Arched breakwaters

An arched breakwater looks like an aqueduct.

Pont du Gard aqueduct

The arches are supported by massive piles, which are made of stone or concrete. 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”[1]. The ratio of pile width over opening between adjacent piles 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.

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 (29 out of 42), especially around Naples (23 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 7.2 m: nearly square. The average horizontal surface is 39 m2. The height cannot be determined on Google Earth.
  • The largest pila is the one found at Nesis: 14 x 14 x 5 m3.

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

This leads us to have a closer look at the most famous ancient arched breakwater located at Puteoli where the opening between adjacent pilae varies from 0.7 to 1.0 pila width, which is close to the values mentioned above for Portus Iulius and Misenum.

[1] 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).

List of known pilae

Ancient name Modern name Country Length (m) Width (m)
Massalia Graecorum, Lacydon Marseille, Vieux Port, place Jules Verne & place Villeneuve-Bargemon (see also nearby Musee des docks: Dolia warehouse) France South
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
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 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 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
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 1000 m3 Italy West 14 14
Imperial Villa of Pausilypon Posillipo, protected by row of pilae Italy West 10 7
Imperial Villa of Pausilypon Porto Marechiano, near Posillipo Italy West 14 5
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
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 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
Apollonia, Sozousa Arsuf 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

Santa Liberata (GE 2013)

Santa Liberata (GE 2013)

Cosa (GE 2017)

Circei (GE 2015)

Caieta (GE 2016)

Misenum, Punta Terrone (GE 2007)

Misenum, Punta di Pennata (GE 2007)

Secca Fumosa (GE 2007)

Castello di Baia (GE 2007)

Portus Iulius (GE 2007)

Portus Iulius (GE 2007)

Nesis (GE 2015)

Pausylipon (GE 2014)

Pausylipon, Porto Marechiano (GE 2014)

Capri, Palazzo a Mare East (GE 2014)

Capri, Palazzo a Mare West (GE 2014)

San Marco di Castellabate (GE 2016)

Scidrus (Sapri) (GE 2015)

Gnathia (Egnazia (GE 2017)

Side (Selimiye) (GE 2017)

Horrea Caelia (Hergla) (GE 2015)

Psamathos (Cherchel) (GE 2003)