552 



HARBOUR 



the moles of the latter city and the works in the 

 lagoons of Venice remain to this day. France 

 was next in the field, embanking, protecting, and 

 deepening the mouths of the rivers along her 

 north-western shores, as at Havre, Dieppe, Dun- 

 kirk, &c. In 1627, during the siege of Rochelle, 

 Metezeau constructed jetties of loose rubble-stone, 

 to prevent access to the city. 



Meanwhile, Britain, whose ocean-commerce is 

 of comparatively recent date, lagged far behind her 

 continental rivals. With few exceptions her ports 

 were absolutely unprotected, or rather uncreated ; 

 and this state of tilings continued until late in the 

 18th century. Two of the few exceptions were 

 Hartlepool, where a harbour was formed about 

 1250, and Arbroath in 1394. In the 17th century, 

 at Whitby and Scarborough rough piers were 

 thrown out, protecting the mouth of the port ; 

 while at Yarmouth a north jetty and subsequently 

 a south one were formed. An ancient mole existed 

 at Lyme Regis, a section of which, from Smiles's 

 Lives of the Engineers, is given below ( see fig. 3 ). 

 But the chief efforts of the early English engineers 

 were directed against the shoals and waves of 



Fig. 2. Dover Harbour in the time of Henry VIII. 



Dover. When, however, Smeaton rose to vindicate 

 the engineering talent of England, things took a 

 different turn ; and now few countries surpass 

 Great Britain in the number of artificially improved 



Fig. 3. 



a, ancient pier at Lyme Regis ; 6, wooden-framed pier, filled 

 with rubble ; c, pier at Havre, with apron ; d, masonry pier, 

 on rubble foundation. 



commercial harbours, or in the just appreciation 

 of their importance. 



In the construction of harbours the great 

 desiderata are sufficient depth of water and 

 perfect security for the vessels likely to frequent 

 them, together with the greatest possible facilities 

 for ingress during any weather ; while the chief 

 obstacles to be surmounted are the action of the 

 waves upon the protecting piers and breakwaters, 

 and the formation of sandbanks and bars, which 

 diminish the depth of water at the entrance and 

 also within. The designs of harbours, as has been 

 already indicated, may be classified under the 

 following heads : ( 1 ) harbours of refuge and 

 anchorage breakwaters; (2) deep-water and tidal 



harbours for commercial purposes ; ( 3 ) piers, either 

 straight, or kanted, or curved; (4) quays or 

 wharves. 



These different works are obviously suited for 

 different localities, and for contending with different 

 exposures. Quays are clearly suited for the most, 

 sheltered situations only, and the engineer must 

 consider, when designing a harbour, which type of 

 harbour will be most economical and effective. In 

 coming to a decision the nature of the traffic, the 

 exposure, and the geological features of the coast 

 must be carefully considered. A good chart or 

 marine survey furnishes valuable evidence as to 

 the force to which harbour- works will be exposed. 

 Among the points to be noted is the line of maximum 

 exposure, or the greatest fetch or reach of open sea, 

 as well as the deptli of water, in front of the har- 

 bour. Thomas Stevenson proved by observation* 

 that the waves increase in the ratio of the square- 

 root of their distance from the windward shore as. 

 measured along the line of exposure, and he gives 

 the following simple formula : Where h = height of 

 wave in feet during a strong gale, and d = length 

 of exposure in miles for distances of, say, 10 miles 

 and upwards, then h = l'5\/rf. The heights so- 

 obtained will be increased when they pass into con- 

 verging channels, and decreased when they pass 

 into expanding channels. The greatest measured 

 height of the waves was by Scoresby in the Atlantic 

 Ocean, where he found billows of 43 feet in height 

 from hollow to crest, and 36 feet was not an uncom- 

 mon height. At Wick, Caithness-shire, waves of 

 about 40 feet have struck the breakwater. Amongst 

 the greatest recorded forces exerted by the waves 

 may be mentioned the breaking or quarrying out 

 of its position in situ of a mass of 13 tons on the- 

 Skerries of Whalsay, in Shetland, at a level of 74 

 feet above the sea this height, of course, being 

 reached by sliding. But the most astonishing feat 

 of which we have any knowledge was at Wick 

 breakwater, where in the winter of 1872 a mass 

 of masonry, concreted together as a monolith, and 

 bound with iron bars 4 inches in diameter, and 

 weighing no less than 1350 tons, was torn from its 

 seat in the work, and thrown to leeward. 



Thomas Stevenson devised an instrument called 

 the Marine Dynamometer for ascertaining numeri- 

 cally the force which is exerted by the waves in 

 the Atlantic and German oceans. He found that 

 the mean of his observations during winter was- 

 more than three times that exerted during summer,, 

 the maximum force recorded being 3J tons per 

 square foot. 



Various local causes materially affect the height, 

 and therefore the force of the waves. In some 

 cases, where a strong current runs past the coast, 

 as at Sumburgh roost in Shetland, it causes a 

 dangerous breaking sea in the current, and while 

 this roost or race continues to rage the coast under 

 lee is comparatively sheltered ; but when the force 

 of the tide is exhausted and the roost disappears, 

 a heavy sea rolls in upon the shore. It is this 

 encounter between the ground-swell waves of 

 the ocean and the current of tide or land water 

 which causes miniature races at the mouths of 

 rivers. 



Another most material element in the question- 

 of exposure is the depth of water in front of 

 the harbour ; for, if that depth be insufficient to 

 admit of the transmission of tne waves, they break 

 or spend themselves before they reach the piers. 

 Thus, Leslie found at Arbroath harbour that the 

 works were not so severely tried by the heaviest 

 waves as by others of lesser size which were not 

 tripped up and broken by the outlying rocks. In 

 the same way, at the river Alne the harbour within 

 the bar is more disturbed by ordinary waves thai* 

 during great storms. It thus appears that the 



