You can't actually see the Gridiron in this photo, but I believe it's still there, under that amazing buildup of silt.
The Gridiron—one of the few photos of it can be seen on the Maritime Mud and Miasma page of About Bristol—was a cheaper alternative to putting your boat in a dry dock. It was a 200-foot long grid (there seems to be some debate over whether the grid itself is actually metal, or in fact timber; I'll have to try to find out), 38 feet wide, nestled here between the north and south entrance locks. A ship captain could sail his vessel over here at high tide, then as the tide gradually receded, the ship—up to 250 feet, as overhanging a bit was apparently okay—would settle onto the gridiron.
At that point an inspection and repair crew could make their way down to the gridiron by ladder and do whatever work they could manage before the tide rose up again.
GPS Coordinates:
51.44864, -2.62319
Location: Hotwells