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Why Council undertakes flood studies

Heavy rainfall can lead to flooding when water levels rise and inundate nearby land. This is a natural process.

Flooding is a serious problem in towns and cities, where it can have significant consequences for individuals and the community. These include damage to property and infrastructure, economic hardship, and trauma and emotional distress.

Flooding in urban and rural NSW costs the NSW economy about $200 million each year, and the human impact is even greater.

Managing flood risk is everyone’s responsibility. Communities need to be prepared for flooding, understand how to respond to flood threats and recover from the impacts of floods when they happen. Council aims to reduce the impacts of floods by taking action before they occur.

Local council has two key responsibilities:

  • Local councils carry out studies to understand flood risk. They keep the community informed about flooding, support emergency management planning, and examine options to manage flood risk.
  • Local councils take flooding into account when controlling the development of flood-prone land, and in carrying out management actions including the investigation, design, construction, operation and maintenance of flood mitigation works.

The key reasons why Council undertakes flood studies:

  • To construct flood mitigation infrastructure.
  • To provide flood awareness and information to the community.
  • To aid agencies in flood emergency management.
  • To inform land use planning and development control.

The objective of flood studies are to improve understanding of flood behaviour and impacts, and better inform management of flood risk in the study area in consideration of the available information, and relevant standards and guideline. Flood studies also provide a sound technical basis for any further flood risk management investigation in the area.

Flood studies are overseen and guided by Council and its steering committee (Coast, Estuary & Floodplain Advisory Sub-committee), which include representatives from key stakeholders and end user groups.

The objective of flood studies are to:

  • Improve understanding of flood behaviour and impacts,
  • Better inform management of flood risk, and
  • Provide a sound technical basis for any further flood risk management (i.e. investigations and works)

Flood studies are overseen and guided by Council and its steering committee (Coast, Estuary & Floodplain Advisory Sub-committee), which includes representatives from key stakeholders and end user groups, including:

  • Council Staff,
  • NP&WS,
  • OEH,
  • SES,
  • NSW Fisheries,
  • RMS Maritime,
  • Crown Lands
  • Oyster & Fishing Industry,
  • Community Members.

Council’s Role in floodplain management

Councils have a significant and vital role in addressing and adapting to known risks. This stems from our responsibilities under the Local Government Act (1993) (among others) regarding land use planning, transport, development assessment and asset or infrastructure management.

The task of managing and mitigating against risks is ongoing and can be contentious, expensive and requires long term commitment from Councils. The task requires sound planning and the framework for decision making on floods at a Council level helps to minimise liability and risks and insures that risks are within acceptable limits and can be adequately managed.

The objective of the floodplain management process is to assist Councils understand and limit potential liability exposures arising from the day-to-day operations and legislative functions of local government. Specifically the floodplain management process framework gives consideration to the legislative requirements incumbent on NSW Councils as local authorities in relation to the management of areas that are potentially affected by flooding.

Inadequate planning instruments (and supporting tools and policies) means that Council could find their planning decisions subject to legal challenge. Legal challenges may take the form of proceedings for administrative review of Council decisions or liability claims for recovery of damages associated with those decisions.

It is also expected that, in an environment of uncertainty associated with increased environmental risks, many ratepayers will more frequently look to Councils to engage with, consult and offer direction in relation to predicted natural disaster related impacts.

Flood study approach

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Councils are required to undertake flood investigations in an evidence-based risk management approach, using the best information available to underpin decisions made in their planning schemes or at the time a development application is lodged.

The objective of flood studies are to improve understanding of flood behaviour and impacts, and better inform management of flood risk in the study area in consideration of the available information, and relevant standards and guideline. Flood studies also provide a sound technical basis for any further flood risk management investigation in the area.

Flood studies are overseen and guided by Council and its steering committee (Coast, Estuary & Floodplain Advisory Sub-committee), which include representatives from key stakeholders and end user groups.

Flood studies are a comprehensive technical investigation of flood behaviour that provides the main technical foundation for the development of a robust floodplain risk management plan. It aims to provide a better understanding of the full range of flood behaviour and consequences. It involves consideration of the local flood history, available collected flood data, and the development of hydrologic and hydraulic models that are calibrated and verified, where possible, against historic flood events and extended, where appropriate, to determine the full range of flood behaviour.

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NSW government legislation stipulates that Local Councils are primarily responsibility for the flood planning, flood mitigation works and the management of development within floodplains. To appropriately manage development within the floodplain Councils are required to complete a strategic plan which considers the risks of development in the floodplain and balances these against the beneficial use of the floodplain by development.

The consideration of a range of environmental, social, economic, financial and engineering issues is paramount in this context and appropriate flood management needs to recognise the full flood risk. This assessment is undertaken via a Floodplain Risk Management Study. The outcome of the study is the Floodplain Risk Management Plan, which details how best to plan for and manage flood risks in the floodplain for the foreseeable future.

Typically these studies and plans cover a range of works and planning measure, including:

  • flood warning and emergency management enhancements;
  • flood protection works (e.g. ring levees) for existing developments;
  • voluntary purchase or house raising of severely flood-affected properties;
  • flood policy, planning and building controls to ensure future development is compatible with existing and future flood risks; and
  • flood education to raise the community’s awareness of flooding so that they are better able to deal with the flood risks they face.

Vital to flood studies is community engagement, which forms part of the preparation, revision and ultimate adoption of a flood study. The NSW government flood policy provides the community with the opportunity to have their say throughout the process. Consultation typically occurs in the early stages of a flood study where the community is asked to provide any flood related information they have (i.e. pictures, flood levels, anecdotal evidence etc). Consultation is also undertaken upon preparation of a draft study and then again when the Floodplain Risk Management Study phase is underway. The floodplain management process, mandated by the NSW government, allows the community’s concerns, suggestions and comments to be considered and all feedback received throughout flood studies is essential to ensure that an accurate flood study is produced.

The Floodplain Risk Management Plan ultimately documents and conveys the decisions on the management of flood risk into the future. Drawing on the investigations undertaken as part of the flood study and Floodplain Risk Management Study, the plan outlines what measures Council will take to manage existing, future and residual flood risk effectively and efficiently. This will include a prioritised implementation strategy, what measures are proposed and how they will be implemented.

  • Flood studies provide the community with an increased understanding of the impacts of floods on the community.
  • Council is able to provide the community with information about the potential risks associated with living in flood prone areas.
  • Council can provide detailed information to prospective purchasers of land within flood prone areas.
  • Development can be assessed against Council adopted flood studies which can reduce the need for individual flood studies for small developments.
  • Insurance premiums can be lowered for those areas which are located in low risk areas of the floodplain or outside flood prone land.

Flood studies are revised by Council every 10 years, on average. As part of these updates predicted flood levels can change. The resultant change in flood levels is normally minor and is generally associated with improvements on computer modelling capability and new data.

The reasons why Council adopted flood models are revised can include:

  • new rainfall and runoff information becomes available;
  • modelling of future climate change impacts;
  • better topographic information;
  • new floods occur which provide additional data to refine the models;
  • better computing capabilities and the science of flood modelling improves; and
  • flood mitigation works may have been carried out, or development within the catchment may have occurred, that was not previously simulated in the models (i.e. the new Pacific Highway).

 

Flood studies which are carried out under State Government guidelines are eligible for grant funding up to a 2:1 basis (i.e. $2 is provided from the state government for every $1 provided by Council). This funding arrangement is also available for the construction of flood mitigation works and implementation of actions from any adopted Floodplain Risk Management Plan.

Without an adopted flood study and plan Council would not be able to receive any state government funding to undertake flood mitigation works. It is common for flood mitigation works to be very expensive, especially when undertaken in already built-up areas. If Council has not completed a flood study and wishes to undertake works (i.e. raising a road or upgrading a culvert) then it will need to be fully funded by Council as we would not be eligible for grant funding from the NSW government. In this case it is likely that valid projects may go un-funded as it would be prohibitive for Council to fully fund the project, given competing budgetary items that Council is responsible for delivering.

Examples of recent flood projects that have been funded by Council and the NSW government:

Camden Haven Floodplain Risk Management Plan (2004):

  • Dunbogan Flood Access Road (Stages 1C & 1B) – total project cost $5M ($3.33M state : 1.66M local)
  • Dunbogan Flood Access Road (Stage 1A) – total project cost $1.5M ($2M state : 1M local)
  • Camden Haven Flood Markers – total project cost $25k ($16.6k state : $8.3k local)
  • Flood Gate North Wall – total project cost $420k ($280k state : $140k local)

Hastings River Floodplain Risk Management Plan (2014):

  • Hastings River Flood Study Update – total project cost $64k ($42.5k state : $21.2k local)
  • Hibbard Precinct Floodway Investigation  – total project cost $114k ($76k state : $38k local)
  • Hastings River Additional Streamflow Gauges  – total project cost $50k ($33.3k state : $16.6k local)

 

The floodplain is classified in many ways. The floodplain is generally broken down into areas of Low to Extreme risk (in terms of hazard) and Flood Storage to Floodway (in terms of the function of the floodplain).

Typically the floodplain is categorised using the 100 year flood event, however various other flood magnitudes can be used depending on the level of risk of the development being proposed. i.e. a machinery shed has a different level of flood constraints applied when compared to a Hospital or Aged Care Facility.

Whilst the 100 year event is commonly referred to and used in flood planning, other floods, up to and including the Probable Maximum Flood (PMF) and various climate change scenarios can be used in flood planning. The PMF is the largest flood that could possibly occur and it is an extremely rare and improbable flood. However records of historical floods in Australia have approached the magnitude of a PMF event, therefore every property within a floodplain is potentially inundated by a PMF, hence it will have some flood risk, even if it is very small. Under current NSW government guidelines, Councils must consider all flood risks.

Note: A 100 year flood is the flood that is expected to occur on average once every 100 years. A better way to think of a 100 year flood is to consider that it has a 1% probability of occurring in any given year. The last 100 year (1%) floods in the Hastings and Camden Haven valleys occurred in 1963 & 1968. Another event of this magnitude could occur at any time, it is a misconception that once a large flood has occurred that there will be a wait of another 99 years before the next flood arrives. Floods do not follow any set pattern and it is possible for multiple 100 year floods to occur within quick succession.

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PROVISIONAL HAZARD MAPPING FOR THE 100 YEAR RECURRENCE FLOOD

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HYDRAULIC CATEGORY MAPPING FOR THE 100 YEAR RECURRENCE FLOOD

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PREDICTED FLOOD DEPTHS AND VELOCITIES AT THE PEAK OF THE DESIGN 100 YEAR RECURRENCE FLOOD

It depends on where your property is and how affected it is by flood events. It also depends on what the data is intended to be used for, e.g. flood planning and development, emergency management, insurance companies, lending institutions.

There are properties within the floodplain that can be inundated on a frequent basis by very minor flood events. Consequently, when large flood events occur these properties are likely to be considered to have a high risk (in terms of structural damage to property, low chance of evacuation etc) and strict controls may be applicable. Insurance and finance on these properties may be impacted, however in many instances prospective lending authorities and insurance companies will inform themselves of a properties flood risk regardless of Council flood study results. These institutions generally identify the nature of the flood risks associated with a certain property and can form their own conclusions on whether they will insure or lend money on a particular property.

On the other hand there are properties that are located on the fringe of floodplains and are only impacted by rarer flood events. These properties are likely to be considered low risk. Planning restrictions may be virtually non-existent and development of the property may occur without hindrance. Similarly where properties are located outside of known flood prone areas, insurance premiums and lending restrictions will be reflective of this fact.

Note: In certain instances properties can have increased premiums imposed where flood data is not available, even if the property is located well away from any flood threat. Where flood studies have not been completed, insurance companies and lending institutions may apply flood premiums purely as a result of the lack of data. The lack of flood information can therefore impact on more properties than it otherwise would have if a flood study had been conducted.

 

  • Brisbane Flood (2010)
  • Toowoomba & Grantham Flood (2011)
  • Grantham Flood (2011)
  • Hastings River Flood (2013)
  • Dungog Flood (2015)
  • Lismore Flood (2017)

Flooding is a fairly common occurrence in Australia. Councils, in conjunction with the NSW government, emergency management agencies (SES) and the local community can reduce risks associated with this natural phenomena by ensuring that we are well prepared.

By undertaking flood studies and implementing flood mitigation works Council can ensure that the existing, future and residual risks associated with human habitation and development on the floodplain can be managed and that impacts on the community when flooding occurs is minimised.

 

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This page was last updated on: 30 January 2019