The water comming here is intermitent and came from the overflow of the 2 sewers constructed on the same time. (one on each side)

Your solution sound good yes. 

Additional info : each culvert has a width of 6.10 m (source lefleat Coordination Senne and the book "Bruxelles sur Senne" D/1971/0478/1 

So 2 culvert and actual draw should be the centerline (despite the illustration in Sewer Museum indicate that the culvert are exactly on each part of the tram line). It's probably figurative and i don't have find accurate source to indicate verifiability. So let's do that and when i find more info on position i will open a another note. 