SCOURING AND SLUICING. 271 



that the harbour or channel was not solely kept open by the 

 influence of the fresh water, which was obviously inoperative 

 on the bottom while it was running out over the salt water. 1 



At Woolwich, in 1824, ships used to be supplied with fresh 

 water pumped up from alongside at the turn of the tide and 

 up to the first quarter of the flood, but the end of the hose was 

 not allowed to be more than two feet below the surface of the 

 water. If this precaution was neglected, and the hose was 

 allowed to reach three feet below the surface, salt water was 

 immediately pumped up. This clearly showed that at Woolwich, 

 at the date referred to, the salt water flowed up the river under 

 the fresh water. 2 



Mr. W. Dyce Cay observes that among the causes of the 

 phenomena of tidal estuaries the difference in the specific gravity 

 of salt and fresh water is important. The fresh water floats like 

 oil on the top of the salt water, and the currents of the two 

 fluids proceed almost independently of one another, having 

 different velocities, and sometimes flowing in different directions. 

 Thus it is that during flood tide a river may continue running 

 out, while the salt water beneath it may be running in to fill 

 the estuary. 3 



Mr. H. Keller, in alluding to the use of fresh water for 

 scouring purposes, says that the effect of such scour does not 

 reach below a depth of 6 to 9 feet, so that its power upon a 

 bar is limited. 4 It would appear that this would depend greatly 

 upon the volume of fresh water discharged, and the sectional 

 area of the channel. 



Avomnouth Dock Entrance. 5 At the mouth of the Avon the 

 water was made to debouch through one channel, simply by the 

 tugs always taking that particular channel close upon low water, 

 and so stirring up the mud, which was carried away by the last 

 of the ebb. 



Opinions appear to have differed as to the possibility of 

 maintaining a fairway to the entrance and the entrance channel 

 itself. On the one hand, it was anticipated that, considering the 

 great amount of mud which the water held in suspension, diffi- 

 culties would be experienced in keeping the entrance clear, and 

 that it would eventually silt up ; on the other hand, it was held 

 that the flow from the Avon, scouring the entrance so near the 



1 M.P.I.C.E., vol. xxi. p. 337. 2 Ibid., p. 339. 3 Rid., vol. Ixvi. p. 68. 



4 Ibid., vol. Ixvii. p. 461. * Ibid., vol. Ixx. p. 36. 



