Draft Overland Flow Flood Study

Draft Sutherland Shire Overland Flow - Flood Study (Terminology) - PDF - 135 KB

Draft Sutherland Shire Overland Flow - Flood Study (Executive Summary) - PDF - 1145 KB

Draft Sutherland Shire Overland Flow - Flood Study (Vol 1 - Report) - PDF - 16764 KB

Draft Sutherland Shire Overland Flow - Flood Study (Vol 2 - Mapping) - PDF - 1850 KB

Additional Information

In NSW, councils are responsible for local stormwater drainage and are primarily responsible for managing flood risk to their communities. The NSW Government provides councils with technical and financial support to manage the flood risk through the Floodplain Management Program provided by DCCEEW (Department of Climate Change, Energy, the Environment and Water). The Program exists to understand and reduce the impacts of flooding and flood liability on communities and individual owners and occupiers of flood prone property.

As part of the Floodplain Management Program, councils develop and implement flood risk management plans consistent with the NSW Flood Risk Management Manual (2023). Flood risk management plans outline how councils plan to manage flood risk in the area including any feasible recommended measures, such as upgrading stormwater drainage systems.

Flood studies are the first key step in developing a flood risk management plan. They develop information about flood behaviour to help Council, Government, and the community understand flooding, so they can be informed about flood risks when making decisions and in responding to flood events.

Flooding in the Sutherland Shire can result from mainstream – riverine flooding where water rises above the banks of rivers and watercourses (in some cases formed channels) and flows over the land. Flooding can also occur due to tidal inundation or ocean storm events in low lying areas near the coast. The most common flooding which residents experience is overland flow flooding. This is where excess stormwater flows across land on its way to the watercourses.

The first step in managing flood risk is to identify where the risks currently exist. Overland flow flood studies show Council where water will flow in rainfall events. These studies can show where overland flow causes a risk to people or property and where roads may close in flash flood events. This information is very important to Council to enable us to manage our stormwater network, ensure that important flow paths remain open and provide advice to landholders on how to protect their properties from overland flooding. The information is also used to guide the development process and to provide important information to emergency services such as the NSW SES.

An overland flow flood study is a study prepared to show where water will flow, and the impacts on   infrastructure and properties across a full range of rainfall events. These events include frequent rainfall events, where the modelling can be used to design pipework, rarer events which may be used to guide planning assessments, and extreme events which are used to show the maximum extent of the floodplain and guide emergency management or plan for critical infrastructure and vulnerable developments such as hospitals.

An overland flow flood study is prepared by using data which shows the landform (topography of an area), the land use which helps work out how much of the rainfall is likely to result in runoff, and information related to Council’s pit and pipe network and major drainage structures.

The data is used to prepare computer models which show how runoff from rainfall will move through the system. The results of the computer model are compared with information collected in previous flood events to check if the model is providing reliable information. Unlike river flood models where there is good gauge data, overland studies rely on data collected by Council officers, SES and residents during flood events to validate the models. This is why feedback from the community is important.

Overland flow flood studies can show the depth, velocity and extent of flow paths for various rainfall events. This helps Council identify where flood risk is present and is the first step in being able to manage and reduce risk for the community.

Overland flow is where water from rainfall flows across the land on its way to major drainage systems, such as creeks and rivers. All properties, even those on hillsides, will have some overland flow in heavy rain. Overland flow is called overland flooding when the flow is deep enough and/or running fast enough that it may pose a risk to life or property.

Overland flow occurs whenever drainage systems are overwhelmed by how heavy the rainfall is or when systems are blocked. Piped drainage is rarely designed to cater for every rainfall event, and once the rainfall exceeds the ability of the pits to admit water or the pipes to carry the flow, the water has to find another route. Ideally this will occur on the edges of roadways where water flows down the kerb and gutter to lower points in the catchment. At low points in a roadway, water may pass over the road verge and enter private property. Sometimes these flow paths are within drainage easements however in other cases, flow may have to find its way around buildings and other obstructions, or in the worst case, enter buildings, garages or basements.

Overland flow flooding, or ‘flash flooding’ often occurs from sudden, severe rain events where there is little or no warning and it is difficult to take action in advance. Overland flow generally does not last for long however it can cause significant disruption to traffic and can cause damage where homes are not built far enough above the flow path or have not been designed to direct flows safely around buildings.

The NSW Government provides the NSW Flood Risk Management Manual and guidelines to councils, consultants, industry and other stakeholders to assist in the preparation of an overland flow flood study.

Sutherland Shire Council engaged BMT, a specialist flood management consultant, to prepare this draft Flood Study. This consultant has undertaken overland flow studies for other councils in NSW in accordance with the NSW Flood Risk Management Manual and guidelines.

The Floodplain Risk Management Sub-Committee oversees the preparation of the study and is attended by community members, Councillors and technical advisers. The technical advisers are represented by DCCEEW, SES and Council staff.

Flood models were established for a range of storm patterns and checked using flood data collected in historic events.

Modelled overland flow behaviour was mapped for many criteria (such as peak depth, peak levels, and hazard) for rainfall events ranging from frequent events to extreme events. Climate change scenarios were also modelled to allow Council to be aware of how risk may change in the future.

Once modelling is completed the mapping is adjusted so that very shallow depths and/or low velocities of flow are not mapped because these do not pose a risk to people or properties. This process is called Filtering. Filtering criteria is used to finalise flood mapping to ensure that flood extents are consistently applied based on a risk management approach. Use of filtering criteria makes sure that properties which receive overland flow that poses a risk are identified so that development can be managed in a way which protects the development without causing increased risk to others.

Our Overland Flow Flood Study represents the first step in addressing overland flow and flooding because it identifies where problems occur for the full range of floods, including frequent, rare and extreme events. Once completed, the Overland Flow Flood Study can then be used to prepare Flood Risk Management Studies and Plans which will allow Council to work out what can be done to reduce flood risk for our communities. Studies prepared under the NSW Floodplain Management Program may be eligible for government funding to assist in carrying out important flood mitigation works.

Figure 1. The Flood Risk Management Program

We have advised impacted property owners that they been identified as potentially flood affected, which is a requirement under State Government legislation.

Any property which is identified as being flood prone has a message placed on the property’s Section 10.7 Planning Certificate – the same as 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.

It is important to note that no new notations will be added to any property’s planning certificate as a result of the draft Overland Flood Study study until it is finalised and adopted by Council.  Any pre-existing notations not related to the draft Study (e.g. based on previous flood studies) will remain.

The Flood Study will have no impact on existing development. You will still be able to use your land in accordance with previous approvals. The findings of the study will be used if you propose to carry out new development on your land. This may require you to meet a minimum floor level so that your home is protected from overland flow or to keep areas of your land open so that flow is not diverted elsewhere or trapped on your land.

Some properties are affected by the Probable Maximum Flood event (PMF) which is a very rare event. There are additional considerations, if sensitive development is proposed on these lots. Sensitive development supports vulnerable users who are less likely to be able to evacuate such as hospitals, childcare centres, and aged care facilities. It does not apply to regular residential development.

Flood studies identify land which may be impacted in relatively rare events. The 1% AEP, annual exceedance probability event is commonly used for planning purposes. This is a flood for which there is a 1 in 100 chance of happening any year. It is an infrequent event but much more likely than winning the lottery. Your home is often your most important asset and using 1% AEP event for planning is the best way to protect any improvements to your property.

Storm events are highly variable and do not occur in a regular pattern. The probability (or chance) of flooding hasn’t changed because of this study, and your property is no more flood affected than it was previously.

Just because you have not personally experienced flooding at your home before does not mean that it is not at risk. Consider that every flood is different and how the flood behaves will depend on a very large number of variables which are considered as far as possible when floods are modelled.

House prices vary continuously with changes in the real estate market and are based on a large range of criteria such as a property’s location, size and age, and are not under the control of Council. Real estate markets also consider access to local amenities, environmental and social benefits. There is no clear data on whether the risk from overland flooding negatively impacts property values. Ultimately, it is the market that determines the value. This study does not prohibit the sale or purchase of property.

The Insurance Council of Australia states that insurers already have extensive data on flood risk. The general insurance industry has developed the National Flood Insurance Database (NIFD) for use by insurers to determine flood risk. Insurers use the NFID and other criteria including building type, location and claims history to calculate insurance premiums.  Further information on flood insurance is available from the Insurance Council of Australia:

Flood insurance explained - Insurance Council of Australia(External link) https://insurancecouncil.com.au/resource/flood-insurance-explained/(External link)

Councils complete flood studies to understand flood risk to their communities. These studies inform Council, government, and the community about flood risk so it can be considered in decisions in response to floods and managing flood risk. The impact on property values and insurance is outside the scope of all flood studies.

The study is not expected to have any impact on your rates. We collect rates from residents and businesses to help fund local infrastructure and services based on the unimproved property values set by the Valuer General.

Council’s stormwater drainage network comprises stormwater pits, pipes and open channels. Like all drainage networks, it is designed to manage minor more frequent storm events so that people are not stepping into puddles every time it rains. In major storm events, runoff that is not captured by the drainage network must flow overland. Our roads and valleys are usually the primary overland flow paths.

It is rarely economical or possible to manage all flows using stormwater drainage. Upgrades of stormwater drainage are sometimes recommended after an overland flow study because they identify where the problem areas occur in the drainage network. Risk management studies (prepared after a flood study) can assess whether drainage upgrades can reduce the frequency of overland flow and improve safety. Ideally water runs down roadways or through parkland and easements when the pits and pipes are overloaded, however overland flow will always find the low spots in the topography and move along those paths. Sometimes these are the areas which would have been creeks or natural drainage depressions before street networks and drainage were installed. If provision was not made at the time to safely manage the flow which occurs in these locations, it is important that they be identified to see if improvements can be made.

Chapter 40 of the Sutherland Shire Development Control Plan (DCP) 2015 lists controls to minimise the impact of flooding on proposed developments, as well as the impact of the proposed development on flooding. These controls relate to floor levels, building components, structural soundness, flood effects, car parking, evacuation, and management and design. Different controls may apply depending on the nature of the proposed development and the nature of flooding.

These controls are developed to provide protection to investment you make on your land and ensure that the problems are not shifted to your neighbours by obstructing flow paths. Flood related development controls do not stop you being able to develop your land, they aim to ensure that development is sustainable and resilient and correctly considers flood risk.

A 1% annual exceedance probability (AEP) flood represents a 1% chance of occurring in any year. This is a large but rare flood. It was previously known as the 100-year recurrence flood. Many parts of Australia have experienced 1% floods in the past few years and some areas such as Lismore experienced very rare floods. Just because you have had a 1% flood in the past does not mean that one cannot occur again or that it will not happen for another 100 years. Multiple 1% floods can happen any year. It is sometimes easier to think of this as if the raffle tickets are put back in the bag after the first draw, your chances remain the same even if you won the first prize.

A probable maximum flood (PMF) is the largest flood which can be expected to occur. While extremely rare, a few floods in Australia have approached the magnitude of a PMF.  The mapping which shows the PMF extent shows the full extent of a flood plain. Areas outside of the PMF extent are considered flood free. The PMF flood is mainly used to consider safety of evacuation routes and to decide where sensitive development such as hospitals should be located. The PMF flood is extreme and does not have the same restrictions as the floods used for setting flood planning areas such as the 1% AEP flood.