Flood planning

Council commissioned a review of potential property price impacts which concluded that adoption of new flood maps would unlikely  impact property prices and also that previous flood maps, which were present in Council’s Local Environment Plan (LEP), had no impact on property values.

The Waverley LGA Flood Study outlined that most of the water where inundation could occur, aside from Bronte Gully and Tamarama Gully, in the Waverley LGA should be considered as ‘stormwater’ for the purposes of insurance. In Australia, all insurers have adopted a consistent definition of a flood as:

‘The covering of normally dry land by water that has escaped or been released from the normal confines of any lake, river, creek or other natural watercourse, whether or not altered or modified; or any reservoir, canal or dam’. Stormwater is not considered a ‘flood’ for insurance purposes.

Simplistically, stormwater damage is associated with water travelling to a watercourse or water body, while flood damage is associated with water travelling from a watercourse or water body.

Insurance premiums for specific properties and addresses are determined by individual insurance companies based on their assessment of risk and probability of damage – based on best available research, as well as historical events in an area – and are outside of Council’s control.

Council undertook research which concluded that there was no clear correlation between flood risk related planning controls and mapping and increased insurance premiums and that a variety of factors influence insurance premiums provided by insurers.

The Insurance Council of Australia recommends that if you believe a flood premium has been incorrectly charged that you investigate the following:

If you have evidence that an insurer has incorrectly assessed risk of flooding (e.g. a Council flood study, floor level survey, site-specific flood report or similar), please contact the insurer directly to discuss. Many major insurers have dedicated flood premium review processes in place and welcome information that helps improve the accuracy of their flood risk assessments. The Insurance Council of Australia (ICA) can also assist in reviewing information if an insurer cannot. Providing the insurer or ICA documentation will assist in this discussion.

It is also important to shop around if you are not satisfied by the premium or cover offered by your insurer.

Flood modelling used by Waverley Council has been prepared in accordance with contemporary flood modelling techniques considering different rainfall intensity events. In accordance with industry standards, the study defined the 20% Annual Exceedance Probability (AEP) event (formerly referred to as a 1 in 5 year rainfall event), the 5% AEP event (formerly referred to as a 1 in 20 year rainfall event), the 1% AEP event (formerly referred to as a 1 in 100 year rainfall event) and the Probable Maximum Flood (PMF) event (the largest flood that could conceivably occur at a given location based on the meteorological and hydrologic conditions conceivably possible for the region).

The flood maps determined from these modelling exercises formed the basis of the flood study mapping and associated planning controls. These are the theoretical flood extents which are statistically expected to occur in any given year based on the best available information today.

For context in relation to the weather events described above, the storms in February 2022 were approximately equivalent to a 5% AEP event. It is likely that the flood event in November 1984 in the Waverley LGA was approximately equivalent to a 1% AEP event, when many properties within the LGA experienced flooding from the intense rainfall which occurred. Rarer more severe flooding can occur. Despite living in your property for many years, it is not unreasonable for you not to have witnessed these more significant rainfall events.

There are two types of flood risk: mainstream flooding, where water comes out of a creek, river or other watercourse/waterbody, and overland flow, where rainfall runoff is making its way towards a watercourse/waterbody.

The Waverley LGA predominantly has overland flow, where rainfall runoff is making its way towards a watercourse/waterbody. This is the type of flooding which is likely to have been identified as impacting upon your property if your house is on a hill.

Overland flow is generally shallow, can be fast flowing, occurs during heavy rain events and passes soon after the rain stops.

There is minimal mainstream flooding in the Waverley LGA, restricted primarily to Tamarama Gully and Bronte Gully. Note, for insurance purposes, overland flow flooding is primarily not considered ‘flooding’. You can read more about insurance and flooding in the above FAQs on insurance.

The flood modelling outputs from the Waverley LGA Flood Study were translated into low, medium and high flood risk precincts for the purposes of applying planning controls. The planning controls differ based on the sensitivity of the land use to flood risks and the location of the land within the floodplain.

The medium and high flood risk precincts are defined based on the 1% AEP event (formerly referred to as a 1 in 100-year rainfall event) and the hazard classification (which relates to the depth of water and the speed/velocity in which it travels) this category has planning controls generally apply to most types of development. The low flood risk precinct (FRP) is land defined as being between the 1% AEP and the probable maximum flood (PMF, the worst possible flood that could occur). The low FRP exists predominantly so planning controls exist for sensitive uses which need extra consideration (such as schools, hospitals, aged care homes etc.).

Council has commenced the Floodplain Risk Management Study and Plan (FRMSP). This will involve more detailed investigation into the areas of more frequent and problematic flooding in the LGA and identification of ways of mitigating/reducing these flood impacts. This process will be guided by Council's Floodplain Risk Management Committee. Interested residents were invited to have an opportunity to express their interest in participating in the committee.

Common to urban areas across Australia, Council stormwater assets which people would be familiar with (kerb and gutter, stormwater entry pits and underground pipes) are typically designed to cater for smaller, more frequent rainfall events, in the order of a 20% AEP event (formerly referred to as a 1 in 5 year rainfall event). In some areas of Waverley Council this stormwater infrastructure is designed to cater for larger storm events up to a 10% AEP event (1 in 10 year rainfall event). Rainfall events more severe than this are typically conveyed within roadways and through low lying open space areas (parks and ovals).

The larger stormwater drains and infrastructure (such as those which discharge at our many beaches) are typically owned, managed and maintained by Sydney Water. Council does own and manage a few of these larger stormwater drains as well. These drains receive the flow from all of the surrounding smaller Council drains before discharging to the ocean or local creeks. These drains are also designed to carry only a certain sized event and their capacity is often exceeded during more significant rainfall events with the drain running full and backing up into the catchment. When this occurs, it can mean that water from the Council owned stormwater drains cannot discharge freely and can result in short term ponding and inundation back upstream within the catchment.

Council stormwater assets are not designed to convey all rainfall events. Works undertaken by Council to minimise flood impacts are just one action that can be taken to manage flood risk. Depending on the specific property and the localised flood behaviour the works undertaken may not necessarily eliminate flood risk. Council is however assessing areas where flooding impacts on private property and where possible investigating options to reduce these impacts. Council will also reassess mapped flood risk precincts where large capital works projects have an improved outcome as determined by revised modelling, and update these maps accordingly as part of the Flood Risk Management Study and Plan (FRMSP). This may result in changing category to the flood risk precincts in the future.