CHAPTER VII 



The flood- wave of rivers The roll- wave in the Tees In the 

 Ure In the Nikko torrent On the advance of a flood over 

 dry ground. 



On the Flood-waves of Rivers 



A PROGRESSIVE wave is not an obvious phenome- 

 non in ordinary river floods ; nevertheless, a 

 careful consideration of the matter shows that 

 wave-transmission is an important factor in the 

 disposal of the flood-water of a river. Let us take 

 the case of a river receiving an accession of 

 water from a suddenly swollen tributary. The 

 threatened congestion of water is avoided by 

 wave -transmission down the main stream. The 

 length of the wedge of newly added water is suffi- 

 cient to originate 'a " long " wave, and the wave- 

 velocity down-stream is therefore the greatest that 

 is compatible with the depth, viz., the velocity 

 which a body would acquire if falling in vacua, 

 under the action of gravity, through a height equal 

 to one-half the depth of the stream. If the calcu- 



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