I have set up a model trying to find the headwater on a culvert and have input a flow change to account for the addition of offsite drainage. Upstream of this culvert I have a culvert paired with a lateral weir structure.
1050 cfs is put into the reach upstream of the culvert and lateral weir structure. 450 cfs overtops the lateral weir and the flow is being added to a separate reach. 600 cfs goes through the culvert system and overtops the road. This part of the model seems to be stable and makes sense. The sections downstream of the embankment are consistent.
There is about 370cfs from offsite drainage that comes into the reach from offsite drainage. I have varied the location at which I place this flow change location, but this and subsequent downstream reaches only show a Q total of 520 cfs.
I have increased the discharge at the flow change location to be both larger and smaller, but the whole Q is still not being applied to the sections. It seems that the amount of discharge leaving over the lateral weir is being subtracted out even though these sections are not located near the lateral weir. It seems strange that everything is okay until I add in a flow change. Is there a way to remedy this situation?
I have rebuilt the model to ensure that the file is not corrupted.