Soakaway Design Spreadsheet
This spreadsheet makes iterative soakaway design much quicker and easier, allowing the designer to complete an optimal design in minutes.
A soakaway is a tank or trench where water is stored and allowed to soak into the ground, rather than discharging to an outfall or sewer. Soakaways are used in remote or isolated areas, where the existing drainage is not sufficient and would be expensive or difficult to upgrade, or where a SUDS system is being installed to reduce the runoff from a new development. For more detail, please check out our Soakaway Design Guide.
The quantity of rainfall must be calculated for rain events of different durations to determine the worst case design storm. The infiltration rate then needs to be calculated and multiplied by the surface area of the soakaway to calculate the rate at which the water will soak into the surrounding ground. The quantity of rainfall and the soakaway rate can then be used to determine the maximum water storage volume required. Then a tank of appropriate dimensions can be designed to store the water while it is slowly discharged into the ground. Note that the void ratio of the tank must be taken into account when designing the storage.
This spreadsheet calculates the design runoff flow for a site in accordance with the Flood Estimation Handbook. It then calculates the requirements for a soakaway system and assists the user to design a suitable system. It includes the following functions;
- Calculates the design runoff flow for any site in the UK using actual rainfall data
- Non UK users can specify a storm duration and intensity to calculate the design runoff
- User can alter all runoff parameters including;
- Different surfacing conditions can be modelled by altering the runoff coefficient
- Calculates the design flow into the soakaway system for 9 different storm events from 5 mins to 4 hours
- Next the infiltration rate of the soakaway is calculated using user specified ground information and soakaway dimensions
- The designer can choose to design either a rectangular trench or pit shaped soakaway or a circular perforated manhole shape soakaway
- A plan view and cross section are drawn of the user specified soakaway allowing the designer to easily visualise or even specify the correct soakaway dimensions
- Then the program calculates the storage capacity required for each of the 9 design storm events and returns the worst case
- Additional outflow can be specified
- The program then checks that the user specified design is adequate for the site and storm conditions.
- The program calculates the time required for the soakaway to empty by half, therefore the time before the system is able to handle a new storm event.
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