The photos shown are what we call overflow tunnels. They were put in to carry water from areas like School Lane and Station Road. The three lower photos show a culvert, recently discovered, that goes from the original west bank side of the Great Whyte, towards the main tunnels.
Having said this, there are no details on the Architects Drgs. that show us where they actually go to! and what they were put in for.
The Entrance tunnel pictured is in the main Tunnel and is the Overflow Tunnel from School Lane.
This is at the top of the Main entrance tunnel shown left.
Overflow Tunnel being replaced
These three photos show different angles of the same Culvert