This is the breakwater that shields the mouth of the River Douro from northerly swells. Known as the Pontão da Barra do Douro - Molhe Norte (the last part meaning north pier).
As far as I can tell this pier is relatively new, possibly dating back to 2009. It is generally open to the public, except when it gets stormy, and it's a popular 600 metre stroll to the little lighthouse at the end.
