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STUDIES FOR STUDENTS 



the bottom would be cleaned out, part of the detritus swept to the 

 upper tidal limit and deposited, while a larger and coarser part aided 

 by the downward grade would be swept seaward by the ebb-tide. 



In nature the land slope is never smooth, and the discharge of 

 the ebb-tide is concentrated along those lines which give the quickest 

 egress and the greatest depth of water, the result being the building 

 up of tidal marshes in protected places, the scouring of tidal chan- 

 nels, and the formation of extensive fiats chiefly below the level of 

 low tide. In these ways an originally widely extended littoral zone 

 would be narrowed to a certain stable width for a given height of 

 tide. The above discussion neglects the action of waves, chiefly 

 operative at the upper and lower tidal limits, in exposed situations. 



Fig. 7. — Diagram to illustrate relation of littoral zone and tidal scour to height of 



tide. 



2. The influence of varying height of tide may be seen in Fig. 7. 

 Letting AB and EF be the respective upper limits of tides on a 

 recently invaded shelving shore: the volumes of water flowing past 

 the low- tide limit will be as the areas of the two triangles OAB and 

 OEF. But these being similar, the ratio of the areas will be given 

 by the ratio of the squares of similar sides. Consequently, the vol- 

 ume, and with it the mean velocity, of the water invading the shore 

 varies at the line OE as the square of the height of the tide, OA or 

 OE. But Revy has shown that as the depth of a current increases 

 the bottom velocity approaches more nearly the mean velocity, until 

 in great depths and in strong surface currents they are substantially 

 alike. ^ Therefore on an ideal newly formed shore the bottom velocity 

 past the point of low tide will increase somewhat faster than the square 

 of the height of the variable tidal rise. 



I Op. cit., p. 148. 



