HARBOUR 



HARBOUR 





haroour. Perspective plan showing arrangement oi Dover Harbour as it was oeiore tne ureat War. Ine larger Oasin 

 is the Admiralty harbour, completed in 1909 ; on the left is the Commercial harbour, leading to the tidal basin and docks 



and depth, and face in the right 

 direction in regard to tidal currents 

 and prevailing storms to permit of 

 the safe navigation of ships enter- 

 ing the harbour in the worst 

 weather ; (3) local conditions, 

 which must allow of the construc- 

 tion of the necessary breakwaters. 



In practice the necessity for a 

 harbour in a given locality usually 

 overrides other considerations, but 

 localities where progressive silting 

 occurs should be avoided. Full 

 knowledge of all such local condi- 

 tions as tidal range and currents, 

 prevailing winds and waves, coast 

 erosion and silting, and the effect 

 of artificial obstructions is essen- 

 tial in order to avoid unsatisfac- 

 tory results, e.g. the construction 

 of breakwaters without due regard 

 to currents may cause silting on 

 an extensive scale. The position 

 and width of opening are also im- 

 portant : the position is deter- 

 mined by the direction of prevail- 

 ing storms ; the width is made as 

 narrow as possible to prevent the 

 entry of heavy seas, but must be 

 sufficient for the safe passage of 

 vessels during the heaviest gales. 

 Again, it must be proportioned to 

 the width of the harbour itself, so 

 that waves entering the enclosed 

 space may have ample room in 

 which to expand. 



Sometimes an outer breakwater 

 is constructed upon which the 

 main force of the waves is ex- 

 pended. This forms an outer 

 silting basin, which may also 



serve as a refuge, while the basin 

 within the inner breakwaters or 

 moles constitutes a commercial 

 harbour. In such cases the dis- 

 tance between the openings in the 

 outer and inner breakwaters should 

 be sufficient to allow sailing ships, 

 running before the wind, to 

 shorten sail after passing the outer 

 breakwater and make the inner 

 entrance in safety. 



Harbours of Refuge 



A harbour of refuge may be 

 either natural or artificial, and 

 may be used solely as a refuge for 

 ships in a storm, or may also con- 

 stitute a commercial harbour. 

 The essential features are good 

 anchorage and safe and easy 

 access from the sea at any state of 

 the tide and in any weather. The 

 best known harbours of refuge 

 are the one at Sandy Bay, near Cape 

 Ann, on the coast of Massa- 

 chusetts, and that at the mouth of 

 Delaware Bay, U.S.A. The break- 

 water of the former is over 9,000 ft. 

 long, 72 ft. in height, and 205 ft.wide 

 at the base. Both are rubble mound 

 constructions capped with stone. 



A commercial harbour may be 

 either a natural or artificial har- 

 bour within which docks, quays, 

 wharves, and piers are constructed 

 and equipped with the necessary 

 appliances for the loading and dis- 

 charge of cargoes. Sometimes the 

 inner side of a breakwater is con- 

 structed as a quay or wharf along- 

 side which ships may berth. Dry 

 docks are also sometimes pro- 



vided. Owing to the presence of a 

 bar or sandbanks, or the difficulty 

 of maintaining a sufficient depth 

 of water in the approach channel 

 at low tide, or to other local con- 

 ditions, many harbours can only 

 be entered and left at high tide. 



In the early period of commer- 

 cial enterprise, sites for harbours 

 were frequently selected which 

 afforded natural shelter, and were 

 readily accessible to the small 

 ships then in use. Small rivers and 

 creeks fulfilled the requirements 

 of those days, and led to the 

 establishment of ports inadequate 

 for the accommodation of the 

 bigger vessels built later. The 

 channel was maintained by. the 

 discharge of land water or tidal 

 flow, and to deepen it for the pas- 

 sage of ships of greater draught, 

 training jetties were in many in- 

 stances constructed along each 

 bank, projecting seawards beyond 

 the original entrance, for the pur- 

 pose of concentrating the flow of 

 water and scouring the channel. 



Whilst this purpose was served, 

 these projecting jetties*, often 

 acted as groynes on the fore- 

 shore, checking the travel of 

 shingle and sand, and eventually 

 causing a reduction in depth ot 

 the channel at the outer end by 

 the formation of a bar. This led 

 to the periodical extension of the 

 jetties seawards until a great length 

 was sometimes attained, as in the 

 case of Dunkirk. This method of 

 improvement can, therefore, only 



