How the sizing works
Many councils set on-site detention as a site storage requirement (SSR) in cubic metres per hectare and a permissible site discharge (PSD) in litres per second per hectare. Storage is the SSR multiplied by the site area. One cubic metre is 1,000L.
If you add the PSD and the head over the outlet, the calculator sizes the outlet orifice with the standard orifice equation. Some councils size detention with a drainage model instead, so follow the method your council requires.
Single Mullaly OSD tanks range from 5,000L to 150,000L (the largest sizes for some applications), up to 3.28m in diameter. Larger volumes use several tanks joined with pipework.
Stormwater detention tanks