Tides in Estuaries 



471 



gets into the river. As the foot of the wave progresses only slowly and the 

 water in front of it rises very rapidly, the bore soon develops. Head winds 

 can increase the stowing of the water in front of the foot of the wave. If the 

 bank is located in front of the entrance, then, according to Comoy, the 

 opening is narrow and the cross-section of the river increases upstream. 

 In this case, no bore appears, as for instance like in the Loire. 



Figure 199 gives an example of a longitudinal profile of a bore, which 

 has been taken from a survey made by Bell Dawson, on the Petit-codiac 

 River in Moneton. It shows the bore of 6 August 1898 (fifth tide after spring 

 tide, full moon on 2 August). The wave travelled with a velocity of 14 4 km/h 

 upstream and the picture shows how the water level changed until 23 min 

 after the passage of the bore. If we change the succession in time into a suc- 

 cession of places, the picture represents from the start of the bore on the 

 left to the right edge a distance of 5i km. The heights are indicated at the 

 right in feet. Champion has obtained interesting automatic registrations of 

 the bore in Flixborough on the River Trent. Figure 200 gives these regis- 



20 



30 



40 



50 



60 



70 



Fig. 200. Automatic records of a bore at Flixborough 13-18 September 1931 (Champion 



and Corkan). 



trations of the beginning of the bore for 6 successive days; they clearly show 

 the difference of the bore at spring and neap tide. At spring tide the passage 

 is sharp and within about 10-20 sec the water surface rises nearly \ ft, followed 

 by waves of about 1 min period. At neap tide the bore is hardly noticeable 

 and the rise goes on continuously for 20 min and more. The thorough in- 

 vestigations made on the River Trent have also revealed the variations of 

 the profile of the water surface along the entire river during a whole tidal 

 period and have thus shown the propagation of the bore form. Figure 201 

 gives these profiles at spring tide for eleven successive intervals. It appears 

 that the bore starts to develop in the River Trent and then runs up to 

 Torksey with nearly uniform velocity and increasing intensity. Between 

 Walkerith and Torksey the bore seems to disintegrate. Unfortunately, no 

 observations are available for this interesting area. 



It is easy to see that the increasing asymmetry in the wave profile of the 

 river tides must finally lead to the development of a bore. The front slope 



