250 HYDROGRAPHICAL SURVEYING [chap. ix. 



This consists in the face of the rising tidal wave becoming 

 so steep that it rushes up the estuary in the shape of a sudden 

 wave, sometimes almost a wall of water, breaking as it advances, 

 and the tide thus rises many feet in a few seconds, followed by a 

 still rapid, but more gradual, further rise, so that the whole 

 flood may only last one or two hours, the ebb prevailing for 

 nine or ten hours. 



The main factor in the production of a bore is the retardation 

 of the lower part of the inflowing water by the friction of the 

 bottom in shallow water, and by the action of the down-flowing 

 river, so that a high tide rises quicker than it can flow forward, 

 until its height and momentum enable it to overcome these 

 obstacles by a final grand rush. 



Bores are rare, but whenever encountered the surveyor 

 should investigate the conditions, as but little is known of the 

 details of most of them. 



