Puteoli (now Pozzuoli) was a major Roman port. It was sheltered by the most famous arched breakwater. This breakwater was buried under the modern breakwater (!) but it was still visible in the 19th c. and known as “Molo Caligoliano”:
It can be seen from the dates of these pictures that the arches were still in place in the 19th century.
Paolo Antonio PAOLI, provided the dimensions of the ancient arched structure in his “Antichita di Pozzuoli” in 1768 (with some later editions, including Giuliano DE FAZIO in 1828).
The drawings show 15 pilae (including 2 submerged pilae). The largest pilae of ca. 15 x 15 m are at the offshore end of the structure. The nearshore pila is somewhat smaller: ca. 8 x 12 m. The opening between adjacent pilae (8 to 11 m) varies from 0.5 to 0.9 pila width, which is close to the values found for the arched breakwaters of Portus Iulius and Misenum.
The area north of the structure had to be protected from waves incoming from south and the arched structure cannot have been very efficient as a breakwater. On the other hand, the massiveness and the height of this structure above the sea water level makes it even less acceptable as a simple jetty for loading/unloading ships.