The rain intensity I was previously calculated with Dahlström’s equation (Dahlström, 2010) for all durations, but at extremely throttled outflows, it may be the question of durations that are 24 hours or longer before the required maximum detention volume is obtained.
For such long durations, from one to four days, the intensity is now calculated according to Dahlström (1979). This procedure is in accordance with P110 (Svenskt Vatten, 2016).
This results in lower intensity at design durations ≥ 24 hours than previous method gave, but the same as before in most cases (<24 hours). Before, the maximum required level of flow detention could not be achieved up to 4 days, but the new values for rain intensity (l/s/ha) mean that maximum volume can be achieved at lower outflows. This results in a lower detention volume being required than before at extremely throttled outlets where the design rain duration is ≥ 24 hours.