to absorb runoff;
• ground levels of the rivers, creeks and floodplains;
• the location and size of built objects such as bridges, pipes, levee banks and dams;
• the “roughness” of the rivers, creeks and floodplains: whether a creek or floodplain is covered with dense vegetation or smooth concrete (for example) influences how quickly water can move across it and therefore the depth and velocity of floodwaters;
• downstream water levels (sea levels may include an allowance for climate change); and
• historical flood levels and extent of flooding.

To try to make sure a flood map is as realistic as possible, flood modellers often calibrate their model against one or a number of historical floods. Comparing the results from real information against the modelled information for the same event can help to show where errors in the model are so that these can be corrected. This can be difficult or impossible to do in areas where floods have not been recorded for a long time, the flood scenario is for a flood larger than has been recorded or where data is inadequate from the historical event.

Finally, the model is run over a period of days or weeks using computer software to produce a map. This final map allows a reasonable estimate of where flooding may occur in the scenario chosen. Because all the data entered into the initial model is selected from a very large range of possible figures it is highly unlikely that all of these figures would be exactly matched in a real flood event. This means that while flood maps aim to represent a likely flood event, a flood that happens in reality may be different to that shown on a map.

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