City of Ryde Draft Flood Study 2023

The City of Ryde Draft Flood Study 2023 needs to be reviewed, presented in Council meetings and then adopted for it to be effective. At this stage, it is hard to provide an exact date for the implementation of the Flood Study. However, Council plans to implement this Flood Study in mid-2024. 

Changes to the Flood Tagging status are attributed to factors that include but not limited to:

• Using the latest guideline, Australia Rainfall and Runoff 2019 (ARR19) instead of ARR87. This includes updating the design rainfall that is input into the model.
• The flood models have been updated using best practice modelling techniques. This includes aspects such as reduced grid cell size, updated terrain information and inclusion of recent developments.

In brief, the development will need to be mindful of the constraints and risks of development in flood effected areas. This may require preparation of a Flood Impact Statement, which will need to consider the risks presented by flooding, ensure the development does not exacerbate risks to safety and property damage that may arise from flood impacts and is to be prepared and submitted with the associated planning approval for the works.

For all development in the Flood Planning Area (FPA), a Flood Impact Statement/Flood Study may be required pending on the proximity of the development / exposure to floodwaters. If a property is external to the FPA yet located in the Probable Maximum Flood (PMF) event, a Flood Study may be required pending on the type of development and nature of occupants that may not be at threat to mobilisation during such an event (eg a childcare centre, hospital, etc). The Flood Impact Statement / Report should be prepared in accordance with Section 2.2 of the Stormwater and Floodplain Management Technical Manual and is required to address the various controls related your development proposal. Data files may also be required by Council for review/assessment. 

The result of the Flood report will inform the design of your building in compliance with the planning controls.

Generally the flood planning controls would aim to achieve the following:

(a)  to minimise the flood risk to life and property associated with the use of land,

(b)  to avoid adverse or cumulative impacts on flood behaviour and the environment,

(c)  to enable the safe occupation and efficient evacuation of people in the event of a flood.

The need for a Flood Impact Statement / Report will generally be required for properties and development significantly subject to flooding however Council may give consideration to the scale of the report being reduced, relative to the risks presented by the development.

Council doesn’t put together Flood Studies or Floodplain Risk Management Plans for, or on behalf of insurance companies. Flood Studies are a legal requirement for Council to complete and part of our responsibilities to help manage flood risk in the community. Insurance companies may rely on their own assessment of risk and can use their own methods to identify flood risk.

If you live in an area that is affected by flooding, there may be rules in place that restrict the kind of buildings and developments that can be built. These rules aim to reduce the risk and damage caused by flooding.

The rules for building and development in areas prone to flooding are explained in the Ryde Development Control 2014 Part 8.2. Generally, a flood report will provide details regarding the degree of flood affectation and depending on the level of risks it will be determined how the built form should be designed such as raised floor level, clearance to allow flood water to pass through etc.

A Complying Development Certificate (CDC) cannot be issued for any part of a flood control lot that is identified as any of the following:

• a flood storage area,
• a floodway area,
• a flow path,
• a high hazard area,
• a high-risk area.

If you are a pursuing a CDC on land designated as a flood control lot, you will need a professional engineer to certify that the development complies with the relevant criteria as set out in the (NSW) State Environmental Planning Policy (Exempt and Complying Development Codes) 2008 (Clause 3.5). If a CDC pathway is not possible, then applicant have the option of seeking approval from Council via a Development Application.

The Flood Impact Statement must be prepared by a suitably qualified and practising Drainage Engineer, with skills and experience in the design and analysis of stormwater drainage and runoff.

A Flood Impact Statement report must be prepared in accordance with Section 2.2 of the Stormwater and Floodplain Management Technical Manual and is required to address the various controls related your development proposal.

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 addition to this, the 1% AEP flood is used as the event to define the flood planning area. It is likely that in many parts of the Ryde Council area, a 1% AEP flood event has not been observed in recent years.

If your property has been identified as being susceptible to flooding and overland flow during 1% AEP flood events, a flood report is required to ensure all fencing does not affect the flow of flood water, change the flood behaviour, or increase flood levels on adjacent properties.

This Flood Mapping was produced based on more rainfall data, updated guidelines, newer LiDAR data, refined models, etc. It is an update of the previous Flood Mapping. 

A Flood Report is required to understand the flood behaviour in a post development condition and to provide flood protection measures for the site.

Annual Exceedance Probability (AEP) is a term used to the express the percentage of likelihood of a flood of a given size or larger occurring in a given year. AEP is an alternative to ARI (Average Recurrence Interval) to express the likelihood of occurrence of a flood event. For example, a 1-in-100-year ARI flood or a 1% AEP 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. This does not mean that if a location floods one year that it will not flood for the next 99 years. Nor, if it has not flooded for 99 years that it will necessarily flood the next year.

The PMF is the largest flood that could conceivably occur at a particular location, usually estimated from probable maximum precipitation. Generally, it is not physically or economically possible to provide complete protection against this event. The PMF defines the extent of flood prone land, that is, the floodplain.