Breakwaters (q.v.) or moles (q.v.), suitably placed so as to resist the passage of waves coming in from the open sea, will secure a safe anchorage on their shore side. The entrance to such an anchorage, or to that of any more enclosed harbour, should be so situated that the sides shall not deflect the sea across the entrance. In both cases also it is desirable that any heavy seas that pass into the enclosure shall be allowed the opportunity of wasting themselves on a suitable beach. If the enclosure is walled all round, the incident and reflected waves round the sides may create a dangerous choppy sea in the harbour. There are advantages in having the moles arched; these certainly resist the waves to a less extent, but do not prevent the flow of currents, which when entirely obstructed may cause an immense amount of excavation from one part and deposition of silt and mud in another. It is of great importance that the harbour shall be sufficiently deep, and that it shall not be silted up in the above manner.
Various plans have been adopted to cause a scouring action of outflowing tidal water to keep the basin free from such matter; an instance of such a provision is the large sluicing basin in the Calais harbour. Quays or wharves of different kinds are added to most harbours; they require to be well protected, and are arranged to suit the situation. These and other such requisites of commercial harbours belong to the subject of docks (q.v.).