ON THE ACTION OF WAVES AND CUKKENTS. 397 



Comparing tliis with the results in Experiments I. and III., where the 

 reduced length of the river is only some 50 miles, but in which the rise of 

 tide at tbe mouth of the river was somewhat greater, the effect of the 

 extra 20 miles length in the river is seen to have improved the general 

 and navigable deptb of the river and estuary from the top of the river to 

 a distance of 40 miles down the estuary by from 40 to 30 feet. 



17. The efeds of dredging in the river, Experiment V., in Tank F, from 

 November 19 to December 23, 1890, Plan 3, Plate VIIL— The initial con- 

 ditions of this experiment were the same as those of Experiment IV. in 

 tank F, except that the mean level of the tide was raised to 0-016 above 

 the initial level of the sand, and the period was increased from 22 to 23-3 

 seconds. The experiment was undertaken with the intention of ascertain- 

 ing (1) whether raising the mean level of the tide above the initial level 

 of the sand without altering the rise of tide would prevent the river 

 becoming barred : and, supposing thia did not succeed, (2) to ascertain 

 whether, if the bar which had hitherto formed in the river during the 

 early stages of the experiments in tank F were kept down by dredging- 

 out the sand as it rose above the initial level, the later stages would 

 follow the same course as in tank E. 



The results were remarkable, and bring out the critical character of 

 the conditions at the mouth of the river. 



The experiment was allowed to run 30,000 tides, during which the 

 progress of the sand was much more rapid than in IV., reaching Section 19 

 in (5,000 tides, as against 36,000 in Experiment IV. and 13,000 in Experi- 

 ment E, v., and reaching Section 23 in 16,000. At this point it stuck, 

 and the sand accumulated at the head of the estuary and in the river, 

 which became barred at Section 19, on reaching 30,000 tides. 



It thus appears that lowering the initial level of the sand produced an 

 effect on the first action very nearly equal to increasing the rise of tide 

 by double the amount, but that as the sand distributed itself this effect 

 passed off. 



At 30,000 tides the bar in the river was dredged down to the initial 

 level of the sand, and this level was maintained by daily dredging till 

 70,000 tides had been run, 0-08 cubic foot of sand in all being removed. 



At this stage the sand in the river suddenly shifted up to the top as in 

 Experiments IV. and V., E. The sand at the mouth of the river and top 

 of the estuary falling until the bottom appeared, dredging was discon- 

 tinued. At 95,000 tides the final condition had been reached, which was 

 almost identical over the whole estuary with that of Experiment V. E 

 after 60,000 tides, as shown in Plan 3, Experiment V., E and F. 



The instability of the condition which may prevail at the mouth of a 

 river is thus clearly shown, as well as the useful effect of improving the 

 tideway by dredging in the upper reaches in the river. In three 

 experiments in tank F, I., III., and IV., the river became completely 

 barred, and the estuary became a bay with a stream of land water 

 entering at its top ; in Experiment V. the bar again formed, but on being 

 kept down by di-edging to the level of half-tide till the sand had fallen at 

 the head of the estuary, the river at length prevailed, and the sand was 

 washed out till there was 30 feet of water at low tide. 



The time and amount of sand removed in producing this effect were 

 considerable. The tidal capacity of the river and estuary is 1 cubic foot ; 

 this reduced to a 30-foot tide is 21,700 million cubic yards, or on a 15- 

 foot tide is 5,422 million The amount of dredging, O'OS cubic foot in 



