According to this webinaire of Sewer Museum this is a "bras mort" de la Senne. 

https://www.youtube.com/watch?v=nuIE7tNdqOw (circa 20'20") 

Check/research  if a culvert realy exist here. (société intercommunale de la Senne for construction/ deviation) and Vivaqua if still exist. (not on hydria or Bruxelles environnement open data). 