Tides in Estuaries 



459 



greater than the low-water values. According to Comoy, the velocity of the 

 foot of the wave in the French rivers seems to correspond to the formula 

 v = | (gh) — U, in which h is the depth at low water and U the velocity of 

 the river water (head water) downstream. 



Table 74. Velocity of the crest (high water) and of the 

 trough (low water) of the flood wave in rivers 



The data in Table 74 apply for normal tide waves. At spring and neap tide conditions can be 

 different; especially at neap tide the foot of the wave seems to travel faster upstream, in the lower 

 section of the tide zone, than at spring tide. Thus, this velocity was, on 26 September 1875, in the 

 Gironde between Pointe de Grave and La Marechale, 17-95 m against only 5-72 at spring tide on 

 19 September. However, upstream the difference soon became smaller and the velocity at neap tide 

 was already somewhat smaller than at spring tide (near Bordeaux 4-51 against 4-85). The cause 

 of this remarkable difference is that at neap tide the low water is not so well developed as at spring 

 tide and that the water depth is greater at neap tide than at spring tide. However, v adjusts itself ac- 

 cording to /;. The range of the river tides in most cases first increases somewhat up stream. Then, 

 from a certain point, it decreases again slowly to the tide mark; see as an example Table 75. 



Another characteristic feature of the tide zone is that the peak of the tide 

 wave along the tide zone (the high-water mark) maintains almost the same 

 absolute height from the ocean to the tide mark or rises slightly only in the 

 upper section of the tide zone. Irregularities are due to local widenings or 

 narrowings of the cross-section of the river. Table 75 shows, after Comoy, 

 the elevations of the wave crest above normal datum (NN) of the French 

 general levelling for the various points along the Gironde and the Garonne 



