The volume runoff coefficients and the design runoff coefficients have been investigated through supplementary literature studies and compared, after which revisions of the typical template values and interval values have been performed for some land uses. These changed runoff coefficients have also been compiled in the database.
These adjustments refer to typical recommended values, but both of these types of runoff coefficients can still be changed according to site-specific conditions. This e.g. applies to the land use Forest with increased typical runoff coefficients that have been adjusted after extensive empirical studies in Sweden and the US, but which also show large variations between different locations. More about this can be read in a new section in the Guide, under Design Flow.
In the same chapter there is also news from an international study which indicates that there is no significant relationship between the runoff coefficient and the size of the catchment area, which also means that there is no upper value for the catchment area for employing the rational method in the model, so it can be used without a specific upper limit area, provided that site-specific conditions such as longest runoff transport (concentration) time are carefully considered.