A soakaway should be designed so that the storage area can discharge from full to 50% capacity within 24 hours. This is required for any design in accordance with BRE 365. The soakaway storage area should be able to empty in less than 24 hours so that the soakaway has sufficient capacity to accommodate another smaller storm around 24 hours after the design storm. The soakaway time of emptying is calculated using the following equation;
This requirement also prevents anaerobic conditions being created in the soakaway and underlying soils.
In some cases the soakaway can be designed with a large storage area suitable for the design storm, however if the infiltration rate is too low it will not have sufficient capacity to accommodate following storms. This effectively imposes a minimum infiltration rate depending on the volume of runoff entering the soakaway.
If the soakaway is being designed for long return periods and large volumes of runoff this requirement can become constricting. Where return periods longer than 30 years are specified this requirement can sometimes be relaxed on the basis that storms with return periods greater than 30 years are unlikely to occur in successive days.
Related Spreadsheets from CivilWeb;
Soakaway Design Spreadsheet
This spreadsheet calculates the requirements for a soakaway system and assists the user to design a suitable system.
Attenuation Design Spreadsheet
This spreadsheet calculates the requirements for a attenuation system and assists the user to design a suitable system.
Runoff Calculator Spreadsheet
This spreadsheet calculates the design runoff flow for a site in accordance with the a number of different methods including the Wallingford Procedure.
Full Drainage Design Suite
Full drainage design suite (50% Discount) including 7 spreadsheets;
- Colebrook White Pipe Design
- Manning Pipe Design
- Manning Open Channel Design
- Linear Drainage Design
- Runoff Calculator
- Attenuation Design
- Soakaway Design