Tides in Estuaries 



463 



order of magnitude in the French rivers (Charente near Taillebourg 30 min, 

 12 km upstream near Saintes 40 min, Ardour near Urt 25 min, 13 4 km up- 

 stream near Lannes 30 min). It seems that upstream the head water feeds 

 the ebb current for a longer period of time than could be expected from the 

 wave motion alone, probably because of its gradient. 



Fig. 195. Tide curve and tidal current at Cuxhaven. 



The flood current, on the contrary, alternates more close to high water 

 the more we get upstream; thus, near Cuxhaven it is 1 h 30 min after high 

 water, near Brunshausen and Nienstedten 30 min, near Hamburg 12 min 

 after high water. Table 78, gives the velocities of the tidal current of the 

 Elbe and Weser. The flood current, near the entrance, is generally stronger 

 than the ebb current. In the Elbe the slope of the upper stream starts to 

 make itself felt close to Hamburg; in the Weser no flood stream is noticeable 

 above Vegesack, only a stowing of the water by the tidal wave from 1 to 2 min ; 

 to the contrary, the ebb current has a velocity up to 0-7 m/sec. 



The velocity of propagation of the high water (crest of the flood wave) upstream is for the lower 

 Elbe, according to Table 73 of the order of magnitude of 6 m/sec; the velocity of the flood current, 

 according to Table 78 does not quite reach one-tenth thereof. If c is the mean velocity of the 

 flood current, V the mean velocity with which slack water at the end of the flood current progresses 

 up stream (average velocity of propagation of the flood wave) and T the duration of the flood, 

 then T V is the length of the wave crest. According to Comoy, the water travels upstream during 

 time (T V)l(V-c), so that the distance covered during that time is S = (TVc)l(V-c). This equa- 

 tion has been used to compute how far saline sea water will penetrate up the river. For the Gironde 

 we have T — 22,200 sec, V= 12 and c = 1-9 m/sec, so that S = 50-2 km. The observations showed 

 that sea- water was still present near Pouillac, 51 km from the mouth. For the Elbe near Cuxhaven 



