Does anyone know the purpose of this broad low arch on the Quayside road, Stockton?
Without knowing the exact location this structure could be a part of the cellars system of 1700s brick warehousing. Tapping into the local water table for hundreds of years, shipping took fresh-water from brick and stone reservoirs along the Quay side. i.e. the Finkle Street area (This information was kindly provided by Robert Harbron ) 
Picture Stockton Archive
A Pictorial History of Stockton-on-Tees
There was an arch like this that went under the old co-op coal yard on Thornaby road to Stockton. We went through it a few times when we were kids.
The arch on the Quayside was in the builders yard,where I worked. Next to the Greyhund Hotel. .
Yes – referance this low arch it is still there today, however with the building of the Castlegate Centre and Riverside Road it is now quite modernised. Under the Castlegate Centre there is a natural spring which now has a sump pump that redirects the water into the Tees via this little tunnel. If you are on the Riverside Road across from the old Uptons Store and you look over the railings into the Tees it will be directly below you. Ideally see it from the Teeside Princess. Regards