Little follow up : 
Reason i didn't removed the culvert yet :  https://www.mapillary.com/app/?pKey=1020008419374296 you have a manhole and there's water on it ; i made the survey on a rainy day so it's difficult to say if there was flow on it or not. Another think is that in Bxl there is some culvert going under other river (called Siphon) and as i did't see where exactly go this one : https://www.openstreetmap.org/way/1002228179 there's a possibility that a underground connection exist. Will continue some search in following days.