Attenuation tank sizing
The required capacity for attenuation tanks depends on a number of factors which are unique to each project.

We can model the circumstances on a project using Microdrain software to calculate the volume of water storage required. To undertake this exercise, we need the following information:

• Impermeable drained area (i.e. roads/car parks)
• Max. rate of discharge permitted from the site
• Storm return period, now typically 1 in 100 yrs
• Geographical location
• Approx. depth of drainage system
• Plan area where attenuation is to be located

Floatation
In some situations, particularly where the groundwater level is high relative to the tank, there might be a risk of flotation.

Given the tank dimensions, invert level and groundwater level, we can undertake a check calculation and establish whether any special measures are needed.

Factor of safety against flotation must be equal to or greater than 1.2.