Sizing Your Stormwater Detention Tank: Volume Calculations Explained

If you’re building or renovating, you may have encountered the term “OSD requirements” in your development application. Understanding the volume calculations for On-Site Detention systems is crucial for compliance and effective stormwater management. This guide will explain the key components of Stormwater Detention Tank volumes, how to calculate the detention capacity correctly, and why accurate sizing is essential for meeting council requirements.

What is a Stormwater Detention Tank?

A Stormwater Detention Tank (OSD tank) is designed to temporarily hold rainwater collected from your roof and slowly release it into the stormwater system. This controlled release helps prevent localised flooding during heavy rainfall by reducing the peak flow of water entering the public stormwater system.

Unlike a standard Rainwater Tank that simply stores water for later use, a Stormwater Detention Tank serves an additional purpose:

• It temporarily captures and holds stormwater during heavy rain
• It slowly releases this water into the drainage system at a controlled rate.

Understanding Tank Volumes: What Really Matters

When selecting a Stormwater Detention Tank, it’s crucial to understand the different volume components:

The three key volume components of an OSD tank:

• Empty Space: This is the area at the top of the tank where the overflow is installed. This space is generally not counted in your working volume calculations as water is never held in here and goes straight to stormwater.
• Detention Volume: This section temporarily captures stormwater during heavy rain and slowly drains to the stormwater system through a controlled slow-release outlet (orifice). Council requirements typically specify the minimum detention volume needed.
• Rainwater Reuse (Retention) Volume: This bottom section stores water for everyday use in gardens, toilets, laundry, etc. It’s the water you can actually use on your property.

The Total Tank Volume is the amount in litres from the tank base to the tank roof. The Total Working Volume is the amount in litres from the tank base to the bottom (invert) of the overflow.

In a Stormwater Detention Tank, the Total Working Volume is the combined capacity of the Detention Volume and the Rainwater Reuse Volume, excluding the Empty Space at the top. 

You can calculate the approx. Empty Space volume using the height of the overflow (e.g. wdith x length x 100 for a standard 100mm Overflow).

Why Tank Size Calculations Matter

When purchasing a OSD Tank, it’s important to note that the total tank volume (as advertised) isn’t always equivalent to the working volume available for detention and reuse. As explained above, the empty space at the top of the tank doesn’t usually count toward your detention or reuse calculations.

For example, a 5000L tank might actually provide only 4700L of working volume due to the 300L of empty space at the top. If your council requires a minimum 3000L detention volume plus 2000L for rainwater reuse, this tank would not meet requirements despite its “5000L” label.

Some councils are quite specific about these requirements, even with small differences, so we recommend choosing a slightly larger tank capacity to account for the empty space.

Sizing Your Detention Tank Correctly

When determining the right tank size for your OSD requirements, consider these key factors:

• Council Requirements: Check your development approval for the specific detention and retention volumes required.
• Available Space: Measure the area where you plan to install the tank, noting both the footprint and height constraints.
• Total Working Volume: Ensure the tank provides sufficient working volume for both detention and reuse requirements, accounting for the Empty Space at the top.
• Tank Configuration: Consider whether a longer, shorter tank or a taller, narrower tank would better suit your space constraints.

Expert Advice for Your Stormwater Detention Needs

Understanding stormwater detention systems is essential for homeowners facing OSD requirements. By correctly sizing your tank and ensuring proper installation, you’ll not only meet council requirements but also contribute to sustainable water management and potentially reduce your water bills.

For specific advice on your stormwater detention needs, consulting with a hydraulics engineer who understands your local council requirements is highly recommended.

If you need an OSD tank for your property and would like confirmation of your detention/retention volume calculations, feel free to send our water tank experts a copy of your plans for review and advice.

This article provides general information about stormwater detention systems. Requirements may vary by council area, so always check your specific development approval conditions.