An arched breakwater looks like an aqueduct with a single tier.
The arches are supported by massive piles (opus pilarum), which are made of stone or concrete (opus caementicium). According to Brandon et al. (2014), the Latin word pila designates “a large mass of concrete, generally square in plan, and often a cube or upright rectangular prism in shape”. The ratio of opening between adjacent piers over pier width is as follows on the Pont du Gard:
- Upper level: opening = 1.4 pile widths
- Lower levels: opening = 4.1 pile widths
“Maritime pilae” seem to be more “closed” than aqueducts, i.e. they have a smaller opening over pila-width ratio. This might be explained by their completely different aim which is not to support some kind of road or canal, but to stop wave penetration into the port while providing limited opening for water circulation supposed to reduce sedimentation inside the port.
The method of construction of pilae with marine concrete was described by Vitruvius and tested by Brandon et al. (2014) in Brindisi.
Except in Civitavecchia, no ancient arched breakwater can be seen today, but remains of concrete pilae have been found in many places. A list is presented below, along with pictures of those that can be seen under water on Google Earth. Most of them were listed and studied by Brandon et al. (2014).
The following conclusions can be drawn:
- Most sites with one or more pilae are located in Italy (35 out of 50), especially around Naples (25 sites from Caieta to Sapri), which is no wonder as the pozzolana required for under water pila construction originates from this area.
- The average dimensions of the measured pilae are 9.2 m x 7.3 m: nearly square. The average horizontal surface is 67 m2. The height cannot be determined on Google Earth.
- The largest pila is the one found at Nesis: 14.5 x 14.5 x 8 m.
Various types of alignments can be distinguished from the pictures below:
- single isolated structures (e.g. Punta Fuenti, Fréjus), possibly a foundation for some heavy structure such as a tower or lighthouse,
- rather continuous structures in the open sea, probably part of a vertical breakwater (e.g. Castellabate, Scidrus, Gnathia, Side, Psamathos).
- rather continuous structures in a sheltered area, perhaps forming a massive jetty or quay platform inside a harbour basin protected by a breakwater (e.g. Cosa, Horrea Caelia),
- pilae spaced with regular intervals (say 0.5 to 1.0 pila-width), perhaps the base of arched breakwaters or timber decks, or intervals meant to be filled with rubble dumped into timber formworks placed between the pilae (e.g. Caieta, Misenum, Baia, Portus Iulius, Nesis, Pausylipon). The pictures show that the distance between adjacent pilae is usually less than their width:
- Caieta: opening = 0.3 to 0.4 pila width
- Portus Iulius: opening = 0.7 pila width
- Misenum: opening = 1 to 1.5 pila widths
Several alignments of pilae have been claimed to be remains of arched breakwaters, including the Roman breakwater at Tarragona, but no proof whatsoever was provided, except for Puteoli where many pictures are available and Civitavecchia, still visible.
One last note on arched breakwaters concerns the “Mosaico parietale con scena di porto” which was found at Palazzo Rospigliosi on the Quirinal Hill in Rome. It is supposed to show the Alexandria lighthouse:
The arched structure shown at the lower side of the mosaic clearly is a quite massive arched structure which looks like an arched breakwater, rather than a portico. As it was found in Rome, it might be questionned if this is not the north breakwater of Portus rather than a structure in Alexandria …
 BRANDON, C. & HOHLFELDER, R. & JACKSON, M. & OLESON, J. et al. (2014) « Building for Eternity – The history and Technology of Roman Concrete Engineering in the Sea », Oxbow Books, (327 p).
 MATTEI, G., TROISI, S., AUCELLI, P., PAPPONE, G., PELUSO, F., STEFANILE, M., 2018, “Sensing the Submerged Landscape of Nisida Roman Harbour in the Gulf of Naples from Integrated Measurements on a USV”, Water 2018, 10, 1686, (31 p).
 TERRADO, P., 2019, “El Puerto de Tarraco en Epoca Romana, (siglos II aC – III dC). Fuentes, historiografía y arqueología”, Autoritat Portuària de Tarragona y Arola Editors, (362 p).
List of known pilae
Note that piles made of ashlar (e.g. Fossae Marianae piles) and masses of marine concrete that are not nearly-cubic (e.g. breakwaters of Portus, Antium & Terracina, the quay of Les Laurons and numerous fishponds-piscinae) are not listed hereunder.
|Ancient name||Modern name||Country||Length (m)||Width (m)|
|Tarraco, Tarrakon||Tarragona, Roman breakwater demolished in 1843||Spain|
|Massalia Graecorum, Lacydon||Marseille, Vieux Port, place Jules Verne & place Villeneuve-Bargemon (see also nearby Musee des docks: Dolia warehouse)||France south|
|Forum Julii, Forum Julium||Roman naval base at Frejus, with a pila near the Lanterne d’Auguste||France south||6.75||6.2|
|Domitiana positio, Portus Domitianus||Roman villa at Santa Liberata, on the peninsula of Argentario||Italy west||9-10||8|
|Cosa, Cossae, Portus Herculis Cosanus, Etruscan Cusi, Cuthi||Ansedonia||Italy west||6.5||6|
|Centumcellae||Civitavecchia, Molo del Lazzaretto||Italy west|
|Astura, Storas||Torre Astura||Italy west|
|Port of Circei, Circe||inside Lago di Paola, with access via canal and breakwaters||Italy west||6.5||6|
|Caiete, Caieta, Caeatas, Etruscan Caithi||Spiaggia di Fontania, at Gaeta||Italy west||6||5.5|
|Misenos, Misenum, Misene||Punta Terrone, pilae of the southern breakwater||Italy west||8-9||6-7|
|Misenos, Misenum, Misene||Punta di Pennata, pilae of the northern breakwater||Italy west||12||10|
|Misenos, Misenum, Misene||Punta di Pennata, pilae within the harbour||Italy west|
|Castello Aragonese di Baia, is not a port (?)||Italy west||8.5-10.5||7-7.5|
|Cantieri di Baia, is not a port (?)||Italy west||ca. 8||ca. 7|
|Baiae, Baïes, Portus Baianus, with connection to Lacus Baianus||Baia, two concrete moles over 200 m long||Italy west|
|Villa dei Pisoni, is not a port (?)||Italy west|
|Secca Fumosa is not a port but some kind of platform, with opus reticulatum facing||Italy west||8||8|
|Portus Iulius, Julius, port of Julien, with connection to Lacus Lucrinus||Lucrino, two concrete moles over 200 m long||Italy west||8||8|
|Portus Iulius, Julius, port of Julien, with connection to Lacus Lucrinus||East of eastern breakwater||Italy west||5.5||5|
|Puteoli, Dikaiarcheia, Dicearque, in the Campi Phlegraei volcano district||Pozzuoli, Pouzzoles, Puteoles, in the Campi Flegrei volcano district, pilae of arched mole are under modern breakwater||Italy west||12-15||8-15|
|Puteoli, Dikaiarcheia, Dicearque, in the Campi Phlegraei volcano district||Pozzuoli, Pouzzoles, Puteoles, east of modern breakwater||Italy west||10||10|
|Nesis||Nisida, very large pila of over 1500 m3, with opus reticulatum facing||Italy west||14.5||14.5|
|Imperial Villa of Pausilypon||Gaiola||Italy west|
|Imperial Villa of Pausilypon||Imperial Villa at Posillipo||Italy west||10||7|
|Imperial Villa of Pausilypon||Palazzo degli Spiriti||Italy west||7.5||6|
|Imperial Villa of Pausilypon||Pollion’s villa at Porto Marechiaro||Italy west||14||5|
|Imperial Villa of Pausilypon||Villa Rosebery||Italy west|
|Neapolis||Naples, Piazza Municipio, offshore Roman quay made with wooden caissons||Italy west|
|Capraria, Capreae insula||Bagni di Tiberio, near Marina Grande on the isle of Capri, with lighthouse at Villa Jovis||Italy west||7||4|
|Capraria, Capreae insula||Palazzo a Mare, near Marina Grande on the isle of Capri||Italy west||11||8|
|Capraria, Capreae insula||Scoglio del Monacone, near the isle of Capri||Italy west|
|Seirenoussai nesoi, Anthemoessa insulae, Anthemuse, possible Siren islands, no stopover for Odysseus||isola di Gallo Lungo||Italy west|
|Vietri||Punta Fuenti, near Vietri sul Mare||Italy west||12||10|
|San Marco di Castellabate||Italy west||?||4.5|
|Scidrus||Roman villa at Cammerelle, near Sapri||Italy west||8||5.5|
|Hadrianou Hormos, port of Lupiae, Miltopiae?||Porto Adriano, at San Cataldo near Lecce; concrete poured into ashlar cells||Italy Adriatic||?||12|
|Gnathia||Egnazia, with several pilae, one with opus reticulatum facing||Italy Adriatic||5||3.5|
|port of Hatria, Adria||Torre del Cerrano, with several pilae||Italy Adriatic|
|Side, Sida||Selimiye, with possible ancient lighthouse||TR: South||?||7.5|
|Soles, Soli, Soloi, Pompeiopolis||Mezitli, west of Mersin; concrete poured into ashlar cells||TR: South||?||15|
|Caesarea Palaestinae, Cesaree, Ace, Sebastos||Qesaria, Caesarea Maritima, Roman port of Herod, built from 22 to 10 BC, with Drusion lighthouse; concrete poured into timber caissons||Israel|
|Alexandria, Portus Magnus and its Pharos, home port of Classis Alexandrina fleet||Alexandria, also called « Le Phare », The Pharos; concrete poured into timber caissons||Egypt: Med Sea||15||8|
|Alexandria||Alexandria, SE of Fort Qait Bey||Egypt: Med Sea|
|Leptis Magna, Lepcis Magna, Lepcitani Septimiani||Leptis Magna, Lepcis Magna, with ancient lighthouse, on R Lebda||Libya|
|Thapsus||Ras Dimass, near Bekalta, south of Monastir, large breakwater of the south port, with concrete poured into timber caissons & possible lighthouse||Tunisia|
|Horrea Caelia, Heraklea||Hergla||Tunisia||3||3|
|Carthago, Carthagine, Punic Qart Hadasht, Knyn, port of Salammbo||Carthago, commercial port, see also so-called “Neptune block” on the coast North of the ports||Tunisia||18||9|
|Thapsa, Tipasa||Tipaza, sheltered by two islets||Algeria||10||3|
|Psamathos||isle of Joinville in front of Cherchel, with ancient lighthouse||Algeria||8||6|
Pilae seen on Google Earth