Presumably there's a good reason it's called Greenmarsh Lane, adjacent to Gores Marsh Park. A lot of the names in the area suggest some rather marshy heritage. I'm guessing this drainage is part of what stops it being quite so marshy in the modern day.
In fact, gosh: here's a picture of it during the flood in 1968. The great flood of 1968 apparently killed seven people, and it's worth clicking through to the Evening Post site to take a look at the picture gallery.
These floods were one of the key reasons for the building of the Malago Storm Water Interceptor, whose outfall we've seen in an earlier wander.
GPS Coordinates:
51.43680, -2.61958
Location: Ashton Gate