It is not feasible for all stormwater to be collected in underground drainage systems.The drains required would have to be enormous and expensive.

Drainage systems are generally designed to accommodate stormwater runoff in a piped system to cope with a 20 percent Annual Exceedance Probability (AEP) (1-in-5 year) event. In a one percent AEP event, the piped system quickly fills, and the additional runoff flows along the natural drainage paths towards a larger waterway across properties, roads and reserves.

Drainage system improvements are prioritised for houses, commercial or industrial buildings that are at greater risk of flooding.

Key criteria in prioritising investment includes the potential risk to life, above-floor flooding in homes, the cost of flood mitigation works and the number of properties that benefit from the works.

A flood study is a technical investigation of flooding behaviour, based on records of rainfall, catchment impacts and physical terrain.

It determines how far and high flood waters are expected to reach for floods of various magnitudes.

It also identifies the nature of a range of flood risks and flooding behaviour that require special consideration.

It investigates ways to reduce and manage flood risks and makes recommendations for potential further action.

Areas designated as ‘liable to flooding’ indicate that the land is likely to flood during an event with an Annual Exceedance Probability (AEP) of one percent.

A one percent AEP means that a flood of this size has a one percent chance of occurring in any year.

It is important to note that designating areas liable to flooding does not cause or change the likelihood of flooding but recognises the existing risk.

The designation will not be described on property titles but will be acknowledged in Property Information Certificates issued under the Building Regulations 2018.

The value of a property is determined by many different factors and it is not possible to specify what effect, if any, the identification of land as liable to flooding may have on the market value.

Land liable to flooding indicates there is a flood risk associated with your property and this will need to be considered for building permits and building works.

Flood risk information is also used to prepare planning scheme amendments and inform land use decisions.

Designation of land liable to flooding reflects new information that has become available to the City. 

The City has been progressively improving its flood information for areas liable to flooding, both to designate new areas with a flood risk and removing restrictions on areas that have demonstrated flood risk no longer exists.

We cannot assume a property that has never flooded is safe from flood risk, without investigating the catchment through flood studies.

The resulting flood extents identified are the basis for determining if a flood risk exists. 

Each insurance company has their own process for calculating their premiums, so it is best to speak directly with your insurance provider.

Individual insurers decide what criteria they use to determine flood risk and calculate premiums. This may include historical flood information, claims history and building type.

If your property is identified as liable to flooding it does not mean that building permits and building works will not be approved. The flood information is used to ensure that any building works are carried out in a manner that recognises the areas liable to flooding.

Requirements for any proposed building works depend on the flooding characteristics of the land such as water depth, flow velocity, and the nature of the proposal.

The City and Corangamite Catchment Management Authority can both provide advice on the requirements for works in these areas.

Depending on the nature of the works you may also need to request a Council Consent.

More information can be found on our Council Consents page. 

AEP is a commonly used international drainage and flood management standard.

This standard has been applied in Victoria since the early 1980s and was previously called a ‘1-in-100 year’ flood event. This phrase is misleading as it wrongly implies that the flood event can only occur once every 100 years.

Annual Exceedance Probability is used to define the extent of the overland flow along the drainage system and where flooding from waterways may occur.

A one percent AEP means there is a one percent, or one in 100, chance of the flood event occurring or being exceeded in any year.

An overland flow path is an above ground component of a stormwater network.

Overland flows occur where there is no underground drainage or where the system reaches capacity and cannot manage the stormwater volume.

The excess run-off then flows overland, following low-lying, natural drainage paths generally down to our rivers and oceans.