Attenuation Design Spreadsheet

Attenuation Design

Attenuation systems are large storage facilities included in a drainage system so that rainfall from storm events can be stored onsite and slowly discharged instead of discharging the whole storm event as it happens. These systems are required where there is a limitation to the quantity of rainwater which can be discharged from a site. This is common when discharging from a large site to a small outfall or public sewer.

The quantity of rainfall must be calculated for rain events of different durations to determine the worst case design storm. The design outflow from the system is then 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. Note that the void ratio of the tank must be taken into account when designing the storage.

Check out our detailed guidance for Attenuation Design.


This spreadsheet calculates the design runoff flow for a site in accordance with the Flood Estimation Handbook. It then calculates the requirements for a attenuation system and assists the user to design a suitable system. It includes the following functions;

o   Calculates the design runoff flow for any site in the UK using actual rainfall data

o   Non UK users can specify a storm duration and intensity to calculate the design runoff

o   User can alter all runoff parameters including;

o   Different surfacing conditions can be modelled by altering the runoff coefficient


  • Calculates the design flow into an attenuation system for 9 different storm events from 5 mins to 4 hours
  • The program plots a chart of storage required over storm duration so the designer can easily see the worst case storm events
  • Then the program calculates the storage capacity required for each of the 9 design storm events and returns the worst case

o   Outflow can be specified

o  Next the user can specify the following parameters to design the attenuation system;

o   Number of tanks

o   Dimensions of tanks

o   Void percentage

o   A plan view and cross section are drawn of the user specified attenuation system allowing the designer to easily visualise or even specify the correct system dimensions

o  The program then checks that the user specified design is adequate for the site and storm conditions.


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