40 



increase in water depth would in fact decrease the difference between the 

 tide at the closed end and that at the mouth. 



Consider first the case of an estuary of mean water depth, h = 15 m. 

 Given an estuary length, S. = 108 km, from Eq. 4.2 R = 3.7, for a 

 semi-diurnal tide. Now if h is increased, for example, by 2 m, R is 

 reduced to 2.60 (assuming no change in the estuary length). Further 

 suppose that as a result of the 2 m sea level rise, the tidal range at the 

 mouth increases by 10%, say from 1 m to 1.10 m. Then, by virtue of 

 Eq. 4.2, the range at the closed end will decrease . from 3.2 m to 2.9 m. 



A bay- like water body connected to the sea via an entrance will 

 experience range amplification as the frequency of tidal forcing approaches 

 the natural period of oscillation of the water body. The situation is 

 analogous to the response of a damped harmonic oscillation (Mehta and 

 Ozsoy, 1978). In a number of bays along the U.S. coastlines, for example, 

 the tidal range in the bay is greater than that outside (O'Brien and Clark, 

 1974) . Amplification becomes most pronounced when the forced and natural 

 frequencies are equal. If therefore an increase in water depth due to sea 

 level rise were such as to shift bay response away from resonance, the 

 tidal range relative to that at the mouth could, as illustrated previously, 

 decrease in spite of the opposing trend caused by decreasing bottom 

 friction and increasing tidal admittance with increasing water depth. In a 

 great many inlet/bay systems, however, bottom friction in the inlet channel 

 controls the bay tide; hence in these cases resonance is not a critical 

 factor. 



4.3.2 Superelevation Effect 



In most bays, the tidal mean water level is usually different, often 

 higher than mean sea level. The difference, referred to as bay 

 superelevation, results from a number of physical factors. Mehta and 

 Philip (1986) reviewed these factors, and the physical mechanisms by which 

 they generate superelevation. Representative maximum superelevations 

 corresponding to each cause, as might be found from measurements, were 

 suggested; Table 4.1 gives a summary of the findings. Among the listed 

 causes, sea level rise will directly or indirectly influence inlet/bay 

 geometry, sea tide, salinity, wave penetration and some other factors. 



