Flood management

Flood management Nashwauk Crescent 2005

Floodplain mapping

Why do we do it?

One of the strategies identified in Water Futures, developed as part of our Community Plan in 2008, was to Manage Stormwater to Protect and Minimise Risk to Property Infrastructure and Public Health and Safety.

To help deliver this strategy we have undertaken studies to determine the possible high flow rates and anticipated flood flow paths of watercourses in our city.

We use this information to help inform our development plan, and identify potential stormwater infrastructure improvements.

Improving our flood information

A number of flood studies have been completed and we now have reliable flood information for all our major stormwater catchments.

The studies have a technical focus, so to help our community also understand about flood risk in our area we have produced information sheets. 

The information sheets summarize the study findings and show the flood extents for major floods in each of the catchments, along with information on how to read the flood maps, how we create the maps and the limitations of the maps. 

The information sheets are available by clicking on the study name, or by calling our customer service centre on 8384 0666.

Hackham Creek(PDF, 3MB)

Pedler Creek (McLaren Vale)(PDF, 3MB)

Pedler Creek (Moana)(PDF, 2MB)

Seaford rail (lower Onkaparinga River)(PDF, 3MB)

Christie Creek(PDF, 3MB)

Washpool (Silver Sands)(PDF, 4MB)

Panalatinga Creek(PDF, 3MB)

Sauerbier Creek(PDF, 3MB)

If you aren’t sure if you are within one of these flood mapped areas, you can quickly check by looking at the flood map index (below), which shows the flood mapped extents on a map of the council area. 

Flood mapping areas

We have also developed a series of frequently asked questions (FAQs) to try and help answer any questions you may have about our floodplain mapping.

Note that the use of our flood plain maps should acknowledge these limitations.

View technical information on the process we use to develop the maps, and explanation of technical terms.

If you have any questions and cant find the answer to the question you have in our of frequently asked questions (FAQs)  or the information sheets, please contact our customer service centre on 8384 0666, or email your query to mail@onkaparinga.sa.gov.au

What does it all mean?

Fortunately, as our city has developed, open space has been provided along many of our major watercourses.  As a result, less than 2% of all residential properties in the Council area are identified as “flood prone”. (Note – even if only a small part of a property has shallow floodwaters, we still call it “flood prone”).

As a result of the completion of these studies we will look at ways to minimise risk and hazard associated with the identified flood extents. 

There are many options available for reducing the impact from floods, including:

  • placing controls on development in high flood risk areas, (our development plan now identifies flood prone land)
  • providing flood warning systems where sufficient advance warning notice can be provided to assist with orderly evacuation or preparation,
  • building flood mitigation structures where feasible, and
  • improving community awareness and resilience to ensure the community is prepared and able to respond.

Be prepared

Although our flood mapping helps identify higher risk areas, each flood event is unique.  The severity of flooding in a single event cannot be reliably predicted.

It is possible that localised flash flooding can occur from heavy rain within or outside of the mapped catchments, or floods may be greater, or behave differently from the events we have modelled. 

For these reasons, it is a good idea to be prepared – especially if your property is within the flood mapped areas, but even if it isn’t!

One of the simplest things that can be done to reduce loss or damage from flooding, is to make sure you have your own personal, household emergency flood plan.  To help you develop one of these, we have prepared the guide below. 

Flood protection - prepare your emergency flood plan(PDF, 231KB)

We have also engaged with the State Emergency Service (SES) to deliver their Floodsafe and Stormsafe program in schools and the community throughout the area, to help build community resilience and understanding about flood risk.

The Bureau of Meteorology provides a severe weather warning for potential for flash floods, and floods on most watercourses in our council area, as well as a flood warning service for the Sturt River and Onkaparinga River.  Current information on rainfall and river levels is available on the BOM flood Adelaide rainfall and river conditions page.

In the event of a flood emergency the State Emergency Service (SES) –telephone 132500, is the response authority.

More information about the SES and the Floodsafe and Stormsafe program can be found on the SES website


What does ‘flood prone’ mean?

The flood maps we have prepared show the estimated area and depth of flooding in the catchment in a 1:100 ARI and a range of other ARI flood events.  The 1:100 ARI flood is the generally adopted, national risk management standard for flood management, noting that a 1;100 ARI flood event is something that may not occur in a typical lifetime – it is a significant event! (We have not had even close to a 1:100 ARI flood in any catchment in our City in the last 50 years).

Detailed technical information on how we develop these maps, and what is meant by ARI.

Your property is considered to be “flood prone” or within a “flood management area”, if your property is within the flood extents (the coloured overlay) shown on the flood maps for a 1;100 ARI flood event.  This means there is some probability that surface flows of water may pass over part or all of your property during a flood event of this magnitude.

The degree to which this may occur can only be estimated, and may vary from very shallow, slow moving water entering part of your property, to, in some instances, deep, fast flows passing through the property.  By looking at the depth of water (as per the colour code shown on the map legend) you can get an indication of the degree of flooding that may affect your property.

If your property is within the flood extents shown in the 1:20 ARI flood map, then this means that there is a high probability that during a larger storm event (like the 1:100 ARI flood) , surface flows of water may pass over part or all of your property.  If your property is only shown as being within the flood extents on the 1:100 ARI flood map, then there is still a probability that surface flows of water may pass over part or all of your property in a flood of that magnitude.

Our (now revoked) Development Plan also has some policies within the ‘Hazards’ section for addressing development proposed in ‘“flood management areas” for properties that are identified as flood prone in the 1:100 ARI flood maps. These policies may change and additional policies added as amendments are made to the Development Plan. The policies may apply to a development application and may require additional consideration to any hazard impacts. In addition to this, you may not be able to apply for a ‘complying’ development if you property is identified in a flood management area in the development plan.

How can I tell if my property is flood prone?

There are three ways you can find out if your property is flood prone: 

  1. Firstly, if your house is in the flood prone area identified in any of our recent flood mapping studies, you, as owner or occupier, should have received a letter from us advising of this.
  2. Secondly, you can locate your property on the flood maps provided on our website and if the property is partly or wholly within the coloured flood prone areas on the maps, then your property is considered to be flood prone.
  3. Finally, if you are still unsure if your property is flood prone, you can contact, Customer Services on 8384 0666 and speak with one of our friendly officers.

    Flood mapping example Ward Street


In the example above, all the properties on Ward Street, Gray Street and most of the properties on Gawler Street are flood prone, while only the properties on the east (right hand side) of Saltfleet Street north the roundabout are flood prone, and several properties on the west side, south of the roundabout, are flood prone.  This is because the coloured overlay showing flood depth also covers all or part of these properties.

Even if only a small part of the property is covered by the coloured layer, we still consider the property to be flood prone. 

What do the different colours on the maps mean?

For all of our flood mapping a map legend is used to give a guide as to how deep flood waters may be in any location.

The different colours in the legend indicate the depth of water (in metres) that may be expected.

The colour schemes for the legends vary from study to study.  You should refer to the legend provided on the flood map for your area to identify the extent or depth of flooding expected in your area.

Three legend examples are shown below:

Flood mapping legend examples


Flood depth example:

In the flood mapping extract below, street and property boundaries are shown as a white outline.

By referring to the legend on the right, it can be seen that the flood depth on Ward Street varies from the 0.5m - 1m range (shown in mid blue) in the centre and south eastern end of Ward Street. 

The flood depth progressively gets shallower to the north west near the junction with Gawler Street, to 0.25-0.5m (shown in light green), 0.1 - 0.25m (shown in light brown), and finally 0-0.1m deep (shown in light blue) at the very edge of the flood prone area.  On the eastern side of Gray Street, the depth varies up through 1-1.5m (shown in purple) to 1.5m to 2.5m (shown dark green).

If you need any help reading the map or legend, please give our customer service team a call on 8384 0666. 

Flood mapping example Ward Street

My property is within the flood prone area. Does that mean my property will get flooded?

If your property is within the flood prone areas on the flood plain maps, it is possible that flooding may occur.  Whether floodwaters may enter your dwelling will depend on the depth of flood waters, and a number of other factors associated with the design and construction of your house.

We recommend that you take steps to reduce the risk of damage or harm to your property in the event of a flood. Some practical measures for reducing this risk are detailed in our information sheet(PDF, 231KB)

Further information about flood proofing your home may be available from the State Emergency Service at www.SES.gov.au  (see our information sheet for contact details). 

However, you should also read the limitations to understand the limitations of flood plain mapping. 

How accurate are the maps?

The mapping is intended as a guide only for development and strategic planning purposes and general information.  There are limitations on the accuracy of the maps.  Find further information and to help understand the limitations of flood plain mapping in the next section.

It is not possible to model to a very high degree of accuracy how stormwater will behave as it runs over complex surfaces, or even the type of rain events which will occur, as no two events are ever the same and we cannot predict with any certainty what rainfall events will occur in the future. 

The modelling used to create these maps simulates what may happen in a significant rain event. The modelling cannot take into account unique characteristics of individual properties that can influence flows, including buildings, fences and gardens.  In the development of flood modelling software and flood maps, valued engineering judgements, based on monitoring and scientific experiments, have been made regarding how flows will behave in these circumstances.

If my property is flood prone, will that have an impact on the value of my house or property?

There are many things that can influence the value of a property.  If a property is identified as being flood prone, then the degree to which the property is flood prone may have an impact on the property valuation. This is because:

  1. The Development Plan may ask for additional criteria to be addressed when a development proposal is lodged for your property in the interests of reducing the risk of flood waters damaging buildings, and
  2. Potential purchasers may perceive this risk differently to you.

y property is within the flood prone area. Will that mean I have to pay a premium for flood insurance?

Following the severe flooding in the eastern states in 2011, 2012, the insurance industry and Australian Government have been working on making changes to the cover provided for flooding, with the intent of keeping flood insurance “affordable” and also making new rules on what is a flood and what isn’t, and what is covered by insurance and what is not. 

These discussions were still in progress (as at June 2013)

We suggest that if your property is within a flood prone area, you talk to your insurer(s) to ensure that you have appropriate cover for your property.

If my property is flood prone, will that have any implications for what I can do with my property – eg approval for new buildings or land uses?

Our Development Plan has provisions relating to flood management areas.  The provisions vary to some degree depending on the form of development proposed.

You should discuss your intentions with our duty planning officer to see what provisions may apply.

My property is within the flood prone area. What is Council going to do to prevent my property from being flooded?

Council performs a statutory function under the Local Government Act to take measures to protect its residents from “hazards”, including flooding.  Floods in watercourses are a “natural” event and we cannot stop floods from occurring.  However our service level has been established to minimise the potential for loss and damage due to flooding, through a combination of good development planning, and effective flood protection infrastructure.

Our capital works program provides for funding for stormwater management works including flood protection, subject to Council’s overall budget restraints and priorities. 

Wherever it is feasible to improve local levels of flood protection, we will strive to achieve this in accordance with the resources available.  We design and construct our flood protection infrastructure to provide flood protection for all but the most extreme events.

However, flood protection may not be possible in all instances, or in the short term and that is why we are providing information to residents, and supporting the SES in running the “Floodsafe” and “Stormsafe“ programs across the city, so that you are informed of the things that you can do to limit loss, in the event that a flood does occur.

My property is within the flood prone area. How can I protect my family and property from flood risk or damage?

There are a number of things that property owners can, and should do to reduce the risk of damage or harm from flooding.
We have developed an information sheet(PDF, 231KB) with some suggestions. 

The SES has additional information available either by contacting them on telephone 8463 4171, or visiting their Floodsafe web page.

Additionally, the SES, in conjunction with council, will be running a program of Floodsafe community information sessions in your area, to help raise awareness, and help you to reduce the risk of damage or harm in the event of a flood.

My property is not in the flood prone area. Does that mean I will not be flooded?

Our flood mapping only simulates floods on the larger watercourses in our Council area.  It is impossible to simulate every possible form of storm event that may occur.  If your property is not within a flood prone area, there is statistically a lower chance of flooding occurring from a major flood event in the catchment.

However, even if your property is not within the flood prone areas identified by our flood mapping, due to the highly variable nature of our weather patterns, there is still some possibility that heavy rain may fall, creating local ‘flash flood’ conditions. 

By following the suggestions provided in our information sheet(PDF, 231KB) or through the SES “Floodsafe” and “Stormsafe” programs, you can help minimise the risk of harm or damage in the event of flash flooding.

I have owned this property for many years and it has never flooded here. What has changed that has made my property flood prone now?

In some areas, urban development has resulted in the potential for more stormwater to run off the land than would have been the case before European settlement, and this has been considered in developing the flood maps.

We do know there have been very significant rain and flood events in the past, before European settlement, which have helped shape the form of the area. 

However, compared to the length of time that the world’s climate has been reasonably stable (since the Ice Age many thousands of years ago), we do not have a very long record of rainfall or flooding in our area. 

Based on our statistical analysis and assessment of the relatively short rainfall history we have, there have been very few major events in the area since European settlement. There is a possibility that these major events can still occur, even though we have not seen one since early European settlement.

Our flood mapping provides a scientifically robust estimate of the probability of flooding from major events, but it cannot predict a specific event outcome. 

You should read our limitations for more information.

Will the State Emergency Services (SES) be able to assist in the event of flooding?

We have provided our flood mapping to the SES. Their emergency response will take account of this information to ensure that they can deliver the emergency response required in the event of flooding.

The SES is the first point of contact in the event of a flood emergency – telephone 132 500.

Where do I go for more information about the flood plain mapping?

If you require more information about the flood plain mapping, please contact our Customer Service team on 8384 0666.



Limitations of flood plain mapping

The maps delineate areas which are assessed as being subject to inundation along the watercourse modelled. The maps do not show flooding from local subcatchments or drains.

The maps are based on available survey, hydraulic and hydrological modelling as at the date of the map, to an accuracy sufficient only for broad scale flood risk management and planning.

The inundation patterns are not those of actual historical flood events.  Actual inundation patterns will vary from one flood to another, being affected by earthworks, structures, blockages of drain and structures, the state of vegetation coverage, local intensity and duration of rainfall, and other factors.

The limit of flooding shown is not the boundary between flood prone and flood free land.  Areas outside the limits shown could be inundated by larger floods.

Flood assessment for a particular site will require more detailed interpretation, survey and hydraulic analysis by appropriately qualified and experienced persons.

Detailed technical information on flood plain mapping

Major floods and “flash floods”

The flood studies being reported on here are for the larger watercourse catchments and flooding which may occur generally as the result of an extended period of lighter rainfall, and/or heavy rain. The time from the onset of (continuous) rainfall before flows peak in the watercourse typically varies upwards from 6 hours. 

On the other hand, flash flooding (the term applied to localised flooding occurring in smaller catchments due to short, but very heavy rain) could potentially affect many more properties.  Urban “flash flooding” is very complex to model, and further research and detailed studies are required to identify flood prone land in urban catchments, associated with “flash flood”.

Council’s major flood management service level

Our service level for major flood flows in urban areas is intended to limit risk in the event of a major (1;100) flood, primarily by conveying flood flows via public land (eg the roadway or Council reserves), or by limiting flows over private land to shallow depth and low velocity.  Stormwater drainage infrastructure in the street is generally not designed to deal with these “flash flood” flows.

To protect dwellings and other buildings from risk of flooding in “flash floods” Development controls applied under our Development Plan will require development to be constructed “to prevent the ingress of floodwaters”.  This typically means setting the floor level of the house sufficiently above the estimated flood level. 

The flood mapping process

The techniques and methodology used for flood mapping studies is used across the nation, and is based on industry standards.

Highly developed computer modelling software used widely throughout the world is utilised for our floodplain modelling and mapping.

Its important to note that flood mapping studies are based on statistical analysis, and require interpretation, interpolation and extrapolation of rainfall and run off data.  In many cases, the available data is limited, either by location, or by length of record.  Accordingly there is a margin for variability in results.

In all our studies, specialist consultants with experience and capability in flood modelling and mapping have been engaged.  The projects are overseen and reviewed by representatives from the Bureau of Meteorology and the Natural Resource Management Board.  Technical review has been provided by the hydrologist at the Department of Planning, Transport and Infrastructure and a former flood hydrologist with the Bureau of Meteorology, both nationally recognised for their competence in this field. 

The process used for our studies involves a number of steps and processes:

  • Identification of rainfall intensity, frequency and duration associated with flood events.  We use either continuous records from rain gauges within or close to the catchment, where available, or estimates provided by the BOM
  • Determination by observation, of the rates of run off from the catchment, (ie: how much and how quickly will rainfall flow across the ground). Estimates are based on assessment of a combination of land use type, gradient, soil type and vegetation cover. 
  • Development of a hydrologic model using widely used and tested software to determine flow time and volumes for overland flow using the rainfall estimates and run off coefficients.
  • Development of a digital terrain model (DTM)or catchment surface model using a combination of aerial photography and ground survey to build a three dimensional model of the catchment ground contours to model overland flows.
  • Creation of a hydraulic model using sophisticated modelling software to simulate stormwater flowing over the catchment surface model, to map overland flows paths and depths, including flows along creeklines and through major bridges and culverts.
  • Wherever possible, the models are “calibrated” ie, comparing modelled flows with actual recorded flows and rainfall, to confirm the model simulates the actual flows as closely as possible.  Unfortunately this relies on having historical information available.  Most of our catchments are “ungauged” and no flow information is available, and even long term rainfall information is not available for all catchments.
  • A number of model simulations are run, using different rainfall intensity and duration, to identify the flood extents.  These are mapped and overlayed on each other, and the outer limit of the combined flood flows is adopted as being representative of the 'design flood'.
  • We model flood extents for the 1;100 ARI – as the generally adopted national risk management standard for flood management
  • Additionally we model “flood hazard” for the 1:100 ARI and 1:500 ARI events.  This is a guide to the degree of hazard associated with flood conditions. It is generally determined based on a combination of flow depth and velocity.  The scale takes into account the ability to wade, or drive through flood waters.  Additionally, flood hazard may also be influenced by other factors including time till peak flow and availability of evacuation routes which impact on our ability to prepare for and respond to a flood emergency.
  • The modelling also determines the capacity of major drains, culverts and bridges to help guide planning for future renewal or improvements.

ARI - Average recurrence interval and flood hazard

Average recurrence Interval

The method of describing the severity of a flood is based on statistical terminology, and the presumption that the larger the flood event, the less likely, or less regularly it is likely to occur.  There will typically be rainfall of short duration or low intensity at various times during a normal year.  However, extremely heavy rain, or long periods of continuous rain which overwhelms a stream’s capacity and result in floods, are much less likely to occur – even though they can, and will occur. 

We use the terms “average recurrence interval” (ARI) To describe the statistical frequency of rainfall and flood events.

A 1:1 ARI event, is a rain event which, based on statistical analysis, is considered likely to occur once per year-ie: there is a 100% chance that it will occur during a year.  It is a reasonably foreseeable and regular event.

A 1:20 ARI event has a statistical probability of occurring once every 20 years, or 5 times every 100 years.  This is a rarer event than a 1:1 ARI event, and therefore, expected to be a more significant event, with higher rainfall, or a longer than usual duration of continuous rain. 

A 1:100 ARI event is a much rarer event, and also a much more severe event, likely to result in widespread rainfall, very heavy rainfall or very long duration rainfall, causing rivers to overtop their banks and result in flooding and possibly even physically alter the path of the river.

A 1:100 ARI event is something that may not occur in a typical lifetime – it is a significant event with only around a 50% chance of one occurring in a typical lifespan!  However, statistically there is still a 1% chance of an event such as this occurring in any year.  This is the standard flood “event” we seek to manage flood impacts for, and is the standard our Development Plan identifies for “Flood management areas”.

The ARI is measured over an infinite time, therefore, the mere fact that there has not been a 1:100 event in our area for 100 years does not imply that there will be one.  Likewise, it is feasible for two or more 1:100 year events to occur consecutively.