Flooding

Flooding can cause significant damage to property, infrastructure and pose a risk to life. All councils in Tasmania are required by law to work towards identifying the land in their urban area which has the potential to be affected by flooding. This helps councils to guide urban development, identify potential improvements to stormwater infrastructure, protect property and keep the public safe.

Clarence City Council has developed management plans for its stormwater systems in urban areas and a key component of this process was investigating the extents and effects of flooding.

Flood modelling data has been used to produce a set of flood maps which show the likely extents of inundation for different rainfall events. The flood maps will help council to:

• Protect the safety of the public in future flood events.
• Plan future infrastructure upgrades.
• Implement improved town planning controls to guide future development.
• Determine the minimum floor height for new properties in flood prone areas.

Council engaged several leading Australian engineering consultancies in stormwater modelling (GHD, Cardno and Engeny Water Management) to undertake detailed flood studies of the urban areas in the City of Clarence. These flood studies have adopted the approach recommended by national guidelines for flood modelling in Australia.

Computer models have been set up using historical flood events, which allows council to estimate the extent of urban flooding for a range of different rainfall events. This includes several scenarios including a 5% AEP flood, a 1% AEP flood and other flood scenarios related to climate change (refer below for an explanation of the terms 5% AEP flood and 1% AEP flood).

The flood mapping on council’s website shows the 1% AEP flood, including an allowance for climate change. This is the storm event which is used by council to implement town planning and building controls, as well as plan for the safety of the public.

AEP stands for annual exceedance probability. This is a term used to describe the intensity of a storm event, and the associated flood that it causes. It is used in flood studies to model the effects of different types of storm events.

The AEP is the likelihood of a storm event occurring in any given year and is usually expressed as a percentage (%).

A 1% AEP storm event has a one percent chance of occurring in any given year. This expression has replaced the use of the old terminology of a one in 100-year storm event.

Likewise, a 5% AEP storm event has a five percent chance of occurring in any given year.

The lower the percentage (%), the less likely an event is to occur. Very extreme rainfall events have a low chance of occurring in any given year, whereas less extreme events are more frequent and more likely.

Flood mapping included on council’s website is based on the 1% AEP storm event and includes an allowance for climate change. Rainfall is predicted to be heavier as a result of climate change, which means that flooding may also be more frequent and more severe. This possible increase in rainfall has been considered in council’s flood mapping.

The availability of more accurate information about the flood risk to your property may affect your property’s flood insurance, so you are encouraged to contact your insurance provider to ensure you are adequately covered.

Flood mapping is available on council’s website here.

The flood mapping shows the likely effects of the 1% AEP flood, including an allowance for climate change. This is the storm event which is used by council to implement town planning and building controls, as well as plan for the safety of the public.

Council recommends that you contact your insurer to make sure that you have adequate flood insurance for your property.

Council is working on prioritising projects to mitigate the impact of future flood events. Increasing our understanding of existing and future flood risks allows for improved planning to better prepare for and manage these events into the future.

Council’s flood studies considered a range of different scenarios, including the impact of sea level rise and change in rainfall intensity caused by climate change.

Council always uses the most up-to-date climate change predictions and acknowledges that this may change in the future as more information becomes available.

The flood maps provided on the council website include an allowance for climate change and sea level rise.

Floods do not occur in a regular pattern. There may be long periods where there are no floods, followed by short periods with several floods.

A 1% AEP flood is an extreme event and may not have occurred in your location while you have lived in the area. A 1% AEP storm event has a one percent chance of occurring in any given year.

Council is collecting information and developing an understanding of flooding within the municipality. Therefore, we may not have known that your property was in a flood zone when you purchased your property.

Council can provide advice to any property owners and buyers about whether a property is subject to flooding. If you are concerned, please contact council.

Please note that a Land Information Certificate (also called a Section 337 Certificate) for a property does not currently provide information about flooding.

Underground stormwater infrastructure in Tasmania is typically designed for a 5% AEP flood event. This is considered to provide a reasonable level of protection from flooding at a cost which is sustainable to the community.

In the event of a more severe storm, where the capacity of the underground system is exceeded, overland flow can be expected to occur. In contemporary design, it is best practice to allow for additional capacity in the road reserve, or through easements on private land, for overland flooding to occur without damage to infrastructure and property.

However, older infrastructure was built at a time when there were different design standards, different climate and a lower level of urbanisation. As a result, parts of council’s stormwater drainage network may reach capacity for weather events that happen more often than the 5% AEP and overland flow paths for flooding, once the system capacity is exceeded, may not have been provided for.

Council is continuously working on upgrading its stormwater drainage network to meet the expected level of service.

Please report any concerns about blocked council stormwater drains to council’s customer service team on (03) 6217 9500.

Property owners are responsible for making sure that private stormwater systems are properly connected to council’s stormwater system. If you think there is a blockage in one of the drainage pipes on your property, you should contact a licensed plumber to investigate.

Council has many different roles to play when it comes to flood management. These include:

• Developing and implementing local flood management plans to reduce the adverse impacts of flooding to acceptable levels.
• Incorporating flood mapping and controls into the local planning schemes to manage land use and development (buildings, works and subdivisions) within flood-prone areas.
• Managing local community infrastructure by building, owning, and maintaining public assets.
• Providing for the conservation of significant natural resources and environmental values.

If you need assistance during a flood event, call your local State Emergency Service (SES) on 132 500 or triple zero (000) in life threatening situations.

The flood information will be used by council to prioritise upgrades to stormwater infrastructure and to guide the planning of new and future developments.

Council is currently in the process of completing a flood study for the Coal River catchment. Once this study is complete, additional mapping will be available for the Richmond township and surrounding areas.

You can find flood mapping for the urban areas of the City of Clarence here.

The Bureau of Meteorology provides information on what to do before, during and after a flood here: http://www.bom.gov.au/water/floods/document/What_todo_floods.pdf

Additional information on what you can do to manage and prepare for a flood situation is found on the State Emergency Services and the Tas Alert Services websites following the links below:

• https://www.ses.tas.gov.au/plan-prepare/flood
• http://alert.tas.gov.au/GetReady/Pages/Flood.aspx

You are always welcome to contact council’s customer service team on (03) 6217 9500 should you have any concerns regarding flooding or stormwater within the municipality.