I have modelled a simple bio-retention in ICMv8. The model has one sub-catchment which includes the SuDS control. The overflow from SuDS has been allowed to drain to a node before to an outfall. There is no under-drain modelled.

Assessment of the model results show that there is no flow out from the 'SUDS surface outflow (m3/s) [q_lid_out]'. However, results on the sewer that drains the overflow from SUDS show flow through it, 70% of the sub-catchment flow. Therefore, I would like to understand the reason why 'SUDS surface outflow' value is zero. A review of the timestep log results show that the level on the surface of the SUDS is well within the berm height suggesting capacity within SUDS whereby overflow shouldn't happen at the outfall.

Any suggestions/thoughts are highly appreciated.