45 



It is also interesting to observe from Table 4.2 (for Bremerhaven and 

 Cuxhaven) that the increase in tidal range has been considerably more 

 significant in recent years (N = 25) than what is obtained based on a 

 100-year record (N = 100). At Bremerhaven, the mean tidal range 100 years 

 ago was -3.30 m. Thus the range increased there by -9% during the 

 subsequent 75 years. During the next 25 years the range increased again by 

 about the same percentage . 



Fuhrboter and Jensen noted a trend of rising tidal range approximately 

 over the past century at all ten locations examined. They concluded that 

 this trend is not due to any long-term changes in meteorological 

 conditions, but is possibly due to the morphology of the North Sea, a very 

 shallow water body in which the global rise of the mean water level effect 

 is amplified via a standing wave effect. This possibly suggests a 

 situation in which the natural frequency of the water body approaches the 

 tidal forcing frequency with increasing water depth and changing 

 boundaries . 



Mann (1987) theoretically simulated the response of inlet/bay systems 

 of assumed geometries to a total sea level rise of 1.3 m, corresponding to 

 a 0.3 m rise over the past century and a 1.0 m projected rise. The bay was 

 assumed to be relatively small and deep, with a surface area of 5 x 10 m . 

 The inlet channel was 1,800 m long and 150 m wide. It is illustrative to 

 consider here the case of an initially 1.5 m deep channel. For this 

 shallow system, the ratio of the (semi-diurnal) tidal frequency to the 

 natural frequency is 0.16, which is «1 , thus signifying a 

 friction- dominated (as opposed to resonance -dominated) system. 



In Fig. 4.3, the resulting changes in the mean bay level and bay tidal 

 range are shown. A 1.3 m rise in sea level decreased bay superelevation 

 (head above mean sea level), from 0.27 m to 0.11 m. On the other hand, 

 reduced friction resulted in an increased tidal range. Initially, the high 

 water (HW) and low water (LW) amplitudes of tide relative to mean bay water 

 level were 0.28 m and 0.25 m, respectively. The tidal range was thus 

 0.53 m. After a 1.3 m sea level rise, the amplitudes became 0.66 m and 

 0.56 m (i.e., range 1.22 m) . 



These data on the effect of sea level rise enable the determination of 

 the high water level within the bay initially, and following sea level 



