What is the Maryborough Flood Study?

Council, in partnership with the North Central Catchment Management Authority and Victorian State Emergency Service, has recently completed the Draft Flood Study of Maryborough.

In 2020 Council received a Victorian and Australian Government funding through the Australian Government Natural Disaster Resilience Grants Scheme to undertake the Study.

The Draft Flood Study defines flood risk in Maryborough, contains visual representations of a range of potential flood events, and investigates a range of measures to manage and mitigate the flood risk.

This is the first detailed Flood Study ever completed for Maryborough and will provide better flood information for the town. The outputs from this study will be used to manage future growth of the town as well as for emergency flood response in times of major flood events.

The community was invited to review the Draft Flood Study for Maryborough Township and provide feedback.

A community consultation session on was also held on August 10.

Thank you to everyone who took the time to provide feedback. The feedback will be considered before going to Council for consideration later this year.

North Central CMA is the Floodplain Management Authority by delegation from the Minister responsible for the Water Act 1989. Consequently, North Central CMA is responsible for determining the potential inundation of land from rivers and creeks within the North Central region. Central Goldfields Shire Council is responsible for the provision and maintenance of drainage, levees and the management of any stormwater from its drainage system.

The first stage in managing floodplain risk for a particular catchment involves a flood study, which is a comprehensive technical investigation of flood behaviour for that catchment. These flood studies show the distribution, extent, levels and velocity of floodwaters across sections of the floodplain for a range of different flood events.

Flooding is a natural process intrinsic to all waterways. Flooding occurs periodically as a result of heavy rainfall within a catchment and is generally defined by the runoff from the storm event exceeding the capacity of the bed and banks of a waterway or local drainage system. The effects of flooding in Central Goldfields are magnified by the proximity of urban development to natural or modified creeks and channels resulting in damage to roads, residential, commercial and industrial properties

Flooding remains a significant community issue throughout the whole of our state and country. The average annual cost of damages from flooding in Victoria alone is greater than $100 million. It is widely accepted that the best way to mitigate against the effects of flooding is through appropriate building design and land use planning. To assist these processes the risks posed by flooding need to be properly understood by undertaking flood studies.

Maryborough experiences flash flooding from a number of open drains and waterways. Past flood events have impacted on residential and commercial properties, sporting facilities and other public infrastructure.

Over 1000 properties within Maryborough are currently covered by the Land Subject to Inundation Overlay (LSIO) within the Central Goldfields Planning Scheme. This study offers an opportunity to improve the accuracy of this overlay by using detailed hydraulic modelling and information from past flood events.

This investigation focused on determining flood levels and extents for a range of flood events within the study area and recommending a number of possible structural and non-structural mitigation measures to reduce the future risk of flooding to the Maryborough community.

There is a need to undertake detailed flood modelling for Maryborough to update planning controls and inform future land development planning.

Flash flooding occurs following intense rainfall with resulting flood levels rising to their peak within a very short time, typically between 30 minutes and 2 hours. This tends to occur in steep urbanised catchments such as Maryborough and gives residents very little warning time to prepare.

A 1 in 100-year ARI* flood is a level of flooding that has a 1% chance of occurring in any year. It is considered a very large flood and is used as the minimum design standard for new development in Victoria. If an area experiences a 1 in 100-year flood in a certain year, it does not mean that another 1 in 100 year flood will not occur for another 99 years, it is an average only. The 1 in 100 year flood is technically referred to as the 1% AEP* flood.

* Average Recurrence Interval – is the likelihood of occurrence, expressed in terms of the long-term average number of years, between flood events as large or larger than the design flood event. For example, floods with a discharge as large as or larger than the 100 year ARI flood event will occur on average once every 100 years. This terminology has been superseded by Annual Exceedance Probability.

*Annual Exceedance Probability – is the likelihood of occurrence of a flood of given size or larger occurring in any one year. AEP is expressed as a percentage (%) risk, i.e. a 1% AEP flood has a 1% chance of occurring in any given year.

The PMF is the largest flood that could conceivably occur. It is typically estimated from Probable Maximum Precipitation coupled with the worst flood producing catchment conditions. While it is a rare and improbable occurrence, every property potentially affected by a PMF is considered to be on a floodplain and has some element of flood risk. Under the State Government's Floodplain Development Manual (2005), Councils must consider the full range of risk when managing floodplains.

Australian Height Datum is a standardised way of expressing heigh above (approximately) mean sea level across Australia.

A floodplain is any portion of land that is subject to inundation by floods up to and including the Probable Maximum Flood (PMF) event (or the highest conceivable flood that nature can produce).

Flooding is a natural phenomenon that will always occur. Problems with flooding arise when development occurs in floodplains that does not take full account of flooding. Flooding can rarely be prevented but the risk to life and property posed by flooding can be managed.

The study provides a range of valuable outputs, including:

  • Information and tools to assist emergency management of flooding in Maryborough
  • Improved flood planning information to guide future land use planning and development
  • An investigation of structural options to mitigate the impacts of flooding in Maryborough.

A number of mitigation options have been assessed to determine their effectiveness in reducing flood risk and their feasibility.

The most effective options focus on the Park Road area of Maryborough. This area was identified as the most vulnerable pocket in the township with regards to flood risk with runoff trapped between Park Road and the nearby railway line.

A range of options were assessed to investigate the benefits of modifying and extending the existing channel, constructing a levee along the creek and upgrading culverts. and ex have been assessed in detail which included modelling the full range of design events, costings, damages assessment and benefit cost analysis.

All three options have a high benefit cost analysis and it has been recommended to Council to consider these results and progress one of these options to the next stage of design.

Communities that are aware of their flood risk are the best placed to prepare for, respond to and recover from flooding. Taking the time to have a look at the flood study will help inform you about flood risk in Maryborough, and highlight particular areas and roads most at risk.

In addition, Council is keen to hear your feedback about the recommended mitigation options in the Park Road area before these are investigated further.