42 



Since these in turn influence the mean bay level, in the evaluation, for 

 instance, of the change in tidal range due to sea level change, the 

 associated change in superelevation must be additionally considered in 

 calculating the net water change. 



Mann (1987) examined the superelevation effect resulting from 

 inlet/bay response to tidal forcing. Tide -averaged hydrodynamic equations 

 were developed and it was shown that bottom friction in the inlet channel 

 is the primary cause of superelevation. Stokes drift, tidal current 

 asymmetry and river runoff were identified (in the absence of such effects 

 as those arising from salinity, wind waves, etc.) as the major governing 

 physical processes. Mann considered the case of a small, deep bay 

 connected to the sea via a long inlet channel. The combined effects of 

 tide and superelevation resulting from sea level rise were evaluated, as 

 will be noted in the next section. 



4 . 4 EXAMPLES 



Fiihrboer and Jensen (1985) evaluated long-term sea level trends at ten 

 gages in the German Bight. The evaluation was based on records obtained 

 over a 100-year period from 1884 to 1983. Trends relevant to the present 

 purpose may be illustrated by considering four gages, at Norderney, List, 

 Cuxhaven and Bremerhaven, shown in Fig. 4.2. Of these, the gages at 

 Norderney and List may be considered as "open coast" gages, while 

 Bremerhaven is decidedly up estuary (Weser) . Cuxhaven is at the mouth of 

 estuary (Elbe). Table 4.2 gives relevant results. 



The rate of change of mean tidal range has been calculated in two 

 different ways for each location. The first is the average rate based on 

 the entire 100 -year period (no values were computed for Norderney and List 

 due to insufficient data) . The second is based on the last 25 -year (= N) 

 record, converted to an equivalent 100 -year rate. Comparing Norderney and 

 List to Bremerhaven, it is observed that the tidal rise (N = 25) has been 

 far more significant (threefold) within the estuary than on the open coast. 

 The rise at the estuary mouth is intermediate in magnitude. One likely 

 reason is the effect of reduced bottom friction due to sea level rise. 

 This effect is more pronounced in the shallow estuary than in the deeper 

 sea. 



