38 



Bight (southern North Sea) suggest this type of a trend, as will be noted 

 later. 



Within the estuary itself, increasing the water depth can have a 

 drastic effect on the tidal range. The majority of present day estuaries 

 are of holocene origin, having been formed since the last ice age and 

 accompanying sea level rise. In some, sea level rise has caused the depths 

 to increase while in others, sedimentation rates have been high enough for 

 the depths to have "kept pace" with sea level rise. In a few cases, for 

 example, some estuaries in China (e.g., Qitang) , sedimentation rates have 

 essentially exerted an overwhelming control, causing the depths to decrease 

 in spite of sea level rise, and thereby pushing the mouth seaward. 



While increased water depth would generally increase the estuarine 

 tidal range, the opposite effect could occur, for example, in cases where 

 tidal resonance is a significant factor. This can be illustrated in a 

 simple way by considering the case of a tidal wave entering a frictionless 

 channel closed at the upstream end. In this case, considering complete 

 wave reflection at the closed end, the incident and reflected progressive 

 waves combine to form a standing wave, as shown in Fig. 4.1. The estuary 



is of length i, with the closed end at x = and the mouth at x i. If 



the range of the progressive wave is H, the range of the standing wave at 

 the closed end will be 2H. The standing wave envelope is thus defined by 

 an antinode at the closed end and a node in the sea. It can be shown 

 (Ippen and Harleman, 1966) that the ratio, R, of the amplitude, fjom- ^^ ^^^ 

 closed end to the amplitude, »7-^iji' ^^ ^^^ mouth will be (ignoring bottom 

 friction) 



R = £onL = 1 (4.2) 



^-M I cos (^) I 



Since | cos(27ri/L) | < 1, in general, the tide at the closed end will be 

 higher than that at the mouth. This type of a resonance effect is well 

 known, and occurs in such estuaries at the Bay of Fundy, Canada, and at 

 Cambay in India. Given such a behavior, a situation can arise whereby an 



