Updated and revised functions have been developed for calculating required volumes for flow detention in serial facilities. These take into account factors for flow reduction in both previous and subsequent facilities.
The revisions affect both existing and new projects but do not make any changes in individual facilities and the first facility in a series of facilities.
They provide relatively small changes in subsequent facilities if the flow is not reduced as much between the facilities, but major changes if large flow is passed (no or less throttle) with greater flow detention downstream.
The functions take into consideration that, as before, the flow from the previous facility reduces the outflow up to some longer rainfall duration, after which all lower flows flow further downstream.
If long duration will be dimensioning downstream and there is strong flow reduction downstream, this may result in the upstream facility, in some cases, not will be as effective as you might think for reducing the required detention volume downstream.
This because a flow below the reduced flow could be dimensioning for long duration.
Then you can try to increase the flow reduction factor, which results in shorter design rain duration and reduces the required detention volume.
In practice, this means that you use a flow regulator in the outlet.
All this can now be simulated better than before by using the updated and revised functions.