We have worked with the Vinnova project SECURE – StormwatEr planning for inCreased Urban REsilience, which is a collaboration between StormTac, Tyréns and Luleå University of Technology. It is an international literature study for a comparison of commonly used calculation methods and used design rain as well as return times and climate effects, and of their impact on the design of transport systems and stormwater facilities for flow detention.

The literature study concluded that determination of runoff coefficients is the most uncertain parameter that also has the greatest impact on the resulting design flow and required detention volume. Suggestions have been made for new Design runoff coefficients, including a new proposed method (equations) for calculating new runoff coefficients as a function of return time (years) and rainfall intensity (l/s/ha).

The new typical runoff coefficients were developed from an extended literature study with new and revised runoff coefficients compiled in StormTac Database. The values ​​are now available in the StormTac database and StormTac Web together with comments on recommended minimum and maximum values.