412 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



disturbance to be expected, may be found in the level of the mud 

 below the water line, inasmuch as in very exposed situations mud 

 cannot repose near the surface. 



The absence of mud, however, in any locality, without taking 

 into consideration other conditions, proves nothing; local currents 

 may sweep it away, or the geological formation of the district may 

 not produce it ; on the other hand, its presence seems to afford both 

 a delicate and certain test of the lowest limits to which the disturb- 

 ance generated at the surface has reached. Therefore, if in front of 

 a proposed work mud is found within a few fathoms of the surface 

 of the water, there is in that fact good ground for considering that 

 the work will never be assailed by a very heavy sea. 



Facts appear to indicate fairly conclusively that the disturbing 

 effect of waves is in many cases of more importance than that of 

 any ordinary current. Even when the material composing the bed 

 is particularly favourable to movement by current one or two heavy 

 gales will cause a greater amount of disturbance than an ordinary 

 tidal current in several months. 



Chopping Seas. Choppy water may be the result of two equal 

 swells moving in opposite directions, or it may be due to the super- 

 position of a number of swells of different amplitudes, heights, and 

 directions, to currents and other causes. Chopping waves rise and 

 fall vertically, and have twice the height of each of the two swells 

 of which they are the components, and the same length. This double 

 height explains why they break much more readily than travelling 

 waves or swell. 



Chopping is the natural motion of water in a limited basin or 

 area, whilst swell is the natural motion of water of an indefinite 

 extent. 



Ground Swell. When a strong gale has been blowing for some 

 time a considerable forward motion is imparted to the water greater 

 at the surface than at the bottom. Out of the influence of the 

 wind the forward motion of both wave and water is maintained by 

 momentum; but the propelling force being absent, the motion of 

 the fluid becomes less at the surface than at the bottom, making 

 what is known as ground swell. 



Strains on Dock Gates. The primary strains acting on dock gates 

 are : Water pressure, reaction at the mitre- and heel-posts, and reac- 

 tion parallel to the gate. 



The subject has been ably treated both mathematically and 

 graphically by Mr. W. R. Brown 1 and Mr. Blendy, 2 in papers laid 

 before the Institution of Civil Engineers, and previously in papers 



1 M.P.I.C.E., vol. xxxi. p. 317. Ibid., vol. Iviii. p. 154 



