Flood Studies are required in order to provide solutions to existing flood problems in developed areas as well as ensuring new development is compatible with the flood hazard and does not create additional flooding problems in other areas. Flooding causes more damage in Australia than any other natural disaster; many lives are lost and there is a huge financial burden on communities.
 
A flood study is a technical investigation of flood behavior within the study area which defines the extent, depth and velocity of the flood waters as well as the variation in flood hazard. It defines the nature of potential personal danger and potential damage to property resulting from flooding by providing information on the distribution of flood waters across the section of land that is prone to flooding.
 
The major components of a flood study involve determining discharge (hydrologic aspects) and water levels, velocities, etc (hydraulic aspects) for floods of varying severity.
 
A number of analytical tools can be used in flood studies, depending on the data available, the flow situation, the nature and extent of development, and the level of detail required. Generally detailed studies are required in both urban and rural areas, because knowledge of flood characteristics is required to deal with existing problems, future development and the continuing flood risk.
 
Your local council can advise you if your property is affected by flooding and provide you with relevant information which may include a previous Flood Study undertaken. This flood study would normally include maps which show the various flooding characteristics across the local government area. In addition, Council will provide Flood Certificates on request, for a fee, for individual properties. The certificates provide specific information about the property that can be used to understand the extent of flooding on the property and to assist in preparation of Flood Risk Management Reports which are required in support of development applications for development or redevelopment on flood control lots in accordance with the Development Control Plan.
 
You face real flood risks if your property is identified by Council as being in a flood affected area. Properties that have been identified as being within the flood planning area are classified as a flood control lot. This means that the land will be subject to flood-related development conditions to reduce the impact of flooding. For example, if your property is identified, you may need to put measures in place to manage flood risk, such as raising the floor level of any new building work. It is important that you discuss this with an experienced civil engineer to get an understanding of what options are available for your site. The civil engineer has the ability to undertake flood modelling and will work with you to achieve the best possible outcome whilst also complying with the council’s development control planning controls.
 
Council is already undertaking a Flood Risk Management Study and Plan to identify and
prioritise measures to manage the risks of flooding, such as controls on new development, review of emergency response and mitigation works like a drainage system upgrade.

 On-site detention is a way to deal with all the water that falls on your property when it rains by setting up temporary stormwater storage that will minimise flooding from runoff. This reduces the impact on both downstream properties and the existing stormwater drainage system. In order to provide relief to the downstream drainage infrastructure, Councils require that the runoff is stored temporarily on site and released at a controlled rate which the downstream stormwater system can adequately handle. This is called on-site detention.
The existing stormwater system was constructed decades ago and therefore was not designed to cater for the amount of runoff generated from increased recent development. As a result, the existing drainage infrastructure cannot cater for the increase in flows from development which can cause local flooding issues for downstream properties.
On-site detention is an effective method of minimising the impact of stormwater runoff on the existing drainage system which may be caused by re-development of your site. An experienced engineer can implement on-site detention into your development using different systems that can control the runoff from your property.

How would I know if on-site detention is required for my development?
Many Councils are requiring on-site detention to be included as part of the stormwater management for redevelopments and on-site detention requirements are generally determined and enforced by your local council who have specific development controls that apply to the on-site detention design and guidelines that determine the types of development that do and do not require on-site detention.
As a general rule of thumb, all new developments require on-site detention. Exceptions to this general rule may be (but not limited to):

• Minor alterations and additions
• Flood affected sites
• Sites where drainage is near significant water body; example creek, channel, harbour, river.

 
How is on-site detention sized and what options are available?
Various methods and analysis can be undertaken to determine the on-site detention design. The objective is to limit the discharge (controlled) to a minor storm event such as the 1 in 5 year storm and store the volume for the 1 in 20 year or 1 in 100 year storm depending on the specific council guidelines for your area.
Analysis of the design can be undertaken through modelling software and/or hydraulic calculations of rainfall runoff methods. It is advised that you engage an experienced engineer to model the most cost-effective solution for your development.
There are different on-site detention system options available which are dependent on the council. Each on-site detention system has their own advantages and cost implications for your development. These options are:

• Below ground
• Concrete pit/tank
• Rainwater tank
• Above ground
• Basin
• Rainwater tank

If I require on-site detention, what are the next steps?
It is important that you engage a suitably qualified civil/stormwater engineer who can explain the various options that are available for your development. The civil engineer will be able to prepare a Stormwater Concept Plan/Report which would need to be submitted with any Development Application or Complying Development Submission.
Donovan Associates have suitably qualified engineers with 40 years of experience in civil/stormwater design and on-site detention design. Our engineers will communicate with council and will liaise with the property owner, architects, developers and other stakeholders in the early stages of the development to ensure that the requirements and potential on-site detention design is addressed and optimised for your development.