Luis,
Many thanks for your help, and sorry for the delay in my reply.
I have downloaded HEC-RAS version 6.1 and tried it out with my lateral weir.
In the results table I get a flow over the weir (Q Weir). The tailwater connection is set to ‘Out of the System’; however, the discharge in the channel downstream of the weir is not reduced.
Also, for one profile in my flow plan, the flow over the weir is much greater than the flow rate entered in the plan: the total flow rate for the profile is 1.62 m3/s but the flow leaving over the weir is apparently 3.17 m3/s, 196% of the total flow in the cross-section. How can this be?