470 



Tides in Estuaries 



America the phenomenon in the estuary of the Amazon River has gained 

 great fame; it is called there Pororoca. This Pororoca originates in the mouth 

 of river and in many of the numerous tributaries and canals, where it makes 

 the Canal do Norte sometimes absolutely unnavigable. For a good descrip- 

 tion of a Pororoca see Krummel (1911, vol. II, p. 301). Large bores occur 

 in the estuary of the Narbada River into the Gulf of Barygaza (Cambay), 

 in the Hugli near Calcutta, in the Menga, in numerous rivers of the East 

 Indian Islands. Particularly famous is the bore in the Chien-tang-kiang, which 

 was studied by Admiral Moore (1888, 1893). The bore in the Amazon 

 River is perhaps the greatest one, and it frequently attains heights of 

 25 ft (8 m). Seen from the high dykes, it has the appearance of a several- 

 mile long waterfall which travels upstream with velocities of 12-13 knots 

 (6-5 m/sec) and produces a roar which is audible at a distance of 22 km 

 Figure 198 gives a picture of a bore on the Chien-tang-kiang which we owe 

 to Prof. G. Schott. 



The principal characteristic of a bore is the quick rise in level. This wall 

 of water, sometimes a straight wall, sometimes curved concave, rushes up- 

 stream; the water in the back overtakes the water at the front side of the 

 wave. The height of the bore varies a great deal: we have already mentioned 

 that it attains 8 m (25 ft) in the Chien-tang-kiang, in the Amazonas and the 

 Ganges it attains 5-6 m, in the Trent 1 \-2 m, in the French rivers it remains 



Arrival of Bore 

 I2 h 29 m 50 s 



Rapid rise 



29 31 33 35 m 40 r 



Neap tide 



45 r 



Fig. 199. Longitudinal profile of a bore in the vicinity of Moneton on the Petit-codiac 

 River. 6 August 1898 (5th tide after spring tide). 



normally below 1 m. The bore occurs at the time of the syzygies (spring 

 tides), but there are also cases in which it occurs at every tide (as for instance 

 in the Chien-tang-kiang). According to Comoy, the formation of a bore 

 seems to be dependent on the fact that the capacity of the channel is too 

 small to take care of the extra influx of water. If a bank is located at the 

 mouth of the river (as for the Seine), the tide enters only slowly, the banks, 

 which were dry at ebb, become covered with water and a large water-mass 



