The method is very simple to use, including only 3 parameters which can all be obtained from the maps published in the Flood Studies Report.

AREA is simply the area of the catchment being considered.

SAAR is the Standard Annual Average Rainfall value. This describes the average wetness of the catchment which is used to estimate the antecedent wetness of the catchment during the design storm event. Further guidance on this factor is given here.

SOIL is the Winter Rainfall Acceptance Potential (WRAP) soil index for the catchment being considered. Further guidance on this factor is given here.

Applications of FSSR 6

The FSSR 6 method is included in the CivilWeb Rainfall & Runoff Calculator spreadsheet. The equation is valid for small catchments between 0.5km² and 20km². Caution should be used for any sites smaller than 50ha. For very small areas the runoff value for a 50ha site should be calculated, then this should be scaled down to suit the required catchment size. This is considered a better approximation than using the equation directly for very small sites and this is how the Rainfall & Runoff Calculator spreadsheet works for small catchments.

In practice it was found that the equation was over simplified and produced results no more accurate than the more complex Flood Studies Report equation for all catchments, though it does have the advantage of being much simpler and easier to use for small catchments. This method is not generally used today with the IH 124 method and the ADAS 345 method preferred for small catchments. The FSSR 6 method is included in the CivilWeb Rainfall & Runoff Calculator spreadsheet for use analyzing existing drainage designs where the FSSR 6 method has been used and for comparison with other methods.