Ancient Port Structures

The main elements of a port are its breakwater(s) to reduce wave action inside a protected basin, where quays or jetties, with some mooring devices, are available for loading/unloading ships. Hence, a breakwater and a quay have to be built using available construction materials and methods, and a basin has to be dredged and maintained at adequate depth.

For a general overview of ancient and modern port structures, please refer to “Ancient Port Structures, An engineer’s perspective“.

From our Catalogue, we know that for around 6000 ancient coastal settlements, ports and harbours, we have around 650 ports (only 12%) with at least one of the structures listed below. The following port structures were found in ancient ports:

Abbr. Type of structure Nb
BW Breakwater, sometimes also called mole 380
QU Quay (masonry with berthing on one side), pier or jetty (masonry with berthing on two sides), and landing stage (jetty on piles) 375
PL Pila, made of hydraulic concrete containing pumiceous volcanic ash (pozzolana) 51
MO Mooring device (bollard, pierced block) 83
CN Canal (for navigation or basin flushing and/or desiltation) 70
SL Slipway to take ships in/out of the water 140
SH Shipshed (always including a slipway) 86
SY Shipyard (neoria, navalia) (incl. arsenals) 56
EX Man-made basin excavated in the rock (e.g., Carthage’s circular cothon) 36
LK Limen Kleistos, “closable” harbour with a narrow entrance 88
PH Lighthouse 174
HO Warehouse 88

Some rubble-mound breakwaters have been luckily preserved and survived two millennia of wave attack, but most of the ancient breakwaters were destroyed by wave action and remains are found under water as “submerged breakwaters”. Careful examination of historical Google Earth images enables us to see quite a few breakwater remains in shallow waters.
As the process of destruction of breakwaters by waves was not all that clear, further analysis was undertaken by the author, focussed on the worst possible wave conditions, considering that they will eventually occur in the long term. In other cases, an approach based on a “design wave” must be used.

Breakwater destruction by wave action is not the only way for breakwaters to be submerged. Subsidence is another possibility because coastal structures were often built on layers of loose sand provided by longshore sand transport along the coast. Such layers might have been compacted by the overload and by wave-induced liquefaction due to repeated storms. Earthquake-generated liquefaction is another option for subsidence as it is likely to affect large areas covered with cohesionless water-saturated sand. Last but not least, tectonic subsidence involves crustal movements of the earth which may be horizontal, vertical or combined.

Vitruvius‘s “de Architectura” dated around 20 BC, is the major ancient text left about maritime construction methods. Unfortunately, no drawings  are available, so that his descriptions are not all that clear to us. The three of his methods are considered here in some detail with help of various sketches prepared by previous architects and engineers.
See: OLESON, J. et al. “Building for Eternity”, 2014 .
See also: ,
and our list of ancient Greek terms on maritime structures.

A question might be asked why the ancient engineers did not invent reinforced concrete, e.g. by means of chains placed inside the mortar. As steel is subject to corrosion and therefore to increase of its volume, that induces cracking of the concrete, the ancients may not have found it such a good idea (NB: the oldest modern reinforced concrete structures are around one century old and are not in a good condition today, e.g. Tour Perret in Grenoble, France). Another part of the answer might be that as the ancients had vaults, they did not use overhanging structures that require reinforced concrete. However, massive structures like walls and towers needed to be reinforced at their base in order to provide internal cohesion. It appears that courses of bonding tiles were used for this purpose. It can be shown from available testing results that the initial shear strength of lime mortar on tiles and bricks is somewhat larger than on natural stones. Hence, each course of tiles placed inside the stone masonry acts like a modern tie beam made of reinforced concrete.

Pilae are massive piles (opus pilarum), which are made of stone or concrete (opus caementicium) which have been used as a base for arched structures like aqueducts and bridge piers. Many of them can still be seen on Google Earth pictures and some, like the one at Nisida, have been studied in detail. It is proposed here that several alignments of maritime pilae may have been the base of arched breakwaters.

Pierced stones can be used as mooring devices when the hole has a horizontal axis. Holes with a vertical axis are believed to be used for derricks like those used onboard ships.

Defensive chains strechting across a harbour entrance are mentioned by several ancient authors, including Vitruvius who explains that chains are suspended by means of machinery placed inside towers located on each side of the harbour entrance. Considering the forces involved, the length and the weight of the chain was obviously limited.

Silting up of harbours was always a major concern and that is still the case for modern port engineers. One should remember that waves are the driving force of the so called “littoral drift” (longshore sand transport along the coast). As the aim of breakwaters is to reduce wave penetration into the port, sand will settle down. Hence structures including arches are not efficient to stop waves while letting sand passing through. That simply does not work!

Around 65% of the natural tombolos found on the coasts of the Mediterranean Sea are ancient settlements. A tombolo is a sandy isthmus connecting the mainland at a right angle to an offshore island or obstacle. A bell-shaped salient occurs when the offshore island is smaller, or further away from the initial coastline, than for a tombolo. If we define L as the length of the island and D as the distance from the initial shoreline, a tombolo occurs if the ratio L/D > 0.65.