I have added an overland flow path model (from ground model 5*5m) to an existing sewer system model. After error correction and model validation, when analyzing simulation results of a rain event, the conduits of the overland type are all filled with water (10% of the height of the conduit), which I guess it is some kind of baseflow, for model stability? However, this water remains there during the whole simulation and is not flowing (velocity and discharge are always zero). Even if the water in the upstream node is rising about the ground level, no flow is initiated in the overland flow paths. The water is there from the beginning of the simulation until the end.

Furthermore the height of the water in the upstream node of the overland flow path (a sewer system manhole) is influenced by the height of the (rectangular) overland flow path. Nevertheless the water height always remains below the top of the conduit. The results of the simulation of the same rain event in three different models. The difference between the last two models is only the height of the rectangular conduit.

No flow paths flow paths with height of 10m (rectangular) flow paths with height of 1m (rectangular)
Max volume that comes out of node 5m3 4.70m3 3.70m3
Max water height in the same node 0.31m 0.283m 0.10m

Any suggestions about why is there always stationary water in the overland flow paths (10% of the height of the flow path)? Why is there never flow in the overland flow paths, even if the water rises out of the sewer system above the ground level? Why does the height of the water in the node of the sewer system change when the height of the flow paths changes (but always remains higher than the height of the rising water)? And how can I solve these problems?

Thank you for taking the time to answer these questions.