The designer must fully assess the storm characteristics of the location of his site and derive a suitable depth of rainfall for the period of the storm’s duration. Generally, M5-60 storms are chosen and are seen as a reasonably practicable return period to use.

Design rainfall should be expressed in metres for the soakaway calculation. In areas where flooding is likely or there is no facility for an overflow in the event of an unusually heavy storm, the designer should consider increasing the Factor of Safety in Design Options.

New to Version 4 is the Z1, Z2 factor calculation.

This feature has been added to reflect the methodology adopted in BRE 365 and is used to derive a statistical probability of rainfall depths for various locations in the UK.

The form below shows how it works.

Simply select the Z1 factor from your rainfall duration and ratio r.