Engineering Wonders of the World
Volume I

DOCKS. 177 port of London open to ocean-going vessels of deep draught. Some important ports on estu- aries with large tidal ranges—Cardiff and New- port, for example—are accessible at high water only. The breakwater at Marseilles has been al- luded to. Works which achieve much the same end on a larger scale are to be seen at Dover and Plymouth in Eng- Breakwaters, land> and ftt Colombo and Training Walls etc -labæ Bay abroad. That at Dover is perhaps the most interesting example, because it is an open roadstead transformed into a tidal dock with two entrances. At Havre there are converg- ing breakwaters or training walls, to prevent accretions in the Seine estuary affecting the depth of water in the approach channel. At other but smaller Continental ports the ap- proach is formed by erecting parallel jetties, and deepening the channels so formed by suc- tion dredging. As has been said, tidal docks are open basins in which the water falls and rises with the ebb and flow of the tide. One of the largest ex- amples of the type is the Prince’s Dock at Glasgow. This has a water area of 82 acres. Wet docks are not so easy to describe, although the structural differences between them and tidal docks extend no further than the flood is ebbing. At other ports—Liver- pool, for example—sluice pipes serve the same purpose. At Tilbury a lock with a gate at each end leads from the tidal basin to the main dock. But at Liverpool, where space is extremely valuable, some of ths older docks are entered through Half-tide i , K , Basins. passages closed by a single pair of gates. The drawback of this arrange- ment is that vessels can only enter or leave at the time of high water or a little before it. By opening the gates before high, water the depth in the dock is reduced, but the flood soon makes good the deficiency. The gates must be closed, of course, before the ebb be- gins. In some instances, however, the incon- venience of this arrangement is removed by interposing between the exit of the tidal basin and the entrance to the main dock a half- tide basin, which serves to pass several vessels at once into or out of main docks at the half tide—in other words, before high water. In some instances—in the port of London, at any rate—the entrance to a half-tide basin is through a double gate lock. A basin thus protected is used for the purpose of collecting all the outward bound vessels before high water. Some time before the tide is full the water level in the basin is reduced to that of iv . n . t’he entrances. In the case of Wet Docks. the Tilbury Docks, which are typical, vessels leaving the river at a con- venient angle enter what is called a tidal basin. As a rule the depth of water in modern tidal basins is slightly greater than that of th© approach channel, the object of this being to enable a vessel to get into dock even after the tide has begun to ebb in the river. At Til- bury the tidal basin has a depth at low water of spring tides of 24 feet, which, makes the main docks accessible at practically all stages of the tide. As the flood always brings in large quantities of mud, powerful jets of water are used to stir up the matter deposited while (1.408) the river by opening the outer lock. The out- ward bound vessels are passed out, and the incoming vessels brought in to await the open- ing of the inner lock or gates at high water. The most modern practice is, however, to provide a wet dock with lock entrances giving direct access from a tidal basin or from an approach channel. Good ex- amples of this type of en- „ Lock , . Entrances. trance are to be found m our larger naval dockyards. In length and width the lock chamber should be sufficient for the largest vessel frequenting the port. At Ports- mouth and Devonport so ample are the dimen- sions of one or two of the lock chambers that 12 VOL. II.