FAQs

Overland flow flooding is caused by intense rainfall events, which result in flow paths forming or floodwater pooling in low-lying flatter areas. Overland flow flooding is typically shallow depths and short duration (less than a few hours).   Water naturally flows to the lowest point, generally ending up in a creek or a drain. Overland flow paths are the routes taken by rainfall runoff and are not always obvious until they flood.

In June 2016, Council completed and formally endorsed the draft Overland Flow Flood Study for the Hurstville, Mortdale and Peakhurst Wards.  The Flood Study is a technical investigation of flood behaviour, while the Floodplain Risk Management Study and Plan aims to investigate and prioritise strategies to manage flood risk and reduce the impacts of flooding on the community through implementation of these strategies. Impacts can include property damage and financial loss, inconvenience and personal danger.

A 1% AEP (1 in 100 year) flood is a flood event that has the probability of occurring on average once every 100 years, i.e. there is a 1% chance of a flood of this size occurring at a particular location in any given year.  Some parts of Australia have experienced more than one ‘1 in 100 year’ floods within a decade of each other. Within the Floodplain Risk Management Study and Plan the ‘1 in 100 year’ flood is referred to as the 1% AEP flood event.

The Flood Planning Level (FPL) is a height used to set floor levels for property development in flood prone areas. It is generally defined as the defined flood event flood level plus an appropriate freeboard (see explanation of ‘freeboard’ below). This level may be higher for vulnerable land uses (e.g. hospitals or schools).

A freeboard is a height above the defined flood event flood level that is included in the Flood Planning Level to account for factors such as wind, waves from vehicles, unforeseen blockages, other localised hydraulic effects.

Council’s flood modelling determines the extent of flooding throughout the catchment and the identification of properties partially or fully impacted by flooding.  Section 10.7 Planning Certificates identify if flood related development controls apply to the land.  

If the situation is life threatening you should call 000.

For other assistance during an emergency such as flood, storm or tsunami, please contact the NSW State Emergency Service (SES) on 132 500 or visit their website at www.ses.nsw.gov.au.

It is best to be prepared for any flood. The State Emergency Service provides advice on how to manage your flood risk at floodsafe.com.au.

Councils prepare Flood Studies and Floodplain Risk Management Studies and Plans in accordance with the NSW Government's Floodplain Development Manual (2005), and implement associated recommendations with the technical and financial assistance of the NSW Department of Planning Industry and Environment.

This draft plan details the recommendations to reduce the impacts of overland flow flooding to residents and businesses in the study area.

There are three main ways of managing flood risk:

1. Flood modification – this approach looks at changing the flood behaviour using structures such as detention basins or drainage system improvements. 
2. Property modification – this approach includes modification to existing properties and seeks to better manage future development through land use planning.
3. Response modification – this approach focuses on increasing our resilience through better preparedness, improved warning and recovery planning. 

For most people, there is nothing to do in response to this Draft Plan. However, property owners who are planning redevelopment of their property may need to appropriately consider the impacts of overland flood risk during the development process.  This could include the requirement to have the floor levels of the development set at or above the Flood Planning Level.  This ensures that the impacts of flooding are reduced.

The results from this plan will update flood information obtained from previous studies. Almost all properties identified in this plan have previously been identified as flood affected. Overall, there is a slight reduction in the number of properties identified.

Studies show that an actual flood event, rather than a flood planning notation on a Planning Certificate, is more likely to have an effect on property values.

Individual insurance companies typically identify Flood Prone Land and assess risk through their own flood studies, analysis and flood mapping exercises, irrespective of whether Council has undertaken a flood study. These calculations are outside Council’s control. The information is then used to set policies and premiums.

The Probable Maximum Flood (PMF) combines a range of extreme conditions and probabilities to calculate the largest flood that could possibly occur within a catchment. It is extremely rare and is calculated to aid understand the full risk of flooding and used in the design of critical and sensitive facilities such as hospitals, childcares, nursing homes, schools and emergency service facilities.

Floods do not occur in a regular pattern. There may be a period of no floods and a period of several floods. For example, the last time the Brisbane River flooded before the 2011 disaster was in 1974. Residents who moved there in more recent times may never considered flooding an issue until the floods in January 2011. In 1949 and 1950 Kempsey experienced the two worst floods ever recorded, within eight months of each other. Following extensive rain every water course will flood.

Members of the public can make a written submission to Council to raise flood related issues. You can provide your valuable feedback and comments by emailing Council.

Properties can be identified as being flood affected, in accordance with the Council’s policy, regardless of the floor levels in the building. This is to ensure any future developments of the property are appropriate for the Flood Planning Area and not built below the flood level.

Property owners or developers can seek flood level information from Council. Contact customer service on (02) 9330 6400 or mail@georgesriver.nsw.gov.au for more information on a flood advice letter application.

Council’s piped stormwater system is designed to convey frequent minor storms with the aim of reducing day-to-day nuisance flooding, while major storms are conveyed via overland flow paths with the aim of protecting life and property in major events. It is impractical to provide piped drainage systems for major storm events as this is cost prohibitive.

Property owners must maintain the stormwater pipes, gutters, downpipes, gully pits and any other components of the approved stormwater system on a private property in good condition and in compliance with any Council requirements. If your property contains a stormwater easement it must be maintained and kept clear of obstructions to allow the natural flow of stormwater.

Property owners are responsible for maintaining the stormwater system on their property to the kerb and gutter and if maintenance is needed within the footpath area an application to carry out work within the road reserve is required. Applications are available at Council’s Customer Service Centre or on Council’s website.

During a rain event, overland surface flow will follow the lowest point of the land. Accepting natural overland flow from adjoining properties or public land is necessary and the flow of water must not be diverted or redirected from its natural path onto neighbouring properties.

It is important to note that a downstream property owner cannot erect any type of barrier that interferes with the flow path of stormwater. To put it more simply, if your property is located downstream, you must accept the ‘natural’ runoff onto your property.