Understanding the Flood Study

‘Why did Orange City Council do a flood study? ‘

The NSW Government requires local councils to complete these studies from time to time.

A scientific study of flood patterns helps Councils and residents design buildings and complete other projects to reduce the risk of flooding.

A flood study also helps with the council’s work dealing with planning approvals, and managing future land use.

The last time Orange City Council completed a flood study was in 2009. Since then the city has grown significantly, and because of projects such as new stormwater channels and new housing subdivisions, the patterns of flooding had potentially changed. Technology has also changed to enable more accurate modelling.

‘How did Orange City Council do the flood study?’

The flood study was completed by an independent consultant, Lyall & Associates.

Lyall & Associates is a Sydney-based firm specialising in the fields of water resources and environmental engineering, with particular emphasis on catchment/floodplain modelling and Floodplain Management. 

One of the initial steps taken was to survey residents of the existing known flooding areas and to collect historical information and community opinion regarding flooding in the city and drainage.

In 2017 a consultant used the latest technology to measure ground levels of every square metre of the city. Special equipment mounted in a plane was flown over the city and infrared laser equipment, called a LiDAR was used. The laser was used from the height of the plane and is able to measure every change in level with an accuracy of up to 30 mm.

Using this new information, existing data, the latest flood-modelling software and residents’ surveys, the consultant designed a series of maps detailing the probability of flooding in each area of Orange.

As an industry standard reference point, the study uses the potential height of a ‘1 in 100 year’ flood to maps areas that may be affected.

‘I’ve lived here many years and never seen a 1 in 100 year flood.’

The mapping predicts the probability, or the likelihood that there will be a flood in each area of Orange.

The industry standard is to no longer use the term ‘1 in 100 year’ flood. Instead percentage figures are used to describe the probability of a flood.

The probability of flooding is referred to as the Annual Exceedance Probability.

The Annual Exceedance Probability (AEP) is a way of understanding the likelihood of a flood event occurring in any year.

This probability is expressed as a percentage. For example, a large flood (formerly known as a 1 in 100 year flood) which may be calculated to have a 1% chance to occur in any one year, is described as 1% AEP. It is entirely possible, although unlikely, for 1% AEP floods to occur back to back in succeeding years.

So using the above example even though, in 20 years, there hasn’t been a flood at your place, each year there is a 1 per cent chance that a 1% AEP flood will happen.

‘Orange is 862 metres above sea level, if my property floods, the rest of Australia floods.’

Flooding is mostly likely to occur anywhere there is a creek or river, despite the altitude above sea level. 

Flooding occurs whenever a rainfall event within a given catchment is greater than the ability of the creeks and rivers to drain that catchment.

For example, Toowoomba (QLD) is 691m above sea level and it is estimated approximately $2.3 billion worth of damage was done by its 2011 flood.

Orange has experienced several flooding events in the last few decades.

What does the study mean for my property?

‘How can I find out more information?'

There are number of ways you can find out more :

  • Look at the flood study maps online at https://yoursay.orange.nsw.gov.au/flood-study-update
  • Call the Orange City Council Customer Service team on 6393 8000 and make an appointment to meet with a Council staff member.
  • Attend a face-to-face information workshop. A workshop is to be held Thursday 1 August at 6pm in the Civic Theatre forum, Byng St Orange.

The study, which identifies land that’s potentially affected by a flood, is on public exhibition from 26 July to 26 August alongside associated proposed changes to Orange's Development Control Plan (DCP) and Local Environment Plan (LEP).

If you want to discuss specific information with a member of Council’s Planning team (about the DCP or LEP changes) or a member of the Technical Services team (about the flood study) call the council 6393 8000 to make an appointment

‘The flood study maps show two different kinds of potential flood zones, marked on the maps as red and blue. What's the difference?'

There are two kinds of flooding assessed in the flood study maps. Riverine flooding and overland flooding. 

Orange's previous flood study only identified riverine flooding. Areas marked as overland flooding are being identified for the first time.


Riverine flooding occurs in the areas in the neighbourhood of creeks, waterways and channels where floodwaters could spill over the banks of a creek and inundate nearby areas.

These areas are marked on the maps in red and are called ‘Flood Planning Areas’.


The maps of overland flooding describe areas that can be affected after very heavy storms, when water produced by heavy rain, that doesn’t come from a creek, flows from higher land in a neighbourhood, to lower land.

These areas are marked on the maps in blue and are labelled as ‘Land outside a Flood Planning Area which is subject to overland flow deeper than 100 mm.’

While residents living near creeks may expect riverine flooding from time to time, the study of overland flooding attempts to analyse and raise awareness of flooding triggered by heavy storms in a neighbourhood.

What does this mean for my property values?

In general, Council cannot predict the housing market and it is best to seek independent advice, however it is unlikely your property value will change if your floor level is above the 1% AEP level.

If your floor is below the 1 per cent flood level, there may be an impact. 

There are mitigation works a home owner can do to raise the floor height. It’s best to come into Council to speak with a town planner or seek independent advice.

What happens next?

‘Areas of Orange have been identified as being potentially affected by flooding. What is Council going to do about this?’

Once Council has adopted detailed flood maps it will then be able to design and construct projects aimed at reducing the potential of flooding or to lessen its impact. 

These projects can include earth-works such as retention basins, which divert or slow down floodwaters.

Importantly, adopting the flood maps will let Council qualify for funding for these projects from other tiers of government.

After the last study was done in 2009, Council completed flood projects, such as widening the East Orange channel.

Proposed changes for Orange's Planning Assessment system, due to the new flood study

‘What does the flood study mean for Orange City Council’s role in managing planning approvals?’

When someone wants to buy or sell a property, it’s routine that they can apply to the council for a Section 10.7 Planning Certificate. This is required by the conveyancing system.

It’s important to note that, Section 10.7 planning certificates provide a snapshot in time of the planning controls that apply to the land. As such, they do not guarantee that the planning controls will never change.

This lets a potential buyer be aware of a range of information held by the council, such as zoning, whether the area is bush-fire prone or in a heritage conservation area.  The information about whether particular properties are identified in various flood maps is one of the categories of information that is required to be provided.

How does a potential buyer of land find out if a property is identified on a flood map?

Like other councils Orange City Council has a system where people can apply for a document called a Section 10.7 Planning Certificates.

Under NSW legislation a contract for sale of a property must include a S10.7 certificate from the local Council.  There are two types of s10.7 certificate:

S10.7 (2) certificate

Among other things this will include information about :

  • planning instruments
  • development control plans
  • zoning
  • land use
  • heritage items
  • road widenings
  • flood related development controls
  • bush fire prone land and
  • land reserved for acquisition.

This list is specified in Schedule 4 of the Environmental Planning and Assessment Regulation 2000.

S10.7 (5) certificate

This provides the above s10.7 (2) information plus advice on other relevant matters affecting the land that Council may be aware of.

Why is this information provided publicly?

Council has a responsibility to inform the community including future property owners as clearly as we can about potential risks to life and property, and in the case of flood studies, to regulate development in order to limit damage from flooding now and in the future.

Council also has a legal and moral obligation to let current and future owners know about development controls that may apply to specific property.  Any property identified as being flood prone, and that may attract development controls, will have this information on its s10.7 certificate.

Does being identified on a flood study map mean I can’t develop my land?

It is unlikely that development controls will prohibit all development on land on flood prone land.  

However, some factors may need to be taken into consideration in any future development to alleviate risk to life and property.  For example, you may be asked to build on a certain part of your land or elevate your building to a certain height.  Council’s development assessment team can help you understand these considerations.

Being identified as flood prone land is more likely to influence decisions around rezoning land in the future, as it is clearly preferable to direct development away from areas susceptible to flooding.

Will the flood map listing affect the value of my property?

Individual property values are based on many factors and the impact of flood development controls varies greatly from one property to another. For that reason, it’s impossible to predict whether flood information on a planning certificate will affect property values.  

The information may affect one potential buyer’s decision to purchase a property but for another it may have no impact.  Studies have shown that an actual flood event rather than a flood planning information, is more likely to have an effect on property values.

It’s important to note that being listed on a flood map will not change how flood prone a particular property is.

A major advantage of appropriate acknowledgement of a flood risk is that it should ensure that your neighbour, or even other land within the catchment is developed in such a way that it does not divert or increase flood waters onto your property that would disadvantage your property and potentially reduce value.

The flood study maps show only a small part of my property is affected, so why is the whole property now classified as ‘flood prone’?

The flood study maps highlight which properties are expected to be affected by flooding.  

Even if a small portion of the property is affected, the whole property area will be classified as affected and this information mentioned on a S10.7 certificate.  S10.7 certificates apply to the whole property therefore must acknowledge all development controls that affect the property, whether in whole or in part.

However, assessment of the controls during a development application depends on where on the property the proposed development is located. If an application is seeking to build a structure clear of the acknowledged hazard, the fact that another part of the property is flood prone will typically* not affect the assessment.

*other than ensuring that the additional runoff from roofs and paths does not exacerbate flooding for other properties.

‘What if I’m halfway through my renovations when the study is adopted and my property is affected? ‘

Please come and talk with a Council Town Planner. Council’s planners can help you work out a solution.

Existing development consents remain legally valid and you may proceed with your project in accord with the consent conditions. 

However, it may be in your interest to discuss any implications for your property with one of Council’s town planners or engineers in case you wish to modify the approval in light of the new study.