ON THE ACTION OF WAVES AND CURRENTS. 401 



carrying the sand up the estuary in spite of the effect of the ebb to brincr 

 it down. But this in itself was not so much ; it was the large eddy 

 caused by the groin which produced the greatest effect. The water 

 entering on the left of the estuary crossed over to the right, and re- 

 turned along the right bank. In other words, during flood the right side 

 of the estuary for 30 miles from the generator was in back water. This 

 back water also gave the ebb a start down the right bank which rendered 

 the ebb stronger on this side. 



The sand came down rapidly on the right side, and besides was carried 

 over from the left to the right, and formed a bank along the right middle 

 of the estuary, reaching the generator after a very few tides. Round this 

 hank the water circulated, carrying the sand with it up on the left and 

 down on the right, the bank growing all the time. The ripple round this 

 bank was very striking, arranged with the ripple heads all down on the- 

 right side and up on the left. After about 3,000 tides the sand began to 

 pass from the point of this bar in a fine stream aci'oss the open channel, 

 dividing this point from the point of the groin, and commenced the 

 formation of a bank in the generator corresponding to that in the tank. 

 This bank had to be removed from the generator, and after 6,000 tides 

 4 lbs, of sand were so removed. In Experiment V. the first sand removed 

 from the generator was after 120,000 tides had been run. 



The sand also went more rapidly up the river in Experiment VI. 

 than in Experiment V. But this was accounted for by dredging in 

 the river having begun much earlier, after 20,000 tides as against 

 30,000. 



In all 8 lbs. of sand were removed from the river in Experiment VI.,, 

 against 10 lbs. in V., or about 0-004 of the tidal capacity in VI. against 

 008 in V. In both cases the dredging stopped when the sand began to 

 shift up the river after 70,000 tides. 



At 100,000 tides a condition of final equilibrium had been arrived at. 

 The sand in the river was just the same as in V., Plan 3, Experiments V. 

 and VI. in tank F. There is deep water in VI. up to Section 21, 30 miles 

 from the generator, the levels of the sand being much the same from this 

 point up as in V. 



A similar groin was then introduced at Section 16, extending from the 

 left bank to the middle of the estuary. This groin was 4^ miles long and 

 100 feet high to H.W.L., and 50,000 more tides were run, the river all the 

 time slightly improving. Thus having brought deep water up to Section 

 14, or about 44 miles from the generator, a groin extending from the right 

 bank to mid-channel at Section 8, about 25 miles long and 70 feet high, 

 and another from the left bank to mid-channel at Section 5, 2 miles long 

 and 70 feet high, were put in. 



Thefirsteffect of these groins was to raise the sand slightly in the mouth 

 of the river ; but this improved again, and after 50,000 more tides thero 

 was deep water extending from the mouth of the river to the generator, 

 and the river was better than in Experiment V. with the training walls, 

 though not quite so good as before these were put in. 



In the meantime the banks had risen in the estuary below the groins, 

 extending down from nearly H.W.L. to the point of the next groin, 

 whei-e there was a pass with water nearly to the bottom of the tank. 



The sand carried down into the generator during the expei'iment 

 amounted to 69 lbs., or 57 per cent, of the tidal capacity. In Experiment 

 V. 24 lbs. were removed in like manner, or 20 per cent, of the tidal capa- 



1891. D D 



