For areas with a risk of flooding and risk of damage to the infrastructure in cases of impounded flows on the ground and in basements, etc., the flow detention volume is calculated for a rain with a longer return time (e.g. 10, 20. 30, 50 or 100 years) than the return time for which the detention facility is dimensioned (e.g. 2, 5 or 10 years). This volume minus the detention volume at a shorter design return time gives the flood volume at a longer return time. This is done in StormTac Web by entering a so-called secondary (longer) return time in the Design flow box, whereby the results are presented in the report.

Calculation of the flood volume and the increased extra flow that occurs at most during the flood (in addition to the flows for which the facility is dimensioned) is done in StormTac Web, both for an individual flow detention facility via the Flow detention box and in the facility boxes wet pond and biofilter, including for series-connected facilities.