On-Site detention Volume Calculator

Why do we need On-site Detention?

On‑site detention (OSD) is essential for managing stormwater runoff in urban developments, helping prevent downstream flooding, erosion, and overload of existing drainage infrastructure. As sites become more impervious through new buildings, driveways, and pavements, peak runoff increases significantly. OSD temporarily stores this excess stormwater and releases it at a controlled rate, ensuring post‑development flows do not exceed pre‑development conditions. This protects local waterways, reduces council infrastructure upgrades, and supports sustainable, compliant development. By integrating OSD early in the design process, developers can minimise flood risk, meet regulatory requirements, and improve long‑term stormwater performance.

How do I know the Design Rainfall Depth to Calculate the OSD volume?

You need to refer to the IDF curves for the location where the tank is situated, and select the storm event suitable for your design, say 10%AEP for design and 1%AEP for checking to ensure no overflow up to this storm event.  

Once you have the rainfall intensitiy (mm/hr), then convert it to ranfall depth (mm) for the selected critical storm duration. 

Formula: Depth (mm) = i(mm/hr) * t(min) /60

How do I know the critical storm duration for an OSD?

This is based on your site's geometry and the time water takes to reach the outlet from the furthest location, typically known as time of concentration (tc). You can calculate it based on the kinematic wave formulas (or others).

If unknown, for a small site 5mins may be adopted to start with. However, this is a critical parameter to estimate and can vary greatly depending on surface type, path length and slope.

What is the permissible site discharge for an OSD?

This is how much water you can return to the network. Typically set by the authorities. Some Councils adopt a maximum of 25 L/s if discharging to the road surface (via a kerb outlet).  If you are using an orifice plate, you can input how much water the orifice can discharge as an estimation. Alternatively, you can set it to zero and retain all the water for the design event (conservative, resulting bigger tanks). 

Related Calculators

Calculate the time of concentration

Calculate the peak flow runoff 

Calculate an orifice discharge rate