Puteoli

Location: 40.82160N  14.11540E

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”:

Puteoli breakwater fresco at Villa Stabiae, Pompei (1st c.)
(source: http://www.marine-antique.net/Port-de-la-maison-de-Stabie-Pompei)

“Il Designo Bellori”, drawing by Pietro Santi Bartoli after a 3rd c. fresco found at Esquilino (Rome) (now vanished) and published by Bellori in 1673 in his “Fragmenta Vestigii Veteris Romae”.

Puteoli breakwater on a souvenir glass flask known as Fiascetta di Populonia and showing the pilae (4th c.)
(source: http://www.archeoflegrei.it/i-souvenir-di-puteoli/)

Puteoli breakwater on a souvenir glass flask kept at the National Museum of Prague and showing the pilae (4th c.)
(source: https://web.uvic.ca )
See also: Picard, C. (1959) « Pouzzoles et le paysage portuaire », Latomus, T. 18, Fasc. 1, pp. 23-51.

Castrum Puteolanum in the 17th c. (?) (detail)
(source: http://www.archeoflegrei.it/i-castra-flegrei/)

Puteoli breakwater after Paoli (1768)
(source: http://www.archeoflegrei.it/portodiputeoli/)

Puteoli breakwater after Morghen (1769) (source: https://www.e-rara.ch/zut/content/pageview/14428247)

Puteoli breakwater after Hamilton (1776) (source: https://commons.wikimedia.org/wiki/File:William_Hamilton_-_Campi_Phlegraei,_Pozzuoli.jpg)

Puteoli breakwater after Smargiassi (ca. 1840) (source: http://www.artvalue.com/)

Puteoli breakwater after Leitch (1840) (source: http://www.antiquemapsandprints.com)

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).

Puteoli arched breakwater acc. to Paoli (1768) (source: http://www.archeoflegrei.it/portodiputeoli/)
Puteoli arched breakwater acc. to De Fazio (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 varies from 0.7 to 1.0 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.