HARBOUR 



be regarded as a temporary mea- 

 sure, since in its execution the 

 engineer combats a persistent force 

 of nature, which in the end gener- 

 ally prevails. 



An alternative method is to 

 make use of river or other land 

 water for scouring the channels, 

 or where this is not available in 

 sufficient quantity, to construct 

 sluicing basins, i.e. reservoirs with 

 sluice gates, in the vicinity of the 

 channel, in which sea water is 

 impounded at high or spring tide. 

 At low tide the sluices are opened 

 and a volume of water sweeping 

 down the channel scours out the 

 silted material. The jetties are 

 sometimes made to converge to- 

 wards the outlet so as to concen- 

 trate the scouring effect at the 

 entrance where a bar tends to 

 form. This method has been 

 greatly favoured at Belgian and 

 French ports, and in many other 

 localities where silting occurs on an 

 extensive scale. In some cases, 

 where waves of considerable force 

 enter a jetty channel, the channel 

 is widened for a short distance, the 

 jetty is made of open construction 

 for a portion of its length, and a 

 basin Avith a shelving beach is 

 constructed. By this means the 

 waves passing up the channel find 

 an outlet, and, expanding into the 

 basin, spend their force upon the 

 beach. Such an arrangement is 

 known as a wave breaker. 

 Semi-natural Harbours 



Next to a purely natural har- 

 bour, an inlet or creek sheltered 

 on two sides by headlands, and only 

 requiring artificial protection at 

 the entrance, forms the most 

 desirable harbour site, other con- 

 ditions being equal. At Plymouth 

 and Cherbourg, for example, this 

 natural advantage exists, the en- 

 trances being protected by de- 

 tached breakwaters. Such condi- 

 tions are only to be expected on 

 more or less rocky coasts. 



Varieties of the principal types 

 of harbour are numerous, depend- 

 ing upon local requirements and 

 conditions as well as upon financial 

 considerations and limitations. A 

 portion of a large bay may be 

 converted into a harbour by a de- 

 tached breakwater more or less 

 parallel with the coast and one or 

 two mole breakwaters projecting 

 from the shore, or a harbour may 

 be formed by two mole break- 

 waters with outer ends converging 

 or by a single mole breakwater. 



A fine example of a combined 

 harbour of refuge and commercial 

 harbour, the second largest purely 

 artificial harbour in the world, 

 exists at Dover, (ft which the 

 leading features are as follows : 

 Low water area of Admiralty 



3832 



(refuge) basin, 610 acres ; commer- 

 cial basin, 80 acres ; length of Ad- 

 miralty mole breakwater, 4,000 ft. ; 

 E. mole breakwater, 2,942 ft. ; S. 

 (detached) breakwater, 4,212 ft, ; 

 total length of breakwater, 2'1 m., 

 of which about 1 75 m. measures 

 100 ft. high from foundation to 

 the top of the parapets and 60 ft. 

 across the base, and consists of solid 

 concrete and granite. 



The inner faces of the east arm 

 and Admiralty mole extension are 

 tendered, and, in addition to the 

 Prince of Wales's pier, 2,910 ft. long, 

 dividing the Admiralty and com- 

 mercial basins, provide berthing 

 accommodation for shipping. The 

 deck level of all the breakwaters is 

 10 ft. above high (spring) tide, the 

 E. arm and Admiralty mole ex- 

 tention being provided with ad- 

 ditional high sheltering parapets. 

 There are two entrances, one fac- 

 ing E. and the other S., the width 

 being 650 ft. and 740 ft. respec- 

 tively, and the depth 40 ft. at low 

 tide. The tidal range is 18 ft. 9 in., 

 and very strong currents occur on 

 this part of the coast, and it is 

 estimated that with every tide 

 17,000,000 tons of water enter 

 and leave the harbour, of which 

 half enters or leaves in two hours. 



These facts influenced the deci- 

 sion to provide two entrances. 

 The extension works, complete in 

 1909, occupied twelve years to 

 construct, cost 4,000,000, and 

 comprised 1,300,000 cubic yds. of 

 concrete and 1,900,000 cubic ft. 

 of granite. Concrete blocks, weigh- 

 ing from 26 to 40 tons each, were 

 employed in the construction of 

 the breakwaters. 



Buenos Aires Harbour 



An interesting type of a com- 

 mercial harbour is exemplified in 

 the port extension works at Buenos 

 Aires. The scheme embraces four 

 parallel tidal basins from 385 to 643 

 yds. long by 154 yds. wide, formed 

 by intervening and end moles of 

 the same lengths, the whole being 

 protected by a detached break- 

 water of the rubble mound type, 

 If m. long, beginning near the 

 existing dock approach channel 

 and spaced at a distance of 275 

 yds. from the outer ends of the 

 moles. Within the line of the 

 breakwater gantry stagings were 

 built out from the shore by means 

 of which temporary dams were 

 formed, enclosing the whole area of 

 uhe new port works. Water was then 

 pumped out of the enclosed space, 

 and the construction of the moles 

 and basins commenced in the dry. 



The moles vary from 137 yds. to 

 222 yds. in width, and consist of 

 retaining walls with earth filling. 

 A depth of 33 ft. at low water is 

 provided for. The temporary dams 



HARBOUR 



are arranged in sections, so that 

 as each basin with its correspond- 

 ing moles is completed, it may be 

 opened to traffic by admitting 

 the water and removing a section 

 without interfering with the re- 

 mainder of the work. 



A harbour may be provided 

 with one or more entrances ; at 

 Plymouth and Cherbourg the de- 

 tached breakwaters at the en- 

 trances permit of vessels entering 

 at either end. In purely artificial 

 harbours there is usually only one 

 entrance, as the admission of waves 

 through two openings is liable to 

 reduce tranquillity of the water 

 within, unless an outer breakwater 

 and stilling basin are provided. 

 On the other hand, two entrances 

 allow ships the benefit of selection 

 according to the direction of a 

 storm, and are sometimes an ad- 

 vantage where littoral currents 

 prevail. The width varies greatly 

 according to local circumstances, 

 ranging from 100 ft. to several 

 thousand ft. ; thus, where the en- 

 trance faces a comparatively shel- 

 tered position, it may be as wide 

 as the harbour itself, but when it 

 faces in the direction of prevailing 

 storms it is made as narrow as is 

 consistent with safe navigation. 

 Harbour Entrances 



In exposed situations the width 

 of the harbour itself influences the 

 width of opening, since waves, 

 after passing through the entrance, 

 need ample width in which to ex- 

 pand. Again, a deep-water en- 

 trance in an exposed position 

 should be narrower than one in 

 shallow water, since deep waves 

 passing through it are not so 

 readily stilled as shallow waves. 

 Outer and inner breakwaters are 

 sometimes constructed to overlap, 

 so that the openings do not come 

 opposite each other. Ample room 

 should be allowed between them 

 to permit of a sailing ship tacking 

 to make the inner opening after 

 passing through the outer entrance. 



Rocks and dangerous reefs in the 

 vicinity of harbour works are 

 sometimes removed or lowered by 

 blasting. Extensive operations of 

 this character, occupying many 

 years and costing over a million 

 pounds, were carried out to im- 

 prove the East river at New 

 York, an underwater area of some 

 12 acres of rock being lowered. 

 The positions of sandbanks, bars, 

 dangerous reefs, etc., near harbour 

 entrances are marked by light- 

 houses, or, where a light is not 

 essential at night, by beacons. 

 Large lantern lights are also pro- 

 vided on the ends of breakwaters 

 to mark the entrance by night, 

 Buoys are frequently employed to 

 indicate varying depths of water 



