Frequently Asked Flooding Questions

Individual insurers decide what criteria they use to determine flood risk and insurers use data from multiple sources. Council’s flood risk information will often be one of the sources used by insurers.

If you have experienced significant insurance premium increases for flood protection, you may be able to use Council’s flood information to pursue a lower premium cover. Some things you may be able to do:

• Shop around and get quotes from a range of insurance companies, as they all calculate and define flood risk in slightly different ways.

• Request a flood information report for your property. Is your house footprint outside of the flood extent and not shown as flooded? Is the internal floor level for habitable rooms higher than the ground level of your property, raising the contents above the flood level? Take this information to your provider and have a discussion with them. See Property Flood Information Reports and Online Flood Mapping for more information.

   

Dredging was investigated as a potential flood mitigation measure in the previous work of the 2008 Ballina Flood Study Update. The modelling identified there were both positive benefits and negative impacts associated with dredging.

The results showed that whilst the dredging increased the conveyance of water through the channels and in particular, the river entrance, the impacts occurring during ocean storm surge events (reverse in-flow direction) exceeded the benefits during catchment rainfall flooding events (regular out-flow direction) and was not recommended for flood mitigation. As such, dredging was excluded from the adopted Floodplain Risk Management Study (2012) and Floodplain Risk Management Plan (2015).

This is not a new concept. We know of two locations where the sand dunes were breached during previous floods, at Boundary Creek and at German Creek. There are two known accounts of these occurrences dating back to 1890 which breached by natural means and then in 1893 when the dunes were artificially breached by farmers at Boundary Creek.

Construction of an artificial opening was considered as a possible flood mitigation measure in 1954 by the Richmond River Valley Flood Mitigation Committee, however, was not recommended because maintaining the opening was not practical and removing a section of dune would make the sand environment more susceptible to coastal erosion.

Whilst a secondary opening could provide relief from flooding from river to the ocean, it would create opportunity for flooding to occur from the ocean back in during cyclone events or storm surge events. The scale, cost and environmental impacts of this scheme is beyond the scope of what is feasible for Council to implement, which is why this was not adopted in the Ballina Floodplain Risk Management Studies.

The best way to evaluate if your property will be flooded is to compare the floor level of your house in metres AHD (Australian Height Datum) to the mapped flood height (also in metres AHD) to see if it is above or below the mapped flood level.

If you don’t know the level of your floor level in m AHD, you may consider engaging the services of a local surveyor to measure it for you. Council has estimated peak flood levels for various rainfall events which has been mapped and can be accessed on Council’s website.

See the link below on how to access Council’s flood information.

Property Flood Information Reports and Online Flood Mapping

Sea levels are projected to rise by around 20 centimetres by 2050 and up to 60 centimetres by 2100, which means that king tides and high tide levels could affect more properties in the future.

In Ballina’s urban areas, Council requires new development to include filling to both protect properties from flooding and support long-term flood resilience planning. Filling helps ensure that low-lying roads and infrastructure can be raised in the future, while still allowing stormwater from private properties to drain properly to the street. Ground levels of around 1.8 metres AHD or higher may be required to maintain effective drainage.

In rural areas, Council also requires a filled pad with a curtilage (a safe, raised area) around buildings. This approach, developed in consultation with the NSW State Emergency Service (SES), improves safety for properties that may become isolated during floods. A fill pad provides a safer environment for rescue and emergency access than elevated ‘Queenslander’ or pole-style homes.

As part of the floodplain risk management process, flood studies and flood risk management plans are reviewed over time. As these reviews take place, computer models are revised and predicted flood levels and associated flood mapping can change. The resultant change in flood levels and flood mapping however is normally very small. These changes are not expected to occur very often and the intervals between revisions to the maps would normally be many years.

The reasons why the computer models are revised can include;

• new rainfall or ground topography information becomes available;
• new floods occur which provide additional data from which to fine-tune the models;
• better computer models become available as the science of flood modelling improves and computer capabilities increase;
• development within the catchment may have occurred, that was not previously simulated in the models.

Regional flooding happens when major rivers, waterways or the ocean overflow and flood large areas. These events usually develop over a longer period and can affect communities across an entire region.

Local overland flooding, on the other hand, occurs when heavy rain creates local runoff that flows over the ground on its way to a creek, drain or river, rather than spilling over from it. This type of flooding usually happens quickly and affects smaller, more localised areas.

Council’s flood height information is provided in ‘m AHD’ which stands for metres above the Australian Height Datum which is ‘mean sea level’. It is approximately the mid-tide level in Ballina. So when you see a flood level stated as 2.1m AHD, it means the peak flood level is 2.1 metres above mean sea level.

RRVD stands for Richmond River Valley Datum or often referred to as “River Datum’ is a local datum set according to the lowest tide level in the Richmond River. RRVD is the datum commonly used on tide charts and numerous water gauges in the river. It is commonly used in flood forecasting in the Richmond Valley and can easily be confused with m AHD. River Datum varies slightly according to different gauges up and down the Richmond River, however, is about 0.9m below AHD or mean sea level at Ballina and 0.8m below AHD at Wardell. So, when you see a River Datum level of 1.9m in Ballina, it is equivalent to 1.0m AHD

Council’s flood information starts by establishing the ‘baseline’ according to the sea levels and historical rainfall that predates the influence of climate change. A combination of increased rainfall intensity and sea level rise is then added in the flood models according to various time horizons. ‘Near term’ (2020-2040), ‘Mid-term’ (2040-2060) and ‘Long-term’ (2080-2100) climate change factors has been considered.

A 1% Annual Exceedance Probability (AEP) Flood, also known 100 year ARI flood, means you have a 1 in 100 chance that a flood of that size or larger could occur in any one year. It does not mean that if it floods one year, it will not flood again for the next 99 years.

The 1% AEP flood level is generally used as the Design Flood Level for most types of development. 

The Flood Planning Level is a height used to set floor levels for property development in flood prone areas. For the majority of development, it is set based on the level of a 100-year flood (1% Annual Exceedance Probability) with Climate Change increased added.

The flood planning area is the area below the Flood Planning Level and as such subject to flood related development controls.

Freeboard is a factor of safety expressed as the height above the design flood level to compensate for uncertainties in the estimation of flood levels across the floodplain, such as wave action, localised hydraulic behaviour, unforeseen blockages and impacts that are specific event related. Freeboard is usually 0.5m above the design flood level.

The Probable Maximum Flood (PMF) is the largest flood that could possibly occur. It is a very rare and improbable flood. Every property potentially inundated by a PMF will have some flood risk, even if it is very small. Under the State Government changes implemented during 2001, the terms 'flood liable', flood prone' and 'floodplain' have been changed to refer to land inundated by the PMF. Council maps the PMF to meet the requirements under these State Government Requirements