462 



Tides in Estuaries 



holds only when the head water is equally large at spring and neap tides. 

 When the head water increases at spring tide, this point of intersection moves 

 downstream ; if the head water decreases, the point moves upstream. In case 

 of great drought and correspondingly little head water, the surface level lines 

 for neap and spring tide do not intersect at all. 



Table 11. Tides in Elbe and Weser * 



* According to L. Franzius. The heights for the Elbe refer to "Cuxhaven Zero". The heights 

 for the Weser refer to a plane which is 5 m below "Bremen Zero". 



Whereas for localities on ocean shores the mean sea level (average value 

 of the ordinates of the tide curve) has a constant value and varies only very 

 little locally, the line connecting the mean water levels for the localities along 

 the tide zone is a curve varying very much from one locality to another; the 

 mean water at spring tide lying definitely higher than that at neap tide. 

 A decided slope downstream develops only in the upper part of the tide 

 zone (see Table 75). 



Particularly remarkable in the river tides are the tidal currents within the 

 tide zone. There, the flow of the river will be reversed for several hours, 

 which is the most striking feature to the "islander". We can take as a simple 

 example, the Elbe near Cuxhaven, as shown in Fig. 195. The flood current 

 starts about 1 h 30 min after low water, and the ebb current about 1 h 25 min 

 after high water. The currents alternate after (and not at) the water has 

 reached its maximum and minimum heights. The lag after low water seems 

 to maintain itself up stream approximately with the same amount. Accord- 

 ing to Franzius, this lag is in the Elbe near Cuxhaven 1 h 20 min, near 

 Brunshausen 20 min, near Nienstedten equally 20 min, near Hamburg 25 min. 

 According to Comoy, the lag of the ebb current after low water is of the same 



