The method is designed to be simple to use. For this reason only three parameters are used, all of which can be derived from the design maps published in the Flood Studies Report.

AREA is the catchment area 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.

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

The IH 124 Method does not include any new information regarding rainfall intensity. It uses the SAAR value and regional growth curves from the Flood Studies Report.

Applications of IH 124

In the UK, BS 8582-2013 recommends that the IH 124 method is used to estimate greenfield site runoff rates. These rates are then used in the planning and design process to evaluate the hydrological impact of the development.

The method is very simple to use and is based on data taken from a large number of small rural catchments. However, the definition of small being less than 25km² limits the equations use of most developments which are much smaller than this. Caution should be used for any sites smaller than 50ha. For very small areas the CivilWeb Rainfall & Runoff spreadsheet calculates the runoff value for a 50ha site, then scales this value down to suit the required catchment size. This is considered a better approximation than using the equation directly for very small sites.

The catchments studied are drained by a well-defined watercourse, so it is not entirely suitable for all greenfield sites. There is also a lack of a slope component to the equation (besides that implied in the SOIL and SAAR factors) so caution should be used on steeply sloped sites.