What is the water flow going through the ditch? was it at or near the top to begin with? Could be that the flow was not really contained in the first place...

Initially, the flow was extremely wide IE 500' wide. I believe the issue is that the slope for the channelized ditch is very shallow and it is a backwater condition. We can't upsize the culverts going under the road so we have a tailwater condition, plus the land to get from one point to the other only allows for about .5% slope which is flat. Two conditions not very favorable to keep the storm from over topping the road.

Is the model sub or super critical? Back water can be controlling. What is the scenario for your ditch? Are you encroaching in a floodplain? Maybe provide a bit more information.

It looks more like a backwater issue and the existing flow was not contained in the existing channel. You would need to improve the entrance condition of the culvert to see additional drop in WSE on your new channel.

Without seeing the model, this was my first guess after reading your initial question:

Quote (slange)

backwater condition

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