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. 

In today’s world, people with technical backgrounds, including engineers, are being frequently challenged to explain to people with little or no technical insights or expertise details of their projects. Explaining a project to the everyday person requires a different mindset to connect with them and encourage them to action or decision-making.
 
The question becomes,  how do you present a message that non-technical clients will understand? How do we get our client to feel comfortable and engage in discussion?
 
Communication is the key to creating rewarding interpersonal relationships with clients and it is important that clients receive information that they can easily understand. This promotes greater collaboration and ensures a meaningful connection is established. Understanding the value of the technical side of things does not necessarily mean that the client will also; therefore, the engineer will need to explain in plain terms to the client. Their message must be simple!
 
It is extremely important to ensure that your architect, engineers, builders and all other contractors are all on the same page and have a vested interest in achieving the outcome of developing and completing your dream home.
 
From an engineering perspective, we understand that the technical aspect that goes into creating the design can be a very challenging conversation to understand. We also appreciate the fact that when people know they are going to get exactly what they want and feel comfortable they are in good hands with someone with experience, with someone who cares, they value the interaction much more and it no longer feels like a transaction but rather, a partnership.
 
There are added benefits of creating partnerships between an engineer and yourself and this can include:

• Maximising the potential value of your development by understanding the engineering aspect and how it integrates into the architectural design. For example, the location and design of any stormwater systems in the rear of your site could limit the design of any future development (such a construction of a pool, or granny flat). If you discuss this or advise the engineer to allow for this, then you could increase your property’s value and not have to modify an existing major stormwater system.
• Understanding the cost implications of the stormwater system construction and the different options that are available.
• Knowing the products available to achieve the stormwater design outcomes and how they can be incorporated into the design. For example, having a below ground rainwater tank as opposed to an above ground for the aesthetic appearance for your development.
• Understand the council requirements that apply to your development.

 
 
Let us mediate between you and your builder, architect, lawyer and engineers to get your project completed quickly, efficiently and in line with all the relevant legal codes.