Using a stormwater detention system on your property

With Australia’s ever-increasing urban population, it’s never been more important for all residential and commercial developments to effectively manage rainwater.

Most local councils require stormwater detention systems in all developments throughout many of the country’s built-up areas. Equally, most Australian states require a rainwater tank to be installed in all new dwellings to supplement mains water use.

Whether you’re a commercial developer, or are simply wanting to improve your own home’s water use, at some point you will need to address the need for rainwater retention and detention.

Let’s look at your options.

What is stormwater detention?

As the number of new homes and other land developments increase, the less ground surfaces are available to soak up rainwater. Pavements, roads, rooftops and driveways all cover areas of ground, making them impenetrable by water.

As a result, uncontrolled rainwater can pool where it’s not wanted, causing damage to property and infrastructure, especially during heavy downpours.

Detention systems work to draw that rainwater runoff away from the street and into the complex stormwater network of drains, pipes and tunnels.

While a rainwater tank is designed to hold water until it’s needed for use, a detention tank is intended to remain empty except during periods of heavy rainfall. The tanks are designed to slowly drain water into the connected stormwater system over time.

To help share the burden of responsible stormwater management, your local council might require you to install a stormwater detention tank on your property. A 200m2 roof will collect 1000 litres from a 5mm downpour. A detention tank stops this water from running straight into the drains, which helps to avoid flooding in your area.

Which types of properties require stormwater detention?

Specifics vary from city to city, however these guidelines give a good indication on whether you need to install a detention system:

  • All commercial, industrial and special use (eg community, education, recreational) buildings or structures.
  • Town houses, villas, home units or other strata subdivisions. These may use a single on-site detention system for the total site area as long as it’s located on common property and the body corporate is responsible for maintenance.
  • Dual occupancy lots. Each lot within the dual occupancy must have its own on-site detention system, which each individual lot owner is responsible for its maintenance.

Stormwater detention options for your property

A detention system has a dual function: it needs to control the flow rate that’s discharged from the site, and it also needs to store excess rainwater to stop flooding.

Your builder will work with a qualified engineer to design an appropriate detention system for your property. They will take into account several possible solutions depending on your site, including:

  • Dual use detention/retention tanks. These have 2 compartments – one for rainwater collection (kept full), one for stormwater runoff (kept empty).
  • Underground tanks. These are often used in conjunction with a rainwater tank, working to receive excess water from an overflowing collection tank.
  • Surface holding areas. Pools, trenches, soakage pits and basins can all be incorporated into a water detention system.

The size of the tank and drain orifice outlet are engineered to allow a particular flow rate that suits the requirements of the area you’re building in.

Design calculations for your detention system

Your detention system must be able to store runoff caused by a storm event up to 100 year average recurrence interval (ARI) for the site. It must also control the rate of discharge to ensure downstream stormwater assets can handle the extra run-off.

To achieve this, the system must be designed to meet two key requirements:

Permissible Site Discharge (PSD)

PSD is the maximum rate of discharge for the total site that the existing downstream stormwater system can handle.

Site Storage Requirement (SSR)

SSR is the minimum storage volume that is needed to temporarily store and offset the excess stormwater runoff due to the development.

To determine your PSD and SSR, your builder will use the following information about your site:

  • Location
  • Total site area (m2 )
  • Existing pre-development impervious area (m2)
  • Proposed post-development impervious area (m2).

 Combining your detention tank with a rainwater collection tank

Water collection tanks are used for collecting rainwater for later use. Typically rainwater tanks provide water for the garden, toilet and laundry. But they can also be used to supply drinking water provided that you carry out routine rainwater tank maintenance and take proper precautions with the water stored in your tank.

Having a detention tank attached to your collection tank allows any overflow from your rainwater tank to be safely passed into the stormwater system.

ASC Water Tanks carry a wide range of rainwater tank solutions for residential, commercial and industrial areas. We service all of Victoria and other major Australian cities from our warehouse in Hallam.

If you’d like some advice or guidance on a stormwater detention system for your property, give us a call on 1300 229 606.

Figure 1. Example of how to setup a retention/detention system for underground water tanks.

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