198 HARBOUR CONSTRUCTION. 



according to the height of the waves, the depth of the water, the 

 size and specific gravity of the materials of which it is composed, 

 and the level up to which it is carried in relation to the water- 

 line. Such mounds form artificial boulder beaches, or sunken 

 reefs, according to the level up to which they are brought ; and 

 they assume the forms which the laws of nature enforce, nothing 

 more being necessary, in constructing a breakwater of this type, 

 than to throw the stone into the sea, and allow the waves to do 

 the rest of the work. 



The slopes of mounds which rise above low water, or in 

 which the surface is not far below it, are often many years in 

 coming to. a state of rest, seeing that each storm greater than 

 those preceding it draws down the material, and causes it to 

 assume a flatter slope for a greater depth. 



The sudden change which takes place in the slope of a 

 mound on its weather side, at a level varying from about 10 feet 

 to 15 feet below low water of spring tides, indicates very clearly 

 the depth to which storm-waves have power to disturb the 

 rubble, seeing that below this depth the material assumes a 

 slope nearly corresponding with its natural angle of repose, say 

 1 in 1, or 1 in 1 J ; whereas above that point, and up to the level 

 of high water, the slope is very much flatter being, in the 

 Holyhead breakwater, as flat as 1 in 12 (Fig. 41). 



Other things being equal, the form which the action of 

 storm- waves imparts to the rubble in breakwaters of this class 

 affords a reliable means of judging of the relative exposure of 

 their sites. 



Plymouth, Kingstown, and Table Bay breakwaters, also the 

 outer portion of the one at Portland, are examples of mound 

 breakwaters without superstructures. 



At Plymouth and Kingstown the slopes are faced with heavy 

 stone pitching down to low water of spring tides ; they are not, 

 therefore, liable to much disturbance above that level. Below 

 low water, the rubble in the Plymouth breakwater has taken 

 a slope which varies from 1 in 3- to 1 in 5 to a depth of about 

 10 feet, and from 1 in 1^ to 1 in 1J from that level to the bottom. 

 There is an almost level benching, between 80 feet and 90 

 feet wide, which extends seaward from the toe of the pitched 

 slope at about the low-water level. The inclination of the 

 pitched slope on the weather side is 1 in 5, and on the harbour 

 side, 1 in 1J. 



