Timothy Lightoler's plan of Liverpool, 1765 - prepared to show a design proposal for George's Dock (opened 1771). He also included existing parts of the dock system including Canning Dock, Old Dock and Salthouse Dock.
What is usual is that there appears to be two drains leading off the head of the Old Dock, perhaps existing, but could represent a proposed scheme? One connects to Salthouse Dock and the other leads to the proposed George's Dock.
I've no idea what these were for? Maybe with the streets being newly paved they collected rainwater and disposed it into the docks?