Understanding Stormwater Detention Tanks: A Homeowner’s Guide

If you’re building or renovating, you may have encountered the term “OSD requirements” in your development application. These three letters – standing for On-Site Detention – often cause confusion for homeowners. This guide will explain what Stormwater Detention Tanks are, how they work, and why they might be essential for your property.

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 may serves an additional purpose:

1. It temporarily captures and holds stormwater during heavy rain
2. 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:

1. Empty Space: This is the area at the top of the tank where the overflow is installed. This space is not counted in your working volume calculations as it’s designed to handle overflow that goes straight to stormwater.
   
   
2. Detention Volume: This middle section temporarily captures stormwater during heavy rain and slowly drains to the stormwater system through a controlled outlet. Council requirements typically specify the minimum detention volume needed.
   
   
3. 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 Working Volume is the combined capacity of the Detention Volume and the Rainwater Reuse Volume, excluding the Empty Space at the top.

Why Tank Size Calculations Matter

Many homeowners make the mistake of assuming the total tank volume (as advertised) equals the working volume available for detention and reuse. However, as explained above, the Empty Space at the top of the tank doesn’t count toward your detention or reuse calculations.

For example, a 7000L tank might actually provide only 6700L of working volume due to the 300L of Empty Space at the top. If your council requires a minimum 4000L detention volume plus 3000L for rainwater reuse, this tank would be insufficient despite its “7000L” label.

Sizing Your Detention Tank Correctly

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

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

Need Help with Your Stormwater Detention Requirements?

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 professional who understands both council requirements and available tank options is highly recommended.

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