I remember why I chose the bad solution : the bridge.
A bridge means open space under it. A culvert means earth around it so no space under the bridge. So to have a bridge and a culvert is incoherent too.
The bridge + the canal is coherent, the bridge+the canal+the culvert is incoherent. I preferred to remove the culvert rather than the bridge+the canal.

So now the question is : should we remove the bridge ?