FAQs

Severe flooding, triggered by sudden and intense rainfall, leads to overland or flash flooding. Floodwaters can cause significant damage to homes, businesses, and human life. Flood Studies increase our understanding of flood behaviour, providing us with improved tools to guide future land use planning, respond to emergencies and prioritise infrastructure.

The NSW State Government's Flood Prone Land Policy requires Councils to identify and manage flood prone land.

NSW Local Government have responsibility for identifying and then managing the risk to life and property from flooding and have a duty of care to disclose this information to the community. Local Governments must act in accordance within the NSW Flood Prone Land Policy, which aims to reduce the impact of flooding and flood liability on individual owners and occupiers of flood prone property. One of the most important steps in this process is increasing our community's awareness of flooding so that people are better able to understand and plan for the flood risks they face.

A flood study is a comprehensive technical investigation of flood behaviour. it defines the nature and extent of overland flooding across a study area by providing information on flood extents, discharges, levels, and velocities for different flood events.

This study is the first stage of the NSW State Government's Flood Prone Land Policy, undertaken prior to a Floodplain Risk Management Study & Plan which involves the investigation of flood risk and mitigation strategies.

The study will identify which properties have the potential to be inundated and inform future mitigation.

Overland flooding (also referred as flash flooding) occurs during rainfall events where drainage systems reach capacity and excess water will flow over land.

This type of flooding is generally quick to occur and subside.

This might be observed as water trapped within low points, flow travelling along roads, surcharging pits, local parks/basins filling up or water travelling through private and public properties. 

The State Government defines the guidelines on how to conduct Flood Studies. To prepare the study, Council has engaged an independent consultant specialising in floodplain management and flood studies and will draw upon the advice of the NSW Department of Environment and Planning.

The preparation of a flood study involves the collection and review of all available flood-related information for the study area, followed by the development of computer flood models to simulate the runoff resulting from excess rainfall and its distribution across the catchment.

The computer models will be calibrated against collected information to reproduce historic floods, then will be utilised to simulate a range of hypothetical floods ranging in severity.

A draft Flood Study report and mapping will be then produced, summarising the outcomes of all phases of the investigation.

The stages of a flood study are generally divided into the following:

• Stage 1: Data Collection, gap analysis and Community Survey - We are here.
• Stage 2: Hydrological and Hydraulic Model Development and Validation.
• Stage 4: Design event modelling, model simulations and mapping.
• Stage 4: Draft Flood Study Report including flood tagging.
• Stage 5: Community Consultation.
• Stage 6: Finalise Flood Study Report.

If there is significant rain at the top of the hill, the rain will run downhill through low lying gullies, channels and surface depressions. The speed and depth of floodwaters may be less at the top of the catchment than further down the hill, but still be significant enough to pose a risk of flood damage and significant erosion. 

For most people, there is nothing to do in response to this flood study, as there is no immediate change to the situation. However, property owners who are planning redevelopment of their property may need to take some additional steps as part of the consent process because flood related development set at or above the Flood Planning Level. This study will allow Council to provide this information to the property owners or potential purchasers.

Storm events vary widely and do not occur in a consistent pattern. Flood modeling is based on certain criteria and methodologies. Modelled flood levels may differ from observed levels across the study area because some large and rare floods have often not been experienced in living memory.

Flood Study usually models 1% AEP event and Probable Maximum Flood Event to determine the risk to property which are rarer flood than the frequent flood events. Property owners may have not experienced similar rarer flood.

A Section 10.7 Planning Certificate provides information on any controls, requirements and restrictions that may impact how a property is used, such as land zoning, permissible and prohibited land uses, and controls for development or hazards such as flooding. Under NSW legislation, a contract for sale of a property must include a current s10.7 certificate from the relevant local Council.

There are two types of s10.7 certificate:

• s10.7(2) certificate: contains the information specified in Schedule 4 of the Environmental Planning and Assessment Regulation 2000. Among other things, this will include information relating to relevant planning instruments and development control plans, zoning and land use, heritage items, road widening and realignments, flood-related development controls, land reserved for acquisition and bushfire prone Land.
• s10.5(5) certificate: provides the above s10.7(2) information plus advice on other relevant matters affecting the land of which Council may be aware. This additional flood related information, details of environmental reports, specific planning controls, tree preservation information and more.

Under the regulation, answers to the following questions are provided in a planning certificate for flood risk to a site:

1. If the land or part of the land is within the flood planning area and subject to flood related development controls.
2. If the land or part of the land is between the flood planning and the probable maximum flood and subject to flood related development controls. Here, development control is applicable to sensitive, critical and hazardous landuses.

The Flood Planning level (FPL) is the height used to set floor levels for houses in flood prone areas. It is generally defined as the "1% AEP flood level" plus and appropriate freeboard (see explanation of 'freeboard' below).

The FPL can be calculated as:

FPL = 1% AEP flood level + freeboard

Freeboard is included in the Flood Planning level as a buffer, to account for factors such as foreseen blockages, waves from vehicles (such as cars, SES rescue tracks), other localised hydraulic effects and uncertainties in the modelling and determination of flood levels. Freeboard is typically 0.5m above a flood level.

It's unlikely that development controls will prohibit all development on flood prone land. However, some factors may need to be taken into consideration in any future development to alleviate risk to life and property. For example, you may be asked to build on a certain part of your land or elevate your building to a certain height or avoid building in a section where risk is high. Council's Development Services team can help you understand these considerations.

The results from this study will update flood information obtained from previous studies. The majority of the affected properties identified in this study have previously been marked as being flood prone. An actual flood event, rather than a flood planning notation on a Planning Certificate, is more likely to impact property values.

Individual insurance companies possess extensive data on flood risk obtained through their own flood studies and analyses which is already reflected in the cost of premiums, irrespective of whether Council has undertaken a flood study.

Further information on flood insurance is available from Flood Management Australia.

The study is not expected to have any impact on your rates. Council collects rates based on the value of each parcel of land within the LGA determined by the Valuer General.

Council's network of stormwater pits, pipes, and open channels are designed to manage minor, more frequent storm events, while excess runoff during major storms must be conveyed via overland flow paths, such as roads and valleys. This aims to maximise the protection of life and property in major events. The next stage of these studies will focus on additional mitigation works Council and other stakeholders e.g., Sydney Water and Transport NSW should explore into the future.

The impacts of overland flood risk may need to be appropriately considered during the development process. Different controls may be applicable, depending on the nature of the proposed development and the extent of flooding on the land. These controls could relate to floor levels. building components, car parking, or other aspects of management and design. This ensures that the impacts of flooding on proposed developments are minimised. Council Engineers can provide support to understand these controls.

A 1% annual exceedance probability (AEP) flood represents a flood which has a 1% chance of occurring within any year. This is a large and rare flood. It was previously referred to as the 1-in-100-year flood. 

A probable maximum flood (PMF) is the largest possible flood that could conceivably occur at a particular location and is therefore extremely rare. The PMF defines the maximum extent of flood prone land, that is, the floodplan. While extremely rare, a few floods in Australia have approached the magnitude of a PMF. It is calculated to aid in the design of critical facilities such as hospitals, nursing homes, schools and emergency service facilities. the PMF is also modelled so that emergency managers can understand the maximum possible extent of flooding that could ever occur in a location and plan accordingly.

Impacted property owners will be informed as having been identified as potentially flood affected, as required under State Government legislation.

Any property identified as being flood prone will have a flood notation on its Section 10.7 Planning Certificate, indicating that flood related development controls apply to the land - which is similar for other potential hazards, including bushfire, landslip, coastal hazards or contamination. This notation is a flag that indicates that flood related development controls apply to the land. This simply means that if there is a development proposal on the land, then it would need to consider the flood risk so that the occupants can live safely on the land, minimising the flooding risk to the property.

This is generally done in two stages:

• During the public exhibition of the draft flood tagging and report, a draft note is added in the planning certificate to warn any potential buyer that a draft flood study is currently underway and this site might be affected by future flood related development controls.
• After the adoption of the flood study, a final note is added to the planning certificate confirming whether the site is located within flood planning area, PMF and whether a flood related development control is applicable for a site.

As a result, property owners planning future redevelopment of their property will need to appropriately consider the impacts of overland flooding during the development process.