s5a is the neighbour of s3c, its bigger fresher cousin. Hidden on the riverbank of the Irwell the outfall looks like a portal into a time gone by. The initial pipe is a brick egg around 4ft high, a tree has grown out from the brickwork at the outfall, further lending to the suggestion of it's age.
The egg must of been the original overflow pipe for the CSO that lies within, unfortunately a 10m stretch is all that remains of the older system.
A manhole chamber marks the transition to 3/4ft RCP, after a short distance another chamber houses a curved weir. On the other side a channel joins a pipe coming from the CSO chamber, to one that enters a dropshaft not far from the weir.
The CSO chamber had a processor and being in here was slightly unnerving, as a penstock hung over the way back into the overflow.
From what I understand the setup serves to divert any overflow from the CSO down the dropshaft into a detention tank. When this gets full the CSO chamber penstock could be closed, maybe if there was storage capacity further down the system. If left open the flow will fill up the dropshaft, over the weir and down into the river.